The Marine Life Information Network

Global warming (extreme)

Extreme emission scenario (by the end of this century 2081-2100) benchmark of:

• A 5°C rise in SST and NBT (coastal to the shelf seas),

• A 6°C rise in surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 5°C rise in surface air temperature in intertidal habitats exclusive to Scotland (Global warming pressure definitions).

Evidence

Alaria esculenta

The distribution of seaweeds is climatically defined (Kain, 1979, Van Den Hoek, 1982, Breeman, 1990, Lüning, 1990). Northern boundaries are set by winter temperatures that are lethal or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which makes them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020).

Alaria esculenta is a polar to cold-temperate species that has been recorded from Brittany, France to Northern Norway (Birkett et al., 1998b). Sea temperature regulates metabolism and reproduction and defines the regional distribution of Alaria esculenta (Fredersdorf et al., 2009). The southern limit of Alaria esculenta has been defined at the 20°C isotherm (Munda & Lüning, 1977; Fredersdorf et al., 2009). However, it is more common north of the 16°C isotherm (Munda & Lüning, 1977; Van Den Hoek, 1982). As a result of this upper temperature threshold, Alaria esculenta is largely absent from the southern North Sea and English Channel where summer temperatures exceed 16°C and has already shown signs of contracting its range in the UK as a result of ocean warming (Mieszkowska, 2016).

Munda & Lüning (1977) observed that temperatures of 16 to 17°C, sustained over 2 weeks in Helgoland, Germany, were lethal to resident Alaria esculenta. Experimental observations showed that acute exposure to ≥21°C is lethal to Alaria esculenta causing bleaching and disintegration (Sundene, 1962; Fredersdorf et al., 2009). At its northern range edge (Svalbard), it is a prominent macroalga on sublittoral fringe bedrock.  At these northern latitudes, average summer temperature can reach 5°C, with an average annual sea temperature of 3°C (1980-2014, Beszczynska-Möller & Dye, 2013). Experimental observations conducted by Fredersdorf et al. (2009) found the optimal temperature for sporophyte photosynthesis was within the range of 13 to 17°C but the optimal temperature for Alaria esculenta germination was 2 to 12°C (Fredersdorf et al., 2009).

Alaria esculenta is nearing its southern limits in the UK, with this species only occurring as far south as Brittany, France. Sea surface temperatures (SST) around the UK currently fall between 6 to 19°C (Huthnance, 2010). The available evidence suggests that the effects of ocean warming may occur throughout the year.  If winter temperatures exceed 12°C this is likely to inhibit germination, whilst summer temperatures greater than 20°C will cause mortality of Alaria esculenta.

Yesson et al. (2015) reported no significant change in Alaria esculenta abundance in the UK from 1974 to 2010, and no relationship between abundance and summer and winter sea surface temperatures. Despite this, they noted the contrast between their findings and those of previous research that reported declines in Alaria esculenta around the UK and Ireland, and suggested that if temperatures increase in the future, then abundances could change.

Laminaria digitata

The distribution of kelp is strongly influenced by climatic conditions; therefore, kelp species are extremely sensitive to the ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, while southern limits are set by high lethal summer temperatures or winter temperatures too high for the induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which make them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Laminaria digitata is distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight & Thompson, 2008). Its distribution suggests that the species would tolerate chronic temperature change (i.e. by 2°C for a year). However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. On French coasts, Laminaria digitata is at the southern limit of its range where sea surface temperatures are closer to the upper thermal limit for this species. These populations are known as ‘trailing-edge’ populations, where the species’ geographic ranges are contracting due to ocean warming (Merzouk & Johnson, 2011). Trailing-edge kelp populations are known to be more sensitive to temperature increases than range-centre populations because they are already living close to or at the limit of their thermal tolerance (Liesner et al., 2020; Hereward et al., 2020; Smale, 2020; Leathers et al., 2024).

The thermal optimum of Laminaria digitata is between 5 and 15°C (Dieck, 1992; Arzel, 1998; Franke et al., 2021; OBIS, 2026), while cell damage and mortality have been reported to occur at 22 to 24°C (Sundene, 1964; Lüning, 1980; Bolton & Lüning, 1982; Dieck, 1992). Reproductive ability has been reported to decline to 20% at 18°C (Arzel, 1998). Spore production only occurs between 5 and 10°C and is the most temperature-sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the long-term. A minimum of 10 weeks a year between 5 and 18°C is needed to ensure spore formation and hence reproduction (Bartsch et al., 2013).

Optimum temperatures for Laminaria digitata may differ between life stages. Martins et al. (2017) found that gametophyte growth was optimal at 10 to 18°C under long-day photoperiods. Gametogenesis occurred in temperatures ranging from 5 to 15°C, and was fastest between 10 and 15°C. Sporophyte recruitment was highest at 5°C. Maximum sporophyte development occurred only when long photoperiods were combined with nutrient‑enriched conditions reflecting natural late‑spring environments. Martins et al. (2017) suggested that warming summer conditions are likely to promote vegetative gametophyte growth but suppress gametogenesis, delaying recruitment until cooler autumn–spring periods. This shift could alter annual recruitment patterns and reduce the formation of juvenile sporophytes during increasingly stressful warm summers.

Temperature changes at the benchmark level may affect reproductive success in Laminaria digitata. In the Arctic, predicted future summer sea surface temperatures (9°C) may reduce reproductive success and sporophyte recruitment in Laminaria digitata compared to current (5°C) conditions (Silva et al., 2022). Laminaria digitata gametophytes in this region survive sufficiently (32%) in 2°C in complete darkness over the winter and then become fertile in the spring under moderate light and temperature (5°C) conditions. However, under high light conditions (100 μmol photons/m²/s), reproduction and sporophyte recruitment were reduced. This suggests that under global warming scenarios, where temperatures are higher and light availability may be higher due to reduced ice cover, Laminaria digitata are less likely to successfully reproduce. Although UK waters have no ice cover, this combined effect of temperature and light availability could still be relevant in clearer waters.

Gauci et al. (2022) examined whether the thermal history of kelp gametophytes influences reproductive success and juvenile performance in Laminaria digitata. Clonal gametophytes were maintained for three years under either cold (5°C) or warm (15°C) conditions before being induced to reproduce at 10°C. Gametophytes primed at 5°C completed gametogenesis more rapidly and produced significantly more sporophytes (approx. 278 cm²) than those primed at 15°C (approx. 194 cm²). Juvenile sporophytes from cold-primed parents also exhibited greater thermal tolerance, growing faster at both low (0°C) and high (20°C) extremes compared to warm-primed sporophytes, which ceased growth at 20°C after two weeks. Photosynthetic efficiency declined at 20°C for all sporophytes, indicating heat stress. These findings suggest that elevated temperatures may reduce reproductive success.

Trautmann et al. (2024) also found that elevated temperatures are especially stressful for Laminaria digitata under low light conditions. They investigated the effects of Arctic winter warming by exposing excised discs from sporophytes to three months of darkness at 0°C (current mean) and 5°C (future scenario). Photosynthetic efficiency indicated good health throughout the experiment at both temperatures, although values were still significantly lower in the warmer treatment. Carbohydrate reserves showed marked depletion under warming: mannitol decreased by approx. 37% at 0°C but by approx. 65% at 5°C, while laminarin dropped by approx. 40% and approx. 90%, respectively. Pigments also declined more strongly at 5°C, particularly xanthophyll cycle pigments, and C:N ratios fell from approx. 20 to 13.4 at 5 °C versus 17.9 at 0°C. These results suggest that Laminaria digitata tolerates prolonged darkness well, but experiences accelerated metabolic demand and resource depletion under warmer winter conditions. While Arctic conditions differ to those experienced by this Laminaria digitata biotope around the UK, this evidence still suggests that temperatures at 5°C above the usual could increase energy depletion in the kelp, especially under light-limited conditions.

Temperatures experienced by Laminaria digitata in the low intertidal zone (the upper depth range limit of this biotope) are highly weather‑dependent and often differ substantially from subtidal seawater conditions. During low tide, fully emersed individuals are exposed directly to ambient air temperatures that depend on weather, which varies on short- and long-term temporal scales. Emersed marine organisms may experience desiccation stress, which could potentially worsen thermal stress. For Laminaria digitata populations in tide‑pools, solar radiation can heat shallow water and cause evaporation, further elevating temperature, whereas overcast conditions, wind, and precipitation can cause cooling. Additional factors such as substratum heat retention, shading, tidal timing, and diurnal cycling may also act as stressors. Together, these processes create highly variable and sometimes extreme thermal environments for kelps in the low intertidal zone.

King et al. (2018) investigated stress physiology of Laminaria digitata under simulated low tide exposures to temperature changes by using 1-hour exposures for four consecutive days. The experimental design included a control temperature of 15°C, with immersion treatments at 24, 28, and 32°C to simulate summer tidal pool conditions, and at 0 and 4°C to represent winter tide pools. Emersion treatments were conducted at 24, 28, and 32°C to simulate summer atmospheric heatwaves, and at –5°C to replicate severe winter frost. Stress was quantified by photosynthetic efficiency (F_V/F_M) and heat shock protein (Hsp70) expression. Measuring Hsp70 expression showed that the maximum water temperature tolerable by Laminaria digitata under laboratory conditions was 24°C, and 28°C was the point at which proteins can no longer be synthesised to defend the kelp against heat stress. Laminaria digitata exhibited progressive declines in photosynthetic efficiency under consecutive immersed heat shocks, with resilience eroding after the third exposure at 24°C and complete mortality by the fourth exposure at 28°C. All excised discs of Laminaria digitata tissues became non-viable after two exposures to 32°C. In the emersion experiment, Laminaria digitata showed greater recovery from exposures at 24°C and 28°C, though recovery was incomplete after repeated shocks. In the 32°C treatment, photosynthetic efficiency continued to decline over the three days following the last heat exposure. At -5°C, Laminaria digitata regained near-control levels of photosynthetic efficiency after 72 hours. Water content data from the emersion treatments revealed that Laminaria digitata showed more water loss with each repeated exposure, but water content consistently returned to normal within 24 hours of each exposure. Overall, this study showed that cumulative stress, rather than single exposures, critically determines thermal limits. Laminaria digitata was shown to be highly vulnerable to repeated elevated air temperatures and is comparatively resistant and resilient to repeated exposures to extreme cold air temperatures.

As shown by King et al. (2018), air temperatures during low tide emersion have a significant effect on the health of Laminaria digitata. During emersion, Laminaria digitata experience desiccation, which could potentially exacerbate the stress effects of elevated air temperatures during atmospheric heatwaves. Hereward et al. (2020) examined the effects of realistic atmospheric heatwave scenarios on Laminaria digitata populations at their trailing-range edge in southwest England. The study simulated consecutive low tide emersion events in spring (March) and autumn (October), exposing kelp tissue discs to air temperatures elevated by +7.5°C, +11.5°C, and +15.5°C above seasonal ambient controls (March: 10.5°C, October: 14.5°C). Each heatwave simulation involved daily 1-hour emersion exposures for four consecutive days, followed by a three-day recovery period in seawater. In spring, photosynthetic efficiency (F_V/F_M) remained close to control values in the +7.5°C and +11.5°C treatments and declined after exposure to +15.5°C followed by a full recovery, indicating resilience to short-term heat stress. Tissue bleaching was moderate (up to approx. 50% at +15.5°C), and although relative water content (RWC) declined during emersion, partial recovery occurred. In autumn, however, all warming treatments caused severe stress. Photosynthetic efficiency declined sharply after day three and reached zero by the end of the experiment, indicating non-viable tissue. Bleaching was extensive (up to 100% at +15.5°C), and RWC dropped significantly with little recovery. Field surveys corroborated these findings, showing minimal bleaching in spring and summer but high prevalence in autumn and early winter. The authors concluded that Laminaria digitata at its trailing-edge persists close to its physiological limits for emersion stress in autumn, making populations highly vulnerable to increasingly frequent and intense atmospheric heatwaves under projected climate change scenarios.

Filbee-Dexter et al. (2016) showed that mean kelp biomass (Laminaria digitata and Saccharina latissima) in Nova Scotia had declined by 85 to 99% from 1949 to 2014. In addition, the percentage cover of these kelps had declined by 89% in the seven sites that were surveyed spanning 110 km of coastline. Over this period, summer sea temperatures had increased on average by 1.58°C, and the number of days in which temperatures exceed the thermal thresholds for kelp tissue weakening (>14°C) and mortality (>18°C for two weeks or 20°C for ≥ one week) had also increased. In addition, exceptionally high mean and maximum temperatures were recorded between 2010 and 2012. However, Krumhansl et al. (2024) later reported an overall increase in Laminaria digitata and Saccharina latissima cover in Nova Scotia from 2000 to 2022. They suggested that Laminaria digitata was able to recover faster than other kelps which were lost during the 2010 to 2012 warm period, and that Laminaria digitata is now the dominant kelp. In Narragansett Bay, Rhode Island, only one Laminaria digitata individual was observed across six surveys which were conducted seasonally from December 2017 to November 2018 (Feehan et al., 2019). Baseline surveys from 1980 to 1981 showed that Laminaria digitata was one of the kelp species which dominated these rocky reefs. Feehan et al.’s surveys showed that 91 to 94% of the reefs were covered in turf-forming algae by 2018 and that kelp biomass was close to zero. This decline was attributed to a 0.3°C increase in sea surface temperatures each decade and an increase in the number of weeks where sea surface temperatures exceeded 22°C.

Kelp forests, including populations of Laminaria digitata, across the coastline of New England, USA, have experienced population shifts since the start of the 21st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled the effects of temperature change and sea urchin density on kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south. Maximum summer near-surface seawater temperatures in southern Maine commonly exceeded 20°C and were on average approx. 5.6°C warmer than those observed in northeast Maine. Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024), and temperatures (20°C) at which Saccharina latissima erodes faster than it grows (Lee & Brinkhuis, 1986 cited in Suskiewicz et al., 2024). In addition, reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study period (Suskiewicz et al., 2024). Although forests (Laminaria digitata and Saccharina latissima) had broadly returned to Maine in the late 20th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest. Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area unsuitable for kelp (Suskiewicz et al., 2024). 

Around the British Isles, sea surface temperatures have increased by 0.7°C from 1974 to 2010 (Yesson et al., 2015b). Long-term surveys across this period revealed mixed trends for Laminaria digitata: 64% of sites showed increases in abundance, 34% showed declines, and 2% remained stable. Regional patterns indicated declines in southern areas (English Channel, Celtic Sea) and increases in northern regions, particularly the northern North Sea. Abundance correlated positively with summer sea surface temperature but negatively with winter temperature, suggesting warmer summers may enhance growth while milder winters could disrupt cold-adapted reproductive stages. Therefore, increasing sea surface temperatures may drive regional shifts rather than an overall decline, with northern populations increasing or remaining stable while southern edge populations face greater vulnerability. Combining predicted sea surface temperatures over the next century with the current distribution of Laminaria digitata, Merzouk & Johnson (2011) predict an expansion of its northern limits and localised extinctions across its southern range edge (Mid Bay of Biscay, Northern France and southern England; Birkett et al., 1998). Under RCP 4.5 and RCP 8.5 climate change scenarios, Laminaria digitata in the Northwest Atlantic will also experience a range contraction due to maximum summer sea surface temperatures exceeding 21.5°C, and that their poleward expansion is limited by geography (Khan et al., 2018). Similarly, Hill et al. (2025) reported that even under the least extreme climate change scenario, Laminaria digitata may lose 3.9% of its range by 2100 in the East Atlantic and 28.3% of its range in the West Atlantic. Under the most extreme scenario, losses of 15.3 and 94.7% are expected in the East and West Atlantic, respectively.

Sensitivity Assessment

Alaria esculenta is competitively inferior to Laminaria digitata in the northeast Atlantic (Hawkins & Harkin, 1985) where it is confined to more exposed shores and can colonize sheltered environments only if Laminaria digitata is removed. As temperatures rise to up to 20°C, it is expected that the competitive ability of this species will decrease further, while as temperatures rise above 20°C both Alaria esculenta and Laminaria digitata will be lost, with more, warm adapted, algae taking their place. Brodie et al. (2014) predict that Alaria esculenta will be completely lost from Boreal habitats by the end of this century, whilst all kelp species will be lost from the Lusitanian region, replaced by smaller, fleshy algae.

Sea surface temperatures around the UK are currently between 6 to 19°C (Huthnance, 2010). Under the middle emission scenario, Alaria esculenta is likely to be lost from most of the UK, although it may manage to maintain some populations in Scotland, where current summer temperatures often reach 14°C, leading to potential summer temperatures of 17°C. Laminaria digitata is likely to be lost in southern parts of the UK, where summer temperatures will exceed 20°C, but will likely remain present around the rest of the UK. Resistance is assessed as ‘Low’ and resilience as ‘Very Low’, as the loss is likely to be a long-term decline due to the long-term nature of ocean warming. Therefore, this biotope is assessed as having ‘High’ sensitivity to ocean warming under a middle emission scenario.

For the high emission scenario and extreme scenario, sea temperatures may rise by 4 to 5°C to give potential southern summer temperatures of 23 to 24°C and northern summer temperatures of 18 to 19°C. Under these scenarios, it is likely that Alaria esculenta will be lost almost completely from the UK, and Laminaria digitata will retreat northwards, with large losses across England, Ireland, and Wales. Therefore, resistance is assessed as ‘None’, and resilience is assessed as ‘Very low’. Overall, this biotope is assessed as having ‘High’ sensitivity to ocean warming for the high emission scenario and the extreme scenario.  

Global warming (high)

High emission scenario (by the end of this century 2081-2100) benchmark of:

• A 4°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 3°C rise in surface air temperature in intertidal habitats exclusive to Scotland. 

Evidence

Alaria esculenta

The distribution of seaweeds is climatically defined (Kain, 1979, Van Den Hoek, 1982, Breeman, 1990, Lüning, 1990). Northern boundaries are set by winter temperatures that are lethal or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which makes them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020).

Alaria esculenta is a polar to cold-temperate species that has been recorded from Brittany, France to Northern Norway (Birkett et al., 1998b). Sea temperature regulates metabolism and reproduction and defines the regional distribution of Alaria esculenta (Fredersdorf et al., 2009). The southern limit of Alaria esculenta has been defined at the 20°C isotherm (Munda & Lüning, 1977; Fredersdorf et al., 2009). However, it is more common north of the 16°C isotherm (Munda & Lüning, 1977; Van Den Hoek, 1982). As a result of this upper temperature threshold, Alaria esculenta is largely absent from the southern North Sea and English Channel where summer temperatures exceed 16°C and has already shown signs of contracting its range in the UK as a result of ocean warming (Mieszkowska, 2016).

Munda & Lüning (1977) observed that temperatures of 16 to 17°C, sustained over 2 weeks in Helgoland, Germany, were lethal to resident Alaria esculenta. Experimental observations showed that acute exposure to ≥21°C is lethal to Alaria esculenta causing bleaching and disintegration (Sundene, 1962; Fredersdorf et al., 2009). At its northern range edge (Svalbard), it is a prominent macroalga on sublittoral fringe bedrock.  At these northern latitudes, average summer temperature can reach 5°C, with an average annual sea temperature of 3°C (1980-2014, Beszczynska-Möller & Dye, 2013). Experimental observations conducted by Fredersdorf et al. (2009) found the optimal temperature for sporophyte photosynthesis was within the range of 13 to 17°C but the optimal temperature for Alaria esculenta germination was 2 to 12°C (Fredersdorf et al., 2009).

Alaria esculenta is nearing its southern limits in the UK, with this species only occurring as far south as Brittany, France. Sea surface temperatures (SST) around the UK currently fall between 6 to 19°C (Huthnance, 2010). The available evidence suggests that the effects of ocean warming may occur throughout the year.  If winter temperatures exceed 12°C this is likely to inhibit germination, whilst summer temperatures greater than 20°C will cause mortality of Alaria esculenta.

Yesson et al. (2015) reported no significant change in Alaria esculenta abundance in the UK from 1974 to 2010, and no relationship between abundance and summer and winter sea surface temperatures. Despite this, they noted the contrast between their findings and those of previous research that reported declines in Alaria esculenta around the UK and Ireland, and suggested that if temperatures increase in the future, then abundances could change.

Laminaria digitata

The distribution of kelp is strongly influenced by climatic conditions; therefore, kelp species are extremely sensitive to the ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, while southern limits are set by high lethal summer temperatures or winter temperatures too high for the induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which make them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Laminaria digitata is distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight & Thompson, 2008). Its distribution suggests that the species would tolerate chronic temperature change (i.e. by 2°C for a year). However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. On French coasts, Laminaria digitata is at the southern limit of its range where sea surface temperatures are closer to the upper thermal limit for this species. These populations are known as ‘trailing-edge’ populations, where the species’ geographic ranges are contracting due to ocean warming (Merzouk & Johnson, 2011). Trailing-edge kelp populations are known to be more sensitive to temperature increases than range-centre populations because they are already living close to or at the limit of their thermal tolerance (Liesner et al., 2020; Hereward et al., 2020; Smale, 2020; Leathers et al., 2024).

The thermal optimum of Laminaria digitata is between 5 and 15°C (Dieck, 1992; Arzel, 1998; Franke et al., 2021; OBIS, 2026), while cell damage and mortality have been reported to occur at 22 to 24°C (Sundene, 1964; Lüning, 1980; Bolton & Lüning, 1982; Dieck, 1992). Reproductive ability has been reported to decline to 20% at 18°C (Arzel, 1998). Spore production only occurs between 5 and 10°C and is the most temperature-sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the long-term. A minimum of 10 weeks a year between 5 and 18°C is needed to ensure spore formation and hence reproduction (Bartsch et al., 2013).

Optimum temperatures for Laminaria digitata may differ between life stages. Martins et al. (2017) found that gametophyte growth was optimal at 10 to 18°C under long-day photoperiods. Gametogenesis occurred in temperatures ranging from 5 to 15°C, and was fastest between 10 and 15°C. Sporophyte recruitment was highest at 5°C. Maximum sporophyte development occurred only when long photoperiods were combined with nutrient‑enriched conditions reflecting natural late‑spring environments. Martins et al. (2017) suggested that warming summer conditions are likely to promote vegetative gametophyte growth but suppress gametogenesis, delaying recruitment until cooler autumn–spring periods. This shift could alter annual recruitment patterns and reduce the formation of juvenile sporophytes during increasingly stressful warm summers.

Temperature changes at the benchmark level may affect reproductive success in Laminaria digitata. In the Arctic, predicted future summer sea surface temperatures (9°C) may reduce reproductive success and sporophyte recruitment in Laminaria digitata compared to current (5°C) conditions (Silva et al., 2022). Laminaria digitata gametophytes in this region survive sufficiently (32%) in 2°C in complete darkness over the winter and then become fertile in the spring under moderate light and temperature (5°C) conditions. However, under high light conditions (100 μmol photons/m²/s), reproduction and sporophyte recruitment were reduced. This suggests that under global warming scenarios, where temperatures are higher and light availability may be higher due to reduced ice cover, Laminaria digitata are less likely to successfully reproduce. Although UK waters have no ice cover, this combined effect of temperature and light availability could still be relevant in clearer waters.

Gauci et al. (2022) examined whether the thermal history of kelp gametophytes influences reproductive success and juvenile performance in Laminaria digitata. Clonal gametophytes were maintained for three years under either cold (5°C) or warm (15°C) conditions before being induced to reproduce at 10°C. Gametophytes primed at 5°C completed gametogenesis more rapidly and produced significantly more sporophytes (approx. 278 cm²) than those primed at 15°C (approx. 194 cm²). Juvenile sporophytes from cold-primed parents also exhibited greater thermal tolerance, growing faster at both low (0°C) and high (20°C) extremes compared to warm-primed sporophytes, which ceased growth at 20°C after two weeks. Photosynthetic efficiency declined at 20°C for all sporophytes, indicating heat stress. These findings suggest that elevated temperatures may reduce reproductive success.

Trautmann et al. (2024) also found that elevated temperatures are especially stressful for Laminaria digitata under low light conditions. They investigated the effects of Arctic winter warming by exposing excised discs from sporophytes to three months of darkness at 0°C (current mean) and 5°C (future scenario). Photosynthetic efficiency indicated good health throughout the experiment at both temperatures, although values were still significantly lower in the warmer treatment. Carbohydrate reserves showed marked depletion under warming: mannitol decreased by approx. 37% at 0°C but by approx. 65% at 5°C, while laminarin dropped by approx. 40% and approx. 90%, respectively. Pigments also declined more strongly at 5°C, particularly xanthophyll cycle pigments, and C:N ratios fell from approx. 20 to 13.4 at 5 °C versus 17.9 at 0°C. These results suggest that Laminaria digitata tolerates prolonged darkness well, but experiences accelerated metabolic demand and resource depletion under warmer winter conditions. While Arctic conditions differ to those experienced by this Laminaria digitata biotope around the UK, this evidence still suggests that temperatures at 5°C above the usual could increase energy depletion in the kelp, especially under light-limited conditions.

Temperatures experienced by Laminaria digitata in the low intertidal zone (the upper depth range limit of this biotope) are highly weather‑dependent and often differ substantially from subtidal seawater conditions. During low tide, fully emersed individuals are exposed directly to ambient air temperatures that depend on weather, which varies on short- and long-term temporal scales. Emersed marine organisms may experience desiccation stress, which could potentially worsen thermal stress. For Laminaria digitata populations in tide‑pools, solar radiation can heat shallow water and cause evaporation, further elevating temperature, whereas overcast conditions, wind, and precipitation can cause cooling. Additional factors such as substratum heat retention, shading, tidal timing, and diurnal cycling may also act as stressors. Together, these processes create highly variable and sometimes extreme thermal environments for kelps in the low intertidal zone.

King et al. (2018) investigated stress physiology of Laminaria digitata under simulated low tide exposures to temperature changes by using 1-hour exposures for four consecutive days. The experimental design included a control temperature of 15°C, with immersion treatments at 24, 28, and 32°C to simulate summer tidal pool conditions, and at 0 and 4°C to represent winter tide pools. Emersion treatments were conducted at 24, 28, and 32°C to simulate summer atmospheric heatwaves, and at –5°C to replicate severe winter frost. Stress was quantified by photosynthetic efficiency (F_V/F_M) and heat shock protein (Hsp70) expression. Measuring Hsp70 expression showed that the maximum water temperature tolerable by Laminaria digitata under laboratory conditions was 24°C, and 28°C was the point at which proteins can no longer be synthesised to defend the kelp against heat stress. Laminaria digitata exhibited progressive declines in photosynthetic efficiency under consecutive immersed heat shocks, with resilience eroding after the third exposure at 24°C and complete mortality by the fourth exposure at 28°C. All excised discs of Laminaria digitata tissues became non-viable after two exposures to 32°C. In the emersion experiment, Laminaria digitata showed greater recovery from exposures at 24°C and 28°C, though recovery was incomplete after repeated shocks. In the 32°C treatment, photosynthetic efficiency continued to decline over the three days following the last heat exposure. At -5°C, Laminaria digitata regained near-control levels of photosynthetic efficiency after 72 hours. Water content data from the emersion treatments revealed that Laminaria digitata showed more water loss with each repeated exposure, but water content consistently returned to normal within 24 hours of each exposure. Overall, this study showed that cumulative stress, rather than single exposures, critically determines thermal limits. Laminaria digitata was shown to be highly vulnerable to repeated elevated air temperatures and is comparatively resistant and resilient to repeated exposures to extreme cold air temperatures.

As shown by King et al. (2018), air temperatures during low tide emersion have a significant effect on the health of Laminaria digitata. During emersion, Laminaria digitata experience desiccation, which could potentially exacerbate the stress effects of elevated air temperatures during atmospheric heatwaves. Hereward et al. (2020) examined the effects of realistic atmospheric heatwave scenarios on Laminaria digitata populations at their trailing-range edge in southwest England. The study simulated consecutive low tide emersion events in spring (March) and autumn (October), exposing kelp tissue discs to air temperatures elevated by +7.5°C, +11.5°C, and +15.5°C above seasonal ambient controls (March: 10.5°C, October: 14.5°C). Each heatwave simulation involved daily 1-hour emersion exposures for four consecutive days, followed by a three-day recovery period in seawater. In spring, photosynthetic efficiency (F_V/F_M) remained close to control values in the +7.5°C and +11.5°C treatments and declined after exposure to +15.5°C followed by a full recovery, indicating resilience to short-term heat stress. Tissue bleaching was moderate (up to approx. 50% at +15.5°C), and although relative water content (RWC) declined during emersion, partial recovery occurred. In autumn, however, all warming treatments caused severe stress. Photosynthetic efficiency declined sharply after day three and reached zero by the end of the experiment, indicating non-viable tissue. Bleaching was extensive (up to 100% at +15.5°C), and RWC dropped significantly with little recovery. Field surveys corroborated these findings, showing minimal bleaching in spring and summer but high prevalence in autumn and early winter. The authors concluded that Laminaria digitata at its trailing-edge persists close to its physiological limits for emersion stress in autumn, making populations highly vulnerable to increasingly frequent and intense atmospheric heatwaves under projected climate change scenarios.

Filbee-Dexter et al. (2016) showed that mean kelp biomass (Laminaria digitata and Saccharina latissima) in Nova Scotia had declined by 85 to 99% from 1949 to 2014. In addition, the percentage cover of these kelps had declined by 89% in the seven sites that were surveyed spanning 110 km of coastline. Over this period, summer sea temperatures had increased on average by 1.58°C, and the number of days in which temperatures exceed the thermal thresholds for kelp tissue weakening (>14°C) and mortality (>18°C for two weeks or 20°C for ≥ one week) had also increased. In addition, exceptionally high mean and maximum temperatures were recorded between 2010 and 2012. However, Krumhansl et al. (2024) later reported an overall increase in Laminaria digitata and Saccharina latissima cover in Nova Scotia from 2000 to 2022. They suggested that Laminaria digitata was able to recover faster than other kelps which were lost during the 2010 to 2012 warm period, and that Laminaria digitata is now the dominant kelp. In Narragansett Bay, Rhode Island, only one Laminaria digitata individual was observed across six surveys which were conducted seasonally from December 2017 to November 2018 (Feehan et al., 2019). Baseline surveys from 1980 to 1981 showed that Laminaria digitata was one of the kelp species which dominated these rocky reefs. Feehan et al.’s surveys showed that 91 to 94% of the reefs were covered in turf-forming algae by 2018 and that kelp biomass was close to zero. This decline was attributed to a 0.3°C increase in sea surface temperatures each decade and an increase in the number of weeks where sea surface temperatures exceeded 22°C.

Kelp forests, including populations of Laminaria digitata, across the coastline of New England, USA, have experienced population shifts since the start of the 21st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled the effects of temperature change and sea urchin density on kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south. Maximum summer near-surface seawater temperatures in southern Maine commonly exceeded 20°C and were on average approx. 5.6°C warmer than those observed in northeast Maine. Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024), and temperatures (20°C) at which Saccharina latissima erodes faster than it grows (Lee & Brinkhuis, 1986 cited in Suskiewicz et al., 2024). In addition, reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study period (Suskiewicz et al., 2024). Although forests (Laminaria digitata and Saccharina latissima) had broadly returned to Maine in the late 20th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest. Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area unsuitable for kelp (Suskiewicz et al., 2024). 

Around the British Isles, sea surface temperatures have increased by 0.7°C from 1974 to 2010 (Yesson et al., 2015b). Long-term surveys across this period revealed mixed trends for Laminaria digitata: 64% of sites showed increases in abundance, 34% showed declines, and 2% remained stable. Regional patterns indicated declines in southern areas (English Channel, Celtic Sea) and increases in northern regions, particularly the northern North Sea. Abundance correlated positively with summer sea surface temperature but negatively with winter temperature, suggesting warmer summers may enhance growth while milder winters could disrupt cold-adapted reproductive stages. Therefore, increasing sea surface temperatures may drive regional shifts rather than an overall decline, with northern populations increasing or remaining stable while southern edge populations face greater vulnerability. Combining predicted sea surface temperatures over the next century with the current distribution of Laminaria digitata, Merzouk & Johnson (2011) predict an expansion of its northern limits and localised extinctions across its southern range edge (Mid Bay of Biscay, Northern France and southern England; Birkett et al., 1998). Under RCP 4.5 and RCP 8.5 climate change scenarios, Laminaria digitata in the Northwest Atlantic will also experience a range contraction due to maximum summer sea surface temperatures exceeding 21.5°C, and that their poleward expansion is limited by geography (Khan et al., 2018). Similarly, Hill et al. (2025) reported that even under the least extreme climate change scenario, Laminaria digitata may lose 3.9% of its range by 2100 in the East Atlantic and 28.3% of its range in the West Atlantic. Under the most extreme scenario, losses of 15.3 and 94.7% are expected in the East and West Atlantic, respectively.

Sensitivity Assessment

Alaria esculenta is competitively inferior to Laminaria digitata in the northeast Atlantic (Hawkins & Harkin, 1985) where it is confined to more exposed shores and can colonize sheltered environments only if Laminaria digitata is removed. As temperatures rise to up to 20°C, it is expected that the competitive ability of this species will decrease further, while as temperatures rise above 20°C both Alaria esculenta and Laminaria digitata will be lost, with more, warm adapted, algae taking their place. Brodie et al. (2014) predict that Alaria esculenta will be completely lost from Boreal habitats by the end of this century, whilst all kelp species will be lost from the Lusitanian region, replaced by smaller, fleshy algae.

Sea surface temperatures around the UK are currently between 6 to 19°C (Huthnance, 2010). Under the middle emission scenario, Alaria esculenta is likely to be lost from most of the UK, although it may manage to maintain some populations in Scotland, where current summer temperatures often reach 14°C, leading to potential summer temperatures of 17°C. Laminaria digitata is likely to be lost in southern parts of the UK, where summer temperatures will exceed 20°C, but will likely remain present around the rest of the UK. Resistance is assessed as ‘Low’ and resilience as ‘Very Low’, as the loss is likely to be a long-term decline due to the long-term nature of ocean warming. Therefore, this biotope is assessed as having ‘High’ sensitivity to ocean warming under a middle emission scenario.

For the high emission scenario and extreme scenario, sea temperatures may rise by 4 to 5°C to give potential southern summer temperatures of 23 to 24°C and northern summer temperatures of 18 to 19°C. Under these scenarios, it is likely that Alaria esculenta will be lost almost completely from the UK, and Laminaria digitata will retreat northwards, with large losses across England, Ireland, and Wales. Therefore, resistance is assessed as ‘None’, and resilience is assessed as ‘Very low’. Overall, this biotope is assessed as having ‘High’ sensitivity to ocean warming for the high emission scenario and the extreme scenario.  

Global warming (middle)

Middle emission scenario (by the end of this century 2081-2100) benchmark of:

• A 3°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf.

• A 2°C rise in surface air temperature in intertidal habitats exclusive to Scotland. 

Evidence

Alaria esculenta

The distribution of seaweeds is climatically defined (Kain, 1979, Van Den Hoek, 1982, Breeman, 1990, Lüning, 1990). Northern boundaries are set by winter temperatures that are lethal or summer temperatures too low for growth and/or reproduction, whilst southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which makes them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020).

Alaria esculenta is a polar to cold-temperate species that has been recorded from Brittany, France to Northern Norway (Birkett et al., 1998b). Sea temperature regulates metabolism and reproduction and defines the regional distribution of Alaria esculenta (Fredersdorf et al., 2009). The southern limit of Alaria esculenta has been defined at the 20°C isotherm (Munda & Lüning, 1977; Fredersdorf et al., 2009). However, it is more common north of the 16°C isotherm (Munda & Lüning, 1977; Van Den Hoek, 1982). As a result of this upper temperature threshold, Alaria esculenta is largely absent from the southern North Sea and English Channel where summer temperatures exceed 16°C and has already shown signs of contracting its range in the UK as a result of ocean warming (Mieszkowska, 2016).

Munda & Lüning (1977) observed that temperatures of 16 to 17°C, sustained over 2 weeks in Helgoland, Germany, were lethal to resident Alaria esculenta. Experimental observations showed that acute exposure to ≥21°C is lethal to Alaria esculenta causing bleaching and disintegration (Sundene, 1962; Fredersdorf et al., 2009). At its northern range edge (Svalbard), it is a prominent macroalga on sublittoral fringe bedrock.  At these northern latitudes, average summer temperature can reach 5°C, with an average annual sea temperature of 3°C (1980-2014, Beszczynska-Möller & Dye, 2013). Experimental observations conducted by Fredersdorf et al. (2009) found the optimal temperature for sporophyte photosynthesis was within the range of 13 to 17°C but the optimal temperature for Alaria esculenta germination was 2 to 12°C (Fredersdorf et al., 2009).

Alaria esculenta is nearing its southern limits in the UK, with this species only occurring as far south as Brittany, France. Sea surface temperatures (SST) around the UK currently fall between 6 to 19°C (Huthnance, 2010). The available evidence suggests that the effects of ocean warming may occur throughout the year.  If winter temperatures exceed 12°C this is likely to inhibit germination, whilst summer temperatures greater than 20°C will cause mortality of Alaria esculenta.

Yesson et al. (2015) reported no significant change in Alaria esculenta abundance in the UK from 1974 to 2010, and no relationship between abundance and summer and winter sea surface temperatures. Despite this, they noted the contrast between their findings and those of previous research that reported declines in Alaria esculenta around the UK and Ireland, and suggested that if temperatures increase in the future, then abundances could change.

Laminaria digitata

The distribution of kelp is strongly influenced by climatic conditions; therefore, kelp species are extremely sensitive to the ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, while southern limits are set by high lethal summer temperatures or winter temperatures too high for the induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which make them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Laminaria digitata is distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight & Thompson, 2008). Its distribution suggests that the species would tolerate chronic temperature change (i.e. by 2°C for a year). However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. On French coasts, Laminaria digitata is at the southern limit of its range where sea surface temperatures are closer to the upper thermal limit for this species. These populations are known as ‘trailing-edge’ populations, where the species’ geographic ranges are contracting due to ocean warming (Merzouk & Johnson, 2011). Trailing-edge kelp populations are known to be more sensitive to temperature increases than range-centre populations because they are already living close to or at the limit of their thermal tolerance (Liesner et al., 2020; Hereward et al., 2020; Smale, 2020; Leathers et al., 2024).

The thermal optimum of Laminaria digitata is between 5 and 15°C (Dieck, 1992; Arzel, 1998; Franke et al., 2021; OBIS, 2026), while cell damage and mortality have been reported to occur at 22 to 24°C (Sundene, 1964; Lüning, 1980; Bolton & Lüning, 1982; Dieck, 1992). Reproductive ability has been reported to decline to 20% at 18°C (Arzel, 1998). Spore production only occurs between 5 and 10°C and is the most temperature-sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the long-term. A minimum of 10 weeks a year between 5 and 18°C is needed to ensure spore formation and hence reproduction (Bartsch et al., 2013).

Optimum temperatures for Laminaria digitata may differ between life stages. Martins et al. (2017) found that gametophyte growth was optimal at 10 to 18°C under long-day photoperiods. Gametogenesis occurred in temperatures ranging from 5 to 15°C, and was fastest between 10 and 15°C. Sporophyte recruitment was highest at 5°C. Maximum sporophyte development occurred only when long photoperiods were combined with nutrient‑enriched conditions reflecting natural late‑spring environments. Martins et al. (2017) suggested that warming summer conditions are likely to promote vegetative gametophyte growth but suppress gametogenesis, delaying recruitment until cooler autumn–spring periods. This shift could alter annual recruitment patterns and reduce the formation of juvenile sporophytes during increasingly stressful warm summers.

Temperature changes at the benchmark level may affect reproductive success in Laminaria digitata. In the Arctic, predicted future summer sea surface temperatures (9°C) may reduce reproductive success and sporophyte recruitment in Laminaria digitata compared to current (5°C) conditions (Silva et al., 2022). Laminaria digitata gametophytes in this region survive sufficiently (32%) in 2°C in complete darkness over the winter and then become fertile in the spring under moderate light and temperature (5°C) conditions. However, under high light conditions (100 μmol photons/m²/s), reproduction and sporophyte recruitment were reduced. This suggests that under global warming scenarios, where temperatures are higher and light availability may be higher due to reduced ice cover, Laminaria digitata are less likely to successfully reproduce. Although UK waters have no ice cover, this combined effect of temperature and light availability could still be relevant in clearer waters.

Gauci et al. (2022) examined whether the thermal history of kelp gametophytes influences reproductive success and juvenile performance in Laminaria digitata. Clonal gametophytes were maintained for three years under either cold (5°C) or warm (15°C) conditions before being induced to reproduce at 10°C. Gametophytes primed at 5°C completed gametogenesis more rapidly and produced significantly more sporophytes (approx. 278 cm²) than those primed at 15°C (approx. 194 cm²). Juvenile sporophytes from cold-primed parents also exhibited greater thermal tolerance, growing faster at both low (0°C) and high (20°C) extremes compared to warm-primed sporophytes, which ceased growth at 20°C after two weeks. Photosynthetic efficiency declined at 20°C for all sporophytes, indicating heat stress. These findings suggest that elevated temperatures may reduce reproductive success.

Trautmann et al. (2024) also found that elevated temperatures are especially stressful for Laminaria digitata under low light conditions. They investigated the effects of Arctic winter warming by exposing excised discs from sporophytes to three months of darkness at 0°C (current mean) and 5°C (future scenario). Photosynthetic efficiency indicated good health throughout the experiment at both temperatures, although values were still significantly lower in the warmer treatment. Carbohydrate reserves showed marked depletion under warming: mannitol decreased by approx. 37% at 0°C but by approx. 65% at 5°C, while laminarin dropped by approx. 40% and approx. 90%, respectively. Pigments also declined more strongly at 5°C, particularly xanthophyll cycle pigments, and C:N ratios fell from approx. 20 to 13.4 at 5 °C versus 17.9 at 0°C. These results suggest that Laminaria digitata tolerates prolonged darkness well, but experiences accelerated metabolic demand and resource depletion under warmer winter conditions. While Arctic conditions differ to those experienced by this Laminaria digitata biotope around the UK, this evidence still suggests that temperatures at 5°C above the usual could increase energy depletion in the kelp, especially under light-limited conditions.

Temperatures experienced by Laminaria digitata in the low intertidal zone (the upper depth range limit of this biotope) are highly weather‑dependent and often differ substantially from subtidal seawater conditions. During low tide, fully emersed individuals are exposed directly to ambient air temperatures that depend on weather, which varies on short- and long-term temporal scales. Emersed marine organisms may experience desiccation stress, which could potentially worsen thermal stress. For Laminaria digitata populations in tide‑pools, solar radiation can heat shallow water and cause evaporation, further elevating temperature, whereas overcast conditions, wind, and precipitation can cause cooling. Additional factors such as substratum heat retention, shading, tidal timing, and diurnal cycling may also act as stressors. Together, these processes create highly variable and sometimes extreme thermal environments for kelps in the low intertidal zone.

King et al. (2018) investigated stress physiology of Laminaria digitata under simulated low tide exposures to temperature changes by using 1-hour exposures for four consecutive days. The experimental design included a control temperature of 15°C, with immersion treatments at 24, 28, and 32°C to simulate summer tidal pool conditions, and at 0 and 4°C to represent winter tide pools. Emersion treatments were conducted at 24, 28, and 32°C to simulate summer atmospheric heatwaves, and at –5°C to replicate severe winter frost. Stress was quantified by photosynthetic efficiency (F_V/F_M) and heat shock protein (Hsp70) expression. Measuring Hsp70 expression showed that the maximum water temperature tolerable by Laminaria digitata under laboratory conditions was 24°C, and 28°C was the point at which proteins can no longer be synthesised to defend the kelp against heat stress. Laminaria digitata exhibited progressive declines in photosynthetic efficiency under consecutive immersed heat shocks, with resilience eroding after the third exposure at 24°C and complete mortality by the fourth exposure at 28°C. All excised discs of Laminaria digitata tissues became non-viable after two exposures to 32°C. In the emersion experiment, Laminaria digitata showed greater recovery from exposures at 24°C and 28°C, though recovery was incomplete after repeated shocks. In the 32°C treatment, photosynthetic efficiency continued to decline over the three days following the last heat exposure. At -5°C, Laminaria digitata regained near-control levels of photosynthetic efficiency after 72 hours. Water content data from the emersion treatments revealed that Laminaria digitata showed more water loss with each repeated exposure, but water content consistently returned to normal within 24 hours of each exposure. Overall, this study showed that cumulative stress, rather than single exposures, critically determines thermal limits. Laminaria digitata was shown to be highly vulnerable to repeated elevated air temperatures and is comparatively resistant and resilient to repeated exposures to extreme cold air temperatures.

As shown by King et al. (2018), air temperatures during low tide emersion have a significant effect on the health of Laminaria digitata. During emersion, Laminaria digitata experience desiccation, which could potentially exacerbate the stress effects of elevated air temperatures during atmospheric heatwaves. Hereward et al. (2020) examined the effects of realistic atmospheric heatwave scenarios on Laminaria digitata populations at their trailing-range edge in southwest England. The study simulated consecutive low tide emersion events in spring (March) and autumn (October), exposing kelp tissue discs to air temperatures elevated by +7.5°C, +11.5°C, and +15.5°C above seasonal ambient controls (March: 10.5°C, October: 14.5°C). Each heatwave simulation involved daily 1-hour emersion exposures for four consecutive days, followed by a three-day recovery period in seawater. In spring, photosynthetic efficiency (F_V/F_M) remained close to control values in the +7.5°C and +11.5°C treatments and declined after exposure to +15.5°C followed by a full recovery, indicating resilience to short-term heat stress. Tissue bleaching was moderate (up to approx. 50% at +15.5°C), and although relative water content (RWC) declined during emersion, partial recovery occurred. In autumn, however, all warming treatments caused severe stress. Photosynthetic efficiency declined sharply after day three and reached zero by the end of the experiment, indicating non-viable tissue. Bleaching was extensive (up to 100% at +15.5°C), and RWC dropped significantly with little recovery. Field surveys corroborated these findings, showing minimal bleaching in spring and summer but high prevalence in autumn and early winter. The authors concluded that Laminaria digitata at its trailing-edge persists close to its physiological limits for emersion stress in autumn, making populations highly vulnerable to increasingly frequent and intense atmospheric heatwaves under projected climate change scenarios.

Filbee-Dexter et al. (2016) showed that mean kelp biomass (Laminaria digitata and Saccharina latissima) in Nova Scotia had declined by 85 to 99% from 1949 to 2014. In addition, the percentage cover of these kelps had declined by 89% in the seven sites that were surveyed spanning 110 km of coastline. Over this period, summer sea temperatures had increased on average by 1.58°C, and the number of days in which temperatures exceed the thermal thresholds for kelp tissue weakening (>14°C) and mortality (>18°C for two weeks or 20°C for ≥ one week) had also increased. In addition, exceptionally high mean and maximum temperatures were recorded between 2010 and 2012. However, Krumhansl et al. (2024) later reported an overall increase in Laminaria digitata and Saccharina latissima cover in Nova Scotia from 2000 to 2022. They suggested that Laminaria digitata was able to recover faster than other kelps which were lost during the 2010 to 2012 warm period, and that Laminaria digitata is now the dominant kelp. In Narragansett Bay, Rhode Island, only one Laminaria digitata individual was observed across six surveys which were conducted seasonally from December 2017 to November 2018 (Feehan et al., 2019). Baseline surveys from 1980 to 1981 showed that Laminaria digitata was one of the kelp species which dominated these rocky reefs. Feehan et al.’s surveys showed that 91 to 94% of the reefs were covered in turf-forming algae by 2018 and that kelp biomass was close to zero. This decline was attributed to a 0.3°C increase in sea surface temperatures each decade and an increase in the number of weeks where sea surface temperatures exceeded 22°C.

Kelp forests, including populations of Laminaria digitata, across the coastline of New England, USA, have experienced population shifts since the start of the 21st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled the effects of temperature change and sea urchin density on kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south. Maximum summer near-surface seawater temperatures in southern Maine commonly exceeded 20°C and were on average approx. 5.6°C warmer than those observed in northeast Maine. Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024), and temperatures (20°C) at which Saccharina latissima erodes faster than it grows (Lee & Brinkhuis, 1986 cited in Suskiewicz et al., 2024). In addition, reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study period (Suskiewicz et al., 2024). Although forests (Laminaria digitata and Saccharina latissima) had broadly returned to Maine in the late 20th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest. Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area unsuitable for kelp (Suskiewicz et al., 2024). 

Around the British Isles, sea surface temperatures have increased by 0.7°C from 1974 to 2010 (Yesson et al., 2015b). Long-term surveys across this period revealed mixed trends for Laminaria digitata: 64% of sites showed increases in abundance, 34% showed declines, and 2% remained stable. Regional patterns indicated declines in southern areas (English Channel, Celtic Sea) and increases in northern regions, particularly the northern North Sea. Abundance correlated positively with summer sea surface temperature but negatively with winter temperature, suggesting warmer summers may enhance growth while milder winters could disrupt cold-adapted reproductive stages. Therefore, increasing sea surface temperatures may drive regional shifts rather than an overall decline, with northern populations increasing or remaining stable while southern edge populations face greater vulnerability. Combining predicted sea surface temperatures over the next century with the current distribution of Laminaria digitata, Merzouk & Johnson (2011) predict an expansion of its northern limits and localised extinctions across its southern range edge (Mid Bay of Biscay, Northern France and southern England; Birkett et al., 1998). Under RCP 4.5 and RCP 8.5 climate change scenarios, Laminaria digitata in the Northwest Atlantic will also experience a range contraction due to maximum summer sea surface temperatures exceeding 21.5°C, and that their poleward expansion is limited by geography (Khan et al., 2018). Similarly, Hill et al. (2025) reported that even under the least extreme climate change scenario, Laminaria digitata may lose 3.9% of its range by 2100 in the East Atlantic and 28.3% of its range in the West Atlantic. Under the most extreme scenario, losses of 15.3 and 94.7% are expected in the East and West Atlantic, respectively.

Sensitivity Assessment

Alaria esculenta is competitively inferior to Laminaria digitata in the northeast Atlantic (Hawkins & Harkin, 1985) where it is confined to more exposed shores and can colonize sheltered environments only if Laminaria digitata is removed. As temperatures rise to up to 20°C, it is expected that the competitive ability of this species will decrease further, while as temperatures rise above 20°C both Alaria esculenta and Laminaria digitata will be lost, with more, warm adapted, algae taking their place. Brodie et al. (2014) predict that Alaria esculenta will be completely lost from Boreal habitats by the end of this century, whilst all kelp species will be lost from the Lusitanian region, replaced by smaller, fleshy algae.

Sea surface temperatures around the UK are currently between 6 to 19°C (Huthnance, 2010). Under the middle emission scenario, Alaria esculenta is likely to be lost from most of the UK, although it may manage to maintain some populations in Scotland, where current summer temperatures often reach 14°C, leading to potential summer temperatures of 17°C. Laminaria digitata is likely to be lost in southern parts of the UK, where summer temperatures will exceed 20°C, but will likely remain present around the rest of the UK. Resistance is assessed as ‘Low’ and resilience as ‘Very Low’, as the loss is likely to be a long-term decline due to the long-term nature of ocean warming. Therefore, this biotope is assessed as having ‘High’ sensitivity to ocean warming under a middle emission scenario.

For the high emission scenario and extreme scenario, sea temperatures may rise by 4 to 5°C to give potential southern summer temperatures of 23 to 24°C and northern summer temperatures of 18 to 19°C. Under these scenarios, it is likely that Alaria esculenta will be lost almost completely from the UK, and Laminaria digitata will retreat northwards, with large losses across England, Ireland, and Wales. Therefore, resistance is assessed as ‘None’, and resilience is assessed as ‘Very low’. Overall, this biotope is assessed as having ‘High’ sensitivity to ocean warming for the high emission scenario and the extreme scenario.  

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C (Marine heatwave pressure definitions).

Evidence

Alaria esculenta

Lebrun et al. (2025) studied marine heatwave (MHW) effects on three kelp species including Alaria esculenta. In their mesocosm experiment, water flow was supplied directly from the ocean, ensuring that the control treatment reflected natural conditions. Control temperatures ranged from approx. 7.5 to 10°C. Additional treatments included a constant high-temperature treatment (HT; +1.8°C above control), a single 13-day MHW treatment (1MH; +2.8°C above control), and a treatment simulating two consecutive 5-day MHWs (2MH; +3.9°C above control). No mortality was reported, but physiological responses varied. Net photosynthetic rates declined significantly between the beginning and end of the experiment in all treatments, with a significant decline occurring by the midpoint of the experiment in the HT treatment. Photosynthetic efficiency (F_V/F_M) remained above 0.7 throughout, indicating good physiological condition. Chlorophyll-a content differed significantly between the beginning and end of the experiment only in the 2MH treatment, while significant differences in C:N ratios were detected only between the 1MH and 2MH treatments at the end of the experiment. Growth rates did not differ among treatments. Overall, these findings suggest that Alaria esculenta remained physiologically healthy and maintained growth under the simulated MHW conditions, although some changes in photosynthetic performance, pigmentation, and C:N ratios occurred.

Laminaria digitata

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persists for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Marine heatwaves are known to cause significant impacts to kelp forests, particularly if a population is found towards the edge of its southern limit (Smale et al., 2019). 

Photosynthetic efficiency (measured as F_V/F_M) is a widely used metric for measuring physiological stress in photosynthetic organisms (King et al., 2018; Trautmann et al., 2024), with F_V/F_M values below 0.7 being widely accepted as an indication of physiological stress (Bass et al., 2023). Burdett et al. (2019) collected Laminaria digitata individuals from Plymouth Sound, near the upper limit of their thermal range, and subjected them to three-day marine heatwave (MHW) simulation treatments (ambient: approx. 16, approx. 18, and approx. 20°C) with a ramping up period of two days. Photosynthetic efficiency remained stable for the duration of the experiment, but net oxygen production and non-photochemical quenching increased significantly, which indicate physiological stress. However, the lack of declining photosynthetic efficiency could be due to the experiment design. The most extreme temperature treatment in this case was below the known upper thermal limit for this species (22°C) and the heatwave exposures were short in duration. In addition, the Laminaria digitata individuals were collected from near their southern limit where they may be more adapted to temperatures close to 20°C.

Davey et al. (2025) found that Laminaria digitata were somewhat resistant to heat shock treatments at 20°C for 72 hours compared to ambient temperature treatments (10°C). While growth, gross primary productivity, photosynthetic efficiency, and phenolic content were not significantly affected, other metrics for physiological stress were. Photosynthetic capacity differed between the treatments. Maximum relative electron transport rate (rETRmax) was 12.8 in the ambient treatment compared to 18.9 in the heat shock treatment. Minimum saturating irradiance (Ek) was 59 and 91 μmol photons/m²/s for the ambient and heat shock treatments, respectively, both peaking after 48 to 72 hours. This study (Davey et al., 2025) and Burdett et al. (2019) both highlight the importance of measuring a variety of physiological responses to thermal stressors by showing short-term sublethal effects of warming.

The duration and intensity of marine heatwave (MHW) events have a significant interactive effect on kelp physiology. Leathers et al. (2024) reported signs of cumulative physiological stress in Laminaria digitata at 18°C over 28 days, and at 22°C over 14 days. Photosynthetic efficiency (F_V/F_M) remained steady at approx. 0.73 under control conditions, indicating good health. Under extreme MHW, photosynthetic efficiency fell dramatically from approx. 0.73 at the start to approx. 0.40 at the end of a 14-day MHW, partially recovering to approx. 0.47 after five days, but never returning to pre-MHW levels. Relative growth rate (RGR) also significantly declined under extreme conditions compared to control. Although moderate MHWs did not affect photosynthetic efficiency during 14-day MHWs, longer exposures (28 days) led to further photosynthetic efficiency reductions, indicating cumulative stress. Bleaching severity similarly increased with intensity and duration.

Responses to increased temperatures can vary by light availability and by season. Bass et al. (2023) investigated the combined effects of 4-week MHWs, light, and season on Laminaria digitata, Laminaria hyperborea, and Laminaria ochroleuca. Control (ambient) temperatures were 10°C and 18°C for the spring and summer treatments, respectively. Moderate MHW treatments were 2°C higher than ambient, and strong MHW treatments were 4°C higher than ambient. Low light treatments were at approx. 8 μmol photons/m²/s, and high light treatments were at approx. 75 μmol photons/m²/s. In the spring experiment, Laminaria digitata showed an increase in biomass and surface area under all combinations of temperature and light treatments, although the increase was significantly lower in the low light treatments. In addition, photosynthetic efficiency remained above 0.69 in all combinations of treatments, indicating good health. In the summer experiment, biomass and surface area showed minimal changes in all temperatures in the high light treatment. Photosynthetic efficiency only changed significantly in the highest temperature treatment, but did not indicate stress. However, in the low light treatment, a significant decline in biomass (approx. 20%) was observed in the +2°C treatment, and all plants were reported to have completely disintegrated in the +4°C treatment by the end of the 4-week experiment. Surface area marginally increased in the ambient temperature treatment with low light, but decreased by approx. 40% in the +2°C treatment. Photosynthetic efficiency remained stable in the ambient temperature treatment but declined significantly (approx. 0.25) in the +2°C treatment, although still at a healthy level. These findings indicate that Laminaria digitata is more sensitive to summertime MHWs, especially in light-limited conditions.

King et al. (2018) investigated stress physiology of Laminaria digitata under simulated low tide exposures to temperature changes by using 1-hour exposures for four consecutive days. The experimental design included a control temperature of 15°C, with immersion treatments at 24°C, 28°C, and 32°C to simulate summer tidal pool conditions, and at 0°C and 4°C to represent winter tide pools. Emersion treatments were conducted at 24°C, 28°C, and 32°C to simulate summer atmospheric heatwaves, and at –5°C to replicate severe winter frost. Stress was quantified by photosynthetic efficiency (F_V/F_M) and heat shock protein (Hsp70) expression. Measuring Hsp70 expression showed that the maximum water temperature tolerable by Laminaria digitata under laboratory conditions was 24°C, and 28°C was the point at which proteins can no longer be synthesised to defend the kelp against heat stress. Laminaria digitata exhibited progressive declines in photosynthetic efficiency under consecutive immersed heat shocks, with resilience eroding after the third exposure at 24°C and complete mortality by the fourth exposure at 28°C. All excised discs of Laminaria digitata tissues became non-viable after two exposures to 32°C. In the emersion experiment, Laminaria digitata showed greater recovery from exposures at 24°C and 28°C, though recovery was incomplete after repeated shocks. In the 32°C treatment, photosynthetic efficiency continued to decline over the three days following the last heat exposure. At -5°C, Laminaria digitata regained near-control levels of photosynthetic efficiency after 72 hours. Water content data from the emersion treatments revealed that Laminaria digitata showed more water loss with each repeated exposure, but water content consistently returned to normal within 24 hours of each exposure. Overall, this study showed that cumulative stress, rather than single exposures, critically determines thermal limits. Laminaria digitata was shown to be highly vulnerable to repeated elevated air temperatures and is comparatively resistant and resilient to repeated exposures to extreme cold air temperatures.

While King et al. (2018) found that the critical thermal maxima (the point at which proteins can no longer be synthesised to defend the kelp against heat stress) for Laminaria digitata was 28°C, a later study showed that the thermal maxima can be lower if they have previously experienced MHWs. King et al. (2024) measured the critical thermal maxima (CTmax) in Laminaria digitata after they had been subjected to 4-week MHW simulations. After the MHW simulations, excised kelp discs were placed in seawater tanks where temperature was ramped up by 2°C every day until total physiological failure (measured by photosynthetic efficiency). Photosynthetic efficiency (F_V/F_M) of the discs from the control (14°C) and moderate MHW (18°C) treatments was measured at 0.73 and 0.74, respectively, indicating healthy physiological status. However, discs from the extreme MHW (22°C) treatment showed signs of stress before the CTmax trial, with photosynthetic efficiency measured at 0.62. During the CTmax trial, photosynthetic efficiency remained at healthy levels in the discs from the control and moderate MHW treatments until 26°C was reached, after which it declined sharply. Discs from the extreme MHW treatment declined in photosynthetic efficiency once 24°C had been reached. Kelp disc mortality occurred at approx. 30, approx. 28 and approx. 26°C in the control, moderate MHW and extreme MHW treatments, respectively.

Experiments with Nova Scotia populations of Laminaria digitata also showed sensitivity to increases in temperature (Wilson et al., 2015). Individuals were exposed for nine weeks to temperatures ranging from typical spring-summer conditions (12, 16, and 20°C) to elevated summer temperatures (23°C) and extreme heatwave scenarios (26 and 29°C). Laminaria digitata showed its greatest growth at 12°C, but mortality occurred in all treatments at 20°C and above. At 20°C, survival declined to approximately 80% after two weeks and to 60% by the third week, with remaining individuals persisting for the rest of the experiment. At 23°C, survival dropped to 80% in the second week and reached zero by week three. Under heatwave conditions (26 and 29°C), no individuals survived beyond the first week.

In Baja California, Mexico, an extreme heat even between 2014– 2016, led to both a decrease in density of Macrocystis pyrifera and a decrease in the number of fronds per individual in Baja California, Mexico (Arafeh-Dalmau et al., 2019). In addition, there was a significant change to the understorey algal composition, and half of the fish and invertebrates associated with this habitat disappeared. The same heatwave, coupled with a loss of starfish through disease and an increase in urchin grazing, led to the loss of >90% of Macrocystis pyrifera from 350 km of coastline in northern California (Rogers-Bennett & Catton, 2019).

In Western Australia, marine heatwaves have extremely affected marine ecosystems with widespread loss of Ecklonia radiata, which has changed biotope structure with many kelp forests being replaced by turf algae (Filbee- Dexter & Wernberg, 2018). Similarly, marine heatwaves have also led to the local extinctions of bull kelp Durvillaea antarctica from the coastlines of New Zealand, which then provided space for the invasive kelp Undaria pinnatifida to colonise (Thomsen et al., 2019).

Under experimental conditions, Nepper-Davidson et al. (2019) exposed a northern (Denmark) population of Saccharina lattisima to a simulated three-week heatwave of three different intensities; 18, 21 and 24°C. When exposed to heatwaves of 18 and 21°C there was a decrease in photosynthesis and growth. When 24°C was simulated, 91% of sporophytes were dead within a week, and the fronds of the few survivors were disintegrating, so the experiment was terminated (Nepper-Davidsen et al., 2019). 

Sensitivity Assessment

Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England and 19°C in Scotland. Under the middle emission scenario, both Alaria esculenta and Laminaria digitata are likely to be lost from the southern parts of the UK (see Global warming pressure). Any remaining populations of Alaria esculenta are likely to suffer severe mortality as a result of a heatwave of this magnitude. Therefore, resistance has been assessed as ‘None’. As a further heatwave is likely to affect this habitat before full recovery, resilience has been assessed as ‘Low’. Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England and 21.5°C in Scotland. Under the high emission scenario, Alaria esculenta is likely to be almost wiped out in the UK. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Marine heatwaves (middle)

Middle emission scenario benchmark:  A marine heatwave occurring every three years, with a mean duration of 80 days, with a maximum intensity of 2°C. 

Evidence

Alaria esculenta

Lebrun et al. (2025) studied marine heatwave (MHW) effects on three kelp species including Alaria esculenta. In their mesocosm experiment, water flow was supplied directly from the ocean, ensuring that the control treatment reflected natural conditions. Control temperatures ranged from approx. 7.5 to 10°C. Additional treatments included a constant high-temperature treatment (HT; +1.8°C above control), a single 13-day MHW treatment (1MH; +2.8°C above control), and a treatment simulating two consecutive 5-day MHWs (2MH; +3.9°C above control). No mortality was reported, but physiological responses varied. Net photosynthetic rates declined significantly between the beginning and end of the experiment in all treatments, with a significant decline occurring by the midpoint of the experiment in the HT treatment. Photosynthetic efficiency (F_V/F_M) remained above 0.7 throughout, indicating good physiological condition. Chlorophyll-a content differed significantly between the beginning and end of the experiment only in the 2MH treatment, while significant differences in C:N ratios were detected only between the 1MH and 2MH treatments at the end of the experiment. Growth rates did not differ among treatments. Overall, these findings suggest that Alaria esculenta remained physiologically healthy and maintained growth under the simulated MHW conditions, although some changes in photosynthetic performance, pigmentation, and C:N ratios occurred.

Laminaria digitata

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persists for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Marine heatwaves are known to cause significant impacts to kelp forests, particularly if a population is found towards the edge of its southern limit (Smale et al., 2019). 

Photosynthetic efficiency (measured as F_V/F_M) is a widely used metric for measuring physiological stress in photosynthetic organisms (King et al., 2018; Trautmann et al., 2024), with F_V/F_M values below 0.7 being widely accepted as an indication of physiological stress (Bass et al., 2023). Burdett et al. (2019) collected Laminaria digitata individuals from Plymouth Sound, near the upper limit of their thermal range, and subjected them to three-day marine heatwave (MHW) simulation treatments (ambient: approx. 16, approx. 18, and approx. 20°C) with a ramping up period of two days. Photosynthetic efficiency remained stable for the duration of the experiment, but net oxygen production and non-photochemical quenching increased significantly, which indicate physiological stress. However, the lack of declining photosynthetic efficiency could be due to the experiment design. The most extreme temperature treatment in this case was below the known upper thermal limit for this species (22°C) and the heatwave exposures were short in duration. In addition, the Laminaria digitata individuals were collected from near their southern limit where they may be more adapted to temperatures close to 20°C.

Davey et al. (2025) found that Laminaria digitata were somewhat resistant to heat shock treatments at 20°C for 72 hours compared to ambient temperature treatments (10°C). While growth, gross primary productivity, photosynthetic efficiency, and phenolic content were not significantly affected, other metrics for physiological stress were. Photosynthetic capacity differed between the treatments. Maximum relative electron transport rate (rETRmax) was 12.8 in the ambient treatment compared to 18.9 in the heat shock treatment. Minimum saturating irradiance (Ek) was 59 and 91 μmol photons/m²/s for the ambient and heat shock treatments, respectively, both peaking after 48 to 72 hours. This study (Davey et al., 2025) and Burdett et al. (2019) both highlight the importance of measuring a variety of physiological responses to thermal stressors by showing short-term sublethal effects of warming.

The duration and intensity of marine heatwave (MHW) events have a significant interactive effect on kelp physiology. Leathers et al. (2024) reported signs of cumulative physiological stress in Laminaria digitata at 18°C over 28 days, and at 22°C over 14 days. Photosynthetic efficiency (F_V/F_M) remained steady at approx. 0.73 under control conditions, indicating good health. Under extreme MHW, photosynthetic efficiency fell dramatically from approx. 0.73 at the start to approx. 0.40 at the end of a 14-day MHW, partially recovering to approx. 0.47 after five days, but never returning to pre-MHW levels. Relative growth rate (RGR) also significantly declined under extreme conditions compared to control. Although moderate MHWs did not affect photosynthetic efficiency during 14-day MHWs, longer exposures (28 days) led to further photosynthetic efficiency reductions, indicating cumulative stress. Bleaching severity similarly increased with intensity and duration.

Responses to increased temperatures can vary by light availability and by season. Bass et al. (2023) investigated the combined effects of 4-week MHWs, light, and season on Laminaria digitata, Laminaria hyperborea, and Laminaria ochroleuca. Control (ambient) temperatures were 10°C and 18°C for the spring and summer treatments, respectively. Moderate MHW treatments were 2°C higher than ambient, and strong MHW treatments were 4°C higher than ambient. Low light treatments were at approx. 8 μmol photons/m²/s, and high light treatments were at approx. 75 μmol photons/m²/s. In the spring experiment, Laminaria digitata showed an increase in biomass and surface area under all combinations of temperature and light treatments, although the increase was significantly lower in the low light treatments. In addition, photosynthetic efficiency remained above 0.69 in all combinations of treatments, indicating good health. In the summer experiment, biomass and surface area showed minimal changes in all temperatures in the high light treatment. Photosynthetic efficiency only changed significantly in the highest temperature treatment, but did not indicate stress. However, in the low light treatment, a significant decline in biomass (approx. 20%) was observed in the +2°C treatment, and all plants were reported to have completely disintegrated in the +4°C treatment by the end of the 4-week experiment. Surface area marginally increased in the ambient temperature treatment with low light, but decreased by approx. 40% in the +2°C treatment. Photosynthetic efficiency remained stable in the ambient temperature treatment but declined significantly (approx. 0.25) in the +2°C treatment, although still at a healthy level. These findings indicate that Laminaria digitata is more sensitive to summertime MHWs, especially in light-limited conditions.

King et al. (2018) investigated stress physiology of Laminaria digitata under simulated low tide exposures to temperature changes by using 1-hour exposures for four consecutive days. The experimental design included a control temperature of 15°C, with immersion treatments at 24°C, 28°C, and 32°C to simulate summer tidal pool conditions, and at 0°C and 4°C to represent winter tide pools. Emersion treatments were conducted at 24°C, 28°C, and 32°C to simulate summer atmospheric heatwaves, and at –5°C to replicate severe winter frost. Stress was quantified by photosynthetic efficiency (F_V/F_M) and heat shock protein (Hsp70) expression. Measuring Hsp70 expression showed that the maximum water temperature tolerable by Laminaria digitata under laboratory conditions was 24°C, and 28°C was the point at which proteins can no longer be synthesised to defend the kelp against heat stress. Laminaria digitata exhibited progressive declines in photosynthetic efficiency under consecutive immersed heat shocks, with resilience eroding after the third exposure at 24°C and complete mortality by the fourth exposure at 28°C. All excised discs of Laminaria digitata tissues became non-viable after two exposures to 32°C. In the emersion experiment, Laminaria digitata showed greater recovery from exposures at 24°C and 28°C, though recovery was incomplete after repeated shocks. In the 32°C treatment, photosynthetic efficiency continued to decline over the three days following the last heat exposure. At -5°C, Laminaria digitata regained near-control levels of photosynthetic efficiency after 72 hours. Water content data from the emersion treatments revealed that Laminaria digitata showed more water loss with each repeated exposure, but water content consistently returned to normal within 24 hours of each exposure. Overall, this study showed that cumulative stress, rather than single exposures, critically determines thermal limits. Laminaria digitata was shown to be highly vulnerable to repeated elevated air temperatures and is comparatively resistant and resilient to repeated exposures to extreme cold air temperatures.

While King et al. (2018) found that the critical thermal maxima (the point at which proteins can no longer be synthesised to defend the kelp against heat stress) for Laminaria digitata was 28°C, a later study showed that the thermal maxima can be lower if they have previously experienced MHWs. King et al. (2024) measured the critical thermal maxima (CTmax) in Laminaria digitata after they had been subjected to 4-week MHW simulations. After the MHW simulations, excised kelp discs were placed in seawater tanks where temperature was ramped up by 2°C every day until total physiological failure (measured by photosynthetic efficiency). Photosynthetic efficiency (F_V/F_M) of the discs from the control (14°C) and moderate MHW (18°C) treatments was measured at 0.73 and 0.74, respectively, indicating healthy physiological status. However, discs from the extreme MHW (22°C) treatment showed signs of stress before the CTmax trial, with photosynthetic efficiency measured at 0.62. During the CTmax trial, photosynthetic efficiency remained at healthy levels in the discs from the control and moderate MHW treatments until 26°C was reached, after which it declined sharply. Discs from the extreme MHW treatment declined in photosynthetic efficiency once 24°C had been reached. Kelp disc mortality occurred at approx. 30, approx. 28 and approx. 26°C in the control, moderate MHW and extreme MHW treatments, respectively.

Experiments with Nova Scotia populations of Laminaria digitata also showed sensitivity to increases in temperature (Wilson et al., 2015). Individuals were exposed for nine weeks to temperatures ranging from typical spring-summer conditions (12, 16, and 20°C) to elevated summer temperatures (23°C) and extreme heatwave scenarios (26 and 29°C). Laminaria digitata showed its greatest growth at 12°C, but mortality occurred in all treatments at 20°C and above. At 20°C, survival declined to approximately 80% after two weeks and to 60% by the third week, with remaining individuals persisting for the rest of the experiment. At 23°C, survival dropped to 80% in the second week and reached zero by week three. Under heatwave conditions (26 and 29°C), no individuals survived beyond the first week.

In Baja California, Mexico, an extreme heat even between 2014– 2016, led to both a decrease in density of Macrocystis pyrifera and a decrease in the number of fronds per individual in Baja California, Mexico (Arafeh-Dalmau et al., 2019). In addition, there was a significant change to the understorey algal composition, and half of the fish and invertebrates associated with this habitat disappeared. The same heatwave, coupled with a loss of starfish through disease and an increase in urchin grazing, led to the loss of >90% of Macrocystis pyrifera from 350 km of coastline in northern California (Rogers-Bennett & Catton, 2019).

In Western Australia, marine heatwaves have extremely affected marine ecosystems with widespread loss of Ecklonia radiata, which has changed biotope structure with many kelp forests being replaced by turf algae (Filbee- Dexter & Wernberg, 2018). Similarly, marine heatwaves have also led to the local extinctions of bull kelp Durvillaea antarctica from the coastlines of New Zealand, which then provided space for the invasive kelp Undaria pinnatifida to colonise (Thomsen et al., 2019).

Under experimental conditions, Nepper-Davidson et al. (2019) exposed a northern (Denmark) population of Saccharina lattisima to a simulated three-week heatwave of three different intensities; 18, 21 and 24°C. When exposed to heatwaves of 18 and 21°C there was a decrease in photosynthesis and growth. When 24°C was simulated, 91% of sporophytes were dead within a week, and the fronds of the few survivors were disintegrating, so the experiment was terminated (Nepper-Davidsen et al., 2019). 

Sensitivity Assessment

Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England and 19°C in Scotland. Under the middle emission scenario, both Alaria esculenta and Laminaria digitata are likely to be lost from the southern parts of the UK (see Global warming pressure). Any remaining populations of Alaria esculenta are likely to suffer severe mortality as a result of a heatwave of this magnitude. Therefore, resistance has been assessed as ‘None’. As a further heatwave is likely to affect this habitat before full recovery, resilience has been assessed as ‘Low’. Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England and 21.5°C in Scotland. Under the high emission scenario, Alaria esculenta is likely to be almost wiped out in the UK. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Ocean acidification (high)

High emission scenario benchmark: a further decrease in pH of 0.35 (annual mean) and corresponding 120% increase in H+ ions, seasonal aragonite saturation of 20% of UK coastal waters and North Sea bottom waters, and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, occurring at a depth of 400 m by the end of this century 2081-2100 (Ocean acidification pressure definitions).

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on the emission scenario. Marine autotrophs will generally benefit from ocean acidification, through an increase in the availability of aqueous CO₂ for photosynthesis (Koch et al., 2013). Most species of kelp appear to be under-saturated in respect to carbon dioxide, although they can generally utilise HCO₃ and have external carbonic anhydrase for extracellular dehydration of HCO₃ to CO₂ (Koch et al., 2013).

Under experimental conditions, Iñiguez et al. (2016a) found that although photosynthesis remained stable in Alaria esculenta in response to increasing CO₂, the growth rate increased. Similarly, Gordillo et al. (2015) found heightened growth rates in Alaria esculenta when exposed to increased CO₂, although this increase was not significant and less pronounced than in Saccharina lattisima.

Research on other kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012, Fernández et al., 2015, Nunes et al., 2015, Iñiguez et al., 2016b, a), except for one study, which found that ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016). Whilst there is currently no research on the impact of ocean acidification on Laminaria digitata, it is likely that this species will either benefit or not be negatively impacted by ocean acidification.

Corallina officinalis is a highly calcified, erect, red algae. Results of experimental CO₂ enrichment suggest that this species could be significantly negatively affected by future ocean acidification. Hofmann et al. (2012) found that growth and photosynthesis decreased as a result of a 0.3 unit decrease in pH. Further investigation showed that skeletal CaCO₃ decreased with increasing CO₂ at levels expected for both the middle emission and high emission scenarios, although this decrease was small (< 2%) (Hofmann et al., 2013). Yildiz et al. (2013) showed that although CaCO₃ decreased in Corallina officinalis as a result of ocean acidification, photosynthesis increased. When ocean acidification was combined with an increase in UV radiation, this led to an increase in growth rate. They summarised that a decrease in CaCO₃ content may not be negative but may lead to this species absorbing and using light differently.

Sensitivity assessment. Kelp forests occur in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). The kelps Alaria esculenta and Laminaria digitata are not expected to suffer negative impacts from future acidification, and whilst calcium carbonate content of Corallina officinalis may reduce as a result of acidification, this species is expected to survive future CO₂ enrichment of the oceans. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’.

Ocean acidification (middle)

Middle emission scenario benchmark: a further decrease in pH of 0.15 (annual mean) and a corresponding 35% increase in H+ ions with no coastal aragonite undersaturation and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, at a depth of 800 m by the end of this century, 2081-2100. 

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on the emission scenario. Marine autotrophs will generally benefit from ocean acidification, through an increase in the availability of aqueous CO₂ for photosynthesis (Koch et al., 2013). Most species of kelp appear to be under-saturated in respect to carbon dioxide, although they can generally utilise HCO₃ and have external carbonic anhydrase for extracellular dehydration of HCO₃ to CO₂ (Koch et al., 2013).

Under experimental conditions, Iñiguez et al. (2016a) found that although photosynthesis remained stable in Alaria esculenta in response to increasing CO₂, the growth rate increased. Similarly, Gordillo et al. (2015) found heightened growth rates in Alaria esculenta when exposed to increased CO₂, although this increase was not significant and less pronounced than in Saccharina lattisima.

Research on other kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012, Fernández et al., 2015, Nunes et al., 2015, Iñiguez et al., 2016b, a), except for one study, which found that ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016). Whilst there is currently no research on the impact of ocean acidification on Laminaria digitata, it is likely that this species will either benefit or not be negatively impacted by ocean acidification.

Corallina officinalis is a highly calcified, erect, red algae. Results of experimental CO₂ enrichment suggest that this species could be significantly negatively affected by future ocean acidification. Hofmann et al. (2012) found that growth and photosynthesis decreased as a result of a 0.3 unit decrease in pH. Further investigation showed that skeletal CaCO₃ decreased with increasing CO₂ at levels expected for both the middle emission and high emission scenarios, although this decrease was small (< 2%) (Hofmann et al., 2013). Yildiz et al. (2013) showed that although CaCO₃ decreased in Corallina officinalis as a result of ocean acidification, photosynthesis increased. When ocean acidification was combined with an increase in UV radiation, this led to an increase in growth rate. They summarised that a decrease in CaCO₃ content may not be negative but may lead to this species absorbing and using light differently.

Sensitivity assessment. Kelp forests occur in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). The kelps Alaria esculenta and Laminaria digitata are not expected to suffer negative impacts from future acidification, and whilst calcium carbonate content of Corallina officinalis may reduce as a result of acidification, this species is expected to survive future CO₂ enrichment of the oceans. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’.

Sea level rise (extreme)

Extreme scenario benchmark: a 107 cm rise in average UK sea-level by the end of this century (2018-2100) (Sea-level rise pressure definitions).

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Jackson & McIlvenny (2011) predicted that under a 30 cm sea-level rise, between 10 - 27% of the extent of intertidal rocky shores in Scotland would be lost, whilst under a 190 cm sea-level rise, between 26 - 50% would be lost. Using a modelling-based approach, Kaplanis et al. (2019) found that in San Diego County loss of intertidal habitat would be most extreme within the first metre of sea-level rise, with 29.9% of intertidal rocky shore lost as a result of a 20 cm sea-level rise, and 77.7% as a result of a 100 cm sea-level rise.

Sensitivity assessment. This biotope is found on steep and vertical rock on the lower shore and shallow subtidal, and therefore an increase in sea level height of 50, 70 and 107 cm could have severe repercussions for the extent of this biotope. Beds may be able to expand their range and migrate upwards to compensate for sea-level rise, if not constrained by lack of suitable habitat (IPCC, 2019). In this assessment we have assessed on a worst-case-scenario basis, assuming that landward migration is not possible, which is likely to lead to the depth distribution of Alaria esculenta shrinking significantly in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery. Alaria esculenta is generally limited to a depth of 1-2 m, as below this depth it is usually outcompeted by Laminaria hyperborea. As depth increases due to sea-level rise, Laminaria hyperborea is expected to extend its distribution shorewards.

Under the middle emission, high emission and extreme scenarios (50, 70 and 107 cm sea-level rise), it is expected that between 25 - 75% of this biotope is likely to be lost.  Therefore, for all three scenarios, resistance has been assessed as ‘Low’, and resilience as ‘Very low’.  Therefore, sensitivity is assessed as ‘High’.

Sea level rise (high)

High emission scenario benchmark: a 70 cm rise in average UK sea-level by the end of this century (2018-2100). 

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Jackson & McIlvenny (2011) predicted that under a 30 cm sea-level rise, between 10 - 27% of the extent of intertidal rocky shores in Scotland would be lost, whilst under a 190 cm sea-level rise, between 26 - 50% would be lost. Using a modelling-based approach, Kaplanis et al. (2019) found that in San Diego County loss of intertidal habitat would be most extreme within the first metre of sea-level rise, with 29.9% of intertidal rocky shore lost as a result of a 20 cm sea-level rise, and 77.7% as a result of a 100 cm sea-level rise.

Sensitivity assessment. This biotope is found on steep and vertical rock on the lower shore and shallow subtidal, and therefore an increase in sea level height of 50, 70 and 107 cm could have severe repercussions for the extent of this biotope. Beds may be able to expand their range and migrate upwards to compensate for sea-level rise, if not constrained by lack of suitable habitat (IPCC, 2019). In this assessment we have assessed on a worst-case-scenario basis, assuming that landward migration is not possible, which is likely to lead to the depth distribution of Alaria esculenta shrinking significantly in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery. Alaria esculenta is generally limited to a depth of 1-2 m, as below this depth it is usually outcompeted by Laminaria hyperborea. As depth increases due to sea-level rise, Laminaria hyperborea is expected to extend its distribution shorewards.

Under the middle emission, high emission and extreme scenarios (50, 70 and 107 cm sea-level rise), it is expected that between 25 - 75% of this biotope is likely to be lost.  Therefore, for all three scenarios, resistance has been assessed as ‘Low’, and resilience as ‘Very low’.  Therefore, sensitivity is assessed as ‘High’.

Sea level rise (middle)

Middle emission scenario benchmark: a 50 cm rise in average UK sea-level by the end of this century (2081-2100).

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Jackson & McIlvenny (2011) predicted that under a 30 cm sea-level rise, between 10 - 27% of the extent of intertidal rocky shores in Scotland would be lost, whilst under a 190 cm sea-level rise, between 26 - 50% would be lost. Using a modelling-based approach, Kaplanis et al. (2019) found that in San Diego County loss of intertidal habitat would be most extreme within the first metre of sea-level rise, with 29.9% of intertidal rocky shore lost as a result of a 20 cm sea-level rise, and 77.7% as a result of a 100 cm sea-level rise.

Sensitivity assessment. This biotope is found on steep and vertical rock on the lower shore and shallow subtidal, and therefore an increase in sea level height of 50, 70 and 107 cm could have severe repercussions for the extent of this biotope. Beds may be able to expand their range and migrate upwards to compensate for sea-level rise, if not constrained by lack of suitable habitat (IPCC, 2019). In this assessment we have assessed on a worst-case-scenario basis, assuming that landward migration is not possible, which is likely to lead to the depth distribution of Alaria esculenta shrinking significantly in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery. Alaria esculenta is generally limited to a depth of 1-2 m, as below this depth it is usually outcompeted by Laminaria hyperborea. As depth increases due to sea-level rise, Laminaria hyperborea is expected to extend its distribution shorewards.

Under the middle emission, high emission and extreme scenarios (50, 70 and 107 cm sea-level rise), it is expected that between 25 - 75% of this biotope is likely to be lost.  Therefore, for all three scenarios, resistance has been assessed as ‘Low’, and resilience as ‘Very low’.  Therefore, sensitivity is assessed as ‘High’.

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

Alaria esculenta

Alaria esculenta is a northern/boreal species that has been recorded from Brittany, France to Northern Norway (Birkett et al., 1998). Its approximate mid-range is within southern Norway (60° to 65° North) (Birkett et al., 1998), so these biotopes are in Alaria esculenta’s southern range. Throughout the UK, northern to southern Sea Surface Temperature (SST) ranges from 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013).

Sea temperature regulates Alaria esculenta metabolism and reproduction (Fredersdorf et al., 2009). Their southern limit has been defined at the 20°C isotherm (Munda & Lüning, 1977; Fredersdorf et al., 2009), but it is common north of the 16°C isotherm (Munda & Lüning, 1977). As a result of this upper temperature threshold, Alaria esculenta is largely absent from the southern North Sea and English Channel where summer temperatures can exceed 16°C. According to OBIS (2026), Alaria esculenta occurs in waters ranging from -5 to 15°C, with most records in the 10 to 15°C range.

Much of the research on Alaria esculenta responses to increased temperatures was based on Arctic populations where temperatures are far cooler than those experienced by this biotope in the UK. The temperature treatments used in these studies are reflective of the current and predicted future temperatures (under global warming scenarios) of the study region. As this biotope occurs throughout the UK, caution is needed when basing assessments on evidence from colder Arctic regions, where characteristic species may have different physiological responses and thermal tolerances to populations occurring around the UK where Alaria esculenta are living closer to the upper limit of their thermal tolerance.

Munda & Lüning (1977) observed temperatures of 16 to 17°C sustained over two weeks in Helgoland, Germany, were lethal to resident Alaria esculenta. Experimental observations showed that acute exposure to ≥21°C is lethal to Alaria esculenta causing bleaching and disintegration (Sundene, 1962; Fredersdorf et al., 2009). Experimental observations conducted by Fredersdorf et al. (2009) found the optimal temperature for sporophyte photosynthesis was within the range of 13 to 17°C. However, the optimal temperatures for Alaria esculenta germination is 2 to 12°C (Fredersdorf et al., 2009). Park et al. (2017) reported higher growth rates at 10 to 15°C than at 5°C in Alaria esculenta gametophytes sourced from Arctic waters. In addition, they reported gametophyte mortality at 20°C. Zacher et al. (2019) found that in Arctic populations, gametogenesis and sporophyte growth was reduced at 15°C compared to 9 to 10°C. They also found that co-cultivation with Laminaria digitata enhanced growth.

Sporophytes show rapid recovery of growth and photosynthetic activity after the return of light following polar night, where kelps spend months in total darkness, but this can be diminished by elevated temperatures by as little as 4°C (Gordillo et al., 2022; Cobos et al., 2026). Martins et al. (2022) investigated whether photoperiod conditions during gametophyte development influence the response of Alaria esculenta sporophytes to temperature. Sporophyte growth was highest at 6°C and declined steadily with increasing temperature, with minimal growth at 21 to 22°C. The photoperiod experienced during the gametophyte stage influenced subsequent sporophyte performance, particularly at 6°C, where growth increased under shorter day lengths. However, elevated temperatures generally reduced growth across all photoperiod treatments. These findings demonstrate that warming strongly limits Alaria esculenta growth and can interact with environmental conditions experienced during earlier life stages to affect later performance.

Park et al. (2017) examined the thermal tolerance of Arctic Alaria esculenta gametophytes from Spitsbergen by culturing them at 5, 10, 15, and 20°C. Gametophytes survived and grew best at 10 to 15°C, with greater germling size and cell numbers than at the control temperature (5°C), while all individuals died at 20°C. Photosynthetic performance remained stable across 5 to 15°C but collapsed at 20°C. These results suggest that Arctic Alaria esculenta can maintain growth and photosynthetic function at temperatures 5 to 10°C above those currently experienced in its natural habitat, although temperatures of 20°C exceed its thermal tolerance.

A 5°C increase in temperature was enough to result in an approximate 50% loss of Alaria esculenta biomass over simulated Arctic winter conditions where kelps spend months in total darkness (Gordillo et al., 2022). Kelps lost 21% more biomass during the prolonged darkness at 8°C than those kept at 3°C. Towards the end of the dark period, Alaria esculenta produced new blade tissue in the 3°C treatment, which continued to grow for two months after light returned. However, in the 8°C treatment, only the holdfast and midrib were left, and growth was unable to resume. In a similar experiment, Cobos et al. (2026) also found that elevated temperatures negatively affected Alaria esculenta during the polar night. In the 2-week light period following the dark period, Alaria esculenta growth resumed in both 1 and 5°C treatments, but more so in the cold treatment. These studies show that warming can have a strong negative effect on Alaria esculenta even in colder and darker areas, and this effect can persist even after light levels return to normal.

Some studies have shown that the effects of temperature on Alaria esculenta can vary depending on other pressures. Silva et al. (2022) showed that warming partially offset the negative effects of high light on gametophyte density under low-nutrient conditions. However, under high-nutrient conditions, warming intensified the negative effects of high light. Moreover, Gordillo et al. (2016) showed that higher CO₂ levels partially offset some negative physiological effects of warming. Furthermore, Zacher et al. (2016) showed that warming by itself increased Alaria esculenta germination and sporophyte growth and that warming can partially offset the negative effects of grazing and sedimentation.

Laminaria digitata

Laminaria digitata is distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight & Thompson, 2008). Its distribution suggests that the species would tolerate chronic temperature change (i.e. by 2°C for a year). However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. On French coasts, Laminaria digitata is at the southern limit of its range where sea surface temperatures are closer to the upper thermal limit for this species. These populations are known as ‘trailing-edge’ populations, where the species’ geographic ranges are contracting due to ocean warming (Merzouk & Johnson, 2011). Trailing-edge kelp populations are known to be more sensitive to temperature increases than range-centre populations because they are already living close to or at the limit of their thermal tolerance (Liesner et al., 2020; Hereward et al., 2020; Smale, 2020; Leathers et al., 2024).

The thermal optimum of Laminaria digitata is between 5 and 15°C (Dieck, 1992; Arzel, 1998; Franke et al., 2021; OBIS, 2026), while cell damage and mortality have been reported to occur at 22 to 24°C (Sundene, 1964; Lüning, 1980; Bolton & Lüning, 1982; Dieck, 1992). Reproductive ability has been reported to decline to 20% at 18°C (Arzel, 1998). Spore production only occurs between 5 and 10°C and is the most temperature-sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the long-term. A minimum of 10 weeks a year between 5 and 18°C is needed to ensure spore formation and hence reproduction (Bartsch et al., 2013).

Optimum temperatures for Laminaria digitata vary between life stages. Martins et al. (2017) found that gametophyte growth was optimal at 10 to 18°C under long-day photoperiods, while gametogenesis occurred at 5 to 15°C and was fastest at 10 to 15°C. Sporophyte recruitment was highest at 5°C and maximum development required long photoperiods plus nutrient-enriched conditions. Warming summers may therefore promote vegetative gametophyte growth but suppress gametogenesis, delaying recruitment until cooler autumn-spring periods and reducing juvenile sporophyte formation during warm summers.

In Arctic populations, Silva et al. (2022) found that predicted future summer temperatures of 9°C reduced reproduction and sporophyte recruitment compared with current 5°C conditions. Gametophytes survived winter darkness at 2°C and became fertile under moderate spring light and temperature, but reproduction and recruitment declined under high light. This suggests that warming, particularly where increased light availability accompanies reduced ice cover, could reduce successful recruitment. Although UK waters lack sea ice, similar temperature-light interactions may still be relevant in clearer waters.

Genetic diversity plays a critical role in thermal tolerance in kelps. Liesner et al. (2022) demonstrated that inbred lineages of Laminaria digitata (produced by self-fertilization) showed significantly lower survival under heat stress compared to outbred lineages, (produced by crossing gametes from different parents). Inbred Arctic sporophytes died within seven days at 19°C, whereas outbred temperate lineages maintained growth and photosynthetic performance at 19 to 20.5°C.

Thermal tolerance may also vary by population. Martins et al. (2020) compared Arctic and North Sea Laminaria digitata gametophytes exposed to sudden or gradual heat stress from 15 to 25°C. Thermal stress reduced performance in both populations, but North Sea gametophytes showed higher growth and greater sporophyte recruitment after high-temperature exposure. Both populations were highly sensitive to sudden 8-day exposure at 22.5°C, which caused poor recovery and minimal sporophyte formation, but tolerated gradual warming to 25°C, indicating that acclimation rate strongly influences upper thermal limits. Recovery temperature also affected outcomes, with 5°C favouring recovery after moderate heat stress and 15°C favouring recovery after more severe exposure. Overall, both populations were vulnerable to sustained temperatures above approx. 22°C, but the North Sea population showed greater resilience to warming than the Arctic population.

Schimpf et al. (2022) found that thermal responses in Laminaria digitata vary with latitude and life stage. Gametophytes from all populations had a similar upper survival limit of 24°C, but trait responses differed. After heat priming at 20 to 22°C, southern populations showed higher sporophyte formation during recovery than northern populations. In low-temperature experiments, survival and sporophyte formation showed signs of local adaptation: Spitsbergen gametophyte survival declined with increasing temperature, while Quiberon survival improved, and sporophyte formation peaked at 6 to 9°C in Quiberon and 9 to 12°C in Spitsbergen. These findings suggest that upper survival limits may be broadly conserved, while growth and reproductive responses are locally adapted. Benchmark-level warming may therefore reduce recruitment success and potentially growth in temperate and southern populations.

Simonson et al. (2015a) found that warming caused progressive tissue degradation in Laminaria digitata. Early damage was evident at 14°C, including holes in the medulla, which may increase the risk of stipe breakage, and thinning of the meristoderm, which may impair photosynthesis and growth. Degradation increased with temperature: at 18°C, significant tissue loss occurred after two weeks, and tensile strength declined after three weeks. At 21°C, tensile strength and extensibility declined by 40 to 70% within one week, tissue loss was four to eight times higher than normal erosion rates, and complete mortality occurred within a week.

Leathers et al. (2024) found that marine heatwave intensity and duration interact to affect Laminaria digitata physiology. Cumulative stress was evident after 28 days at 18°C and after 14 days at 22°C, with extreme MHWs causing sharp declines in photosynthetic efficiency, incomplete recovery, reduced relative growth rate, and increased bleaching. Moderate 14-day MHWs had little effect, but longer exposure reduced photosynthetic efficiency, indicating that prolonged warming can drive cumulative physiological stress.

Responses to increased temperature may vary with light availability and season. Bass et al. (2023) found that 4-week marine heatwaves had limited effects on Laminaria digitata in spring, with biomass and surface area increasing under all temperature and light treatments and photosynthetic efficiency remaining high. In summer, responses depended strongly on light. Under high light, responses were minimal even under warmer treatments. Under low light, biomass declined by approx. 20% under the moderate MHW treatment, surface area decreased by approx. 40%, and all plants disintegrated under the strong MHW treatment. These findings indicate that Laminaria digitata is more sensitive to summertime MHWs, particularly under light-limited conditions.

In contrast, Müller et al. (2008) found that warming can intensify UV radiation stress in Laminaria digitata. Arctic populations germinated successfully at 2 to 12°C but failed at 18°C, whereas Helgoland populations performed best at 7 to 18°C. UV-B was the most damaging factor, reducing germination by up to 99% and greatly suppressing spore release. Combined exposure to higher temperatures and UV radiation produced the strongest negative effects on reproduction, indicating that warming exacerbates UV-B stress.

Schmid et al. (2021) found that growth was strongly influenced by temperature, with the highest relative growth rate (approx. 1.92%/day) occurring at 15°C under high light (90 μmol photons/m/s). At 20°C, growth ceased entirely under medium and high light. These results suggest that while Laminaria digitata performs optimally under moderate temperatures and high light, warming at the benchmark level could severely limit growth.

Temperatures experienced by Laminaria digitata in the low intertidal zone (the upper depth range limit of this biotope) are highly weather-dependent and often differ substantially from subtidal seawater conditions. During low tide, fully emersed individuals are exposed directly to ambient air temperatures that depend on spatiotemporally variable weather. Emersed marine organisms may experience desiccation stress, which could potentially worsen thermal stress. For Laminaria digitata populations in tide-pools, solar radiation can heat shallow water and cause evaporation, further elevating temperature, whereas overcast conditions, wind, and precipitation can cause cooling. Additional factors such as substratum heat retention, shading, tidal timing, and diurnal cycling may also act as stressors. Together, these processes create highly variable and sometimes extreme thermal environments for organisms in the low intertidal zone.

King et al. (2018) simulated repeated low-tide exposures to extreme temperatures. Using four consecutive days of 1-hour heat and cold treatments, they found that Laminaria digitata thermal tolerance declined with repeated exposure. Hsp70 expression indicated that 24°C was the upper limit for tolerable heat stress, while 28°C exceeded the kelp's capacity to synthesise protective proteins. Photosynthetic efficiency declined progressively under repeated heat shocks, with complete mortality after four exposures at 28°C and after two exposures at 32°C. In contrast, kelp recovered well from repeated cold exposure, returning to near-control photosynthetic levels after 72 hours at -5°C. Overall, the study showed that cumulative heat stress, rather than single exposure events, determines thermal limits, highlighting the species' vulnerability to repeated warming and relative resilience to extreme cold.

While King et al. (2018) identified 28°C as the critical thermal maximum (CTmax) for Laminaria digitata, King et al. (2024) showed that prior exposure to marine heatwaves (MHWs) can reduce this threshold. Following a 4-week simulation, kelp exposed to an extreme MHW (22°C) displayed reduced photosynthetic efficiency before CTmax testing and began to decline physiologically at 24°C compared with 26°C in the control and moderate MHW treatments. Mortality occurred at approx. 30°C, 28°C, and 26°C in the control, moderate, and extreme MHW treatments, respectively.

King et al. (2019) investigated thermal tolerance in Laminaria digitata across its East Atlantic range, comparing trailing-edge populations in southwest England with range-centre populations in northern Scotland. Heat shock experiments (8 to 32°C) showed that all populations expressed hsp70 under thermal stress, but trailing-edge populations had higher thermal thresholds (Tpeak ≈24°C, Toff ≈28°C) than range-centre populations (Tpeak ≈16 to 20°C, Toff ≈20 to 24°C). These differences were consistent across seasons, suggesting local adaptation rather than short-term acclimation. Genetic analyses also revealed strong population structure and limited gene flow, supporting the existence of distinct thermal ecotypes with greater heat tolerance at the species' southern range edge.

Building on the findings of King et al. (2018), Hereward et al. (2020) investigated the effects of repeated low-tide emersion heatwaves on Laminaria digitata. Kelp tissue was exposed to air temperatures of 7.5 to 15.5°C above seasonal controls for four consecutive days in spring and autumn. Spring treatments caused only temporary reductions in photosynthetic efficiency, moderate bleaching, and recovery after re-immersion, indicating resilience to short-term heat stress. In contrast, autumn treatments resulted in severe declines in photosynthetic efficiency, extensive bleaching, and poor recovery, with all warmed tissues becoming non-viable by the end of the experiment. Field surveys showed similar seasonal patterns, suggesting that trailing-edge populations operate close to their physiological limits in autumn and are particularly vulnerable to increasingly frequent atmospheric heatwaves.

Experiments with Nova Scotia populations of Laminaria digitata also showed sensitivity to increases in temperature (Wilson et al., 2015). Individuals were exposed for nine weeks to temperatures ranging from typical spring-summer conditions (12, 16, and 20°C) to elevated summer temperatures (23°C) and extreme heatwave scenarios (26 and 29°C). Laminaria digitata showed its greatest growth at 12°C, but mortality occurred in all treatments at 20°C and above. At 20°C, survival declined to approx. 80% after two weeks and to 60% by the third week, with remaining individuals persisting for the rest of the experiment. At 23°C, survival dropped to 80% in the second week and reached zero by week three. Under heatwave conditions (26 and 29°C), no individuals survived beyond the first week.

At sites where sea temperature is artificially increased as a result of anthropogenic activity (e.g. effluent output), local extinction of the biotope may occur (Raybaud et al., 2013), especially if combined with high UK summer sea temperatures in southern examples of this biotope (Bartsch et al., 2013). In southern examples of this biotope, Laminaria digitata may also be outcompeted by its Lusitanian competitor Laminaria ochroleuca which is regionally abundant across the south UK coastline (Smale et al., 2014).

Sensitivity Assessment

The available evidence suggests that the effects of an increase in temperature would be seasonally variable. A 5°C increase in temperature for one month during the summer may cause significant mortality and reproductive failure in Alaria esculenta, while a 2°C increase in temperature for one year may limit germination. It is also likely that southern populations of Laminaria digitata will be sensitive in instances where benchmark-level increases in temperature could become much higher than the usual summertime maximum temperatures, therefore exceeding the species thermal maxima. Lower shore populations may also be most vulnerable due to the combination of emersion and high summer temperatures. Therefore, resistance is ‘None’, resilience is ‘Medium’, and sensitivity is ‘Medium’.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

Alaria esculenta

Alaria esculenta is a northern/boreal species that has been recorded from Brittany, France to Northern Norway (Birkett et al., 1998). Sea temperature has been cited as an influential abiotic stressor; responsible for regulating metabolism and reproduction, plus defining the regional distribution of Alaria esculenta (Fredersdorf et al., 2009). At Alaria esculenta’s northern range edge (Svalbard) it is a prominent macroalgae on sub-littoral fringe bedrock. At these latitudes, average summer temperature can reach 5°C, and average annual sea temperature 3°C (1980 to 2014, Beszczynska-Möller & Dye, 2013). Experimental observations conducted by Fredersdorf et al., (2009) found the optimal temperature for sporophyte photosynthesis was within the range of 13 to 17°C, however, the optimal temperatures for Alaria esculenta germination are 2 to12°C (Fredersdorf et al., 2009). According to OBIS (2026), Alaria esculenta occurs in waters ranging from -5 to 15°C, with most records in the 10 to 15°C range.

Alaria esculenta has an approximate mid-range within southern Norway (60° to 65° North) (Birkett et al., 1998), and as such UK biotopes (IR.HIR.KFaR.Ala and IR.HIR.KFaR.AlaAnCrSp) have a southerly distribution when considering the geographic distribution of Alaria esculenta. Throughout the UK northern to southern Sea Surface Temperature (SST) ranges from 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013).

Laminaria digitata

The thermal optimum of Laminaria digitata is between 5 and 15°C (Dieck, 1992; Arzel, 1998; Franke et al., 2021; OBIS, 2026), while cell damage and mortality have been reported to occur at 22 to 24°C (Sundene, 1964; Lüning, 1980; Bolton & Lüning, 1982; Dieck, 1992). Reproductive ability has been reported to decline to 20% at 18°C (Arzel, 1998). Spore production only occurs between 5 and 10°C and is the most temperature-sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the long-term. A minimum of 10 weeks a year between 5 and 18°C is needed to ensure spore formation and hence reproduction (Bartsch et al., 2013).

Gauci et al. (2022) examined whether the thermal history of kelp gametophytes influences reproductive success and juvenile performance in Laminaria digitata. Clonal gametophytes were maintained for three years under either cold (5°C) or warm (15°C) conditions before being induced to reproduce at 10°C. Gametophytes primed at 5°C completed gametogenesis more rapidly and produced significantly more sporophytes (approx. 278 cm²) than those primed at 15°C (approx. 194 cm²). Juvenile sporophytes from cold-primed parents also exhibited greater thermal tolerance, growing faster at both low (0°C) and high (20°C) extremes compared to warm-primed sporophytes, which ceased growth at 20°C after two weeks. Photosynthetic efficiency declined at 20°C for all sporophytes, indicating heat stress. These findings suggest that cold priming enhances reproductive success and warm priming does not improve heat tolerance.

King et al. (2018) examined the response of Laminaria digitata to extreme cold stress under simulated low tide conditions. Laboratory experiments included immersion treatments at 0°C and 4°C to replicate winter tidal pool conditions, and emersion treatments at -5°C to simulate severe winter frost. Laminaria digitata showed no changes in photosynthetic efficiency (F_V/F_M) in the 4°C immersion treatment. Changes were observed in the 0°C treatment, but these changes were not significant. After emersion at -5°C, photosynthetic efficiency declined after each exposure and did not fully recover between exposures, indicating an erosion of resilience. However, after the last exposure on the fourth day, photosynthetic efficiency gradually returned to near-control levels within 72 hours, indicating strong resilience to frost. Overall, the study concluded that Laminaria digitata is comparatively resistant and resilient to repeated exposures to extreme cold temperatures, in contrast to its vulnerability to heat stress.

Sensitivity Assessment

A 5°C decrease in temperature for one month or a 2°C decrease for one year is unlikely to significantly affect populations of the characterising species of KFaR.Ala and its sub-biotopes. Resistance is therefore ‘High’, resilience is ‘High’, and sensitivity is ‘Not Sensitive’.

Salinity increase (local)

Benchmark. An increase in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition).

Evidence

The biotope has only been recorded from conditions of full salinity (Connor et al., 2004; JNCC, 2015, 2022). Intertidal examples of these biotopes will naturally experience fluctuations in salinity where evaporation increases salinity and inputs of rainwater reduces salinity. Species found in the intertidal likely have some form of behavioural or physiological adaptations to changes in salinity.

Lüning (1990) suggest that kelps are stenohaline, their general tolerance to salinity as a phenotypic group covering 16 to 50 PSU over a 24-hour period. Optimal growth probably occurs between 30 to 35 PSU (MNCR category: Full Salinity) and growth rates are likely to be affected by periodic salinity stress.

Alaria esculenta

According to OBIS (2026), Alaria esculenta occurS in waters ranging from 15 to 40 PSU, with most records in the 30 to 35 PSU range. Karsten (2007) tested the photosynthetic ability of Alaria esculenta under acute 2 and 5-day exposures to salinity treatments ranging from 5 to 60 PSU. A control experiment was also carried at 34 PSU. Between 10 to 50 PSU, Alaria esculenta showed high photosynthetic ability at 83 to 94% of the control. Hypersaline treatments with 55 to 60 PSU led to a 30% reduction in photosynthetic ability, approx. 70% of the control level. At 5 PSU, Alaria esculenta showed a low photosynthetic ability at 15.8% of the control. After 5 days at 5 PSU, all Alaria esculenta specimens were bleached and none survived. Karsten (2007) suggested that Alaria esculenta photosynthetic ability is highly affected by acute exposure to hyposaline conditions (<10 PSU). The effect of long-term salinity changes (>5 days) or the effect of salinity >60 PSU on Alaria esculenta’s photosynthetic ability was not tested. The experiment was conducted in the Arctic, and the authors suggest that at extremely low water temperatures (1 to 5°C) macroalgal acclimation to rapid salinity changes could be slower than at temperate latitudes. It is therefore possible that Alaria esculenta maybe be able to acclimate to salinity changes more effectively and quicker in UK waters, however evidence for this is limited.

Laminaria digitata

Kelps are tolerant to short-term daily fluctuations in salinity and are recorded as tolerating 5 to 60 PSU. However, they are much less tolerant to long-term changes with growth and photosynthetic rates declining either side of 20 to 45 PSU (Gordillo et al., 2002; Karsten, 2007). Axelsson & Axelsson (1987) indicated that damage of the plants’ plasma membranes occurs when salinity is below 20 or above 50 PSU. Localized, long-term reductions in salinity to below 20 PSU may result in the loss of kelp beds in affected areas (Birkett et al., 1998b). Despite this tolerance, Laminaria digitata is considered to be a stenohaline species, and this biotope is only found in conditions of full salinity (Connor et al., 2004). Other species probably outcompete Laminaria digitata at the limits of its salinity tolerance, so this biotope is unlikely to occur in salinities above 40 PSU. According to OBIS (2026), Laminaria digitata occurs in salinities ranging from 20 to 40 PSU, with most records in the 30 to 35 PSU range.

Sensitivity Assessment

Both characteristic species have been shown to tolerate salinities above 40 PSU, with Alaria esculenta showing photosynthesis in experiments with salinities up to 50 PSU (Karsten, 2007) and Laminaria digitata tolerating salinities up to 45 to 50 PSU (Axelsson & Axelsson, 1987; Gordillo et al., 2002; Karsten, 2007), all of which are within the range of the pressure benchmark level. However, this biotope has been recorded exclusively in full salinity (30 to 40 PSU) (Connor et al., 2004; JNCC, 2022). Therefore, a change from full to hypersaline conditions (>40 PSU) for a year could result in changes to the biotope. As a precaution, a resistance of ‘Medium’ is suggested, with a resilience of ‘Medium’ and a resultant sensitivity of ‘Medium’. However, in the absence of direct evidence, the overall confidence in the assessment is 'Low'.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition detail).

Evidence

The biotope has only been recorded from conditions of full salinity (Connor et al., 2004; JNCC, 2015, 2022). Intertidal examples of these biotopes will naturally experience fluctuations in salinity where evaporation increases salinity and inputs of rainwater reduces salinity. Species found in the intertidal likely have some form of behavioural or physiological adaptations to changes in salinity.

Lüning (1990) suggest that kelps are stenohaline, their general tolerance to salinity as a phenotypic group covering 16 to 50 PSU over a 24-hour period. Optimal growth probably occurs between 30 to 35 PSU (MNCR category: Full Salinity) and growth rates are likely to be affected by periodic salinity stress.

Alaria esculenta

According to OBIS (2026), Alaria esculenta occurs in waters ranging from 15 to 40 PSU, with most records in the 30 to 35 PSU range.

Karsten (2007) tested the photosynthetic ability of Alaria esculenta under acute 2 and 5-day exposures to salinity treatments ranging from 5 to 60 PSU. A control experiment was also carried at 34 PSU. Between 10 to 50 PSU, Alaria esculenta showed high photosynthetic ability at 83 to 94% of the control. Hypersaline treatments with 55 to 60 PSU led to a 30% reduction in photosynthetic ability, approx. 70% of the control level. At 5 PSU, Alaria esculenta showed a low photosynthetic ability at 15.8% of the control. After 5 days at 5 PSU, all Alaria esculenta specimens were bleached and none survived. Karsten (2007) suggested that Alaria esculenta photosynthetic ability is highly affected by acute exposure to hyposaline conditions (<10 PSU). The effect of long-term salinity changes (>5 days) or the effect of salinity >60 PSU on Alaria esculenta’s photosynthetic ability was not tested. The experiment was conducted in the Arctic, and the authors suggest that at extremely low water temperatures (1 to 5°C) macroalgal acclimation to rapid salinity changes could be slower than at temperate latitudes. It is therefore possible that Alaria esculenta maybe be able to acclimate to salinity changes more effectively and quicker in UK waters, however evidence for this is limited. Moreover, Sundene (1962) found that Alaria esculenta sporophytes grew poorly below 25 PSU.

Lebrun et al. (2024) investigated the combined effects of warming, salinity, and light on four kelp species, including Alaria esculenta. For six weeks, kelps collected from Kongsfjorden, Norway, were exposed to an unmanipulated control treatment which had ambient seawater flowing into the tank at roughly 5.3°C, 33.8 PSU, with an irradiance of 47.8 µmol photons/m²/s, and three additional treatments: T1 = moderate warming (+3.3°C), reduced salinity (-2.5 PSU, and reduced light (-25% irradiance); T2 = stronger warming (+5.3°C), greater salinity reduction (-5 PSU), and lower light (-40% irradiance); and T3 = stronger warming alone (+5.3°C) without changes in salinity or irradiance. These conditions were chosen to reflect future global warming scenarios in which warming increases ice-melt, causing an addition of freshwater and suspended particles into the ocean. Chlorophyll-a declined in the control, reflecting normal seasonal changes, and showed similar patterns in T1 and T3. However, chlorophyll-a remained significantly higher in T2 (stronger warming with reduced salinity and light). Growth rates and carbon:nitrogen ratios were stable across the experiment. These results suggest that Alaria esculenta could potentially be resistant to the combined effects of global warming, reduced salinity, and reduced light.

Laminaria digitata

Birkett et al. (1998b) suggested that kelps are stenohaline, in that they do not tolerate wide fluctuations in salinity and require regular salinities of 30 to 35 PSU to maintain optimum growth rates. Growth rate may be adversely affected if the kelp plant is subjected to periodic salinity stress. Laminaria digitata tolerates a large salinity range within a 24-hour period (5 to 60 PSU; Karsten, 2007). In the study by Karsten (2007), kelp thalli were kept at constant salinities for five days, with their photosynthetic rates measured after two and five days. At the extremes of this range, decreases in photosynthetic rates are evident, particularly at low salinities (Gordillo et al., 2002). The lower salinity limit for Laminaria digitata lies between 10 and 15 PSU. On the Norwegian coast, Sundene (1964) found healthy Laminaria digitata plants growing between 15 and 25 PSU. According to OBIS (2026), Laminaria digitata occurs in salinities ranging from 20 to 40 PSU, with most records in the 30 to 35 PSU range.

Axelsson & Axelsson (1987) indicated that damage of the plants’ plasma membranes occurs when salinity is below 20 or above 50 PSU. Localized, long-term reductions in salinity, to below 20 PSU, may result in the loss of kelp beds in affected areas (Birkett et al., 1998b). Lebrun et al. (2024) found that reduced salinity (20 to 25 PSU) combined with elevated temperatures (+4°C higher than ambient) and reduced light availability (approx. 20% of incident PAR, equivalent to strong turbidity or shading) did not affect any photosynthetic or biochemical metrics.

Mortensen (2017) measured diurnal carbon dioxide exchange rates (CER) in Laminaria digitata and Saccharina latissima at salinities of 34, 24, 16 and 10 PSU to assess how low‑salinity conditions typical of Norwegian fjords affect performance. Laminaria digitata showed reduced CO₂ uptake at lower salinities, with daily CER falling by about 20%, 30% and 40% at 24, 16 and 10 PSU, respectively. However, unlike Saccharina latissima, the reduction at 10 PSU was fully reversible, indicating that Laminaria digitata can recover after exposure to very low salinity.

Red algae vary in their resistance to reduced or variable salinity, so that changes in salinity may change the species composition of the red algal community. In laboratory experiments, maximum rates of photosynthesis and respiration in Palmaria palmata were observed at a salinity 32 PSU (Robbins, 1978) although photosynthetic rates were high down to a salinity of 21 PSU. Palmaria palmata is likely to be tolerant of small changes in salinity because as an intertidal species it is regularly exposed to precipitation. Corallina officinalis inhabits rock pools and gullies from mid to low water. Therefore, it is likely to be exposed to short-term hyposaline (freshwater runoff and rainfall) and hypersaline (evaporation) events. However, its distribution in the Baltic is restricted to increasingly deep water as the surface salinity decreases, suggesting that it requires full salinity in the long-term (Kinne, 1971). Some of the fauna, including Halichondria panicea, are tolerant of a wide variety of salinity habitats from reduced to full salinity and are therefore unlikely to be affected by a drop in salinity at the benchmark level.

Sensitivity Assessment

IR.HIR.KFaR.Ala plus associated sub-biotopes have been recorded exclusively in full salinity (30 to 40 PSU) (Connor et al., 2004; JNCC 2022). Karsten (2007) suggests that at salinities ranging from 10 to 50 PSU, Alaria esculenta photosynthetic ability was high. However, their experiment duration was only 5 days while the pressure benchmark duration is 1 year. At 5 PSU, Alaria esculenta showed a dramatic decline in photosynthetic ability and after 5 days specimens bleached and did not survive. Sundene (1962) noted that Alaria esculenta sporophytes grew poorly below 25 PSU. A decrease of one MNCR salinity scale to “Reduced” (18 to 30 PSU) may reduce growth rates but not cause mortality. Regarding Laminaria digitata, the evidence shows that they could survive a change to variable or reduced salinity conditions (i.e. 18 to 30 PSU) for one year. Due to the evidence of physiological effects of reduced salinity on Alaria esculenta over short timescales, and since this biotope is only found in full salinity, a precautionary resistance of ‘Medium’ is suggested with ‘High’ resilience (as it is only Alaria esculenta that would need to recover), and therefore ‘Low’ sensitivity.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s and 0.2 m/s for more than one year (Water flow pressure definition). 

Evidence

Alaria esculenta dominates the sublittoral fringe in areas exposed to severe wave action or where water surges along the sides of gullies/steep bedrock faces (Lewis, 1964; Connor et al., 2004). The high wave exposure that defines IR.HIR.KFaR.AlaAnCrSp & IR.HIR.KFaR.Ala plus associated sub-biotopes damages other laminarians and generally excludes them. In less wave exposed locations Alaria esculenta is outcompeted by other laminarians, e.g. Laminaria digitata and Laminaria hyperborea (Connor et al., 2004). This biotope is recorded within moderately strong (0.5 to 1.5 m/sec) to weak (<0.5m/sec) tidal streams. Increased tidal flows (>3 m/sec) may increase Alaria esculenta dislodgement (Birkett et al., 1998).

Increased tidal flow may remove fronds of Corallina officinalis, although calcification is thought to be an adaptation to mechanical damage (Littler & Kauker, 1984). Increases in water flow rate may facilitate the colonization of filter-feeding organisms within the understorey and IR.HIR.KFaR.Ala.Myt may dominate over IR.HIR.KFaR.Ala.Ldig. Decreases in water flow are likely to have the opposite effect (Connor et al. 2004).

Laminaria digitata

Laminaria digitata’s flexible stipe and low-profile holdfast allow them to flourish in areas with strong water currents. In Lough Ine in Ireland, it forms dense forests in the fast-flowing water of the Rapids where water speeds vary from 4 to 6 knots (approx. 2 to 3 m/s) (Ebling et al., 1948). Laminaria digitata is also found in very strong flows (>3.87 m/s) although it is often outcompeted by Alaria esculenta. The biotope is not found in areas where sand scour occurs (associated with high water flow rates).

Laminaria digitata partially achieves survival in a range of water flow conditions due to its blade morphology, which varies with flow, becoming narrower and more digitate as water flow rate increases (Sundene, 1964). In a laboratory study, this morphological adaptation was attributed to longitudinal stress with exposure to this stress over six weeks resulting in narrower blades and a significantly higher rate of cell elongation, compared to plants that had not experienced the same stress. This study also suggested that plasticity would serve to decrease the risk of thallus damage in areas of greater exposure or in stormier conditions (Gerard, 1987).

Millar et al. (2020) examined how different hydrodynamic conditions specifically waves versus currents affect the growth of Laminaria digitata in the low‑intertidal zone. By comparing three contrasting flow environments (low current/low wave, high current/low wave, and high wave/low current), they found that variation in growth was driven entirely by hydrodynamic forces. Growth in high‑current environments was substantially enhanced: meristematic growth increased by 45%, and whole‑blade growth by 25%, relative to high‑wave sites. The authors suggest that currents promote more sustained, unidirectional water movement, whereas waves exert large, intermittent forces that impose drag and bending stress, reducing growth efficiency.

Kregting et al. (2015) found that in winter, the velocity of unidirectional (tidal current) and oscillatory water movement (waves) had no influence on the growth rate of Laminaria digitata. However, in summer, when nitrate concentrations are low, increased water motion can enhance the supply of nitrates to the kelp and increase growth. Nitrate uptake rates were lowest at velocities <5 cm/s and increased with velocity, regardless of the type of water motion. However, oscillatory motion increased nitrate uptake rates by 20 to 50% compared to unidirectional flow at the same speed. These findings suggest that it is the velocity of water speed and the supply of nutrients, rather than the type of water flow, that are important for macroalgal growth. They also show seasonal variation in the effects of this pressure on this biotope.

Spore dispersal is in part governed by the local hydrodynamic regime; increased turbulence is associated with an increase in biotope connectivity and therefore a loss of spores from the local system. A decrease in wave and current mediated water flow is identified by lower connectivity with other sites and a higher settlement rate within the local biotope (Robins et al., 2013). Therefore, an increase in water flow could result in loss of spores from the local biotope, which if not balanced by a spore influx from another geographically different population, could result in the demise of the biotope’s health; with a shift in the age structure of the population and a death of young algae. A decrease in the level of water flow is unlikely to have a detrimental effect because the species often grow in areas of low water movement where it may form extensive loose-lying populations (Burrows, 1958; cited in White & Marshall, 2007).

Sensitivity Assessment

The biotope occurs from weak to moderately strong tidal flow and Laminaria digitata can tolerate flow rates >3.87 m/s. Hence, the characteristic species will probably not be affected by a change of 0.1 to 0.2 m/s in peak mean spring bed flow velocity. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is assessed as ‘Not Sensitive’ at the benchmark level. Prolonged increase in flow could result in loss of the biotope due to reduced local recruitment and competition from other species.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year, or 2) an increase in relative sea level or decrease in high water level for ≥1 year. (Emergence regime change pressure definition).

Evidence

An increase in emergence will result in an increased risk of desiccation. Increased immersion may allow IR.HIR.KFaR.Ala biotopes to extend higher up the shore. However, Alaria esculenta forests will come under increased competition with Laminaria hyperborea in the shallow infralittoral. In this scenario IR.HIR.KFaR.Ala biotope distribution may shift on the shore, but the biotope structure will remain.

Alaria esculenta may extend into the lower eulittoral in extremely wave exposed conditions. However, these marginal populations have a reduced age range in comparison to subtidal populations due to desiccation increasing mortality of Alaria esculenta at low tide. An increase in desiccation is likely to remove Alaria esculenta. The resultant loss of canopy would expose Corallina officinalis turf and macrofaunal crust to desiccation and/or damage by high light intensity (bleaching). Hawkins & Harkin (1985) noted that encrusting corallines and Corallina officinalis often die when their protective algal canopy is removed. Severe damage was noted in Corallina officinalis as a result of unusually hot and sunny weather in the UK summer 1983 (Hawkins & Hartnoll, 1985). 

Laminaria digitata is relatively resistant to desiccation, surviving up to 40% water loss (Dring & Brown, 1982). The desiccation tolerance of Laminaria digitata allows beds to extend a further 2 cm into the eulittoral zone where grazing pressure is removed (Southward & Southward, 1978). When exposed to the air, macroalgal canopies can buffer the effects of high temperatures and water loss on organisms below their fronds, with substratum temperatures on average 8 to 10°C lower under the canopy than bare rock, additionally decreasing water loss by >45% (Bertness et al., 1999). Harris et al. (2025) also reported this buffering effect in Fucus serratus canopies during low tide emersion. They recorded differences of up to 13.8°C between below- and above-canopy temperatures. Below-canopy individuals showed virtually no reduction in photosynthetic efficiency or water loss while their above-canopy counterparts experienced significant, cumulative thermal stress. It is therefore possible that smaller, below-canopy Laminaria digitata may have some level of protection from desiccation stress.

Migné et al. (2015) investigated the photosynthetic activity and productivity of intertidal macroalgae, including Laminaria digitata, under natural conditions of emersion (air exposure) and immersion (submersion). They found that Laminaria digitata exhibited very low photosynthetic activity and productivity during emersion, with electron transport rates (ETR) up to 16 times lower than upper shore species such as Pelvetia canaliculata. Under air exposure, effective quantum yield (a measure of photosynthetic efficiency) dropped to critical levels (<0.1), and respiration often exceeded photosynthesis, resulting in net carbon loss. In contrast, Laminaria digitata performed much better when submerged, showing higher ETR and positive carbon flux. These results indicate that Laminaria digitata is poorly adapted to aerial exposure and physiologically optimized for underwater photosynthesis, explaining its dominance in lower-shore habitats where emersion is brief.

An increase in the benchmark level for air exposure may result in the depression of the biotope’s upper limit, as this species’ lower limit is set by competition with Laminaria hyperborea (Hawkins & Harkin, 1985). The upper, landward limits of Laminaria digitata biotopes are generally set by competition with the brown algae Fucus serratus (Hawkins & Harkin, 1985); therefore, a decrease in the benchmark level for air exposure may result in the extension of the biotope’s upper limit. The main driver of competition between Fucus serratus and Laminaria digitata is based on the ability of Fucus serratus to control its respiration rates based on its desiccation rates, which Laminaria digitata is unable to do. Therefore, longer periods of emergence may result in a compression of Laminaria digitata’s extent as it is outcompeted by Fucus serratus at its upper limit. The kelp is able to resist both an increase and decrease in emergence; however, this resistance is based on the free movement of this species within its environmental optima, shifting up or down the shore. Therefore, if an obstacle to movement perpendicular to the shoreline (e.g. sea defence) is then combined with a change in the emergence regime this biotope could undergo compression of its range and potentially local loss.

Sensitivity Assessment

Resistance to this pressure is considered ‘Low’, resilience is ‘Medium’, so the sensitivity of this biotope is ‘Medium’.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year (Wave action pressure definition). 

Evidence

Alaria esculenta dominates the sublittoral fringe in areas exposed to severe wave action or where water surges along the sides of gullies (Lewis, 1964). A decrease in local wave height will increase spatial competition from other laminarians (Connor et al., 2004; JNCC, 2022). Increased wave exposure may remove fronds of Corallina officinalis, but calcification is thought to an adaptation to mechanical damage (Littler & Kauker 1984) and the fronds grow as a compact (short) turf in wave exposed conditions.

IR.HIR.KFaR.Ala and its sub-biotopes occur in high energy sites from very exposed to moderately exposed. The sub-biotopes are separated based on wave exposure. KFar.Ala.Myt occurs predominantly on sub-littoral fringe bedrock in very exposed conditions. In extremely wave exposed conditions KFaR.Ala.Myt can extend to 15 m below chart datum where Alaria esculenta replaces Laminaria hyperborea as the assemblage dominant, but as depth increases Tubularia spp. becomes more abundant and Mytilus abundance decreases (Bégin et al., 2004, Connor et al., 2004; JNCC, 2022). KFaR.Ala.Ldig occurs predominately at very exposed to moderately wave exposed sites, where Laminaria digitata can spatially compete with Alaria esculenta, and represents an intermediate community between more exposed KFaR.Ala.Myt and pure Laminaria digitata (Ldig) (Connor et al., 2004; JNCC, 2022).

Laminaria digitata

The structure of kelps enables them to survive a range of wave conditions from wave exposed to sheltered conditions (Connor et al., 2004; Harder et al., 2006). Physiological differences between kelps are evident between low wave exposure and medium-high wave exposure. The greatest wet weight of Laminaria digitata occurs at low wave exposure (mean significant wave height <0.4 m) decreasing by a mean of 83% in medium to high wave exposures (mean significant wave height >0.4m; Gorman et al., 2013). At medium to high levels of wave exposure, Laminaria digitata biomass has been shown to decrease by 83% in the field (Wernberg & Thomsen, 2005). In areas of high wave exposure, Laminaria digitata may extend its upper limits into the lower eulittoral zone.

Laminaria digitata erosion rates are generally higher on more exposed shores (Gilson et al., 2023). However, this biotope can be found on extremely exposed shores (JNCC 2022).

The morphology of Laminaria digitata differs between sites with differing levels of wave exposure. Experiments showed that Laminaria digitata individuals translocated from exposed to sheltered sites resulted in frond widening, while individuals translocated from sheltered to exposed sites became thinner and more streamlined (Sundene, 1964; Gerard, 1987). This morphological plasticity is evident during the spore stage; because of this, it is suggested that if wave height is increased or decreased the kelp will adapt morphologically over time to optimise its survival in the new environment.

Millar et al. (2021) compared the mechanical properties, cellular composition, and tissue architecture in Laminaria digitata across sites dominated by waves, currents, or relatively benign flow conditions. They found that kelps from more energetic environments (high waves or strong currents) generally possessed stronger yet more extensible blades than those from sheltered sites. Extensibility was greatest in the meristematic region (the main body of the kelp from which the blades protrude), where enhanced proportions of medulla cells (cylindrical-shaped cells) increased tissue flexibility, allowing blades to stretch and deform safely under high forces. By contrast, the distal blade tips were structurally stronger. Although cellular proportions did not differ significantly between kelps from different hydrodynamic regimes, kelps from high-energy sites showed significantly thicker tissues, suggesting that tissue thickening, rather than major changes in cellular composition, is the key strategy for resisting drag, preventing breakage, and maintaining attachment under extreme hydrodynamic loads. Similarly, Savard-Drouin et al. (2024) found that Laminaria digitata from sites with higher wave exposure had longer and thicker stipes and thicker blades with increased branching.

Kregting et al. (2015) found that in winter, the velocity of unidirectional (tidal current) and oscillatory water movement (waves) had no influence on the growth rate of Laminaria digitata. However, in summer, when nitrate concentrations are low, increased water motion can enhance the supply of nitrates to the kelp and increase growth. Nitrate uptake rates were lowest at velocities <5 cm/s and increased with velocity, regardless of the type of water motion. Oscillatory motion did however increase nitrate uptake rates by 20 to 50% compared to unidirectional flow at the same speed. These findings suggest that it is the velocity of water speed and the supply of nutrients, rather than the type of water flow, that are important for macroalgal growth. They also show seasonal variation in the effects of this pressure on this biotope.

In contrast, Millar et al. (2020) showed that wave movement reduces the growth efficiency in Laminaria digitata. By comparing three contrasting flow environments (low current/low wave, high current/low wave, and high wave/low current), they found that growth in high‑current environments was substantially enhanced: meristematic growth increased by 45%, and whole‑blade growth by 25%, relative to high‑wave sites. The authors suggest that currents promote more sustained, unidirectional water movement, whereas waves exert large, intermittent forces that impose drag and bending stress, reducing growth efficiency.

Storm-induced increases in wave action can be detrimental to Laminaria digitata biotopes. On the Atlantic coast of Nova Scotia, Hurricane Earl generated extreme wave heights of up to 25 m and strong bottom currents which caused a large-scale defoliation of kelp beds in shallow subtidal zones (Filbee-Dexter & Scheibling, 2012). Saccharina latissima and Laminaria digitata were stripped of blades, leaving only stipes and fragments, resulting in a 46% average loss of kelp canopy cover across the surveyed sites. The strong bottom currents also caused the displacement of urchins (Strongylocentrotus droebachiensis). In addition, coralline and filamentous red algae cover increased after the storm due to the loss of kelp.

The associated assemblage of the biotope also influences Laminaria digitata’s ability to withstand increases in wave action. The encrusting epiphyte Membranipora membranacea reduces the ability of individual kelps to withstand wave action, increasing frond breakages and reducing the maximum toughness and extensibility of the kelp blade materials (Krumhansl et al., 2011).

Sensitivity Assessment

This biotope occurs very exposed locations to moderately exposed. An increase in wave exposure by one MNCR wave exposure category for one year from very exposed to extremely exposed could favour a shift towards KFaR.Ala.Myt which sometimes occurs in extremely exposed sites.

A decrease in wave exposure at moderately exposed examples of this biotope would reclassify the biotope as sheltered. Sheltered conditions may favour a transition to a different biotope. However, it is unlikely that this biotope would be affected within a year (as per the pressure benchmark) unless the canopy were removed by another pressure, as kelp lifespans are longer than a year.

Therefore, at the pressure benchmark level, this biotope is considered to have ‘High’ resistance to a decrease in wave exposure category, with ‘High’ resilience by default (no disturbance to recover from) and is therefore ‘Not Sensitive’ to this pressure. In response to an increase in wave exposure, this biotope’s resistance, resilience, and sensitivity are all assessed as ‘Medium’.

Note: Kain (1975) demonstrated that recruitment rates vary with wave exposure in several kelp species including those that characterise these biotopes. It is therefore possible that a decrease in wave exposure for more than one year could change the relative abundances of Alaria esculenta and Laminaria digitata, resulting in a change in biotope. However, a change on this scale is beyond that of the pressure benchmark.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant Transitional metal or organometal (e.g. TBT) contaminants via uncontrolled releases or incidental spills (Transitional metals and organometals pressure definition).

Evidence

This pressure is Not Assessed but evidence is presented where available. Very few studies have investigated whether these contaminants are harmful to kelp. Most studies on this topic instead focus on implications for human health and other trophic levels in kelp forest ecosystems.

Several transition elements have been detected in Alaria esculenta tissues, including arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), and nickel (Ni) (Roleda et al., 2019; Kreissig et al., 2021; da Silva Junior et al., 2023; Jönsson & Nordberg, 2024), but their harmful effects on Alaria esculenta were not investigated.

Maulvault et al. (2015) investigated toxic elements in seafood samples around Europe, including Laminaria digitata, bivalves, and fish. The highest levels of arsenic in the study (41 mg per kg) were detected in Laminaria digitata. Cadmium and copper have been shown to significantly decrease photosynthetic efficiency in Laminaria digitata to 0.65 (Anacleto et al., 2017), which is below the commonly accepted indicative value for stress, 0.7 (Bass et al., 2023).

Further studies have quantified total arsenic (AsTOT) and inorganic arsenic (iAs) in Laminaria digitata and Ascophyllum nodosum (Ronan et al., 2017). Laminaria digitata contained 36 to 131 mg/kg of dry weight (DW) total arsenic, similar to Ascophyllum nodosum (38 to 111 mg/kg DW), but unlike Ascophyllum nodosum, which consistently contained <1% inorganic arsenic, Laminaria digitata exhibited substantially higher iAs concentrations ranging from 2.2 to 87 mg/kg DW. Moreover, the proportion of inorganic arsenic increased markedly from the stipe towards the distal blade tips, with >50% of AsTOT present as iAs in the middle and decaying distal blade regions. This pattern suggests both physiological accumulation and possible remobilisation of iAs in ageing tissues.

Raab et al. (2025) found that Laminaria digitata contains very high proportions of inorganic arsenic (iAs), averaging approx. 61.6 mg/kg compared to the closely related Laminaria hyperborea which contained only trace iAs (approx. 0.36 mg/kg) despite comparable total arsenic levels. This demonstrates that Laminaria digitata is particularly prone to accumulating toxic inorganic arsenic, whereas Laminaria hyperborea predominantly converts arsenic into less harmful organic forms. This high proportion of iAs in the total arsenic of Laminaria digitata was also reported by Ender et al. (2019), who found that total arsenic in Laminaria digitata was 117 mg/kg, of which 53% was iAs. Different cultivation settings can also influence trace metal accumulation. Ratcliff et al. (2016) compared the trace metal content of Laminaria digitata grown in three environments: wild populations, kelp cultivated alone (mono‑cultivation), and kelp cultivated adjacent to organic Atlantic salmon farms in an IMTA system. They measured a range of metals including As, Ba, Cd, Co, Cu, Mn, Sb, V and Zn. IMTA cultivation led to elevated concentrations of Cu, Mn and V in kelp compared with mono‑cultivated plants, indicating some influence of the adjacent fed‑aquaculture operation. However, metal concentrations in IMTA‑grown kelp remained within the natural range seen in wild populations with the exception of arsenic, which exceeded legislative limits in some samples. This study was also the first to report V and Sb in Laminaria digitata.

Cereja et al. (2026) studied the effects of copper on the golden kelp Laminaria ochroleuca and Mytilus edulis. They found that Laminaria ochroleuca significantly reduces copper (Cu) concentrations in the surrounding water. This effect was strong enough to increase Mytilus edulis survival compared to the treatment without Laminaria ochroleuca. However, this also resulted in a decrease in chlorophyll-a concentration, an increase in phaeophytin, and tissue bleaching, indicating physiological stress. After 15 days of exposure, Cu accumulation led to kelp mortality (Cereja et al., 2026).

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including metal contamination) affected six marine habitats (including intertidal and subtidal reefs). A 30 to 50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009) also highlighted that macroalgal communities are relatively tolerant to contamination, but that metal- and nutrient-impacted intertidal communities can have low diversity assemblages which are dominated by opportunistic and fast-growing species (Johnston & Roberts, 2009 and references therein).

Mercury (organic > inorganic) is highly toxic to macrophytes (Bryan 1984; Cole et al. 1999). Mercury and copper were lethal at 0.05 mg/l and 0.1 mg/l respectively and toxic at 0.05 mg/l and 0.01 mg/l respectively in Laminaria hyperborea. Zinc and cadmium were lethal at 5 mg/l and 10 mg/l respectively. The presence of alginates in kelp tissue is thought to sequester heavy metals in a biologically unavailable form. It is likely that laminarians such as Alaria esculenta are relatively tolerant of heavy metals except at high concentrations at high levels.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant hydrocarbon or polyaromatic hydrocarbon (PAH) contaminants via uncontrolled releases or incidental spills (Hydrocarbon & PAH pressure definition).

Evidence

This pressure is Not Assessed but evidence is presented where available.

The mucilaginous coating on kelp fronds is thought to protect them from coatings of oil. Hydrocarbons in solution reduce photosynthesis and may be algicidal. Reduction in photosynthesis is dependent on the type of oil, its concentration and length of exposure, oil-water mixture and irradiance in experimental trials (Lobban & Harrison, 1994). Subtidal populations are only exposed to oil emulsions or oil adsorbed particles. Kelps are relatively insensitive to dispersants (Birkett et al. 1998) e.g. Laminaria digitata exposed to diesel oil at 0.130 mg/l reduced growth by 50% in a 2-year experiment. No growth inhibition was noted at 0.03 mg/l and the plants recovered completely in oil free conditions. Corallina officinalis, however, exhibited dramatic bleaching after the Sea Empress oil spill and died after the Torrey Canyon spill (Crump et al. 1999; Smith 1968). Encrusting corallines and Corallina officinalis recovered from the Sea Empress spill quickly, bleaching only affecting the fronds or surface of crustose forms. Grazing gastropods, e.g. limpets are highly intolerant of oil spillage and if not killed are narcotinized and washed offshore and/or consumed by predators. The lower littoral populations are likely to be most vulnerable to oil spill and sublittoral fringe would be particularly affected at low tide. Although Alara esculenta may not be affected severely, the articulated coralline turf may be lost but recover quickly although the red algae may be intolerant. Grazers such as limpets, barnacles and meiofaunal crustaceans may also be lost from the community.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more synthetic compound contaminants via uncontrolled releases or incidental spills (Synthetic compound contamination pressure definition).

Evidence

This pressure is Not Assessed but evidence is presented where available.

There is currently Insufficient Evidence of harmful effects of synthetic compound contamination in the kelps that characterise this biotope. Studies that investigate the presence of these compounds in seafood, including kelps, focus on implications for human health (see Alvarez-Munoz et al., 2015) rather than their effects on the species or ecosystems themselves. However, Anacleto et al. (2017) did investigate the effects of a range of pollutants on Laminaria digitata health but found no significant reduction in photosynthetic activity in response to pesticides (diflubenzuron and lindane).

Cole et al. 1999 suggest that macrophytes are generally intolerant of herbicides such as atrazine, simazine, diuron and linuron e.g. atrazine was lethal to Laminaria hyperborea sporophytes at 1mg/l and suppressed growth at 0.01 mg/l (Hopkin & Kain, 1978). Smith (1968) noted that Corallina officinalis was killed in areas of heavy spraying after the Torrey Canyon oil spill and affected at 6 m depth in areas of high wave action. High water specimens were more affected than low water specimens, presumably because they are emersed for longer and had more contact with oil and dispersants. Hiscock & Hoare (1975) noted that almost all red algal species and many animal species were absent from Amlwch Bay in North Wales adjacent to an acidified halogenated effluent. Red algae have also been found to be sensitive to oil spill dispersants (O'Brien & Dixon, 1976). Gastropods are known to be highly sensitive to endocrine disrupters such as TBT. Crustaceans (e.g. amphipods, isopods, ostracods, copepods and barnacles) are also susceptible to endocrine disruption by synthetic chemicals. It is therefore likely that some taxa within IR.HIR.KFaR.Ala and its associated sub-biotopes, especially grazing invertebrates and meiofauna will be intolerant of synthetic chemical contamination.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels (Radionuclides contamination pressure definition).

Evidence

No Evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant "other" substances (solid, liquid or gas) contaminants via uncontrolled releases or incidental spills (Introduction of other substances pressure definition). 

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status) (deoxygenation pressure definition).

Evidence

Reduced oxygen concentrations have been shown to inhibit both photosynthesis and respiration in macroalgae (Kinne, 1977). Despite this, macroalgae are thought to buffer the environmental conditions of low oxygen, thereby acting as a refuge for organisms in oxygen depleted regions especially if the oxygen depletion is short-term (Frieder et al., 2012). A rapid recovery from a state of low oxygen is expected if the conditions are transient, which is likely given the wave exposure levels of IR.HIR.KFaR.Ala and its sub-biotopes. If dissolved oxygen concentrations drop below 4 mg/l, negative effects on these organisms may occur, with more severe adverse effects expected below 2 mg/l (Cole et al., 1999). Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly. However, small invertebrate epifauna may be lost, causing a reduction in species richness.

Sensitivity Assessment

No evidence of mortality was found in any characteristic species. Sublethal effects were found, such as reduced photosynthesis and respiration in macroalgae. However, at the pressure benchmark level (1-week exposure to oxygen concentrations of ≤2 mg/l), resistance and resilience of these biotopes are considered to be ‘High’ and so sensitivity is assessed as ‘Not Sensitive’.

Nutrient enrichment

Benchmark. Increased levels of the elements nitrogen, phosphorus, silicon, and iron in the marine environment compared to background concentrations (Nutrient enrichment pressure definition).

Evidence

Eutrophication is associated with loss of perennial algae and replacement by mussels or opportunistic algae (Fletcher 1996). Johnston & Roberts (2009) conducted a meta-analysis that reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including intertidal and subtidal reefs). A 30 to 50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009) also highlighted that macroalgal communities are relatively tolerant to contamination, but that metal and nutrient impacted intertidal communities can have low diversity assemblages which are dominated by opportunistic and fast-growing species (Johnston & Roberts, 2009 and references therein).

Alaria esculenta

Silva et al. (2022) demonstrated that nutrient availability influences the response of Alaria esculenta gametophytes to temperature and light. They used a full factorial experiment to test the combined effects of nutrient availability, temperature, and light intensity on Alaria esculenta gametophytes. The experiment crossed three irradiance levels (0.5, 35 and 100 μmol photons m²/s), two temperatures (5°C control and 9°C future-warming treatment), and two nutrient regimes: low nutrients (2 μM N and 0.2 μM P) and an unspecified high-nutrient treatment simulating increased inputs from mixing, river discharge, and permafrost melt. Gametophyte density was generally lower under high nutrient conditions than under low nutrient conditions, regardless of temperature. Under high nutrient conditions, densities were broadly similar across temperatures and low and medium light treatments, while high light resulted in reduced densities, particularly at 9°C. These findings indicate that responses of Alaria esculenta gametophytes to higher nutrient availability may be exacerbated by higher temperatures and light availability.

Laminaria digitata

High ambient levels of phosphate and nitrogen enhance spore formation in a number of laminarians (Nimura et al., 2002), but will eventually inhibit spore production, particularly at the limits of temperature tolerances as seen in Saccharina latissima (studied as Laminaria saccharina; Yarish et al., 1990). Laminaria digitata seems to follow this trend with a growth peak occurring in conjunction with nutrient upwelling from deeper waters in Norway (Gévaert et al., 2001). Macroalgal growth is generally nitrogen-limited in the summer, as illustrated by the growth rates of Laminaria digitata between an oligotrophic and a eutrophic site in Arbroath, Scotland (Davison et al., 1984). Laminaria digitata does not accumulate the significant internal nutrient reserves seen in some other kelp. Higher growth rates have been associated with algae situated close to sewage outfalls. However, after removal of sewage pollution in the Firth of Forth, Laminaria digitata became abundant on rocky shores from which they had previously been absent (Read et al., 1983). Enhancement of coastal nutrients is likely to favour those species with more rapid growth rates including turf-forming algae (Gorgula & Connell, 2004) which could explain Laminaria digitata absence from the Firth of Forth. In addition, epiphytic abundance and biomass on Laminaria longicruris increase under a eutrophic regime decreasing the ability of individual algae to photosynthesise and withstand pressure from water movement (Scheibling et al., 1999).

Sensitivity Assessment

Evidence shows that Alaria esculenta gametophyte density can be reduced by elevated nutrient levels (Silva et al., 2022), and this effect can be more pronounced in clearer waters with more available light. However, no evidence was found of nutrient loading negatively affecting the sporophyte stage of Alaria esculenta, so it may only affect the recruitment of juveniles rather than the health or survival of mature kelps. Laminaria digitata seems to be resistant to this pressure. As a precaution, due to the effects of elevated nutrients on Alaria esculenta gametophytes, resistance is assessed as ‘Medium’, resilience as ‘High’, and sensitivity as ‘Low’ in a worst-case scenario where elevated nutrient levels persist for over a year. If the pressure is removed within the timescale of Alaria esculenta’s recruitment cycle, the biotope would be considered as ‘Not Sensitive’. Confidence in this assessment is ‘Medium’ due to the lack of further evidence.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr (Organic enrichment pressure definition).

Evidence

Organic enrichment is associated with eutrophication, increased siltation and turbidity (Fletcher 1996). Eutrophication is associated with loss of perennial algae and replacement by mussels or opportunistic algae (Fletcher 1996). Johnston & Roberts (2009) conducted a meta-analysis that reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected six marine habitats (including intertidal and subtidal reefs). A 30 to 50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009) also highlighted that macroalgal communities are relatively tolerant to contamination, but that contaminated intertidal communities can have low diversity assemblages which are dominated by opportunistic and fast-growing species (Johnston & Roberts, 2009 and references therein). Due to the high wave exposure that this biotope, it is likely that additional organic input to the system may be dispersed out of the biotope’s local vicinity (Johnston & Roberts, 2009).

Sensitivity Assessment 

The wave exposure levels of this biotope would likely disperse organic matter away from the biotope. Resistance and resilience are deemed to be ‘High’, so the biotope is deemed to be ‘Not Sensitive’.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site (Physical loss pressure definition). 

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata, or vice versa (Physical change in subtratum type pressure definition).

Evidence

If rock substrata were replaced with sedimentary substrata this would represent a fundamental change in habitat type, which Alaria esculenta would not tolerate (Birkett et al., 1998b). The biotope would be lost.

Sensitivity assessment. Resistance to the pressure is considered ’None‘, and resilience ’Very low‘ or ‘None’. The sensitivity of this biotope to change from hard rock or artificial substrata to sedimentary or soft rock substrata  is assessed as ’High’.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification) (Physical change in sediment type pressure definition). 

Evidence

Not relevant to hard rock biotopes.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock) (Removal of substratum pressure definition). 

Evidence

Not relevant to hard rock biotopes.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat) (Surface abrasion/disturbance pressure definition).

Evidence

Alaria esculenta

If exposed to moorings, groundings, or passing fishing gear, the resultant abrasion may result in the physical removal of a proportion of the Alaria esculenta canopy depending on the scale of the impact, although no evidence of these specific disturbances was found. However, Ronowicz et al. (2022) found morphological differences in Alaria esculenta between 5 and 10 m depth at sites of varying levels of glacial influence. At the site with the most glacial influence (frequent iceberg scour and the highest load of suspended sediment), Alaria esculenta plants were significantly shorter, had reduced biomass, and had less morphological variability within the population. The authors observed no significant difference in these metrics between the non-impacted and intermediate-impacted (high sediment input alone) sites, suggesting that ice scour was the cause of the reduced size of Alaria esculenta at the highest-impacted site.

Laminaria digitata

Low-level disturbance (e.g. solitary anchors) is suggested as being unlikely to cause harm to the biotope as a whole due to their small impact footprint. In a review of the effects of trampling on intertidal habitats, Tyler-Walters & Arnold (2008) found no information on the effects of trampling on Laminaria species but they reported that laminarians are robust species but that trampling on blades at low tide could potentially damage the blade or growing meristem.

Traditionally Laminaria digitata was used on agricultural lands as fertilizers; now Laminaria spp. are used in a range of different products, with their alginates used in the cosmetic, pharmaceutical and agri-food industries (Kervarec et al.,1999; McHugh, 2003). Collection of Laminaria digitata by mechanical harvesting (trawling) is not done in the UK, (Netalgae, 2012). Trawling, used to harvest Laminaria hyperborea in Norway results in whole alga being removed from the substratum, and substantial scouring of the substratum, indicating that the use of trawls in a Laminaria digitata biotope is likely to detrimentally affect the biotope, regardless of the target species. In France, Laminaria digitata is harvested with a ‘Scoubidou’, a curved iron hook which is mechanically operated. This device is considered to be selective; only harvesting individuals older than 2 years (Arzel, 2002). France reportedly harvests 75,000t kelp, mainly consisting of Laminaria digitata annually (FAO, 2007). Davoult et al. (2011) suggested that the maintenance of a sustainable crop of Laminaria digitata was possible if the industry continues employing small vessels evenly dispersed along the coastline. This could protect against habitat fragmentation and buffer over exploitation (Davoult et al., 2011). A fallow period of 18 to 24 months has been suggested for Laminaria digitata in France, where competition between the juvenile sporophytes of Laminaria digitata and Saccorhiza polyschides was indicated as a threat to the continued harvesting effort of Laminaria digitata (Engelen et al., 2011).

Canopy removal of Laminaria digitata has been shown to reduce shading, resulting in the bleaching of sub-canopy algae (Hawkins & Harkins, 1985). Harvesting may also result in habitat fragmentation, a major threat to this biotope’s ecosystem functioning (Valero et al., 2011).

Harvesting and fishing trawls scour and abrade the seabed, dislodging macroalgae and their associated assemblages from the substratum. The impact footprint and recovery period to artificial abrasion by bottom trawling are dependent on the trawl’s characteristics including; duration, type and size. There is little evidence in the literature concerning natural or low-level bedrock abrasion.

Wiktor Jr et al. (2022) surveyed Laminaria digitata forests around Spitsbergen. The west and east sides of the island had contrasting thermal regimes: the warmer, Atlantic (west) side had temperatures ranging from 2°C in the winter to 7°C in the summer, while temperatures on the Arctic (east) side of the island ranged from -1.8°C in the winter to 2°C in the summer. At depths down to 5 m, kelp canopies (including Laminaria digitata) on the warm, west side were dense, intact, and covered approx. 30.5% of the surveyed area. However, on the colder eastern side of the island which experiences heavy ice cover and scouring, canopy cover in this depth range was significantly lower (approx. 20.3%), and kelp thalli were uniformly trimmed or damaged. Within 5 to 10 m depth, no significant differences were detected in kelp coverage or damage between the two sites. The authors therefore concluded that the reduced and damaged canopies of the east side of the island were caused by ice scouring.

At Hansneset, Kongsfjorden, reduced ice scouring from increasing summer sea surface temperatures (approx. 2°C between 1996 and 2014) was suggested to have caused a significant increase in kelp biomass (Bartsch et al., 2016). Between 1996 to 1998 and 2012 to 2014 survey periods, kelp biomass at 2.5 m depth had increased dramatically to 14 kg fresh weight/m², an 8.2-fold rise compared to the 1996 to 1998 survey results. This increase was driven primarily by Laminaria digitata, which formed dense stands with up to 70 individuals/m², mostly aged between 2 and 8 years, and exhibited a leaf area index of nearly 10 at this depth. The depth zone of maximum kelp biomass also shifted upwards from around 5 m in the late 1990s to  approx. 2.5 m in the 2012 to 2014 surveys. These changes were attributed to reduced ice scouring and a longer open-water period, which have increased kelp survival and growth in shallow zones. This indicates that kelps, especially Laminaria digitata, are able to recover in instances where abrasion pressure is stopped.

Sensitivity Assessment

Both characterising species are deemed to have ‘Low’ resistance to abrasion. This biotope is deemed to have ‘Medium’ resilience and therefore ‘Medium’ sensitivity.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat) (Sub-surface penetration pressure definition).

Evidence

Not relevant to hard rock biotopes.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale, e.g. from clear to intermediate for one year (Suspended sediment pressure definition).

Evidence

Suspended Particle Matter (SPM) concentration has a linear relationship with sub surface light attenuation (Kd) (Devlin et al., 2008). Light penetration influences the maximum depth at which kelp species can grow, and it has been reported that laminarians grow at depths at which the light levels are reduced to 1% of incident light at the surface. Maximal depth distribution of laminarians varies from 100 m in the Mediterranean to only 6 to 7 m in the silt-laden German Bight. In European Atlantic waters, the depth limit is typically 35 m. The kelp Ecklonia radiata has shown a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient = 0.1 to 0.2/m; Staehr & Wernberg, 2009). An increase in water turbidity would therefore likely affect the photosynthetic ability of Alaria esculenta and Laminaria digitata and over time, possibly decrease their abundance and density.

Alaria esculenta

Alaria esculenta is not found in areas of siltation and sediment scour (Birkett et al., 1998). Increased siltation and sediment scour inhibits photosynthesis and algal growth, interfere with spore or larval recruitment plus smother germlings and gametophytes (Fletcher, 1996). However, the high degree of wave exposure that typically defines this biotope is likely to clear suspended sediments relatively quickly. If low water clarity is persistent and wave exposure decreased, then low energy silted kelp biotopes (IR.LIR.K) may proliferate. Once siltation returns to its pre-disturbance level the biotope is likely to recover its canopy within a year and the rest of the community in no more than five years.

Increased turbidity is likely to reduce the depth to which Alaria esculenta can grow. However, an increase of one level in WFD water clarity scale for a period of one year is unlikely to affect the population since Alaria esculenta’s lower limit, is generally determined by competition from other laminarians rather than light penetration.

Melting glaciers in the Arctic due to global warming releases turbid, fresh-melt waters, and inorganic material into the water column. In a study by Ronowicz et al. (2018), this had no effect on Arctic kelps (including Alaria esculenta and Laminaria digitata), but it did significantly affect the composition of faunal communities associated with the holdfast of the kelp. Species richness remained similar across sites and levels of disturbance, but the dominant taxa were different. In the lowest impacted sites, kelp holdfast communities were dominated by Hydrozoa, while the highest impacted sites were dominated by Bryozoa.

Ronowicz et al. (2022) investigated Alaria esculenta morphological differences between 5 and 10 m depth at sites of varying levels of glacial influence. At the site with the most glacial influence (frequent iceberg scour and the highest load of suspended sediment), Alaria esculenta plants were significantly shorter, had reduced biomass, and had less morphological variability within the population. The authors observed no significant difference in these metrics between the non-impacted and intermediate-impacted (high sediment input alone) sites, suggesting that ice scour was the cause of the reduced size of Alaria esculenta at the highest-impacted site. Interestingly, the intermediate-impacted site had the greatest overall blade length and biomass values across the three levels of impact. Moreover, these metrics were greater at 10 m depth than at 5 m. While the levels of suspended sediment were not quantified, these morphological adaptations could serve as indirect evidence of resistance to suspended sediments and a reduction of light availability.

Laminaria digitata

An increase in SPM results in a decrease in sub-surface light attenuation. The absence of Laminaria digitata in the Firth of Forth was suggested to be caused by the outflow from a sewage treatment plant, which increased the turbidity of the water and thus decreased photosynthetic activity, although the effect of turbidity was probably coupled with increased nutrient levels (Read et al., 1983). In locations where water clarity is severely decreased, Laminaria species experience a significant decrease in growth from the shading of suspended matter and/or phytoplankton (Lyngby & Mortensen 1996, Spilmont et al., 2009).

Sensitivity Assessment

An increase in water clarity from clear to intermediate (10 to 100 mg/l) represents a change in light attenuation of approx. 0.67 to 6.7 Kd/m and could result in a significant reduction in photosynthesis of kelps. Therefore, the dominant kelp species may suffer a significant decline and resistance to this pressure is assessed as ‘Low’ as a precaution Resistance is ‘Medium’, resulting in a sensitivity of ‘Medium’.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Alaria esculenta is not found in areas of siltation and sediment scour (Birkett et al. 1998). Increased siltation and sediment scour inhibits photosynthesis and algal growth, interfere with spore or larval recruitment plus smother germlings and gametophytes (Fletcher 1996). However, the high degree of wave exposure that typically defines IR.HIR.KFaR.Ala.Ldig is likely to clear suspended sediments relatively quickly. If low water clarity is persistent and wave exposure decreased, then low energy silted kelp biotopes (IR.LIR.K) may proliferate. Once siltation returns to its pre-effect level the biotope is likely to recover its canopy within a year and the rest of the community in no more than five years. Increased siltation will also increase turbidity.

Melting glaciers in the Arctic due to global warming releases turbid, fresh-melt waters and inorganic material into the water column. In a study by Ronowicz et al. (2018), this had no effect on Arctic kelps (including Alaria esculenta and Laminaria digitata), but it did significantly affect the composition of faunal communities associated with the holdfast of the kelp. Species richness remained similar across sites and levels of disturbance, but the dominant taxa were different. In the lowest impacted sites, kelp holdfast communities were dominated by Hydrozoa, while the highest impacted sites were dominated by Bryozoa.

Sedimentation has additional negative effects on the zoospores of brown algae, with spores attaching to the only substratum available. Hence, fine sediment could interfere with recruitment, by preventing and deterring spores’ attachment to a hard substratum; resulting in their subsequent loss due to waves and currents (Devinny & Volse, 1978, Norton, 1978; Bartsch et al., 2008). Field observations reveal that kelp is associated with accelerating sediment deposition and additionally prevent sediments being washed away because of their influence on local water current by increasing drag and thus particulate fall out (Airoldi, 2003 and references therein). However, this sediment is associated with the holdfasts of the kelp and not the fronds. At higher levels of wave exposure, whiplash by kelp is common, and this, in turn, reduces sediment accumulation at these sites (Kennelly 1989; Melville & Connell, 2001; cited in Airoldi, 2003).

Alaria esculenta

Picard et al. (2022) investigated the effects of different levels of sediment loading on Alaria esculenta zoospores in petri dishes. They placed seawater with different concentrations of sediment into the dishes: 0g (control), 1.5 and 2.5g of sediment in 50 ml seawater, which corresponded to approx. 0, 75, and 100% sediment coverage of the dish surface. The authors stated that the highest sediment level provided 100% cover of the dishes in a single-grain layer, therefore it was estimated that the sediment layer depth was 63 to 125 μm. Dishes were maintained in gently moving seawater and were kept in darkness (to exclude turbidity effects) for 89 hours. At the end of the experiment, attached spore density, gametophyte density, and the proportion of attached spores that had germinated (development ratio) were measured. While sediment levels had an insignificant effect on attached spore density and gametophyte density, the latter still saw a decrease from 14 gametophytes/mm² in the no sediment treatment to 10 gametophytes/mm² in the maximum sediment treatment. However, spore development ratio did significantly decline with sediment loading (from 38 to 24% between the no sediment and maximum sediment treatments, respectively). In summary, this study showed that a sediment layer of 63 to 125 μm was enough to significantly reduce the development of gametophytes, and that the impacts of sediment on Alaria esculenta recruitment success occurs after the settlement stage (Picard et al., 2022).

Zacher et al. (2016) investigated the combined effects of warming, grazing, and sedimentation on early life stages of Alaria esculenta. In their full factorial experiment, they tested temperatures of 5 and 9°C with and without the grazing gastropod Margarites helicinus, and three levels of sedimentation (no sediment, 1 g, and 10 g per petri dish). They measured the proportion of germinated spores after five days and counted the density of juvenile sporophytes after 14 days. The authors observed a statistically significant interaction between temperature and grazing, but no interaction was observed between sedimentation and the other variables. By itself, however, sedimentation had a significant negative effect on germination. In the 5°C grazed treatment, germination was the same between no sediment and 1g sediment, but it was significantly lower with 10g sediment. The 9°C treatment showed similar patterns with and without grazers as the 5°C ungrazed treatment, where germination declined with increasing levels of sedimentation, but the decline was less pronounced in the 9°C treatments. Sedimentation also negatively affected the density of sporophytes across all temperature and grazing levels. Sporophyte density was zero in all 10g sediment treatments apart from in the grazed treatments.

Laminaria digitata

An experiment by Roleda et al. (2008) illustrated potential benefits to low levels of siltation including UV protection for Saccharina latissima for short periods. When burial under a variety of sediment types was extended beyond 7 days, symptoms of bleaching, tissue loss and diminished photosynthetic function were exhibited (Roleda & Dethleff, 2011). A layer of fine-grained sediment (0.1 to 0.2 cm thick) caused rotting of Saccharina latissima and 25% mortality after 4 weeks of coverage in a laboratory experiment. Saccharina latissima is considered to be more silt tolerant than Laminaria digitata, suggesting that in locations of low wave and current mediated water flow, sedimentation is a threat to this biotope (Lyngby & Mortensen 1996). However, this study was carried out on disc samples from the thalli placed in petri dishes with no current flow. It is unlikely that this biotope would be found in such conditions of low flow, therefore the relevance of this study is questionable, and it is unclear how a whole plant would respond to siltation.

Sensitivity Assessment

While the evidence shows that ‘light’ deposition of up to 5 cm of sediment (as per the pressure benchmark) could have a severe impact on Alaria esculenta, the high degree of wave exposure that defines this biotope is likely to clear 5 cm of deposited sediment quickly (within one tidal cycle). Even the least exposed examples of IR.HIR.KFaR.Ala.Ldig could be resistant to smothering due to their presence on steep and vertical rocks. If low water clarity is persistent and wave exposure decreased, then low energy silted kelp biotopes (IR.LIR.K) may proliferate. Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and the biotope is probably ‘Not Sensitive’ at the benchmark level.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Alaria esculenta is not found in areas of siltation and sediment scour (Birkett et al., 1998). Increased siltation and sediment scour inhibits photosynthesis and algal growth, interfere with spore or larval recruitment plus smother germlings and gametophytes (Fletcher 1996). However, the high degree of wave exposure that typically defines this biotope is likely to clear suspended sediments relatively quickly. If low water clarity is persistent and wave exposure decreased, then low energy silted kelp biotopes (IR.LIR.K) may proliferate. Once siltation returns to its pre-effect level the biotope is likely to recover its canopy within a year and the rest of the community in no more than five years. Increased siltation will also increase turbidity.

Melting glaciers in the Arctic due to global warming release turbid, fresh-melt waters and inorganic material into the water column. In a study by Ronowicz et al. (2018), this had no effect on arctic kelps (including Laminaria digitata), but it did significantly affect the composition of faunal communities associated with the holdfast of the kelp. Species richness remained similar across sites and levels of disturbance, but the dominant taxa were different. In the lowest impacted sites, kelp holdfast communities were dominated by Hydrozoa, while the highest impacted sites were dominated by Bryozoa.

Sedimentation has additional negative effects on the zoospores of brown algae, with spores attaching to the only substratum available. Hence, fine sediment could interfere with recruitment, by preventing and deterring spores attachment to a hard substratum; resulting in their subsequent loss due to waves and currents (Devinny & Volse, 1978, Norton, 1978; Bartsch et al., 2008). Field observations reveal that kelp is associated with accelerating sediment deposition and additionally prevent sediments being washed away because of their influence on local water current by increasing drag and thus particulate fall out (Airoldi, 2003 and references therein). However, this sediment is associated with the holdfasts of the kelp and not the fronds. At higher levels of wave exposure, whiplash by kelp is common, and this, in turn, reduces sediment accumulation at these sites (Kennelly 1989; Melville & Connell, 2001; cited in Airoldi, 2003).

Laminaria digitata

An experiment by Roleda et al. (2008) illustrated potential benefits to low levels of siltation including UV protection for Saccharina latissima for short periods. When burial under a variety of sediment types was extended beyond 7 days, symptoms of bleaching, tissue loss and diminished photosynthetic function were exhibited (Roleda & Dethleff, 2011). A layer of fine-grained sediment (0.1 to 0.2 cm thick) caused rotting of Saccharina latissima and 25% mortality after 4 weeks of coverage in a laboratory experiment. Saccharina latissima is considered to be more silt tolerant than Laminaria digitata, suggesting that in locations of low wave and current mediated water flow, sedimentation is a threat to this biotope (Lyngby & Mortensen 1996). However, this study was carried out on disc samples from the thalli placed in petri dishes with no current flow. It is unlikely that this biotope would be found in such conditions of low flow, therefore the relevance of this study is questionable, and it is unclear how a whole plant would respond to siltation.

While 5 cm of sediment coverage may be transported from the biotope relatively quickly, a deposition of 30 cm is likely to remain in place for a longer period of time, especially in wave sheltered examples of the biotope. Therefore, heavy siltation may have a greater effect on the health of the biotope, resulting in smothering of the epifauna and flora, the red algae community and holdfast fauna in particular.

Sensitivity Assessment

While the evidence shows that ‘heavy’ deposition of up to 30 cm of sediment (as per the pressure benchmark) could have a severe impact on Alaria esculenta, the high degree of wave exposure that defines this biotope is likely to clear the deposited sediment quickly (within one tidal cycle). Even the least exposed examples of IR.HIR.KFaR.Ala.Ldig could be resistant to smothering due to their presence on steep and vertical rocks. If low water clarity is persistent and wave exposure decreased, then low energy silted kelp biotopes (IR.LIR.K) may proliferate. Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and the biotope is probably ‘Not Sensitive’ at the benchmark level.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline) (Litter pressure definition). 

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT (Electromagnetic pressure definition).

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. There have been no studies examining the effect of EMFs on macroalgae. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity, and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure.

Sensitivity Assessment

Given the lack of data at the level of individual biotopes, resistance and resilience to EMFs cannot be robustly assessed. Sensitivity is therefore recorded as ‘Insufficient Evidence’.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Not relevant

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means (Introduced light or shade pressure definition).

Evidence

It is feasible that localised light sources (e.g. post or harbour side lighting) might increase the length of time available for photosynthesis in shallow examples of the biotope that are in the vicinity. The effects of artificial light on kelp are not yet fully understood, but there is now a growing body of evidence to show that artificial light at night (ALAN) is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50m (Davies et al., 2020; Smyth et al., 2021).

ALAN has been shown to change the timing of Ascophyllum nodosum and Fucus serratus reproduction, with receptacles (the reproductive tissues of fucoid macroalgae) continuing to ripen into the winter months instead of peaking in the summer (Moyse et al., 2025). This change in the timing of reproduction could result in gametes being released during suboptimal conditions, such as winter storms, and therefore reduce fertilisation success. Reduced recruitment may lead to shifts in macroalgal assemblages in favour of species which are less sensitive to ALAN, such as Fucus vesiculosus, which seems to be unaffected (Moyse et al., 2025). ALAN can also vary significantly on small spatial scales and therefore affect some macroalgal forests more than others even if they are close to one another. It is therefore possible that ALAN could cause changes in macroalgal assemblages over time.

Shading of the biotope (e.g. by coastal development) could adversely affect the biotope in areas already low in water clarity. This may lead to a decline in Alaria esculenta abundance in shaded areas and a shift in dominance towards shade-tolerant red algae or faunal turfs. Staehr & Wernberg (2009) showed that, across typical coastal water clarity, an increase in the diffuse light attenuation coefficient K_D from 0.1 /m to 0.2 /m (representing a shift from clear to slightly more turbid water) can reduce the depth limit of the kelp Ecklonia radiata by around 50%.

Alaria esculenta

Alaria esculenta gametophytes cultured from sporophytes originating from Spitsbergen, Norway, showed no significant growth and no gametogenesis under natural winter conditions (2°C in complete darkness for 60 days) (Silva et al., 2022). In the first 38 days, gametophyte density decreased by 42%, suggesting a strong lack of resistance to darkness, but density did not continue to decrease for the remaining 22 days of darkness, indicating that gametophyte density was sensitive to darkness only to an extent. Gametophytes did not become fertile after returning to spring conditions (5°C a 12:12-hour light:dark regime for 36 days), suggesting that sporogenesis, reproduction and recruitment must occur during summer or autumn. In their full factorial experiment, Silva et al. (2022) also tested the combined effects of light intensity, temperature, and nutrient availability on Alaria esculenta gametophytes. The experiment crossed three irradiance levels (0.5, 35 and 100 μmol photons m²/s), two temperatures (5°C control and 9°C future-warming treatment), and two nutrient regimes: low nutrients (2 μM N and 0.2 μM P) and an unspecified high-nutrient treatment simulating increased inputs from mixing, river discharge, and permafrost melt. At 5°C under low nutrient conditions, gametophyte density decreased significantly with increasing irradiance. In contrast, at 9°C under low nutrient conditions, densities were similar under low and medium light levels, and higher under high light than at 5°C. Under high nutrient conditions, densities were generally lower overall, but high irradiance still resulted in reduced gametophyte density, particularly at 9°C. These findings indicate that the effects of irradiance may depend on temperature and nutrient availability.

Gordillo et al. (2022) observed an approximate 50% loss of Alaria esculenta biomass at 8°C, which was 21% more than the biomass loss of those at 3°C, in an experiment simulating the arctic winter. Towards the end of the dark period, Alaria esculenta produced new blade tissue in the 3°C treatment which continued to grow for two months after light returned. However, in the 8°C treatment, only the holdfast and midrib were left, and growth was unable to resume, so further measurements were not possible. Physiological responses to prolonged darkness included declines in respiration and photosynthetic performance (both of which were more pronounced with warming), chlorophyll-a, and carbon content. Nitrogen content remained unaffected.

Similarly, Cobos et al. (2026) simulated a transition out of polar night, where kelps spend two months in total darkness, followed by increasing light exposure. In both 1°C and 5°C treatments, biomass fell to 70 to 80% of the starting fresh weight, while respiration photosynthetic parameters, and pigment concentrations declined similarly. However, the loss of nitrogen in 5°C was double that of the 1°C treatment, while the loss of nitrate reserves was much lower. In addition, carbon content decreased slightly instead of increasing. In the 2-week light period following the dark period, Alaria esculenta growth resumed in both treatments, more so in the cold treatment. The 1°C treatment saw rapid photosynthetic reactivation, positive carbon balance, and a strong growth recovery, while the warming scenario resulted in higher respiration costs, greater nitrogen depletion, loss of photosynthetically active tissue, and a much lower growth recovery. This shows that warming can have a strong negative effect on Alaria esculenta even in colder regions, and this effect can persist even after light levels return to normal. This indicates that recovery after periods of darkness may be rapid under colder conditions and more constrained when darkness is combined with elevated temperature.

Ronowicz et al. (2022) found morphological differences between Alaria esculenta plants at 5 and 10 m depth at sites of varying levels of glacial influence. At the site with the most glacial influence (frequent iceberg scour and the highest load of suspended sediment), Alaria esculenta plants were significantly shorter, had reduced biomass, and had less morphological variability within the population. The authors observed no significant difference in these metrics between the non-impacted and intermediate-impacted (high sediment input alone) sites, suggesting that ice scour was the cause of the reduced size of Alaria esculenta at the highest-impacted site. Interestingly, the intermediate-impacted site had the greatest overall blade length and biomass values across the three levels of impact. Moreover, these metrics were greater at 10 m depth than at 5 m. While the levels of suspended sediment were not quantified, these morphological adaptations could serve as indirect evidence of resistance to suspended sediments and a reduction of light availability.

The development of Alaria esculenta gametophytes into sporophytes can also vary with the amount of light they receive during the gametophyte stage. Martins et al. (2022) showed that gametophyte growth increased with increasing daylength to a limit. While continuous light with no dark period resulted in faster growth, it also produced abnormal recruits with irregular form and reduced pigmentation. They also found that the direction of photoperiod effects on Alaria esculenta changed with life stage. Sporophytes that developed from the gametophytes under the increased daylengths grew slower than those that were cultured under the reduced daylengths. In addition, temperature was found to have an interactive effect with light. Elevated temperatures reduced growth, therefore exacerbating the effects of suboptimal light regimes experienced during the gametophyte stage.

Laeseke et al. (2019) investigated whether Alaria esculenta canopies protect kelp spores from harmful solar radiation in Kongsfjorden, Norway. They measured underwater light at 3 m depth beneath different canopy densities (very dense, dense, intermediate, sparse, and no canopy) and then used these measurements for realistic laboratory simulations. Alaria esculenta zoospores were then exposed to each light regime for 4, 8, 16, or 24 hours at ambient seawater temperature (4°C). Germination remained largely unaffected, so it was concluded that natural radiation conditions in Kongsfjorden are unlikely to severely reduce recruitment, despite some physiological parameters being affected.

UV exposure can have an interactive effect with increased CO₂ levels in the water column. Gordillo et al. (2015) investigated the combined effects of UV radiation and ocean acidification on Alaria esculenta from Kongsfjorden. The experiment consisted of two levels of light and acidification which were fully crossed: PAR at 30 μmol m²/s with no UV, and PAR with UV supplementation (2 W/m² UVA and 0.2 W/m² UVB); ambient CO₂ (pH 8.1 to 8.3) and high elevated CO₂ (pH 7.7 to 7.8). Alaria esculenta growth rate increased by 57% in the UV-enriched + elevated CO₂ treatment, while the UV-enriched alone and the elevated CO₂ alone reduced growth compared to the no-UV + low-CO₂ treatment, therefore indicating a synergistic effect. Photosynthetic efficiency was not affected in the low-CO₂ treatments, while the high-CO₂ treatments saw a significant decline after two days, most notably in the UV-enriched + elevated CO₂ treatment, indicating stress. However, after six days of exposure, photosynthetic efficiency recovered almost to pre-exposure levels. Pigment concentrations (chlorophyll-a and carotenoids) declined under all treatments, but losses were greatest under elevated CO₂ alone and were moderated in UV-exposed treatments. Internal carbon content increased under both elevated CO₂ and UV exposure, while nitrogen content and nitrate reductase activity were largely unaffected by treatment. Overall, increases in UV or CO₂ alone cause physiological stress in Alaria esculenta, but when they are combined, they are beneficial.

Lebrun et al. (2024) investigated the combined effects of warming, salinity, and light on four kelp species, including Alaria esculenta. For six weeks, kelps collected from Kongsfjorden, were exposed to an unmanipulated control treatment which had ambient seawater flowing into the tank at roughly 5.3°C, 33.8 PSU, with an irradiance of 47.8 µmol photons/m²/s, and three additional treatments: T1 = moderate warming (+3.3°C), reduced salinity (-2.5 PSU, and reduced light (-25% irradiance); T2 = stronger warming (+5.3°C), greater salinity reduction (-5 PSU), and lower light (-40% irradiance); and T3 = stronger warming alone (+5.3°C) without changes in salinity or irradiance. These conditions were chosen to reflect future global warming scenarios in which warming increases ice-melt, causing an addition of freshwater and suspended sediments and particles into the marine environment. Chlorophyll-a concentration declined in the control, reflecting normal seasonal changes, and showed similar patterns in T1 and T3. However, chlorophyll-a remained significantly higher in T2 (stronger warming with reduced salinity and light). Growth rates and carbon:nitrogen ratios were stable across the experiment. These results suggest that Alaria esculenta could potentially be resistant to the combined effects of global warming, reduced salinity, and reduced light.

Laminaria digitata

Light availability has been found to buffer the effects of thermal stress. Bass et al. (2023) investigated the combined effects of 4-week marine heatwaves, light and season on Laminaria digitata, Laminaria hyperborea, and Laminaria ochroleuca. Control (ambient) temperatures were 10°C and 18°C for the spring and summer treatments, respectively. Moderate marine heatwave treatments were 2°C higher than ambient, and strong marine heatwave treatments were 4°C higher than ambient. Low light treatments were at approx. 8 μmol photons/m²/s, and high light treatments were at approx. 75 μmol photons/m²/s. In the spring experiment, Laminaria digitata showed a 50.5% increase in mean biomass and a 165% increase in mean surface area in all temperature treatments under the high light treatment. In the low light treatments, the increase was significantly less (28%). Photosynthetic efficiency (F_V/F_M) ranged between 0.82 and 0.69 in all treatments during the spring experiment, indicating no physiological stress in Laminaria digitata. In the summer experiment, biomass and surface area showed minimal changes in all temperatures in the high light treatment. Photosynthetic efficiency only changed significantly in the highest temperature treatment but did not indicate stress. However, in the low light treatment, a significant decline in biomass (approx. 20%) was observed in the +2°C treatment, and all plants were reported to have completely disintegrated in the +4°C treatment by the end of the 4-week experiment. Surface area marginally increased in the ambient temperature treatment with low light but decreased by approx. 40% in the +2°C treatment. Photosynthetic efficiency remained stable in the ambient temperature treatment but declined significantly (approx. 0.25), however still within a healthy range in the +2°C treatment. These findings indicate that Laminaria digitata may be more resilient to summertime marine heatwaves if sufficient light is available, and that an introduction of shading could make this biotope less resistant to increases in temperature. An introduction of shading (e.g. development of coastal infrastructure) could have significant consequences for this biotope, especially in combination with other pressures such as elevated temperatures. Trautmann et al. (2024) investigated the effects of temperature (0°C control and 5°C warming) on arctic Laminaria digitata which were subjected to total darkness for three months. Changes in photosynthetic performance differed significantly between the temperature treatments; values indicated good health throughout the experiment at both temperatures, though values declined slightly at 5°C compared to an overall increase at 0°C. Carbohydrate reserves showed marked depletion under warming: mannitol decreased by approx. 37% at 0 °C but by approx. 65% at 5 °C, while laminarin dropped by approx. 40% and approx. 90%, respectively. Pigments also declined more strongly at 5°C, particularly xanthophyll cycle pigments, and C:N ratios fell from approx. 20 to 13.4 at 5°C versus 17.9 at 0°C. These results suggest that Laminaria digitata tolerates prolonged darkness well, but experiences accelerated metabolic demand and resource depletion under warmer winter conditions. While Arctic conditions differ to those experienced by this Laminaria digitata biotope around the UK, this evidence still suggests that temperatures at 5°C above the usual could increase energy depletion in the kelp under light-limited conditions.

In contrast, Schmid et al. (2021) found that Laminaria digitata growth was negatively affected by elevated temperatures and high light conditions. The highest relative growth rate (approx. 1.92%/day) occurred at 15 °C under high light (90 μmol photons/m²/s). At 20°C, growth ceased entirely under medium and high light. Pigment concentrations, particularly chlorophyll-a, were also affected by temperature-light interactions, dropping from approx. 2.09 mg/g dry weight at 15°C/high light to approx. 0.45 mg/g at 20°C/high light. Fatty acid profiles showed similar stress responses: polyunsaturated fatty acids (PUFA) accounted for approx. 50 to 55% of total fatty acids at lower temperatures but declined sharply to approx. 33.4% at 20°C. These results suggest that while Laminaria digitata performs optimally under moderate temperatures and high light.

Similar results were found by Silva et al. (2022), who found that elevated temperature (+4°C above typical temperatures) and high light conditions (100 μmol photons/m²/s) reduced reproduction and sporophyte recruitment in Arctic Laminaria digitata.

Sensitivity Assessment

Alaria esculenta has mixed responses to increases in light, for example due to increased solar radiation, reduced turbidity, the removal of coastal infrastructure, or artificial light sources. Changes in light regimes have also been shown to produce abnormal recruits with lower growth rates (Martins et al., 2022), while increased irradiance can reduce gametophyte density under elevated temperatures and nutrient concentrations (Silva et al., 2022). Elevated UV levels may additionally cause physiological stress (Gordillo et al., 2015). There is currently no evidence of Alaria esculenta or Laminaria digitata’s response to artificial light at night (ALAN). While Laminaria digitata responds well to increases in incident light, evidence from other macroalgae suggests that ALAN can alter reproductive timing and affect assemblage structure (Moyse et al., 2025).

Therefore, the biotope’s resistance to an increase in light is assessed as ‘Medium’ as a precaution due to the potential effects of ALAN on the characterising species’ reproductive cycles, resilience as ‘High’ if the source of the light is removed quickly, but ‘Very Low’ if permanent. Therefore, in a worst-case scenario where an increase in light is permanent, sensitivity is assessed as ‘Medium’.

Shading of the biotope would certainly negatively affect Alaria esculenta and Laminaria digitata. The evidence shows that Alaria esculenta does not grow and cannot produce gametes in prolonged periods of darkness (Silva et al., 2022), whilst also experiencing declines in physiological performance and biomass, and that recovery from darkness can be constrained in higher temperatures (Gordillo et al., 2022; Cobos et al., 2026). Therefore, resistance to shading is ‘Low’ and resilience is ‘High’ if the source of the shading is removed quickly, but ‘Very Low’ if permanent. Therefore, in a worst-case scenario where shading is permanent, sensitivity to shading is assessed as ‘High’.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion (Barrier to species movement pressure definition).

Evidence

Not relevant. This pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of spores but spore dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure (Death for collision pressure definition).

Evidence

Not relevant.  Collision from grounding vessels is addressed under abrasion above.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature (Visual disturbance pressure definition). 

Evidence

Not relevant

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species may result in changes in the genetic structure of local populations, hybridization, or a change in community structure (Translocation pressure definition).

Evidence

No evidence was found

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus) (pathogen or disease pressure definition).

Evidence

There is no evidence in the literature that infection by microbial pathogens results in the mass death of kelp populations and the kelp themselves are known to regulate bacterial infections through iodine metabolism (Cosse et al., 2009).

Laminariocolax aecidioides (syn. Streblonema aecidioides and Gononema aecidioides) is a filamentous brown alga that causes spot disease in kelps (Lein et al., 1991). Laminariocolax sp. (syn. Entonema, Streblonema, or Myrionema) has been found in Alaria esculenta, Laminaria digitata, and Saccharina latissima (studied as Laminaria saccharina) (Levring, 1937; Jaasund, 1965; Jorde, 1975; cited in Lein et al., 1991).

The endophytic brown microalga Microspongium alariae has been described to “infect” the tissues of many kelp species including Alaria esculenta, sometimes causing the appearance of warts (Murúa et al., 2018). However, it is not currently known if this infection is detrimental to the host kelp. Microspongium alariae has been found in Alaria esculenta hosts in Scotland and in Brittany (France).

Evidence for a new group of phaeoviruses (viruses which affect macroalgae) infecting kelps was first reported by McKeown et al. (2017). Within this group (sub-group C) is the type species referred to as LdigV, which targets the nucleus in the cells of Laminaria digitata for its genome replication. This infection results in the degradation of chloroplasts and the assembly of virions in the cytoplasm of vegetative and reproductive cells. McKeown et al. (2017) found that 64.7% of sporophytes and 23.2% of gametophyte mixes (both from all three kelp species sampled) tested positive for the phaeovirus.

Infection of Laminaria japonica sporophytes by Pseudoalteromonas, Vibrio and Halomonas result in the characteristic symptoms of hole-rotten disease (Wang et al., 2008). Additionally, red spot disease may be caused by bacteria of the genus Alteromonas (Sawabe et al., 1998). Hyperplasia or gall growths are often seen as dark spots on Laminaria digitata and have been associated with endophytic brown filamentous algae. It can be inferred from these observations that microbial pathogens may impact growth rates of individuals.

Evidence of fungal pathogens has been reported for Laminaria digitata. Kononenko et al. (2022) reported traces of mycotoxins (toxic compounds produced by fungi) such as ergot alkaloids (produced by Claviceps spp.) and aflatoxins (produced by Aspergillus spp.). However, no signs of infection were reported.

Sensitivity Assessment

There was no evidence of mortality in Alaria esculenta or Laminaria digitata due to microbial infections. Resistance and resilience are ‘High’, so the biotope is deemed to be ‘Not Sensitive’ to this pressure.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale (targeted removal pressure definition).

Evidence

Alaria esculenta has recently received commercial interest as a consumable product called “Sea Vegetables” or “Atlantic Wakame Kelp”. However, no studies examining the effect of commercial extraction of Alaria esculenta biotopes were found. Removal of the algal canopy would expose the understorey fauna and flora to increased desiccation during low tide. Experimental macroalgal canopy removal experiments conducted in the Isle of Man (Hawkings & Harkin, 1985) found that following the removal of the macroalgal canopy the understorey encrusting red algae became bleached and died within a week.

Traditionally Laminaria digitata was used on agricultural lands as fertilizers; now Laminaria spp. are used in a range of different products, with their alginates used in the cosmetic, pharmaceutical and agri-food industries (Kervarec et al.,1999; McHugh, 2003). Collection of Laminaria digitata by mechanical harvesting (trawling) is not done in the UK, (Netalgae, 2012). Trawling, used to harvest Laminaria hyperborea in Norway results in whole alga being removed from the substratum, and substantial scouring of the substratum, indicating that the use of trawls in a Laminaria digitata biotope is likely to detrimentally affect the biotope, regardless of the target species. In France, Laminaria digitata is harvested with a ‘Scoubidou’, a curved iron hook which is mechanically operated. This device is considered to be selective; only harvesting individuals older than 2 years (Arzel, 2002). France reportedly harvests 75,000t kelp, mainly consisting of Laminaria digitata annually (FAO, 2007). Davoult et al. (2011) suggested that the maintenance of a sustainable crop of Laminaria digitata was possible if the industry continues employing small vessels evenly dispersed along the coastline. This could protect against habitat fragmentation and buffer over exploitation (Davoult et al., 2011). A fallow period of 18 to 24 months has been suggested for Laminaria digitata in France, where competition between the juvenile sporophytes of Laminaria digitata and Saccorhiza polyschides was indicated as a threat to the continued harvesting effort of Laminaria digitata (Engelen et al., 2011).

Canopy removal of Laminaria digitata has been shown to reduce shading, resulting in the bleaching of sub-canopy algae (Hawkins & Harkins, 1985). Harvesting may also result in habitat fragmentation, a major threat to this biotope’s ecosystem functioning (Valero et al., 2011).

Sensitivity Assessment

Both characterising species are targeted for commercial use, although evidence of the effects of harvesting Alaria esculenta was lacking. If it is assumed that the effects of Alaria harvesting are similar to those of Laminaria digitata harvesting, then the biotope’s resistance would be assessed as ‘None’, with ‘Medium’ resilience and therefore ‘Medium’ sensitivity.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale (non-targeted removed pressure definition).

Evidence

Incidental/accidental removal of Alaria esculenta is likely to cause similar effects to that of direct harvesting; hence, the same evidence has been used for both pressure assessments.

Alaria esculenta has recently received commercial interest as a consumable product called “Sea Vegetables” or “Atlantic Wakame Kelp”. However, no studies examining the effect of commercial extraction of Alaria esculenta biotopes were found. Removal of the algal canopy would expose the understorey fauna and flora to increased desiccation. Experimental macroalgal canopy removal experiments conducted in the Isle of Man (Hawkings & Harkin, 1985) found that following the removal of the macroalgal canopy the understorey encrusting red algae became bleached and died within a week. Mytilus edulis settlement has also been found significantly higher in close proximity to Alaria esculenta and is thought to increase beneath Alaria esculenta canopies (Bégin et al., 2004).  Therefore, any loss of Alaria esculenta, as a result of commercial extraction, may dramatically affect the understorey community.

Traditionally Laminaria digitata was added to agricultural lands as fertilizers; now Laminaria species are used in a range of different products, with its alginates used in the cosmetic, pharmaceutical and agri-food industries (Kervarec et al., 1999; McHugh, 2003). Laminaria digitata is harvested with a ‘Scoubidou’ (a curved iron hook which is mechanically operated) in France. This device is considered to be selective- only harvesting individuals older than 2 years (Arzel, 2002). France reportedly harvests 75,000t kelp, mainly consisting of Laminaria digitata annually (FAO, 2007). The loss of Laminaria digitata would represent a significant change to IR.HIR.KFaR.Ala.Ldig.

Corallina officinalis is collected for medical purposes; the fronds are dried and converted to hydroxyapatite and used as bone forming material (Ewers et al., 1987). It is also sold as a powder for use in the cosmetic industry. Moderate trampling on articulated coralline algal turf in the New Zealand intertidal (Brown & Taylor, 1999; Schiel & Taylor, 1999) resulted in reduced turf height, declines in turf densities, and loss of crustose bases in some case probably due to loss of the canopy algae and resultant desiccation. Calcification is thought to be an adaptation to grazing and sediment scour (Littler & Kauker 1984). Corallina officinalis produces spores over a protracted period and can colonize artificial substratum within one week in the intertidal (Harlin & Lindbergh 1977; Littler & Kauker 1984). The crustose base enables Corallina officinalis to survive the loss of fronds.

Sensitivity assessment. There is little evidence for the effects of commercial harvesting of Alaria esculenta. If it is assumed that all canopy-forming kelp are removed then resistance would be assessed as ’None‘, resilience would be assessed as ’Medium‘. Sensitivity has been assessed as ’Medium‘. Within IR.HIR.KFaR.AlaAnCrSp and IR.HIR.KFaR.Ala.Myt, monospecific canopies of Alaria esculenta are expected to recover quicker than mixed canopies of Laminaria digitata (as in IR.HIR.KFaR.Ala.Ldig) Sensitivity of the latter would be assessed as follows; resistance ’None‘, resilience as ’High‘, Sensitivity as ’Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890 and has spread into waters around mainland Europe (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 1999, 2018; Hinz et al., 2011; Helmer et al., 2019; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015). It ranges from the Baltic Sea, the Kattegat and Skagerrak, the North Sea coasts of the UK, Germany, and Belgium, through the English Channels and into the Irish sea coasts of Ireland and south Wales with records in east and west Scotland, Northern Ireland, northwest France, Spain and south into the Mediterranean (NBN, 2024; OBIS, 2026).

Abundances at its northern and southern extremes may be low but densities in UK and France are often over 1000 individuals/m² and it may carpet the seafloor in the Solent and Essex. In the UK, it was reported to reach abundances of >1000 individuals/m² (max. 2,748 individuals/m²) in the Milford Harbour Waterway (Bohn et al., 2012), 84 individuals/m² in Portsmouth, 174 individuals/m² in Langstone and 306 individuals/m² in Chichester harbours in 2017 (Helmer et al., 2019). In France, it has been reported to reach >4,700 individuals/m² in the Bay of Marennes-Oleron, France, 11.6 tonnes/ha in Bay of Mont-Saint-Michel, 8.2 tonnes/ha in the Bay of Brest and 2.8 tonnes/ha in the Bay of Saint-Brieuc (Blanchard, 2009; Bohn et al., 2012, 2015; Powell-Jennings & Calloway, 2018).

Its density and ability to spread within and between sites (e.g., bays) depend on the availability of suitable habitat, competition with other species, larval retention within the site, human activities (e.g., dredging), and seasonal temperatures, particularly in the intertidal zone. For example, the Crepidula fornicata population in the Bay of Mont-Saint-Michel grew by 50% between 1996 and 2004, covering 25% of the area at high density (51 to 100% cover), aided by local oyster farming and shellfish dredging (Blanchard, 2009). However, in Arcachon Bay, France, Crepidula fornicata was limited to only 155 tonnes in 1999 and 312 tonnes in 2011 (De Montaudouin et al., 2001, 2018). It was confined to muddy sediments, which accounted for only approx. 8% of the bay and were colonized by Zostera beds. These areas represented just 0.4% of the suspension feeder biomass compared to the oysters Magallana gigas in the bay, and there was no indication of increasing biomass over a 12-year period. In addition, benthic trawling was prohibited in the bay (De Montaudouin et al., 2001, 2018). As a result, De Montaudouin et al. (2018) concluded that Crepidula fornicata was not invasive in the Bay of Arcachon.

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (from 18 to 40 PSU) although it prefers around 30 PSU (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. It has also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020).

In the eastern Solent harbours of Portsmouth, Langstone, and Chichester, 75% to 98% of Crepidula larvae settled on dead Crepidula shells, while approx. 4% settled on stone, 2.5% on live Crepidula, 0.3% oyster shell, 0.6% cockle shell, 0.3% winkle shell and 0.1% periwinkle shell (Preston et al., 2020). In the Milford Harbour Waterway, the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell, or gravel or both but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). However, gravel formed the base of most stacks of Crepidula in the intertidal, which suggested that initial colonization occurred on available hard substrata (i.e., gravel) in the absence of adult shells of Crepidula. The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula functions as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). Bohn et al. (2015) also noted that Crepidula density was low in areas of homogenous fine sediment and absent in areas dominated by boulders.

Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. However, Hinz et al. (2011) recorded Crepidula off the Isle of Wight in the English Channel, at approx. 60 m on rough ground in areas of high tidal flow. Tillin et al. (2020) suggested that the effect of oscillatory wave meditated flow might have a greater effect on Crepidula than tidal flow, presumably due to mobilization of the substratum. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at Swansea east site (Powell-Jennings & Calloway, 2018).

Crepidula fornicata has been recorded from the lower intertidal to approx. 160 m in depth but it most common in the shallow subtidal and low water springs (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2026; Tillin et al., 2020). Bohn et al. (2012, 2013a, 2013b, 2015) suggested that extreme conditions in the intertidal limited its upward distribution due to early post-settlement mortality. It reached its highest densities in the lower shore (below approx. 0.7 m) and was absent from high tidal level (approx. 1.8 m) in the Milford Harbour Waterway (Bohn et al., 2015).

The density of Crepidula populations in the northern Europe (Germany, Denmark, and Norway) is significantly lower (<100 individuals/m²) than in southern waters. Thieltges et al. (2004) reported that the population of Crepidula was affected strongly by cold winters in the Wadden Sea. The winters of 2001 and 2003 resulted in approx. 56 to 64% mortality of intertidal Crepidula and up to 97% on one mussel bed, compared to only 11 to 14% in southern areas without frost. Crepidula almost vanished from the Wadden Sea after the 1978/79 winter and took ten years to recover due to moderate winters which regularly affected the population. Similarly, 25% mortality was observed in Crepidula populations on the south coast of the UK after the extreme 1962/63 winter (Crisp, 1964, Bohn et al., 2012). Thieltges et al. (2003) suggested that global warming way allow Crepidula populations become more abundant in northern Europe. Valdizan et al. (2011) noted higher water temperatures between 2000 to 2001 and 2006 to 2007 together with elevated chlorophyll-a corresponded to an increase in gametogenesis and the duration of broods in Crepidula population in Bournerf Bay, France. They suggested that rising temperatures in northern Europe could increase its reproductive success due favourable breeding temperatures and increased phytoplankton (Valdizan et al., 2011).

There is currently a lack of evidence of Crepidula fornicata colonization on bedrock in the infralittoral or circalittoral. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’ it preferred shell, especially that of conspecifics.

Sensitivity Assessment

There is currently a lack of evidence of Crepidula fornicata colonization on bedrock in the infralittoral or circalittoral. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011), but considered high energy wave action and macroalgal communities would render these habitats unsuitable. At present, there is Insufficient Evidence to suggest that this biotope is sensitive to colonization by Crepidula fornicata.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ecosystem engineer that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009).

A lack of published descriptions and an incomplete historical record has led to the widespread misidentification of Didemnum vexillum and it is often recorded as Didemnum spp. Hence, the native range of the species is not known conclusively (Lambert, 2009; Stefaniak et al., 2012; McKenzie et al., 2017; Holt, 2024). However, molecular data and limited historical evidence have suggested that the species may be native to Japan with its native range possibly extending into continental Asia and north-western Pacific (Stefaniak et al., 2012; Tillin et al., 2020; Holt, 2024). Previously unrecorded populations of a colonial ascidian have been recently identified as Didemnum vexillum (Tillin et al., 2020).

Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; McKenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024).

In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Minchin & Nunn, 2013; Bishop et al., 2015; McKenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Didemnum vexillum has the ability to rapidly overgrow and displace on other sessile organisms such as other colonial ascidians (Ciona intestinalis, Styela clava, Ascidiella aspera, Botrylloides violaceus, Botryllus schlosseri, Diplosoma listerianium and Aplidium spp.), bryozoan, hydroids, sponges (Clione celata and Halichrondria sp.), anemone (Diadumene cincta), calcareous tube worms, eelgrass (Zostera marina), kelp (Laminaria spp. and Agarum sp.), green algae (Codium fragile subsp. fragile), red algae (Plocamium, Chondrus crispus and bush weed Agardhiella subulata), brown algae (Ascophyllum nodosum, Sargassum, Halidrys, Fucus evanescens and Fucus serratus), calcareous algae (Corallina officinalis), mussels (Mytilus galloprovincialis, Perna canaliculus and Mytilus edulis), barnacles, oysters (Magallana gigas, Ostrea edulis and Crassostrea virginica), sea scallops (Placopecten magellanicus), or dead shells (Dijkstra et al., 2007; Gittenberger, 2007; Valentine et al., 2007a; Valentine et al., 2007b; Griffith et al., 2009; Carman & Grunden, 2010; Dijkstra & Nolan, 2011; Groner et al., 2011; Hitchin, 2012; Tagliapietra et al., 2012; Minchin & Nunn, 2013; Gittenberger et al., 2015; Long & Grosholz, 2015; Vercaemer et al., 2015).

Didemnum vexillum has been found colonizing the stipes of Laminaria spp. in the Gulf of Maine (Dijkstra et al., 2007) and in Norway (Legrand et al., 2025). However, it has not been recorded in sites exposed to wave action, that is ‘very wave exposed’, ‘wave exposed’ and ‘moderately wave exposed’ (sensu MNCR, Hiscock, 1996), especially in the intertidal where wave action is not ameliorated by depth (see Hiscock, 1983).

This species requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and can establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock gravel, pebble, cobble, or boulders (Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020). It is therefore likely that the substrate in this biotope is suitable for Didemnum vexillum colonisation. In addition, the depth range at which Laminaria hyperborea biotopes are found (0 to 30 m) overlaps with the depth range that is suitable for suitable for Didemnum vexillum colonization. Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020).

Didemnum vexillum tolerates a wide range of environmental conditions including temperature and salinity (Herborg et al., 2009; Tillin et al., 2020). Didemnum vexillum can withstand a wide range of salinities from 20 to 44 PSU, is commonly found in marine waters around 33 PSU but is unable to survive in salinities below 20 PSU (Bullard & Whitlatch, 2009; Groner et al., 2011; Tillin et al., 2020). It has been recorded in estuarine conditions and tidal lagoons (Dijkstra et al., 2007; Tillin et al., 2020). In the Lagoon of Venice, Mediterranean, Didemnum vexillum is found in a mean salinity value of 30 PSU. It was absent in low salinity, such as the estuary and around the saltmarshes, but well established in the euhaline and tidally well flushed zones of the Lagoon of Venice (Tagliapietra et al., 2012). Similar results were found in Connecticut and Rhode Island where Didemnum vexillum was not found in environments with salinity less than 20 PSU (Bullard & Whitlatch, 2009). However, in the Wadden Sea, colonies of Didemnum vexillum were abundant in salinities between 17.91 to 25.97 PSU (Gittenberger, 2007; Gittenberger et al., 2015).

Didemnum vexillum is a temperate species that can survive a broad temperature range of -2 to 24°C, with an upper survival limit suggested to be 25°C (Bullard et al., 2007; Valentine et al., 2007a; Herborg et al., 2009; Kleeman, 2009; McKenzie et al., 2017; Holt, 2024). It thrives best at 14 to 20°C, with optimal growth temperature between 14 to 18°C during summer months (May, June, September, October) (Gittenberger, 2007; Kleeman, 2009; McKenzie et al., 2017).

Reinhardt et al. (2012) examined the effects of water flow and hydrodynamics on the encrusting and tendril forms of Didemnum vexillum. They reported that a current speed of approx. 7.6 m/s was required to induce fragmentation of tendrils, but that natural tidal flow alone was insufficient to cause fragmentation of tendrils. They suggested that rare instances of wave action such as storms that resulted in wave orbital velocities of approx. 8 m/s or (more likely) human activity could cause fragmentation of tendrils.

Reinhardt et al. (2012) noted that the tensile strength of Didemnum vexillum was an order of magnitude higher than Botrylloides sp. and was similar to that of Alcyonium digitatum. Alcyonium digitatum is reported from sheltered to very wave exposed conditions, but in the sublittoral. Reinhardt et al. (2012) also suggested that seasonal changes in the condition of Didemnum vexillum reduced the tensile strength of colonies and was associated with the period of greater larval production, and implied that fragmentation aided dispersal.

The oscillatory nature of wave-mediated water flow (wave orbital velocities) combined with wave pressure in the lacerating zone, where breaking wave causes multidirectional strong water movement (Hiscock, 1983), would probably dislodge and break up Didemnum vexillum colonies, prevent them from forming suffocating mats, and restrict the colonies to crevices and overhangs. However, it is unclear if moderately wave exposed conditions would be adequate to prevent Didemnum vexillum from developing extensive mats in the summer months when wave action is typically reduced. Hitchin (2012) suggested that the presence of Didemnum vexillum in Whitstable, Kent was contrary to its then known habitat preferences.

Sensitivity Assessment

There is no evidence of Didemnum vexillum colonizing this biotope in the UK. However, it has been recorded in similar kelp habitats in Norway (Järnegren et al., 2023). Didemnum vexillum requires hard substrata for successful colonization, so it could colonize the bedrock that characterizes this biotope. Didemnum vexillum can overgrow sessile organisms, including kelp Laminaria sp. However, no direct evidence was found on how Didemnum vexillum affects kelp or if it contributes to mortality (Järnegren et al., 2023), although epifaunal growth by Membranacea membrancea was reported to reduce the physical strength of Laminaria digitata fronds and make them susceptible to removal by wave action (Krumhansl et al., 2011). In addition, overgrowth by epiphytes contributed to the decline of Saccharina latissima in Norway (Andersen et al., 2011). However, Didemnum vexillum may compete for light and space with kelp and epifauna and could interfere with recruitment, which could lead to the mortality of some epifauna, the loss of kelp, and a reduction in biodiversity. Didemnum prefer sheltered conditions so the wave exposed and tidally swept conditions that characterize this biotope may mitigate its abundance. Therefore, a resistance of ‘Medium’ (some mortality, <25%) is suggested as a precaution. Resilience is likely to be ‘Very low’ as Didemnum vexillum would need to be physically removed to allow recovery. Hence, sensitivity to invasion by Didemnum is assessed as ‘Medium’. However, confidence in the assessment is ‘Low’ due to the lack of direct evidence of damage to kelp beds.

The Pacific oyster, Magallana gigas

Evidence

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Baron, 2010; GBNNSIP, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Baron, 2010; Padilla, 2010). 

It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSIP, 2011, 2012; Humphreys et al., 2014, cited in Alves et al., 2021; Hansen et al., 2023). It was also introduced to the northeast Adriatic Sea (Ezgeta-Balic et al., 2019) and southwest England from France, possibly via fouling on ships (GBNNSIP, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).

Magallana gigas has a high fecundity, a long-lived pelagic larval phase (2 to 4 weeks) and can produce up to 200 million eggs during spawning (Herbert et al., 2012, 2016; Alves et al., 2021; Wood et al., 2021; Hansen et al., 2023). Hence, as a broadcast spawner, it has a high dispersal potential of more than 1000 km (Padilla, 2010; Wood et al., 2021). Although larval mortality can be as large as 99% due to sensitivity to environmental conditions (Alves et al., 2021), adults are long-lived so that populations can survive with infrequent recruitment (Padilla, 2010).

Larval dispersal has facilitated the establishment of populations in various regions, such as the Oosterschelde estuary in the Netherlands and the Scandinavian coastlines, where northward drift on tidal and wind-driven currents has been suggested (Hansen et al., 2023). Offshore structures and aquaculture operations can enhance spread (Wood et al., 2021).

Magallana gigas is an ecosystem engineer and can dramatically change habitat structure when it invades. Once successfully settled, groups of Pacific oysters may form dense aggregations, potentially forming a reef, which in some regions can reach densities of 700 individuals/m² (Herbert et al., 2012, 2016). Once, the density of live or dead Pacific oysters reaches or exceeds 200 individuals/m², little of the underlying substratum remains visible (Herbert et al., 2016). These reefs can stabilize the sediment surface locally (Troost, 2010). When such reefs are formed or, particularly when the species colonizes soft sediments such as mud or sand, it can change and affect local communities, by creating hard substrata for mobile species, which might not otherwise be present before the invasion (Padilla, 2010). However, Hansen et al. (2023) suggested that no immediate ecosystem risk is observed where the Pacific oyster occurs sporadically.

Settlement requires hard substrata, including rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann, 2012, Kochmann et al., 2013; McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). Magallana gigas also attaches to available hard materials in mixed sediment environments such as shingle and sand within otherwise unsuitable mudflats (Spencer et al., 1994; McKinstry & Jensen, 2013; Tillin et al., 2020).

Populations of Magallana gigas have been found wave exposed rocky shores to wave sheltered soft sediment environments and it has been described as a habitat generalist (Troost, 2010; Kochmann, 2012, Kochmann et al., 2013). For example, in Scotland, wild Magallana gigas are mainly located in the lower intertidal on bedrock, bedrock encrusted with barnacles, within bedrock crevices, and large and small boulders (Cook et al., 2014). Patches of Pacific oyster reefs have been recorded on littoral rock in Kent, southern England and on littoral sediments in southern England, the North Sea, and the English Channel (Herbert et al., 2012, 2016; Morgan et al., 2021).

Magallana gigas has been reported from estuaries growing on intertidal mudflats and sandflats, and other soft sediments (Padilla, 2010; Herbert et al., 2016; Cabral et al., 2020). The settlement of spat on hard substrata within sediments has been observed in the estuaries of the River Dart, Exe, Fal, Fowey, Tamar, Teign, and Yealm in Devon and Cornwall, the Menai Straits, Wales and large estuaries of Lough Swilly, Lough Foyle and the Shannon in Ireland, and the Tagus Estuary in Portugal (Spencer et al., 1994; Kochmann, 2012, Kochmann et al., 2013; Cabral et al., 2020). In Lough Swilly, Lough Foyle and the Shannon, the Pacific oyster was often associated with intertidal mud or sandflats (Kochmann et al., 2013). In contrast, the Pacific oysters were absent from sandflat areas in Poole Harbour (McKinstry & Jensen, 2013).

Although shorelines comprised of mainly mud were suggested to be unsuitable for spat settlement (Spencer et al., 1994), the presence of smaller hard substrata, such as shells or pebbles, can enable larvae to settle (Tillin et al., 2020). For example, in the River Teign estuary, Pacific oyster settlement was observed on shell-covered ground mainly attached to mussel shells, and occasionally attached to cockles, stones and common periwinkle (Littorina littorea) shells on a mud flat in the estuarine intertidal zone otherwise mainly comprised of sand and mud (Spencer et al., 1994). In addition, the Blue Lagoon on the north shore of Poole Harbour had the highest abundance of oysters on mud mixed with shingle and shell (McKinstry & Jensen, 2013). Outside of the Blue Lagoon, oysters were also recorded on mixed substrata composed of mud, gravel, and shell (McKinstry & Jensen, 2013). Tillin et al. (2020) concluded that while successful invasions occurred on mudflats, Magallana gigas prefers mixed substrata. Fine mud sediments without hard substrata (such as small stones, gravel, and shell) are unlikely to be suitable (Tillin et al., 2020).

The speed of Magallana gigas reef formation on soft substrata seems to be dependent on the amount of hard substrata present (Troost, 2010). Bergstrom et al. (2021) reported that the presence of Magallana gigas was partially dependent on increasing gravel content up to 15% but remained stable with increasing percentages (measured up to 80%).

While often described as an intertidal and shallow subtidal species, Magallana gigas has been observed across a broader depth range. Although rocky habitats deeper than 10 m are generally considered unsuitable, it has been recorded down to 42 m in the Oosterschelde, Netherlands (Herbert et al., 2012, 2016; Tillin et al., 2020; Smaal et al., 2009).

It frequently occurs between Mean High Water and Mean Low Water in intertidal zones but has also been recorded at 1 to 10 m depth in regions like Sweden, Ireland, and the UK (Kochmann et al., 2013; Herbert et al., 2016; Bergstrom et al., 2021). In Lough Swilly and Lough Foyle, Ireland, oysters were found on shallow subtidal mussel beds and mixed mud and sand habitats (Kochmann, 2012). In the Thames Estuary and parts of Essex and Kent, oysters have also been found subtidally, 2 to 3 m below chart datum (Tillin et al., 2020).

Bergstrom et al. (2021) suggested the optimal depth in the Skagerrak is around 0.5 m, although presence is documented down to 5 m. In Lim Bay (Adriatic Sea), M. gigas occurs in the intertidal and shallow subtidal (down to 1 m), but not beyond 3 m depth (Stagličić et al., 2020). The species has not been recorded below extreme low water on rocky habitats, although it has been found subtidally on soft sediments in some areas (Herbert et al., 2012).

The Pacific oyster prefers wide intertidal areas with shallow gradients; it is generally absent from steep shores (McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). In Ireland and the Solway Firth, it is more commonly found on intertidal shores over 40 to 50 m wide (Kochmann et al., 2013; Cook et al., 2014).

It has been suggested that recruitment is enhanced, and abundances are higher in wave sheltered conditions (Robinson et al., 2005; Ruesink, 2007 cited in Teschke et al., 2020; Tillin et al., 2020). Teschke et al. (2020) found the abundance of Magallana gigas was significantly higher at wave-protected sites within the artificial harbours of Helgoland, North Sea, compared to wave exposed sites outside the harbours. The authors suggested that the successful colonization in wave-protected sites could be due to the relative retention of water masses in the harbours that reduces larval drift and whiplash effect on newly settled larvae. In addition, better growth and higher survival rates were observed at wave-protected sites, whereas mortality rates increased at wave exposed sites, due to the wave exposure causing dislodgement or detachment from the settlement substratum (Teschke et al., 2020; Tillin et al., 2020). Similarly, Bergstrom et al. (2021) noted that the occurrence of high densities of both Ostrea edulis and Magallana gigas decreased with increasing wave exposure.

Magallana gigas can withstand a wide range of salinities (from 11 to 34 PSU), but no oysters were observed in areas on the west Swedish coast which had salinities less than 20 PSU (Wrange et al., 2010; Kochmann, 2012; Chu et al., 1996 cited in Tillin et al., 2020). Bergstrom et al. (2021) noted that in the Skagerrak, native and Pacific oyster densities increased with rising salinity above 15 to 27 PSU. Larvae can survive salinities between 19 to 35 PSU (Troost, 2010; Tillin et al., 2020). Growth of Pacific oysters can occur between 10 to 30 PSU (Troost, 2010).

Carrasco & Baron (2010) suggested that Magallana gigas has successfully adapted to colonize a range of thermal niches. Temperature is important for the life cycle of the Pacific oyster and influences the establishment of feral and wild populations (Alves et al., 2021). Within its native range, Magallana gigas occurs in areas where the sea surface temperatures range from 14.0°C to 28.6°C in the warmest month of the year, and between -1.9°C and 19.8°C in the coldest month (Carrasco & Baron, 2010).

Magallana gigas has a seasonal reproductive cycle (Alves et al., 2021). Spawning occurs in the summer months, when temperatures are 16 to 34°C and larvae require a water temperature of 18°C or above for successful development (Mann 1979; Troost, 2010; Kochmann, 2012; Ezgeta-Balic et al., 2020; Alves & Tidbury, 2022). In Poole, UK, spawning temperatures were estimated at 19.7°C (Alves & Tidbury, 2022). Ezgeta-Balic et al.’s (2020) study indicated that temperatures in the Mediterranean and the Adriatic were favourable for Pacific oyster larval development, with gametogenesis initiated at temperatures from around 10 to 15°C and spawning initiated at around 24°C. However, the lower thermal limit for spawning was recognized as 16°C (Carrasco & Baron, 2010) and once settled, larvae are unable to survive in temperatures below 3°C (Alves & Tidbury, 2022).

Adults can survive in water temperatures up to 40°C and at low tide, freezing air temperatures as low as -17°C, depending on the salinity of the water in their shells (Troost, 2010; Tillin et al., 2020; Hansen et al., 2023). Growth of Pacific oysters occurs between 3 to 40°C (Troost, 2010; Kochmann, 2012).

Dense macroalgal cover is unsuitable for the Magallana gigas (Herbert et al., 2012, 2016; Tillin et al., 2020), being rarely found under macroalgal cover in Northern Ireland, absent from exposed bedrock or large boulders with macroalgae cover in the Solway Firth, Scotland, and absent in Poole Harbour where there was competition with macroalgae (Kochmann, 2012, Kochmann et al., 2013; McKinstry & Jensen, 2013; Cook et al., 2014; Tillin et al., 2020). Fucus cover significantly reduced larval recruitment of the Pacific oyster in the Wadden Sea (Diederich, 2005). Hence, the Pacific oyster is more likely to colonize bare rock, boulders, or mussel beds without macroalgae (Diederich, 2005; Cook et al., 2014). Kochmann et al. (2013) suggested that macrophyte canopies prevent larvae from settling on the rock underneath and macroalgae fronds inhibit settlement and recruitment by exuding metabolites.

Sensitivity Assessment

While most of the evidence suggests the environmental conditions within these biotopes are suitable for Magallana gigas, it is unlikely that they would be able to colonize this biotope without the removal of the macroalgal canopy. In addition, populations may be limited to low densities due to very wave exposed to wave exposed conditions. Therefore, this biotope is assessed as ‘Not Sensitive’ to this pressure.

Wireweed, Sargassum muticum

Evidence

Sargassum muticum is a circumglobal invasive species (Engelen et al., 2015). It is recorded (2015) from Norway to Morocco and into the Mediterranean in the eastern Atlantic and from Alaska to Baja California in the eastern Pacific and from southern Russia to southern China in the western Pacific (Engelen et al., 2015). It colonizes a variety of habitats and can tolerate -1°C to 30°C and survive salinities below 10 PSU. Although fertilization does not occur below 15 PSU and growth of germlings is limited below 10°C it can complete its life cycle if temperatures are over 8°C for at least four months of the year (Engelen et al., 2015). However, its distribution is limited by the availability of hard substratum (e.g., stones >10 cm) and light (Staehr et al., 2000; Strong & Dring 2011; Engelen et al., 2015). It is most abundant between 1 and 3 m below mean water. But it has been recorded at 18 m or 30 m in the clear waters of California. However, it is a poor competitor under low light and only develops dense canopies in shallow areas (Engelen et al., 2015). 

Sargassum muticum was shown to replace and outcompete leathery, canopy-forming macroalgae such as Saccharina latissima, Halidrys siliquosa, and Fucus spp. and, to a lesser degree, understorey species such as Codium fragile, Chondrus crispus and Dictyota dichotoma in Limfjorden, Denmark between 1984 and 1997 (Staehr et al., 2000; Engelen et al., 2015; De Bettignies et al., 2021). The invasion in Limfjorden had stabilized by 2005 although many of the native macroalgal species continued to decline (Engelen et al., 2015). In Limfjorden, the distribution of Sargassum muticum was limited to areas with hard substratum, in particular stones >10 cm in diameter, while smaller stones, gravel and sand were unsuitable. It was most abundant between 1 and 4 m in depth but had low cover at 0 to 0.5 m and 4 to 6 m, in the turbid waters of the Limfjorden. Limfjorden is wave sheltered but wave exposure has been reported to restrict the growth and survival of Sargassum muticum (Staehr et al., 2000). Viejo et al. (1995) reported that Sargassum muticum transplanted to wave exposed shores in Spain experienced >80% breakages within a month and that the growth of undamaged plants was significantly lower than that of plants on sheltered shores. Similarly, Andrew & Viejo (1998) noted that Sargassum muticum was restricted to intertidal rockpools in wave exposed sites in the Bay of Biscay. 

Strong & Dring (2011) used canopy removal experiments to investigate inter- and intra-species competition between Sargassum muticum and Saccharina latissima in the Dorn, Strangford Lough, N. Ireland. The Dorn consists of tidal pools, very sheltered from wave action but with moderately strong tidal streams (1 to 2 knots). Sargassum muticum grew better in mixed stands with Saccharina latissima than in the highest-density monospecific stands examined. However, the growth of Saccharina was not affected by the proportion of Sargassum in mixed stands. They concluded that Saccharina was not impacted significantly by the alien species while Sargassum benefited from growth in mixed stands. Experimental manipulation of subtidal algal canopies in the San Juan Islands, Washington State, USA, showed that Sargassum muticum reduced the abundance of native macroalgae, including the kelp Laminaria bongardiana due to shading. However, the experimental removal of Sargassum resulted in the recovery of native species within about one year (Britton-Simmons, 2004; Engelen et al., 2015). The negative effects of Sargassum muticum on native macroalgae are mainly due to competition for light, rather than changes in nutrient availability, sedimentation or water flow (Britton-Simmons, 2004; Engelen et al., 2015).

Sensitivity Assessment

This biotope is found within the sublittoral with extreme to moderate wave exposure and strong tidal streams. The evidence above suggests that Sargassum muticum prefers wave sheltered, shallow sites in the sublittoral fringe. Competition with Sargassum is probably site-specific and dependent on local conditions, but it is highly unlikely that Sargassum will be able to colonize and survive in this biotope due to the high degree of wave exposure and tidal streams that characterize these biotopes. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and sensitivity is assessed as ‘Not Sensitive’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Sargassum muticum.

Wakame, Undaria pinnatifida

Evidence

Undaria pinnatifida (Wakame or Asian kelp) is a large brown seaweed and an Invasive Non-Indigenous Species (INIS) that could outcompete native UK kelp species (see Farrell & Fletcher, 2006; Thompson & Schiel, 2012; Brodie et al., 2014; Heiser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017; Epstein & Smale, 2018; Kraan, 2017; Epstein et al., 2019a,b; Tidbury, 2020). Undaria pinnatifida originates from Japan but is established currently on the coastlines of New Zealand, Australia, Northern France, Spain, Italy, the UK, Portugal, Belgium, Holland, Argentina, Mexico, and the USA (De Leij et al., 2017). Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016) and has since proliferated along UK coastlines. One year after its discovery at the Queen Anne Battery marina, Plymouth, it became a major fouling plant on pontoons (Minchin & Nunn, 2014). Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound. 

Undaria pinnatifida seems to settle better on artificial substrata (e.g., floats, marinas or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018). James (2017) suggested that Undaria pinnatifida could outcompete native species on artificial substrata (such as marinas and wharf structures). In Plymouth, UK, De Leij et al. (2017) found that natural habitats with dense native macroalgal canopies, such as Laminaria hyperborea, Laminaria ochroleuca, Laminaria digitata and Saccharina latissima had more resistance to Undaria pinnatifida invasion than disturbed or sparse canopies, due to limited space and light availability for Undaria pinnatifida recruits. However, the dense canopies did not always prevent the invasion of Undaria pinnatifida as sporophytes were still recorded within dense Laminaria canopies, so that canopy disturbance was not always required (De Leij et al., 2017; Epstein & Smale, 2018).

Undaria behaves as a winter annual, and recruitment occurs in winter followed by rapid growth through spring, maturity and then senescence through summer, with only the microscopic life stages persisting through autumn. It exhibits multiple dispersal strategies, such as short-range spore dispersal, and long-range dispersal as whole drift plants or fragments. Undaria pinnatifida has spread rapidly across the UK and Europe, resulting in community-wide responses and impacts (Vaz-Pinto et al., 2014; Epstein & Smale, 2017). Its impacts are complex and context-specific, depending on space, time, and taxa present in the introduced location (Epstein & Smale, 2017; Teagle et al., 2017; Tidbury, 2020). 

Undaria pinnatifida has a wide physiological niche meaning it can occur in both coastal and estuarine environments showing tolerance for varying salinities, turbidity and siltation (Heiser et al., 2014; Epstein & Smale, 2018). Undaria pinnatifida has a greater preference for sites sheltered with low wave exposure and weak tidal streams (Heiser et al., 2014; Epstein & Smale, 2018). In natural habitats, Undaria pinnatifida was not recorded if the wave fetch was greater than 642 km but increased in abundance and cover in very sheltered sites (Epstein & Smale, 2018). 

In St Malo, France, there was evidence that Undaria pinnatifida coexisted with Laminaria hyperborea under certain conditions (Castric-Fey et al., 1993). Epstein & Smale (2018) also observed that Undaria pinnatifida was relatively common (abundance of >70 individuals per 25 m transect) at three sites in Devon, UK (Jennycliff, Bovisand and Beacon Cove) where Laminaria spp. were abundant (40 to 79%) or superabundant (>80%), which suggested that Undaria pinnatifida could coexist within refugia amongst areas with dense Laminaria spp. 

In Plymouth Sound, UK, Heiser et al. (2014) observed that Laminaria hyperborea was significantly less abundant at sites with the presence of Undaria pinnatifida, with only approx. 0.5 Laminaria hyperborea individuals per m² present compared to approx. 8 individuals per m² at sites without the presence of Undaria pinnatifida. However, the results from their correlation study only showed that the species were not found together (pers. comm., Epstein, 2021). Whereas exclusion and succession experiments on reefs tell us that Laminaria spp. exclude Undaria pinnatifida, not the other way around. Epstein & Smale (2018) reported that in Devon, UK, persistent, dense, and intact Laminaria spp. canopies in rocky reef habitats exerted a strong influence over the presence/absence, abundance, and percentage cover of Undaria pinnatifida. A dense canopy of native kelp restricts the proliferation of Undaria pinnatifida and disturbance of the canopy is often the key to the recruitment of Undaria pinnatifida. Epstein et al. (2019b) reported that Undaria pinnatifida density and biomass were significantly negatively correlated with the sum of all Laminaria spp. in Plymouth, UK. The evidence indicated that native Laminaria spp. canopies in the UK inhibited Undaria pinnatifida and that Undaria pinnatifida was opportunistic but competitively inferior (Farrell & Fletcher, 2006; Heiser et al., 2014; Minchin & Nunn, 2014; De Leij et al., 2017; Epstein & Smale, 2018; Epstein et al., 2019b). However, Epstein et al. (2019b) also noted that Laminaria hyperborea had a non-significant positive relationship with Undaria pinnatifida due to low densities of Laminaria hyperborea across the study area, resulting in insufficient data. 

In Plymouth Sound (UK), Epstein et al. (2019b) found that within its depth range (+1 to –4 m), Undaria pinnatifida coexisted with seven species of canopy-forming brown macroalgae, including Laminaria hyperborea. De Leij et al. (2017) found that natural habitats with dense native macroalgal canopies, such as Laminaria hyperborea had more resistance to Undaria pinnatifida invasion than disturbed or sparse canopies, due to limited space and light availability for Undaria recruits. However, the dense canopies will not prevent the invasion of Undaria, as sporophytes were still recorded within dense Laminaria canopies, and this suggests that canopy disturbance is not always required. 

Undaria pinnatifida was successfully eradicated on a sunken ship in Chatham Islands, New Zealand, by applying a heat treatment of 70°C (Wotton et al., 2004). However, numerous other eradication attempts have failed and as noted by Fletcher & Farrell (1998), once established Undaria pinnatifida resists most attempts at long-term removal.

Sensitivity Assessment

Undaria pinnatifida has the potential to colonize and coexist in refugia within kelp habitats, especially in shallow examples of their biotopes that are within its depth range (1 to 4 m) and sheltered from wave action. A dense kelp canopy may restrict or slow the proliferation of Undaria pinnatifida. A lower abundance of native macroalgae may result in increased Undaria pinnatifida growth. However, it is highly unlikely that Undaria pinnatifida will be able to colonize or survive the degree of wave exposure that characterizes these biotopes. Therefore, sensitivity is assessed as ‘Not Sensitive’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Undaria pinnatifida.

Other INIS

Evidence

The golden kelp Laminaria ochroleuca is a warm-temperate Lusitanian kelp with a distribution ranging from Morocco to the south of the UK. It was first recorded in the southwest UK in 1946 (Parke, 1948) and is projected to expand further northwards under future climate change scenarios (Franco et al., 2018). A small population was recorded in northwest Ireland in 2018 (Schoenrock et al., 2019), further suggesting ongoing poleward expansion. While not considered a traditional invasive species, its northward expansion into the UK has led to competition with Laminaria hyperborea. In Plymouth Sound, southwest UK, estimates of Laminaria ochroleuca standing stock are now comparable to those of Laminaria hyperborea (Taylor-Robinson et al., 2024; also see Smale et al., 2016 for standing stock of Laminaria hyperborea).

It is suggested that Laminaria ochroleuca may have a competitive advantage over Laminaria hyperborea due to its tolerance of warmer waters. Barrientos et al. (2025) investigated changes in kelp forests in northwest Spain between 1997 and 2023. They found that kelp forests had disappeared or severely declined in density at 29 of 50 sites, and the canopy was now dominated by Laminaria ochroleuca at the surviving sites, while Laminaria hyperborea is almost entirely absent, occurring at only two sites. These changes were linked to sea surface temperature (an average increase of 0.01 to 0.02°C per year over the 26-year study period), which suggested that Laminaria ochroleuca was more resistant to warming and could therefore outcompete Laminaria hyperborea under global warming scenarios.

There is contrasting evidence on the relative resilience of Laminaria ochroleuca and Laminaria hyperborea to storm damage. Pereira et al. (2017) reported no recovery of Laminaria hyperborea populations in the two years following a storm in northern Portugal, whereas Laminaria ochroleuca showed partial recovery. In contrast, Smale & Vance (2015) found that Laminaria hyperborea was highly resistant to severe storms in the UK during the 2013 to 2014 winter season. The breakage of mature Laminaria hyperborea stipes ranged between 2.3 and 6.9%, while broken Laminaria ochroleuca stipes were on average 8.7 times more prevalent. Given this conflicting evidence, it remains unclear whether Laminaria ochroleuca biotopes could displace Laminaria hyperborea biotopes following storm events.

Another potential advantage of Laminaria ochroleuca is its greater average stipe length compared to Laminaria hyperborea, potentially reducing light availability for Laminaria hyperborea recruits in mixed-population forests (Smale et al., 2015). This shading effect may exaggerate the impacts of marine heatwaves on Laminaria hyperborea, as elevated temperatures increase metabolic demands that cannot be met under light-limited conditions (Bass et al., 2023).

The introduction of Laminaria ochroleuca into Laminaria hyperborea forests can have negative impacts on biodiversity. Kelp stipe assemblages differ significantly between the two species due to the texture of the stipe. Laminaria hyperborea stipes are rough and pitted and therefore have a larger surface area, while Laminaria ochroleuca stipes are uniformly smooth. Teagle & Smale (2018) found species from up to 15 different taxonomic groups on Laminaria hyperborea stipes in spring, compared to 2 taxa at most on Laminaria ochroleuca stipes all year round. In addition, the biomass of Laminaria hyperborea stipe assemblages was >3600 more than Laminaria ochroleuca stipe assemblages. Therefore, the proliferation of Laminaria ochroleuca could reduce available habitat space for epibionts that are associated with Laminaria hyperborea biotopes.

Sensitivity Assessment

No evidence was found of Laminaria ochroleuca competing against the macroalgae species that characterise this biotope. KFaR.Ala and its sub-biotopes occur across a range of wave exposure levels, from extremely exposed to moderately exposed. At present, Laminaria ochroleuca is abundant only in KFaR.LhypLoch, which occurs in sheltered to moderately exposed conditions, and is less abundant in KFaR.LhypR.Loch, which occurs in exposed to very exposed conditions (JNCC, 2022). Evidence also suggests that Laminaria ochroleuca is less resistant to wave action than Laminaria hyperborea (see Wave Exposure Changes section in IR.HIR.KFaR.LhypR.Loch). Since Laminaria hyperborea is excluded from KFaR.Ala biotopes by the high wave action of the sublittoral fringe, these conditions are also likely to exclude Laminaria ochroleuca. Resistance is therefore assessed as ‘High’, resilience as ‘High’ by default (no impact to recover from), and sensitivity as ‘Not Sensitive’.