The Marine Life Information Network

Temperature increase (local)

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

Evidence

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, 2025), 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.

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. This indicates that inbreeding reduces resistance, while outbreeding enhances tolerance to warming.

Liesner et al. (2020) explored how thermal conditions during early life stages influence physiological performance in Laminaria digitata sporophytes. Using five genetic lines, they manipulated temperature (5 vs 15°C) during meiospore germination, gametogenesis, and sporophyte recruitment, then assessed juvenile sporophyte performance at both temperatures. They found that gametogenesis and recruitment at 5°C led to significantly higher growth rates in juvenile sporophytes across both temperature treatments, and photosynthetic efficiency (Fv/Fm, a widely used metric for measuring physiological stress in photosynthetic organisms) was higher when juvenile sporophytes were tested at 15°C. Cold early environments also resulted in greater carbon and nitrogen storage, even after exposure to warmer conditions. However, the magnitude and direction of response to temperatures also differed between genetic lines, indicating that genetics could play a role in adapting to thermal stress.

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.

Thermal tolerance may also depend on location. Martins et al. (2020) compared Arctic and North Sea microscopic life stages of Laminaria digitata by maintaining gametophytes at 15°C for three years, then exposing them to either static (sudden) or dynamic (gradually increasing) heat stress from 15 to 25°C. Although thermal stress reduced performance in all gametophytes, the two populations responded differently. North Sea gametophytes showed higher growth and greater sporophyte recruitment following high‑temperature exposure than Arctic gametophytes. Both populations were extremely sensitive to a static 8‑day exposure at 22.5°C, which resulted in poor recovery and minimal sporophyte formation. However, both populations tolerated a dynamic heat stress peaking at 25°C, demonstrating the importance of acclimation rate in shaping upper thermal limits. Recovery temperature also strongly influenced outcomes. Recovery at 5°C improved sporophyte production after moderate heat treatments (15°C and 20°C), whereas recovery at 15°C was more beneficial following severe static (22.5°C) or dynamic (25°C) exposures. 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.

In contrast, Schimpf et al. (2022) found that Arctic Laminaria digitata populations had a greater tolerance to warming than southern populations. Gametophytes exhibited a uniform upper survival temperature of 24°C, regardless of origin, indicating a shared high-temperature threshold. However, trait responses varied: after 14 days of heat priming at 20 to 22°C, southern populations (Quiberon and Roscoff) showed significantly higher sporophyte formation upon recovery at 15°C compared to northern populations such as Tromsø. In low-temperature experiments (0 to 15°C), survival patterns showed signs of local adaptation. Spitsbergen gametophyte survival declined with increasing temperature, while Quiberon gametophytes improved. Sporophyte formation peaked at 6 to 9°C in Quiberon and 9 to 12°C in Spitsbergen, whereas growth rates were similar across regions, with maxima at 12 to 15°C. These findings suggest that while upper thermal limits are universal, different life-stages show latitude-dependent adaptation. If local temperatures were to increase at the benchmark level, Arctic populations would likely benefit from the warming due to enhanced growth and reproduction. However, recruitment success would decline in temperate and southern populations, and southern populations would also experience hindered growth.

Graiff et al. (2025) also suggested that Arctic Laminaria digitata populations may benefit from increases in temperature at the benchmark level. They exposed young, cultured sporophytes from Arctic (Spitsbergen) and cold-temperate (Helgoland) populations to different temperature treatments: Spitsbergen at 4, 10, and 16°C, and Helgoland at 10, 16, and 22°C. Both exhibited peak growth at 10°C and no significant difference in growth at 16°C. Photosynthetic efficiency (Fv/Fm) and carbon fixation rates were highest at 4°C for Spitsbergen and 16°C for Helgoland. Notably, the Helgoland kelps showed significantly higher oxygen production and carbon fixation than Spitsbergen at 10 and 16°C. At 10°C, Spitsbergen kelps had higher total carbon and mannitol content, indicating greater storage compound accumulation at high latitude. With Helgoland’s physiological performance peaking in temperatures from 10 to 16°C, it is possible that a temperature increase at the benchmark level at this latitude and lower latitudes could negatively affect Laminaria digitata.

Simonson et al. (2015a) investigated the effects of four temperature treatments (11, 14, 18, and 21°C) on the tissues of Laminaria digitata. They found early signs of tissue degradation in the 14°C treatment; holes in the medulla (the innermost layer of the stipe), and thinning of the meristoderm (the outermost layer of cells in the stipe and blades). Holes in the medulla increase the risk of stipe breakage due to wave motion, while thinning of the meristoderm affects photosynthesis and growth. These signs of degradation increased in severity with each level of temperature treatment. At 18°C, significant tissue loss occurred after two weeks of exposure, and tensile strength significantly declined after three weeks. Tensile strength and extensibility declined by 40 to 70% after one week in 21°C, and tissue loss was four to eight times greater than normal erosion rates. At this temperature, complete mortality had occurred within a week.

Simonson et al. (2015b) found no significant effect of temperature on phlorotannin (a chemical compound which is produced during stress) content. While phlorotannin content can increase under thermal stress (Hargrave et al., 2017), exposure to sufficiently high temperatures may inhibit its production (Cruces et al., 2012, 2013). Therefore, the absence of elevated phlorotannin levels observed by Simonson et al. (2015b) could indicate a physiological stress response rather than resistance.

Photosynthetic efficiency (measured as Fv/Fm) is a widely used metric for measuring physiological stress in photosynthetic organisms (King et al., 2018; Trautmann et al., 2024), with Fv/Fm 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 (Fv/Fm) 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.

Hargrave et al. (2017) measured growth, photosynthetic efficiency, chemical defences (phenolic compounds and flavonoids) and palatability of Laminaria digitata in response to different temperatures. Blade elongation and biomass accumulation were approx. three times higher in the 12°C treatment than in the 18°C treatment. Photosynthetic efficiency was also highest (approx. 0.65) in the 12 and 15°C treatments, and significantly lower (approx. 0.62) in the 18°C treatment. Phenol concentration (% of dry weight) was roughly 1.25 in the two cooler treatments and 1.75 in the warmest treatment. Phenolic compounds, such as phlorotannins, are produced under stressful conditions including heat stress as a protective mechanism against increased vulnerability (e.g., tissue damage and epiphyte colonisation). In addition, flavonoid content (% dry weight) was roughly 2.5 in the coolest treatment and over 3.1 in the warmer treatments. Flavonoids are secondary metabolites which increase in production when marine algae are under physiological stress. These metabolites protect cells from oxidative stress which occurs under stressful temperatures. Lastly, the reduced palatability, measured by the rate of grazing (mm²/48 hours) by Steromphala cineraria (studied as Gibbula cineraria), corroborated the higher phenolic and flavonoid concentrations in the kelp tissues. Grazing rate was 200 at 12°C (control treatment), 80 at 15°C, and <50 at 18°C.

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.

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.

In contrast, Müller et al. (2008) found that elevated temperatures can exacerbate stress from ultraviolet radiation from sunlight. They investigated the combined effects of temperature and light quality on early life stages of Laminaria digitata from Arctic (Spitsbergen) and cold-temperate (Helgoland) populations. Temperature treatments ranged from 2°C to 18°C, representing Arctic summer conditions and North Sea summer extremes. Arctic populations germinated well at 2 to 12°C but failed at 18°C, while Helgoland populations showed optimal germination at 7 to 18°C. UV-B radiation was the most damaging factor, reducing germination by up to 99% in Arctic Laminaria digitata, and strongly inhibiting spore release (from 19 to 34 eggs mm² under normal light to 1.5 to 4 spores mm² under UV-B). UV-A occasionally enhanced gametogenesis at moderate temperatures but did not offset UV-B damage. Overall, more light (UV exposure) combined with higher temperatures produced the greatest negative effects, while low light and moderate temperatures favoured Arctic populations. These findings indicated that warming exacerbated UV-B stress and severely limits reproduction.

Schmid et al. (2021) investigated the combined effects of light and temperature on growth and biochemical composition in several temperate macroalgae species, including Laminaria digitata. They 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. 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 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, warming at the benchmark level could severely limit growth and reduce biochemical quality.

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°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 (Fv/Fm) 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 (Fv/Fm) 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.

King et al. (2019) examined thermal tolerance in Laminaria digitata across its East Atlantic range, comparing trailing-edge populations in southwest England with range-centre populations in northern Scotland. Using heat shock experiments (8 to 32°C), they measured expression of the heat shock protein gene hsp70 to identify thermal set points: Ton (temperature at which hsps are synthesised), Tpeak (temperature at which maximum hsp expression occurs), and Toff (temperature at which hsp expression stops). All populations activated hsp70 under heat stress, but trailing-edge populations had significantly higher thermal thresholds (Tpeak approx. 24°C, Toff approx. 28°C) than range-centre populations (Tpeak approx. 16 to 20°C, Toff approx. 20 to 24°C), indicating greater tolerance to higher temperatures at the southern edge. This response did not significantly differ between winter and summer tests, suggesting local adaptation rather than short-term acclimation. Genetic analysis revealed strong population structure and limited gene flow between trailing-edge and range-centre populations, which suggests the existence of distinct thermal ecotypes.

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 (Fv/Fm) 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.

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.

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. (2023) 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.

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 response of Laminaria digitata to an increase in temperature can depend on life stage, genetic lineage, location, latitude, emersion regime and other pressures which may exacerbate temperature effects. This pressure is defined as short-term localised changes (of 2°C for a year or 5°C for a month). While northern populations of Laminaria digitata may be more resistant to this pressure, it is likely that southern populations 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. As a precaution, resistance is assessed as ‘Low’ (25 to 75% reduction in habitat components), with ‘High’ resilience (recovery within two years) and ‘Low’ sensitivity. 

Temperature decrease (local)

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

Evidence

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

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

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

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

This biotope occurs in the shallow sublittoral and is therefore exposed to changes in emergence. Increased emergence will expose the biotope to air for longer periods leading to drying. 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. This pressure is a key driver of biotope extent because the upper and lower limits of this species are set by inter-species competition. In the direct footprint of the impact, resistance is probably ‘Low’ (loss of 25 to 75%). Resilience is suggested as ‘Medium’ (2-10 years) due to the scale of the impact. This biotope is therefore considered to have ‘Medium’ sensitivity to the pressure.

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

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.

While these contaminants have been detected in Laminaria digitata, very few studies have investigated whether they are harmful to kelp. Most studies on this topic instead focus on implications for human health.

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.

Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes. Similarly, Hopkin & Kain (1978) demonstrated sub-lethal effects of heavy metals on Laminaria hyperborea gametophytes and sporophytes, including reduced growth and respiration. Sheppard et al. (1980) noted that increasing levels of heavy metal contamination along the west coast of Britain reduced species richness in holdfast fauna except for suspension feeders which became increasingly dominant. Gastropods may be relatively tolerant of heavy metal pollution (Bryan, 1984). Echinus esculentus recruitment is likely to be impaired by heavy metal contamination due to the intolerance of its larvae. Echinus esculentus is long-lived and poor recruitment may not reduce grazing pressure in the short-term. Although macroalgae species may not be killed, except by high levels of contamination, reduced growth rates may impair the ability of the biotope to recover from other environmental disturbances.

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.

Laminaria digitata is less susceptible to coating than some other seaweeds because of its preference for exposed locations where wave action will rapidly dissipate oil. The effects of oil accumulation on the thalli are mitigated by the perennial growth of kelps. No significant effects of the Amoco Cadiz spill were observed for Laminaria populations and the World Prodigy spill of 922 tonnes of oil in Narragansett Bay had no discernible effects on Laminaria digitata (Peckol et al., 1990). Mesocosm studies in Norwegian waters showed that chronic low-level oil pollution (25 µg/L) reduced growth rates in Laminaria digitata but only in the second and third years of growth (Bokn, 1985). Where exposed to direct contact with fresh hydrocarbons, encrusting calcareous algae have a high intolerance. The sensitivities of the faunal components of the kelp bed are not known although amphipods normally suffer high mortality in oil affected areas. Analysis of kelp holdfast fauna after the Sea Empress oil spill in Milford Haven illustrated decreases in the number of species, diversity and abundance at sites nearest the spill (SEEEC, 1998).

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

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 environmental conditions are transient. If levels do drop below 4 mg/l negative effects on these organisms can be expected with adverse effects occurring below 2 mg/l (Cole et al., 1999).

Sensitivity Assessment.  Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly but small invertebrate epifauna may be lost, causing a reduction in species richness. However, in wave exposed conditions, and hence high mixing, the effects of deoxygenation are likely to be short-lived. Therefore, resistance is assessed as ‘High’.  Hence resilience is likely to be ‘High’, and the biotope is considered ‘Not sensitive’ at the pressure benchmark.

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

High ambient levels of phosphate and nitrogen enhance spore formation in a number of Laminaria species (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. The benchmark of this pressure (compliance with WFD ‘good’ status) allows for a slightly less diverse community of red, green and brown seaweeds with the greatest reduction in red species and an increase in the proportion of short-lived species under the WFD criteria for good status. However, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with good status as defined by the WFD.  

Organic enrichment

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

Evidence

Organic deposition may result in siltation (see smothering and siltation change pressure) and subsequent re-suspension of organic particles reducing water clarity (see 'change in suspended solids' pressure). The deposition of sewage effluent into coastal environments resulted in the absence of Laminaria digitata and many other species from the coastline of the Firth of Forth (Read et al., 1983). The use of some kelp species in conjunction with fish aquaculture (to buffer the effects of organic enrichment in the local area) suggests that many commercial kelps  (including Laminaria digitata) are tolerant to local increases in organic enrichment, although the level of enrichment experienced will be dependent on fish species and aquaculture design (Troell et al., 2003). In California, investigations into the effects of sewage effluent into coastal waters concluded that kelp was negatively affected by the organic enrichment of the surrounding waters (North & Schaefer, 1964). The addition of organic matter may also decrease water clarity and increase particulate matter in the water column the effects of these changes are assessed through the pressure ‘Changes in suspended solids’.

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including subtidal reefs). A 30-50% reduction in species diversity and richness were identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009) however also highlighted that macroalgal communities are relatively tolerant to contamination, but that contaminated communities can have low diversity assemblages which are dominated by opportunistic and fast growing species (Johnston & Roberts, 2009 and references therein).

Sensitivity Assessment. Therefore, the biotope is probably resistant of the direct effects of organic enrichment but may be sensitive to the effects of increased turbidity (see water clarity above) and eutrophication. Therefore, a resistance of 'Medium' is suggested to represent a reduction in species richness and an increase in ephemeral green algae. Resilience is probably ‘High' and the biotope is, therefore, considered to have a 'Low' sensitivity at the pressure benchmark.

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

A change in substratum type from bedrock to sedimentary is likely to have a similar effect as a ‘physical loss (to land or freshwater habitat) because kelp requires a stable, hard, substratum on which to settle. No evidence of this biotope occurring on sedimentary substratum was found in the literature. Also, it is scarce on the south-east coast of Ireland, in particular, Counties Wicklow and Wexford, due to lack of hard substratum.

Sensitivity assessment.  This biotope is considered to have a resistance of 'None' to this pressure as sedimentary habitats are unsuitable for Laminaria digitata and the associated attached biological assemblage. It would be unable to recover from this permanent change of habitat so resilience is ‘Very low' and its sensitivity within the direct spatial footprint of this pressure is, therefore ‘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 bedrock biotopes typical of IR.MIR.KR.Ldig. Please note the soft rock sub-biotope IR.MIR.KR.Ldig.Pid and the under-boulder sub-biotope IR.MIR.KR.Ldig.Bo are assessed as 'High' sensitivity to this pressure.

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

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

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 (Laminaria digitata or Saccharina latissima). The authors 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 approximately 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. Abrasion of the substratum could remove a proportion of the kelp and associated red algal community. Therefore, the biotope is considered to have ‘Low’ resistance in the footprint of the pressure (loss of 25 to 75%) while resilience is ‘High’ (within 2 years). Hence, the biotope is assessed as ‘Low’ sensitivity. Please note the soft rock sub-biotope IR.MIR.KR.Ldig.Pid and the under-boulder sub-biotope IR.MIR.KR.Ldig.Bo are assessed as 'Medium' sensitivity to this pressure.

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

The species characterizing this biotope group are epifauna or epiflora occurring on hard rock, which is resistant to subsurface penetration. Therefore, ‘penetration’ is 'Not relevant'. The assessment for abrasion at the surface only is, therefore, considered to equally represent sensitivity to this pressure’. Please refer to ‘abrasion’ above.

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 availability and water turbidity are principal factors in determining depth range at which kelp can be found (Birkett et al., 1998b). 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 percent of incident light at the surface. Maximal depth distribution of laminarians therefore varies from 100 m in the Mediterranean to only 6-7 m in the silt laden German Bight. In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters the depth at which kelp is found may be reduced, or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Lüning, 1990; Birkett et al. 1998b). Laminaria spp. show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient = 0.1-0.2/m; Staehr & Wernberg, 2009). An increase in water turbidity will likely affect the photosynthetic ability of Laminaria digitata, decrease kelp abundance and density.

An increase in SPM results in a increase in subsurface 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 that 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). Blue light is crucial for the gametophytic stages of Laminaria digitata, and several other congenic species (Lüning, 1980).  Dissolved organic materials (yellow substance or gelbstoff) absorbs blue light (Kirk, 1976), therefore changes in riverine input or other land based runoff are likely to influence kelp density and distribution.  In the silt-laden waters around Helgoland, Germany the depth limit for Laminaria digitata growth may be reduced to between 0 m and 1.5 m (Birkett et al. 1998b). 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. A decrease in turbidity is likely to support enhanced growth (and possible habitat expansion) and is therefore not considered in this assessment. However, an increase in turbidity, is likely to result in loss of the deeper extent of the biotope. Therefore, resistance to this pressure is defined as ‘Medium’ and resilience to this pressure is defined as ‘High’. Hence, sensitivity is assessed as ‘Low‘.

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

Suspended Particle Matter (SPM) concentration has a linear relationship with sub surface light attenuation (Kd) (Devlin et al., 2008). Light availability and water turbidity are principal factors in determining depth range at which kelp can be found (Birkett et al., 1998b). 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 percent of incident light at the surface. Maximal depth distribution of laminarians therefore varies from 100 m in the Mediterranean to only 6-7 m in the silt laden German Bight. In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters the depth at which kelp is found may be reduced, or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Lüning, 1990; Birkett et al. 1998b). Laminaria spp. show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient = 0.1-0.2/m; Staehr & Wernberg, 2009). An increase in water turbidity will likely affect the photosynthetic ability of Laminaria digitata, decrease kelp abundance and density.

An increase in SPM results in a increase in subsurface 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 that 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). Blue light is crucial for the gametophytic stages of Laminaria digitata, and several other congenic species (Lüning, 1980).  Dissolved organic materials (yellow substance or gelbstoff) absorbs blue light (Kirk, 1976), therefore changes in riverine input or other land based runoff are likely to influence kelp density and distribution.  In the silt-laden waters around Helgoland, Germany the depth limit for Laminaria digitata growth may be reduced to between 0 m and 1.5 m (Birkett et al. 1998b). 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. A decrease in turbidity is likely to support enhanced growth (and possible habitat expansion) and is therefore not considered in this assessment. However, an increase in turbidity, is likely to result in loss of the deeper extent of the biotope. Therefore, resistance to this pressure is defined as ‘Medium’ and resilience to this pressure is defined as ‘High’. Hence, sensitivity is assessed as ‘Low‘.

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

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. It is feasible that discarded fishing line or plastic netting, or similar discards could tangle on kelp fronds and potentially damage or remove individuals. However, there was no evidence on which to base an assessment. 

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. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms. 

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

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’ to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’

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 regarding the genetic modification of this species was found. Harvesting is carried out on wild kelp stands in a sustainable 5 year cycle (Vea and Ask, 2011), therefore, translocation of this species is unlikely.  In addition, if translocation of populations does occur, a loss in genetic diversity is not regarded as an issue for this species, unless additional pressures result in the isolation and fragmentation of wild coastal populations (Valero et al., 2011). Genetic differentiation in wild populations occurs within 10 km with genetic flow occurring between adjacent species (Billot et al., 2003).  No evidence was found to suggest that Laminaria digitata hybridizes with other species.

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

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

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). 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). If Laminaria digitata, the key characterizing and structuring species of this biotope is removed then the biotope is considered lost and a significant alteration to the biotope classification and character of the habitat is likely.

Debate exists on whether kelp harvesting is detrimental to fish stocks. While some state that no negative consequences of harvesting have been documented (Vea & Ask, 2011); others suggest that as important foraging and nursery grounds for birds and fish, removal will inevitably result in negative consequences (Lorentsen et al., 2010). Canopy removal of Laminaria digitata has been shown to reduce shading, resulting in the bleaching of sub-canopy algae (Hawkins & Harkin, 1985). Harvesting may also result in habitat fragmentation, a major threat to this biotope’s ecosystem functioning (Valero et al., 2011).  In the UK harvesting of Laminaria digitata is currently restricted to manual removal and farming on small scales, it is therefore not surprising that no evidence of how wild UK kelp populations would react to commercial harvesting was found in the literature (Netalgae, 2012).

Sensitivity Assessment. This biotope has a resistance of ‘None’ to this pressure in the footprint of the pressure, as the removal of its key characterizing and structural species, Laminaria digitata would result in the loss of the biotope. However, resilience is assessed as ‘Medium’, giving a sensitivity of ‘Medium’ to this pressure.

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

Removal of Laminaria digitata as by-catch would result in the loss of the biotope.  In healthy macroalgae communities, many species contribute to the balanced condition of the ecosystem. Disrupting this balance may cause top-down consequences for the biotope; for example, overfishing of top predators in Norwegian waters is thought to have caused the urchin bloom, subsequent overgrazing and proliferation of urchin barrens (Steneck et al., 2004).

Sensitivity assessment. Because of the nature of the pressure, resistance to it is considered as ‘None’, because it potentially involves the removal of Laminaria digitata from the biotope, which would result in its loss. Resilience to this pressure is assessed as ‘Medium’. The sensitivity of this biotope to the removal of the non-targeted catch is assessed 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, 2023; OBIS, 2025).

Abundances at its northern and southern extremes may be low but densities in UK and France are often over 1000 /m² and it may carpet the seafloor in the Solent and Essex. In the UK, it was reported to reach abundances of >1000 /m² (max. 2,748 /m²) in the Milford Harbour Waterway (Bohn et al., 2012), 84 /m² in Portsmouth, 174 /m² in Langstone and 306 /m² in Chichester harbours in 2017 (Helmer et al., 2019). In France, it has been reported to reach >4,700 /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 (51–100% 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 approximately 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 been recorded on 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% perwinkle 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, 2025; 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) are significantly lower (<100 / 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 ca 56-64% mortality of intertidal Crepidula and up to 97% on one mussel bed, compared to only 11-14% 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 may 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).

Nehls et al. (2006) noted that the decline in mussel (Mytilus edulis) beds in the Wadden Sea was due to mild winters that favoured non-native oysters (Magellana gigas) and slipper limpets, which co-existed with the mussels.

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.

At present, there is insufficient evidence to suggest that Laminaria digitata biotopes are 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 have 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 the 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 this Laminaria digitata biotope is found (low intertidal to 5 m) overlaps with the depth range that is 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 (Dijistra 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).

Reinhart 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 ca 8 m/s or (more likely) human activity could cause fragmentation of tendrils.

Reinhart 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 Alyconium digitatum. Alyconium digitatum is reported from sheltered to very wave exposed conditions, but in the sublittoral. Reinhart 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, therefore, it could colonize the bedrock and boulders that characterize 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 Laminaria sp. Mortality (Järnegren et al., 2023), although epifaunal growth by Membranacea membrancea was reported to reduce the physical strength of kelp fronds (inc. Laminaria digitata) 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

Wireweed, Sargassum muticum

Evidence

Sargassum muticum is a circumglobal invasive species (Engelen et al., 2015). It is recorded 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, tolerating temperatures from -1°C to 30°C and 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 as long as 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 tide

 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 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 Laminaria digitata dominated biotope is found within the sublittoral zone with at wave exposed to very exposed sites. The evidence above suggests that Sargassum muticum prefers wave sheltered sites in the sublittoral fringe. No evidence of the effects of Sargassum on Laminaria digitata beds was found. Competition with Sargassum is probably site-specific and dependent on local conditions, so it is highly unlikely that Sargassum will be able to colonize and survive in this biotope due to the high degree of wave exposure. 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 has spread to 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).

Undaria pinnatifida behaves as a winter annual. Recruitment occurs in winter, followed by rapid spring growth, maturation in summer, and senescence by late summer, leaving only microscopic stages to persist 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 co-existed 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 co-exist 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 implied 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. 

Epstein et al. (2019b) found that within its depth range (+1 to -4 m), Undaria pinnatifida co-existed 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 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 (De Leij et al., 2017; Epstein & Smale, 2018). 

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 co-exist in refugia within Laminaria sp. dominated 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, native kelp canopy may restrict or slow the proliferation of Undaria pinnatifida. In addition, it is highly unlikely that Undaria pinnatifida will be able to colonize or survive the degree of wave exposure that characterize this biotope. 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