The Marine Life Information Network

Global warming (extreme)

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

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

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

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

• A 5°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

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

Laminaria hyperborea is a cold- temperate kelp species, distributed from the Barents Sea down to the coast of Portugal (Schoschina, 1997). Laminaria hyperborea has an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). At 17°C gamete survival is reduced (Steinhoff et al., 2008) and gametogenesis is inhibited at 21°C (Dieck, 1992). Therefore, Laminaria hyperborea recruitment could be impaired at a sustained temperature increase above 17°C. However, sporophytes can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are also seasonally variable and temperature changes are less tolerated in winter months than summer months (Birkett et al., 1998b). Since 1970 Laminaria hyperborea has undergone a range constriction of ~250 km at its southern edge, with current persisting populations having reduced longevity and less reproductive individuals than those populations from past studies (Fernandez, 2011; Assis et al., 2016). 

There is evidence that climate change is already having an impact on Laminaria hyperborea populations in the English Channel. Poleward range expansion of the warm temperate Laminaria ochroleuca as a result of ocean warming has led to competition with Laminaria hyperborea in UK waters (Smale et al., 2015). Laminaria ochroleuca was not found in the UK last century.  But Laminaria ochroleuca has now increased its range to include the southwest of England (Smale et al., 2015) and the west coast of Ireland (Schoenrock et al., 2019).

During the 2013-2014 Northeast Atlantic storm season, the UK was subjected to some of the most intense storms recorded within the past five years. A study by Smale & Vance. (2015) investigated the impacts of the storms on kelp canopies along the south coast of the UK, findings indicated monospecific canopies of Laminaria hyperborea were unaffected by the storms. However, the storms significantly altered a mixed canopy study site, composed of Laminaria ochroleuca, Saccharina latissima and Laminaria hyperborea. Therefore, if climate change continues to change species composition within kelp forests resistance to storm disturbance could be altered.

Smale et al. (2015) found that Laminaria hyperborea suffered from much higher epiphytic loadings and lower productivity than its competitor Laminaria ochroleuca during the summer months, which reduced its competitive ability. The decreased competitive ability because of ocean warming corresponds to findings by Pessarrodona et al. (2018), who found a decrease in the size of Laminaria hyperborea plants along a north-south gradient in Scotland, with average maximum stipe lengths of over 150 cm, whereas in southern England they were less than 100 cm. Similarly, Smale et al. (2020b) observed clear differences between net primary productivity (NNP) and carbon standing stock of Laminaria hyperborea between the colder northern and warmer southern test sites in the UK, with NNP and standing stock being 1.5 and 2.5 times greater in the northern sites. Identifying ocean temperatures as a lively driver of productivity, with reduced NNP and standing stock observed in warmer waters (Smale et al., 2020b). 

The decrease in productivity in southern England suggests that Laminaria hyperborea is already growing at suboptimal temperatures. Assis et al. (2018) predicted that under the highest emission scenario (RCP 8.5) the biogeographic range of Laminaria hyperborea will move northwards, and this retreat would lead to the species being lost from approximately 30% of the coastline of the UK.

Subtidal red algae are less tolerant of temperature extremes than intertidal red algae, surviving between -2°C and 18-23°C (Lüning 1990; Kain & Norton, 1990).  Temperature increase may affect growth, recruitment, or interfere with reproduction processes. For example, there is some evidence to suggest that blade growth in Delesseria sanguinea is delayed until ambient sea temperatures fall below 13°C.  Blade growth is also likely to be intrinsically linked to gametangia development (Kain, 1987), and maintenance of sea temperatures above 13°C may affect recruitment success.

Sensitivity assessment. Laminaria hyperborea is already growing at suboptimal temperatures in southern UK, based on evidence of decreased productivity comparative to Scotland (Pessarrodona et al., 2018; Smale et al., 2020b), and predictions have estimated Laminaria hyperborean to be lost from the UK by 2100 as a result of warming (Brodie et al., 2014). Sea surface temperatures around the UK currently range from between 6-19°C (Huthnance, 2010).  

Under the middle emission scenario, a rise of 3°C could lead to maximum summer high temperatures in the south of the UK of 22°C. This is above the upper thermal limit of 21°C for Laminaria hyperborea (Bolton & Lüning, 1982), and is likely to lead to loss of this species from the south of England. Furthermore, biomass and plant size are expected to decrease as waters warm, with Scottish Laminaria hyperborea stipe lengths decreasing to lengths observed in southern England, leading to a decline in carbon assimilation, productivity and habitat quality. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as a loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario.

For the high emission scenario and extreme scenario, this northward retreat of the distribution of Laminaria hyperborea is expected to increase. Under the high emission scenario it is expected to be lost from 30% of the coastline around the UK (Assis et al., 2018), and under the extreme scenario even more is projected to be lost. Populations of Laminaria hyperborea that remain around the UK are predicted to become less productive. Therefore, under these scenarios, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very Low’. Therefore, this biotope is assessed as ‘High’ sensitivity to ocean warming under this scenario.

Global warming (high)

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

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

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

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

Evidence

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

Laminaria hyperborea is a cold- temperate kelp species, distributed from the Barents Sea down to the coast of Portugal (Schoschina, 1997). Laminaria hyperborea has an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). At 17°C gamete survival is reduced (Steinhoff et al., 2008) and gametogenesis is inhibited at 21°C (Dieck, 1992). Therefore, Laminaria hyperborea recruitment could be impaired at a sustained temperature increase above 17°C. However, sporophytes can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are also seasonally variable and temperature changes are less tolerated in winter months than summer months (Birkett et al., 1998b). Since 1970 Laminaria hyperborea has undergone a range constriction of ~250 km at its southern edge, with current persisting populations having reduced longevity and less reproductive individuals than those populations from past studies (Fernandez, 2011; Assis et al., 2016). 

There is evidence that climate change is already having an impact on Laminaria hyperborea populations in the English Channel. Poleward range expansion of the warm temperate Laminaria ochroleuca as a result of ocean warming has led to competition with Laminaria hyperborea in UK waters (Smale et al., 2015). Laminaria ochroleuca was not found in the UK last century.  But Laminaria ochroleuca has now increased its range to include the southwest of England (Smale et al., 2015) and the west coast of Ireland (Schoenrock et al., 2019).

During the 2013-2014 Northeast Atlantic storm season, the UK was subjected to some of the most intense storms recorded within the past five years. A study by Smale & Vance. (2015) investigated the impacts of the storms on kelp canopies along the south coast of the UK, findings indicated monospecific canopies of Laminaria hyperborea were unaffected by the storms. However, the storms significantly altered a mixed canopy study site, composed of Laminaria ochroleuca, Saccharina latissima and Laminaria hyperborea. Therefore, if climate change continues to change species composition within kelp forests resistance to storm disturbance could be altered.

Smale et al. (2015) found that Laminaria hyperborea suffered from much higher epiphytic loadings and lower productivity than its competitor Laminaria ochroleuca during the summer months, which reduced its competitive ability. The decreased competitive ability because of ocean warming corresponds to findings by Pessarrodona et al. (2018), who found a decrease in the size of Laminaria hyperborea plants along a north-south gradient in Scotland, with average maximum stipe lengths of over 150 cm, whereas in southern England they were less than 100 cm. Similarly, Smale et al. (2020b) observed clear differences between net primary productivity (NNP) and carbon standing stock of Laminaria hyperborea between the colder northern and warmer southern test sites in the UK, with NNP and standing stock being 1.5 and 2.5 times greater in the northern sites. Identifying ocean temperatures as a lively driver of productivity, with reduced NNP and standing stock observed in warmer waters (Smale et al., 2020b). 

The decrease in productivity in southern England suggests that Laminaria hyperborea is already growing at suboptimal temperatures. Assis et al. (2018) predicted that under the highest emission scenario (RCP 8.5) the biogeographic range of Laminaria hyperborea will move northwards, and this retreat would lead to the species being lost from approximately 30% of the coastline of the UK.

Subtidal red algae are less tolerant of temperature extremes than intertidal red algae, surviving between -2°C and 18-23°C (Lüning 1990; Kain & Norton, 1990).  Temperature increase may affect growth, recruitment, or interfere with reproduction processes. For example, there is some evidence to suggest that blade growth in Delesseria sanguinea is delayed until ambient sea temperatures fall below 13°C.  Blade growth is also likely to be intrinsically linked to gametangia development (Kain, 1987), and maintenance of sea temperatures above 13°C may affect recruitment success.

Sensitivity assessment. Laminaria hyperborea is already growing at suboptimal temperatures in southern UK, based on evidence of decreased productivity comparative to Scotland (Pessarrodona et al., 2018; Smale et al., 2020b), and predictions have estimated Laminaria hyperborean to be lost from the UK by 2100 as a result of warming (Brodie et al., 2014). Sea surface temperatures around the UK currently range from between 6-19°C (Huthnance, 2010).  

Under the middle emission scenario, a rise of 3°C could lead to maximum summer high temperatures in the south of the UK of 22°C. This is above the upper thermal limit of 21°C for Laminaria hyperborea (Bolton & Lüning, 1982), and is likely to lead to loss of this species from the south of England. Furthermore, biomass and plant size are expected to decrease as waters warm, with Scottish Laminaria hyperborea stipe lengths decreasing to lengths observed in southern England, leading to a decline in carbon assimilation, productivity and habitat quality. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as a loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario.

For the high emission scenario and extreme scenario, this northward retreat of the distribution of Laminaria hyperborea is expected to increase. Under the high emission scenario it is expected to be lost from 30% of the coastline around the UK (Assis et al., 2018), and under the extreme scenario even more is projected to be lost. Populations of Laminaria hyperborea that remain around the UK are predicted to become less productive. Therefore, under these scenarios, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very Low’. Therefore, this biotope is assessed as ‘High’ sensitivity to ocean warming under this scenario.

Global warming (middle)

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

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

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

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

Evidence

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

Laminaria hyperborea is a cold- temperate kelp species, distributed from the Barents Sea down to the coast of Portugal (Schoschina, 1997). Laminaria hyperborea has an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). At 17°C gamete survival is reduced (Steinhoff et al., 2008) and gametogenesis is inhibited at 21°C (Dieck, 1992). Therefore, Laminaria hyperborea recruitment could be impaired at a sustained temperature increase above 17°C. However, sporophytes can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are also seasonally variable and temperature changes are less tolerated in winter months than summer months (Birkett et al., 1998b). Since 1970 Laminaria hyperborea has undergone a range constriction of ~250 km at its southern edge, with current persisting populations having reduced longevity and less reproductive individuals than those populations from past studies (Fernandez, 2011; Assis et al., 2016). 

There is evidence that climate change is already having an impact on Laminaria hyperborea populations in the English Channel. Poleward range expansion of the warm temperate Laminaria ochroleuca as a result of ocean warming has led to competition with Laminaria hyperborea in UK waters (Smale et al., 2015). Laminaria ochroleuca was not found in the UK last century.  But Laminaria ochroleuca has now increased its range to include the southwest of England (Smale et al., 2015) and the west coast of Ireland (Schoenrock et al., 2019).

During the 2013-2014 Northeast Atlantic storm season, the UK was subjected to some of the most intense storms recorded within the past five years. A study by Smale & Vance. (2015) investigated the impacts of the storms on kelp canopies along the south coast of the UK, findings indicated monospecific canopies of Laminaria hyperborea were unaffected by the storms. However, the storms significantly altered a mixed canopy study site, composed of Laminaria ochroleuca, Saccharina latissima and Laminaria hyperborea. Therefore, if climate change continues to change species composition within kelp forests resistance to storm disturbance could be altered.

Smale et al. (2015) found that Laminaria hyperborea suffered from much higher epiphytic loadings and lower productivity than its competitor Laminaria ochroleuca during the summer months, which reduced its competitive ability. The decreased competitive ability because of ocean warming corresponds to findings by Pessarrodona et al. (2018), who found a decrease in the size of Laminaria hyperborea plants along a north-south gradient in Scotland, with average maximum stipe lengths of over 150 cm, whereas in southern England they were less than 100 cm. Similarly, Smale et al. (2020b) observed clear differences between net primary productivity (NNP) and carbon standing stock of Laminaria hyperborea between the colder northern and warmer southern test sites in the UK, with NNP and standing stock being 1.5 and 2.5 times greater in the northern sites. Identifying ocean temperatures as a lively driver of productivity, with reduced NNP and standing stock observed in warmer waters (Smale et al., 2020b). 

The decrease in productivity in southern England suggests that Laminaria hyperborea is already growing at suboptimal temperatures. Assis et al. (2018) predicted that under the highest emission scenario (RCP 8.5) the biogeographic range of Laminaria hyperborea will move northwards, and this retreat would lead to the species being lost from approximately 30% of the coastline of the UK.

Subtidal red algae are less tolerant of temperature extremes than intertidal red algae, surviving between -2°C and 18-23°C (Lüning 1990; Kain & Norton, 1990).  Temperature increase may affect growth, recruitment, or interfere with reproduction processes. For example, there is some evidence to suggest that blade growth in Delesseria sanguinea is delayed until ambient sea temperatures fall below 13°C.  Blade growth is also likely to be intrinsically linked to gametangia development (Kain, 1987), and maintenance of sea temperatures above 13°C may affect recruitment success.

Sensitivity assessment. Laminaria hyperborea is already growing at suboptimal temperatures in southern UK, based on evidence of decreased productivity comparative to Scotland (Pessarrodona et al., 2018; Smale et al., 2020b), and predictions have estimated Laminaria hyperborean to be lost from the UK by 2100 as a result of warming (Brodie et al., 2014). Sea surface temperatures around the UK currently range from between 6-19°C (Huthnance, 2010).  

Under the middle emission scenario, a rise of 3°C could lead to maximum summer high temperatures in the south of the UK of 22°C. This is above the upper thermal limit of 21°C for Laminaria hyperborea (Bolton & Lüning, 1982), and is likely to lead to loss of this species from the south of England. Furthermore, biomass and plant size are expected to decrease as waters warm, with Scottish Laminaria hyperborea stipe lengths decreasing to lengths observed in southern England, leading to a decline in carbon assimilation, productivity and habitat quality. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as a loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming under this scenario.

For the high emission scenario and extreme scenario, this northward retreat of the distribution of Laminaria hyperborea is expected to increase. Under the high emission scenario it is expected to be lost from 30% of the coastline around the UK (Assis et al., 2018), and under the extreme scenario even more is projected to be lost. Populations of Laminaria hyperborea that remain around the UK are predicted to become less productive. Therefore, under these scenarios, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very Low’. Therefore, this biotope is assessed as ‘High’ sensitivity to ocean warming under this scenario.

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C. Further detail.

Evidence

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

Laminaria hyperborea is a cold-temperate species of kelp with an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). Germination success can decrease by almost two thirds at temperatures as low as 17°C. Therefore, it is expected that like other kelp species, Laminaria hyperborea will be highly sensitive to marine heatwaves.

Kain (1964) stated that Laminaria hyperborea sporophyte growth and reproduction could occur within a temperature range of 0 to 20°C. Upper and lower lethal temperatures were estimated at between 1 to 2°C above or below the extremes of this range (Birkett et al., 1988). Gamete survival is reduced above 17°C (Kain, 1964 and 1971) and gametogenesis is inhibited at 21°C (Dieck, 1992). It is, therefore, likely that Laminaria hyperborea recruitment would be impaired at a sustained temperature increase of above 17°C. Sporophytes, however, can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are seasonally variable, with more sensitivity to temperature change in winter months than summer months (Birkett et al., 1998). If marine heatwaves (MHWs) occurred during the in autumn and winter during the reproductive phase, the success of recruitment could be reduced (Jacobs et al., 2024).

Laminaria hyperborea has a geographic range from mid-Portugal to Northern Norway (Birkett et al., 1998), and a mid-range within southern Norway (60° to 65° North) (Kain, 1971). The average seawater temperature for southern Norway in October is 12 to 13°C (Miller et al., 2009), and average annual sea temperature, from 1970 to 2014, is 8°C (Beszczynska-Möller & Dye, 2013). In Portugal and the southwest UK, Laminaria hyperborea is near 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 range is contracting due to ocean warming. Trailing edge populations are known to be more sensitive to temperature increases than populations in the centre of their geographic range because they are already living close to or at the limit of their thermal tolerance (Smale, 2020; Hereward et al., 2020; Leathers et al., 2024). The effects of MHWs on this biotope could therefore differ between locations, i.e., in the southwest UK, this biotope would be more sensitive to MHWs than it would be if an MHW of the same duration and intensity occurred in the north of the UK.

Temperature increases beyond the thermal optimum for kelp can negatively affect photosynthesis in kelps. Photosynthetic efficiency (measured as Fv/Fm) is widely used for measuring physiological stress in photosynthetic organisms (Trautmann et al., 2024). Burdett et al. (2019) found that simulated heat spikes (+2°C and +4°C) for three days had no overall effect on Laminaria hyperborea oxygen flux or photosynthetic efficiency, with the latter remaining above 0.72 for all treatments (with 0.7 being the widely accepted value which indicates physiological stress – Bass et al., 2023). However, photosynthetic efficiency responses to heat spikes can vary by season, light availability, and by the degree of warming. Bass et al. (2023) observed a decline in average photosynthetic efficiency of 0.33 in high light conditions and 0.11 in low light conditions, with both values falling below 0.7. The biggest decline was observed in the 22°C treatment, while the control (18°C) and 20°C treatments showed no significant change in photosynthetic efficiency. This interactive effect was also observed by Diehl et al. (2024), where photosynthetic efficiency was reduced significantly only in the coldest (0°C) treatment combined with a long photoperiod (24:0 hours light:dark) treatment. Cold and long light conditions significantly decreased chlorophyll a, accessory pigments and VAZ pigments, which indicates a photoprotective stress response. In the 10°C treatment, these pigments either decreased or showed no change, suggesting that the relatively higher temperature mitigated light stress. Dry weight increased significantly, despite no measurable change in surface area, when the highest temperature (10°C) treatment was combined with moderate (16:8 h) and long (24:0 h) photoperiods. This increase in dry weight was not detrimental to the kelp and was likely due to the accumulation of storage carbohydrates rather than growth. No significant responses were observed in phlorotannin (compounds that protect against light stress) levels. Mannitol (a storage carbohydrate) decreased under the long night treatment, but this effect is expected and not detrimental to the kelp. Laminarin (the other storage carbohydrate that was measured) increased significantly under both light treatments and the two warmer treatments (5°C and 10°C), which is a positive metabolic response.

The characteristic subtidal red algae, are less tolerant of temperature extremes than intertidal red algae, surviving between -2°C and 18 to 23 °C (Lüning 1990; Kain & Norton, 1990). Temperature increase may affect growth, recruitment or interfere with reproduction processes. For example, there is some evidence to suggest that blade growth in Delesseria sanguinea is delayed until ambient sea temperatures fall below 13°C. Blade growth is also likely to be intrinsically linked to gametangia development (Kain, 1987), and maintenance of sea temperatures above 13°C may affect recruitment success.

Laminaria digitata and Laminaria ochroleuca photosynthetic efficiency is negatively affected by simulated MHWs, with more pronounced effects in the highest MHW intensity treatments (King et al., 2024; Leathers et al., 2024). This pattern was also observed by Leathers et al. (2024), who also found that the duration of MHWs had a significant effect on photosynthetic efficiency and bleaching of these two species.

MHWs have been linked to mass mortalities of Saccharina latissima, a cold-water kelp which shares its range with Laminaria hyperborea. In the spring of 2018, a severe category III MHW in southern Norway led to a reduction in kelp cover from above 50% to below 30%. In addition, four strong category II MHWs occurred on the east coast of the USA, which led to a reduction in kelp density from above 40 individuals per m² to less than 5 individuals per m² (Filbee-Dexter et al., 2020).

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

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

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Laminaria hyperborea is unlikely to survive a heatwave of this magnitude and is, therefore, likely to suffer severe mortality in the south. In Scotland, where a significant portion of Laminaria hyperborea populations occur, temperatures are not predicted to rise above 20°C, therefore, Laminaria hyperborea is likely to survive a heatwave of this magnitude.  Therefore, resistance has been assessed as ‘Medium’. As a further heatwave (as defined by the pressure benchmark) is likely to affect this habitat before full recovery, resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘Medium’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England. Laminaria hyperborea is unlikely to survive a heatwave of this magnitude, and as temperatures are likely to reach >21°C in Scotland under this scenario, there is likely to be mortality throughout this species’ UK biogeographic distribution. Therefore, resistance has been assessed as ‘Low’. As a further heatwave is likely to affect this habitat before full recovery (under the pressure benchmark definition), resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Marine heatwaves (middle)

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

Evidence

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

Laminaria hyperborea is a cold-temperate species of kelp with an optimum temperature for growth of 15°C, and an upper temperature limit of 21°C (Bolton & Lüning, 1982). Germination success can decrease by almost two thirds at temperatures as low as 17°C. Therefore, it is expected that like other kelp species, Laminaria hyperborea will be highly sensitive to marine heatwaves.

Kain (1964) stated that Laminaria hyperborea sporophyte growth and reproduction could occur within a temperature range of 0 to 20°C. Upper and lower lethal temperatures were estimated at between 1 to 2°C above or below the extremes of this range (Birkett et al., 1988). Gamete survival is reduced above 17°C (Kain, 1964 and 1971) and gametogenesis is inhibited at 21°C (Dieck, 1992). It is, therefore, likely that Laminaria hyperborea recruitment would be impaired at a sustained temperature increase of above 17°C. Sporophytes, however, can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are seasonally variable, with more sensitivity to temperature change in winter months than summer months (Birkett et al., 1998). If marine heatwaves (MHWs) occurred during the in autumn and winter during the reproductive phase, the success of recruitment could be reduced (Jacobs et al., 2024).

Laminaria hyperborea has a geographic range from mid-Portugal to Northern Norway (Birkett et al., 1998), and a mid-range within southern Norway (60° to 65° North) (Kain, 1971). The average seawater temperature for southern Norway in October is 12 to 13°C (Miller et al., 2009), and average annual sea temperature, from 1970 to 2014, is 8°C (Beszczynska-Möller & Dye, 2013). In Portugal and the southwest UK, Laminaria hyperborea is near 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 range is contracting due to ocean warming. Trailing edge populations are known to be more sensitive to temperature increases than populations in the centre of their geographic range because they are already living close to or at the limit of their thermal tolerance (Smale, 2020; Hereward et al., 2020; Leathers et al., 2024). The effects of MHWs on this biotope could therefore differ between locations, i.e., in the southwest UK, this biotope would be more sensitive to MHWs than it would be if an MHW of the same duration and intensity occurred in the north of the UK.

Temperature increases beyond the thermal optimum for kelp can negatively affect photosynthesis in kelps. Photosynthetic efficiency (measured as Fv/Fm) is widely used for measuring physiological stress in photosynthetic organisms (Trautmann et al., 2024). Burdett et al. (2019) found that simulated heat spikes (+2°C and +4°C) for three days had no overall effect on Laminaria hyperborea oxygen flux or photosynthetic efficiency, with the latter remaining above 0.72 for all treatments (with 0.7 being the widely accepted value which indicates physiological stress – Bass et al., 2023). However, photosynthetic efficiency responses to heat spikes can vary by season, light availability, and by the degree of warming. Bass et al. (2023) observed a decline in average photosynthetic efficiency of 0.33 in high light conditions and 0.11 in low light conditions, with both values falling below 0.7. The biggest decline was observed in the 22°C treatment, while the control (18°C) and 20°C treatments showed no significant change in photosynthetic efficiency. This interactive effect was also observed by Diehl et al. (2024), where photosynthetic efficiency was reduced significantly only in the coldest (0°C) treatment combined with a long photoperiod (24:0 hours light:dark) treatment. Cold and long light conditions significantly decreased chlorophyll a, accessory pigments and VAZ pigments, which indicates a photoprotective stress response. In the 10°C treatment, these pigments either decreased or showed no change, suggesting that the relatively higher temperature mitigated light stress. Dry weight increased significantly, despite no measurable change in surface area, when the highest temperature (10°C) treatment was combined with moderate (16:8 h) and long (24:0 h) photoperiods. This increase in dry weight was not detrimental to the kelp and was likely due to the accumulation of storage carbohydrates rather than growth. No significant responses were observed in phlorotannin (compounds that protect against light stress) levels. Mannitol (a storage carbohydrate) decreased under the long night treatment, but this effect is expected and not detrimental to the kelp. Laminarin (the other storage carbohydrate that was measured) increased significantly under both light treatments and the two warmer treatments (5°C and 10°C), which is a positive metabolic response.

The characteristic subtidal red algae, are less tolerant of temperature extremes than intertidal red algae, surviving between -2°C and 18 to 23 °C (Lüning 1990; Kain & Norton, 1990). Temperature increase may affect growth, recruitment or interfere with reproduction processes. For example, there is some evidence to suggest that blade growth in Delesseria sanguinea is delayed until ambient sea temperatures fall below 13°C. Blade growth is also likely to be intrinsically linked to gametangia development (Kain, 1987), and maintenance of sea temperatures above 13°C may affect recruitment success.

Laminaria digitata and Laminaria ochroleuca photosynthetic efficiency is negatively affected by simulated MHWs, with more pronounced effects in the highest MHW intensity treatments (King et al., 2024; Leathers et al., 2024). This pattern was also observed by Leathers et al. (2024), who also found that the duration of MHWs had a significant effect on photosynthetic efficiency and bleaching of these two species.

MHWs have been linked to mass mortalities of Saccharina latissima, a cold-water kelp which shares its range with Laminaria hyperborea. In the spring of 2018, a severe category III MHW in southern Norway led to a reduction in kelp cover from above 50% to below 30%. In addition, four strong category II MHWs occurred on the east coast of the USA, which led to a reduction in kelp density from above 40 individuals per m² to less than 5 individuals per m² (Filbee-Dexter et al., 2020).

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

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

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Laminaria hyperborea is unlikely to survive a heatwave of this magnitude and is, therefore, likely to suffer severe mortality in the south. In Scotland, where a significant portion of Laminaria hyperborea populations occur, temperatures are not predicted to rise above 20°C, therefore, Laminaria hyperborea is likely to survive a heatwave of this magnitude.  Therefore, resistance has been assessed as ‘Medium’. As a further heatwave (as defined by the pressure benchmark) is likely to affect this habitat before full recovery, resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘Medium’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England. Laminaria hyperborea is unlikely to survive a heatwave of this magnitude, and as temperatures are likely to reach >21°C in Scotland under this scenario, there is likely to be mortality throughout this species’ UK biogeographic distribution. Therefore, resistance has been assessed as ‘Low’. As a further heatwave is likely to affect this habitat before full recovery (under the pressure benchmark definition), resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Ocean acidification (high)

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

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on emission scenario. Marine autotrophs will generally benefit from ocean acidification, through an increase in the availability of aqueous CO₂ for photosynthesis (Koch et al., 2013). 

Most species of kelp, including Laminaria hyperborea, appear to be undersaturated in respect to carbon dioxide, although they can generally utilise HCO₃ and have external carbonic anhydrase for extracellular dehydration of HCO₃ to CO₂(Koch et al., 2013). This was confirmed for Laminaria hyperborea by Olischläger et al. (2012) who found that ocean acidification at levels expected for the end of this century (700 µatm CO₂; a value between the middle and high emission scenario) led to an increase in female gametogenesis and increasing net photosynthesis and growth of sporophytes. 

Research on other kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012, Fernández et al., 2015, Nunes et al., 2015, Iñiguez et al., 2016b, a), except for one study, which found that ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Sensitivity assessment. Kelp forests live in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Laminaria hyperborea is not expected to be impacted by ocean acidification at levels expected for the end of this century. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’.

Ocean acidification (middle)

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

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on emission scenario. Marine autotrophs will generally benefit from ocean acidification, through an increase in the availability of aqueous CO₂ for photosynthesis (Koch et al., 2013). 

Most species of kelp, including Laminaria hyperborea, appear to be undersaturated in respect to carbon dioxide, although they can generally utilise HCO₃ and have external carbonic anhydrase for extracellular dehydration of HCO₃ to CO₂(Koch et al., 2013). This was confirmed for Laminaria hyperborea by Olischläger et al. (2012) who found that ocean acidification at levels expected for the end of this century (700 µatm CO₂; a value between the middle and high emission scenario) led to an increase in female gametogenesis and increasing net photosynthesis and growth of sporophytes. 

Research on other kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012, Fernández et al., 2015, Nunes et al., 2015, Iñiguez et al., 2016b, a), except for one study, which found that ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Sensitivity assessment. Kelp forests live in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Laminaria hyperborea is not expected to be impacted by ocean acidification at levels expected for the end of this century. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’.

Sea level rise (extreme)

Extreme scenario benchmark: a 107 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt. Sea levels have risen 1-3 mm/yr in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). This biotope IR.MIR.KR.LhypT.Ft occurs on extremely exposed, very exposed, exposed and moderately exposed, but wave surged, upper infralittoral bedrock and boulders. Wave surge is reduced with increasing depth; therefore, sea-level rise may reduce the effect of wave surge in the deeper examples or deeper reaches of this biotope. 

The distribution of Laminaria hyperborea is positivity related to wave exposure, and as wave exposure increases, so does biomass and abundance (Pedersen et al., 2012). Total plant biomass and production per unit area double along an exposure gradient, due to larger leaf lengths and increased density at high wave energy sites (Pedersen et al., 2012). Understanding how sea-level rise will affect exposure and tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Light availability and water turbidity are principle factors in determining kelp depth range (Birkett et al., 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. An increase in depth due to sea-level rise is likely to impact both Laminaria hyperborea and any understory algae, negatively impacting this biotope. The most important factors explaining the distribution of Laminaria hyperborea along the Norwegian coast were depth, wave exposure, light exposure and topography (Bekkby et al., 2009). This species requires rocky substrate for attachment. 

Sensitivity assessment. This biotope is recorded from 0 –20 m in depth.  As wave surge diminishes with increased depth, sea-level rise is likely to lead to the density of faunal turf reducing at the deeper reaches of this biotope and transitioning into a biotope characterised by kelp and dense red seaweeds. Tidal streams are also likely to be reduced so that LhypT and its sub-biotopes may transition into kelp biotopes (IR.HIR.KFaR.LhypR.Ft or IR.HIR.KFaR.LhypR.Pk) with a lower faunal diversity or dense foliose red algae (IR.HIR.KFaR.FoR.Dic).LhpT.Pk and/or LhypTX.Pk made be lost as they represent the depth limit of Laminaria hyperborea. This biotope LhypT.Ft may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of suitable substratum or human modified shorelines. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-20 m in depth, it is assumed at a sea-level rise of 0.5m, or 0.7 m (middle to high emission scenarios) would have limited effect but that a 1.07 m rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance is possibly ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

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Sea level rise (high)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt. Sea levels have risen 1-3 mm/yr in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). This biotope IR.MIR.KR.LhypT.Ft occurs on extremely exposed, very exposed, exposed and moderately exposed, but wave surged, upper infralittoral bedrock and boulders. Wave surge is reduced with increasing depth; therefore, sea-level rise may reduce the effect of wave surge in the deeper examples or deeper reaches of this biotope. 

The distribution of Laminaria hyperborea is positivity related to wave exposure, and as wave exposure increases, so does biomass and abundance (Pedersen et al., 2012). Total plant biomass and production per unit area double along an exposure gradient, due to larger leaf lengths and increased density at high wave energy sites (Pedersen et al., 2012). Understanding how sea-level rise will affect exposure and tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Light availability and water turbidity are principle factors in determining kelp depth range (Birkett et al., 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. An increase in depth due to sea-level rise is likely to impact both Laminaria hyperborea and any understory algae, negatively impacting this biotope. The most important factors explaining the distribution of Laminaria hyperborea along the Norwegian coast were depth, wave exposure, light exposure and topography (Bekkby et al., 2009). This species requires rocky substrate for attachment. 

Sensitivity assessment. This biotope is recorded from 0 –20 m in depth.  As wave surge diminishes with increased depth, sea-level rise is likely to lead to the density of faunal turf reducing at the deeper reaches of this biotope and transitioning into a biotope characterised by kelp and dense red seaweeds. Tidal streams are also likely to be reduced so that LhypT and its sub-biotopes may transition into kelp biotopes (IR.HIR.KFaR.LhypR.Ft or IR.HIR.KFaR.LhypR.Pk) with a lower faunal diversity or dense foliose red algae (IR.HIR.KFaR.FoR.Dic).LhpT.Pk and/or LhypTX.Pk made be lost as they represent the depth limit of Laminaria hyperborea. This biotope LhypT.Ft may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of suitable substratum or human modified shorelines. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-20 m in depth, it is assumed at a sea-level rise of 0.5m, or 0.7 m (middle to high emission scenarios) would have limited effect but that a 1.07 m rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance is possibly ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

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Sea level rise (middle)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt. Sea levels have risen 1-3 mm/yr in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). This biotope IR.MIR.KR.LhypT.Ft occurs on extremely exposed, very exposed, exposed and moderately exposed, but wave surged, upper infralittoral bedrock and boulders. Wave surge is reduced with increasing depth; therefore, sea-level rise may reduce the effect of wave surge in the deeper examples or deeper reaches of this biotope. 

The distribution of Laminaria hyperborea is positivity related to wave exposure, and as wave exposure increases, so does biomass and abundance (Pedersen et al., 2012). Total plant biomass and production per unit area double along an exposure gradient, due to larger leaf lengths and increased density at high wave energy sites (Pedersen et al., 2012). Understanding how sea-level rise will affect exposure and tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Light availability and water turbidity are principle factors in determining kelp depth range (Birkett et al., 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. An increase in depth due to sea-level rise is likely to impact both Laminaria hyperborea and any understory algae, negatively impacting this biotope. The most important factors explaining the distribution of Laminaria hyperborea along the Norwegian coast were depth, wave exposure, light exposure and topography (Bekkby et al., 2009). This species requires rocky substrate for attachment. 

Sensitivity assessment. This biotope is recorded from 0 –20 m in depth.  As wave surge diminishes with increased depth, sea-level rise is likely to lead to the density of faunal turf reducing at the deeper reaches of this biotope and transitioning into a biotope characterised by kelp and dense red seaweeds. Tidal streams are also likely to be reduced so that LhypT and its sub-biotopes may transition into kelp biotopes (IR.HIR.KFaR.LhypR.Ft or IR.HIR.KFaR.LhypR.Pk) with a lower faunal diversity or dense foliose red algae (IR.HIR.KFaR.FoR.Dic).LhpT.Pk and/or LhypTX.Pk made be lost as they represent the depth limit of Laminaria hyperborea. This biotope LhypT.Ft may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of suitable substratum or human modified shorelines. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-20 m in depth, it is assumed at a sea-level rise of 0.5m, or 0.7 m (middle to high emission scenarios) would have limited effect but that a 1.07 m rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance is possibly ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

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Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Kain (1964) stated that Laminaria hyperborea sporophyte growth and reproduction could occur within a temperature range of 0 to 20°C. Upper and lower lethal temperatures were estimated at between 1 to 2°C above or below the extremes of this range (Birkett et al., 1988). Gamete survival is reduced above 17°C (Kain, 1964 and 1971) and gametogenesis is inhibited at 21°C (Dieck, 1992). It is, therefore, likely that Laminaria hyperborea recruitment would be impaired at a sustained temperature increase of above 17°C. Sporophytes, however, can tolerate slightly higher temperatures of 20°C. Temperature tolerances for Laminaria hyperborea are seasonally variable, with more sensitivity to temperature change in winter months than summer months (Birkett et al., 1998b).

Laminaria hyperborea is a boreal northern species with a geographic range from mid-Portugal to Northern Norway (Birkett et al., 1998b), and a mid-range within southern Norway (60° to 65° North) (Kain, 1971). The average seawater temperature for southern Norway in October is 12 to 13°C (Miller et al., 2009), and average annual sea temperature, from 1970 to 2014, is 8°C (Beszczynska-Möller & Dye, 2013). In Portugal and the southwest UK, Laminaria hyperborea is near 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 range is contracting due to ocean warming. Trailing edge populations are known to be more sensitive to temperature increases than populations in the centre of their geographic range because they are already living close to or at the limit of their thermal tolerance (Smale, 2020; Hereward et al., 2020; Leathers et al., 2024).

Trailing edge Laminaria hyperborea populations assimilate less carbon than populations in colder waters (Pessarrodona et al., 2018) and therefore accumulate less biomass (Smale et al., 2016). In colder parts of the UK, lamina extension, regrowth, and carbon standing stock were 1.5, 2 and 3 times higher, respectively, than in warmer areas (Smale et al., 2020). Wernberg et al. (2025) observed differences in morphological features between populations at opposing ends of the species’ range, such as stipe height, lamina width, stipe diameter, lamina thickness, and the number of digits. They also found that stipe epiphyte load was far greater in the colder region than in the warmer region (Wernberg et al., 2025), most likely due to greater stipe surface area (Teagle & Smale, 2018).

Temperature increases beyond the thermal optimum for kelp can negatively affect photosynthesis in kelps. Photosynthetic efficiency (measured as Fv/Fm) is widely used for measuring physiological stress in photosynthetic organisms (Trautmann et al., 2024). Burdett et al. (2019) found that simulated heat spikes (+2°C and +4°C) for three days had no overall effect on Laminaria hyperborea oxygen flux or photosynthetic efficiency, with the latter remaining above 0.72 for all treatments (with 0.7 being the widely accepted value which indicates physiological stress – Bass et al., 2023). However, photosynthetic efficiency responses to heat spikes can vary by season, light availability, and by the degree of warming. Bass et al. (2023) observed a decline in average photosynthetic efficiency of 0.33 in high light conditions and 0.11 in low light conditions, with both values falling below 0.7. The biggest decline was observed in the 22°C treatment, while the control (18°C) and 20°C treatments showed no significant change in photosynthetic efficiency. This interactive effect was also observed by Diehl et al. (2024), where photosynthetic efficiency was reduced significantly only in the coldest (0°C) treatment combined with a long photoperiod (24:0 hours light:dark) treatment. Cold and long light conditions significantly decreased chlorophyll a, accessory pigments and VAZ pigments, which indicates a photoprotective stress response. In the 10°C treatment, these pigments either decreased or showed no change, suggesting that the relatively higher temperature mitigated light stress. Dry weight increased significantly, despite no measurable change in surface area, when the highest temperature (10°C) treatment was combined with moderate (16:8 h) and long (24:0 h) photoperiods. This increase in dry weight was not detrimental to the kelp and was likely due to the accumulation of storage carbohydrates rather than growth. No significant responses were observed in phlorotannin (compounds that protect against light stress) levels. Mannitol (a storage carbohydrate) decreased under the long night treatment, but this effect is expected and not detrimental to the kelp. Laminarin (the other storage carbohydrate that was measured) increased significantly under both light treatments and the two warmer treatments (5°C and 10°C), which is a positive metabolic response.

The loss of Laminaria hyperborea in some parts of the UK has been attributed to increasing sea surface temperatures in the last several decades (Yesson et al., 2015b). A reduction in abundance was observed in 187 out of 496 sites between 1974 and 2010. Declines were recorded in the English Channel and West Channel and Celtic Sea, the southernmost regions in the study. The English Channel decline was strongly correlated to the sea surface temperature increase of 1 to 2°C in this period. Northernmost sites (around Scotland) were overall unchanged, while populations on the west coast of Ireland increased in abundance (Yesson et al., 2015b).

Between 1977 and 2007, many cool-water macroalgae including Laminaria hyperborea and many other species found along its distribution have almost disappeared and been replaced by warm-water macroalgae on the west coast of Asturias, northern Spain (Fernández, 2016). Moreover, only 21 out of 50 (42%) locations that were surveyed between 1997 and 2023 on the northwest coast of Spain still had dense kelp forests, of which nine were completely dominated by Laminaria hyperborea (Barrientos et al., 2025). Sea surface temperatures overall increased by around 0.01°C – 0.02°C per year in this region and across this period. In 2023, only eight of the 21 remaining dense forests still had the same canopy-forming species as they did in 1997. Laminaria hyperborea was no longer the dominant kelp in any of these sites, and it only persisted in two sites, which it shared with Laminaria ochroleuca. The persistence of these forests was strongly correlated with winter and summer sea surface temperatures as well as higher wave action (Barrientos et al., 2025). Globally, Laminaria hyperborea has experienced a range contraction of 14% between the 1980s and 2010s (Casado-Amezúa et al., 2019). This range loss is estimated to continue to up to 39.34% under the most extreme greenhouse gas emissions projections (Assis et al., 2016).

Another feature of this biotope, subtidal red algae, are less tolerant of temperature extremes than intertidal red algae, surviving between -2°C and 18 to 23°C (Lüning 1990; Kain & Norton, 1990). Temperature increase may affect growth, recruitment or interfere with reproduction processes. For example, there is some evidence to suggest that blade growth in Delesseria sanguinea is delayed until ambient sea temperatures fall below 13°C. Blade growth is also likely to be intrinsically linked to gametangia development (Kain, 1987), and maintenance of sea temperatures above 13°C may affect recruitment success.

Warming can indirectly affect this biotope through cascading effects in the food chain. For example, it is suggested that the decline of cod (Gadus morhua) in the northeast Atlantic due to fishing, combined with increased sea surface temperatures in the past 50 years, has led to the spread of crabs (Cancer pagurus and Carcinus maenas) into this region. In addition, king crabs have spread into the northeast Atlantic since their introduction to Russia from the Pacific in the 1960s. These species are known predators of urchins, and this is believed to be a contributing factor in the recovery of kelp forests in this region (Christie et al., 2019).

Against the pressure benchmark, the available information suggests that Laminaria hyperborea recruitment processes may be affected and associated red algae communities may decline.

Sensitivity assessment. Overall, a chronic change (2°C for a year) outside the normal range for a year may reduce recruitment and growth, resulting in a minor loss in the population of kelp, especially in winter months or in southern examples of the biotope. However, an acute change (5°C for a month; e.g. from thermal effluent) may result in loss of abundance of kelp or extent of the bed, especially in winter. Therefore, resistance to the pressure is considered ‘Medium’, and resilience ‘Medium’. The sensitivity of this biotope to increases in temperature has been assessed as ‘Medium’.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

Kain (1964) stated that Laminaria hyperborea sporophyte growth and reproduction could occur within a temperature range of 0 - 20°C. Upper and lower lethal temperatures have been estimated at between 1-2°C above or below the extremes of this range (Birkett et al., 1988). Subtidal red algae can survive at temperatures between -2°C and 18-23°C (Lüning, 1990; Kain & Norton, 1990).

Laminaria hyperborea is a boreal northern species with a geographic range from mid-Portugal to Northern Norway (Birkett et al., 1998b), and a mid-range within southern Norway (60° to 65° North) (Kain, 1971). The average seawater temperature for southern Norway in October is 12 to 13°C (Miller et al., 2009), and average annual sea temperature, from 1970 to 2014, is 8°C (Beszczynska-Möller & Dye, 2013). The available information suggests that Laminaria hyperborea and biotope structure would not be affected by a change in sea temperature at the benchmark level.

Laminaria hyperborea populations in the middle and northern regions of their latitudinal range assimilate more carbon than populations in warmer waters (Pessarrodona et al., 2018) and therefore accumulate more biomass. In colder parts of the UK, lamina extension, regrowth, and carbon standing stock were 1.5, 2 and 3 times higher, respectively, than in warmer areas (Smale et al., 2020). Wernberg et al. (2025) observed differences in morphological features between populations at opposing ends of the species’ range, such as stipe height, lamina width, stipe diameter, lamina thickness, and the number of digits. They also found that stipe epiphyte load was far greater in the colder region than in the warmer region (Wernberg et al., 2025), most likely due to greater stipe surface area (Teagle & Smale, 2018).

Sensitivity assessment. Resistance to the pressure is considered ‘High’, and resilience ‘High’. The sensitivity of this biotope to decreases in temperature has been assessed as ‘Not Sensitive’.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Lüning (1990) suggest that “kelps” are stenohaline, their general tolerance to salinity as a phenotypic group covering 16 - 50 psu over a 24 hr period. Optimal growth probably occurs between 30-35 psu (MNCR category- 'Full' salinity) and growth rates are likely to be affected by periodic salinity stress. Birkett et al, (1998) suggested that long-term increases in salinity may affect Laminaria hyperborea growth and may result in loss of affected kelp, and therefore loss of the biotope.

Sensitivity assessment. Resistance to the pressure is considered ‘Low’, and resilience ‘Medium’.  The sensitivity of this biotope to an increase in salinity has been assessed as ‘Medium’.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Lüning (1990) suggest that ‘kelps’ are stenohaline, their general tolerance to salinity as a phenotypic group covering 16 - 50 psu over a 24 hr period. Optimal growth probably occurs between 30-35 psu (MNCR category-Full Salinity) and growth rates are likely to be affected by periodic salinity stress. Birkett et al,. (1998) suggest that long-term changes in salinity may result in loss of affected kelp and, therefore loss of this biotope.

Hopkin & Kain (1978) tested Laminaria hyperborea sporophyte growth at various low salinity treatments. The results showed that Laminaria hyperborea sporophytes could grow ‘normally’ at 19 psu, growth was reduced at 16 psu and did not grow at 7 psu. A decrease in one MNCR salinity scale from Full Salinity (30-40psu) to Reduced Salinity (18-30 psu) would result in a decrease of Laminaria hyperborea sporophyte growth. Laminaria hyperborea may also be out-competed by low salinity tolerant species e.g. Saccharina latissma (Karsten, 2007), or invasive kelp species, e.g. Undaria pinnatifida (Burrows et al., 2014).

If salinity was returned to Full Salinity (30-40 psu) Laminaria hyperborea could out-compete Saccharina latissma and re-establish community dominance in 2-4 years (Leinaas & Christie, 1996), however full habitat structure may take over 10 years to recover (Birkett et al., 1998; Cristie et al., 1998). The ability of Laminaria hyperborea to out-compete Undaria pinnatifida within the UK is however unknown (Heiser et al., 2014), and as such interspecific interaction between Laminaria hyperborea and Undaria pinnatifida is not included within this sensitivity assessment.

Sensitivity assessment. Resistance to the pressure is considered ‘Low’, and resilience ‘Medium’.  The sensitivity of this biotope to decreases in salinity has been assessed as ‘Medium’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

The morphology of the stipe and blade of kelps varies with water flow. In wave exposed areas, for example, Laminaria hyperborea develops a long and flexible stipe and this is probably a functional adaptation to strong water movement (Sjøtun et al., 1998). In addition, the lamina becomes narrower and thinner in strong currents (Sjøtun & Fredriksen, 1995). However, the stipe of Laminaria hyperborea is relatively stiff and can snap in strong currents. Laminaria hyperborea growth can persist in very strong tidal streams (>3 m/s) such as the Menai Strait, Wales, where tidal velocities can exceed 4 m/s (NBN, 2015) and in tidal rapids in Norway (J. Jones, pers. comm.). Strong currents can, however, cause detachment and reduce the settlement of kelp, therefore reducing their abundance (Bekkby et al., 2019).

Norderhaug et al. (2014) investigated the effects of current speed on species richness and diversity on Laminaria hyperborea holdfasts. Species richness was significantly linked to current speed, with the highest richness observed at intermediate current speeds between 0.12 and 0.18 m/s, (defined as “intermediate” based on the range of values observed in their study). It has been suggested that higher current speeds increase nutrient flow in the area, thereby promoting the growth of epiphytic algae on kelp stipes (Bekkby et al., 2015). Moreover, current speed has an interactive effect with wave exposure, where areas with high tidal flow and high wave action have a higher stipe epiphyte density. In contrast, in areas with high tidal flow and low wave activity, the bidirectional flow of water from tidal forces may increase canopy shading due to drag, while orbital and stochastic wave action can allow more light penetration through the canopy and facilitate epiphyte growth (Bekkby et al., 2015).

IR.MIR.KR.LhypT, IR.MIR.KR.LhypTX and their associated sub-biotopes are predominantly found within strong (1.5 to 3 m/s) to moderate (0.5 to 1.5 m/s) tidal streams. The prominent understorey filter feeding community within IR.MIR.KR.LhypT/TX is reliant on strong tidal flow. It is the abundance of filter feeding organisms that separates the tide-swept Laminaria hyperborea biotopes from the not tide-swept biotopes within the same level of tidal flow (e.g. IR.HIR.KFaR.LhypR). A change in peak mean spring bed flow velocity within the tidal streams of 0.5 to 3 m/s is not likely to significantly affect the abundance of Laminaria hyperborea. A decrease in tidal streams may result in a decline of filter-feeding fauna and an increase in red seaweeds within the understorey community, as in the biotope IR.HIR.KfaR.LhypR. A decrease in tidal flow within this range may also decrease urchin dislodgment and increase urchin grazing. An increase in urchin grazing may cause a decline in the understorey community abundance and diversity (as in IR.MIR.KR.Lhyp.GzFt/Pk). Conversely, increased water flow rates may reduce the understorey epiflora, which may be replaced by an epifauna dominated community (e.g. sponges, anemones and polyclinid ascidians) as in the biotope IR.HIR.KFaR.LhypFa. The composition of the holdfast fauna may also change, e.g. energetic or sheltered water movements favour different species of amphipods (Moore, 1985). Large increases in water flow (e.g. >3 m/s) may increase the dislodgement/loss of Laminaria hyperborea from the biotope and may cause an increase in the abundance of the ephemeral kelps Saccharina latissima or Alaria esculenta, which are both fast growing species and tolerant of fast water movement (Birkett et al., 1998b).

Sensitivity assessment. Water movement is a key environmental characteristic of IR.MIR.KR.LhypT and IR.MIR.KR.LhypTX. However, these biotopes are found within a broad range of tidal streams (0.5 to 3 m/s). A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year is not likely to significantly affect the community. Therefore, resistance to the pressure is assessed as ‘High’, resilience ‘High’, and sensitivity as ‘Not Sensitive’ at the benchmark level.

Large and dramatic changes in tidal streams (>3 m/s) may increase the abundance and dominance of ephemeral kelp species Alaria esculenta and Saccharina latissima, and may result in loss of the biotope. Changes of this dramatic nature are, however, outside of the scope of this habitat sensitivity assessment.

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. Further detail

Evidence

The upper limit of the Laminaria hyperborea bed is determined by wave action and water flow, desiccation, and competition from the more emergence resistant Laminaria digitata. Laminaria hyperborea exposed at extreme low water are very intolerant of desiccation, the most noticeable effect being bleaching of the frond and subsequent death of the meristem and loss of the plant.  An increase in wave exposure (See below- water flow), as a result of increased emergence, has been found to exclude Laminaria hyperborea from shallow waters due to dislodgement of the sporophyte or snapping of the stipe (Birket et al., 1998).  Hence, an increase in emergence is likely to lead to mortality of exposed Laminaria hyperborea and the associated habitat.

An increase in water depth/decreased emergence (at the benchmark level) may increase the upper depth restriction of Laminaria hyperborea forest variants within this biotope group. However, limited light availability at depth will decrease the lower extent of Laminaria hyperborea, and may therefore result in a shift from forest to park biotope variants at depth. Further increases in depth will cause a community shift to that characterized by circalittoral faunal species, however this is beyond the scope of the benchmark.

Sensitivity assessment. Resistance to the pressure is considered ‘Low’, and resilience ‘Medium’.  The sensitivity of this biotope to changes in tidal emergence has been assessed as ‘Medium’.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Laminaria hyperborea fronds, being almost exclusively sub tidal, would not come into contact with freshly released oil, but only to sinking emulsified oil and oil adsorbed onto particles (Birket et al., 1998). The mucilaginous slime layer coating of laminarians may protect them from smothering by oil. Hydrocarbons in solution reduce photosynthesis and may be algicidal. However, Holt et al,. (1995) reported that oil spills in the USA and from the 'Torrey Canyon' had little effect on kelp forests. Similarly, surveys of subtidal communities at a number sites between 1-22.5m below chart datum, including Laminaria hyperbora communities, showed no noticeable impacts of the Sea Empress oil spill and clean up (Rostron & Bunker, 1997). An assessment of holdfast fauna in Laminaria showed that although species richness and diversity decreased with increasing proximity to the Sea Empress oil spill, overall the holdfasts contained a reasonably rich and diverse fauna, even though oil was present in most samples (Sommerfield & Warwick, 1999). Laboratory studies of the effects of oil and dispersants on several red algae species, including Delesseria sanguinea (Grandy 1984; cited in Holt et al., 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. Holt et al., (1995) concluded that Delesseria sanguinea is probably generally sensitive of chemical contamination. Overall the red algae are likely to be highly intolerant to hydrocarbon contamination. Loss of red algae is likely to reduce the species richness and diversity of the biotope and the understorey may become dominated by encrusting corallines; however, red algae are likely to recover relatively quickly.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

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). Further detail

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).  In addition, the biotope occurs in areas of moderate to extreme wave action, so is likely to be continuously aerated. 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 2mg/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. Furthermore, wave exposure is likely to constantly aerate the affected area. While de-oxygenation may not directly affect Laminaria hyperborea, small invertebrate epifauna may be lost, causing a reduction in species richness. Therefore resistance has been assessed as ‘Medium’.  Resilience is likely to be ‘High’, and sensitivity is assessed as ‘Low’ at the benchmark level.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Holt et al. (1995) suggest that Laminaria hyperborea may be tolerant of nutrient enrichment since healthy populations are found at ends of sublittoral untreated sewage outfalls in the Isle of Man. Increased nutrient levels, e.g. from sewage outfalls, have been associated with increases in abundance, primary biomass and Laminaria hyperborea stipe production, but with simultaneous decrease in species numbers and diversity (Fletcher, 1996).

Laminaria hyperborea forests near high-effluent salmon farms show different stipe-associated community compositions to those near low-effluent farms and reference sites with no nearby aquaculture activity (Haugland et al., 2021). Bryozoan biomass was significantly higher at the high-effluent sites, whereas stipes at the low-effluent and reference sites were predominantly colonized by epiphytic macroalgae. At the high-effluent sites, the reduced epiphytic community was dominated by Ectocarpus spp., leading to lower heterogeneity within the stipe assemblage and reduced habitat heterogeneity. This suggests that changes in dissolved inorganic nitrogen could potentially shift this biotope from being fauna-dominated to an algae-dominated one (e.g. IR.HIR.KFaR.LhypR).Increased nutrients may result in phytoplankton blooms that increase turbidity (see above), and may favour sea urchins, e.g. Echinus esculentus, due their ability to absorb dissolved organics, potentially increasing grazing pressure leading to loss of understorey epiflora/fauna, reduced kelp recruitment, and possibly to the formation of urchin barrens. Therefore, although nutrients may not affect kelps directly, indirect effects such as turbidity, siltation and competition may significantly affect the structure of the biotope.

Sensitivity assessment. The above evidence suggests that increased nutrients may benefit Laminaria hyperborea kelp beds but alter the associated community, possibly resulting in changes in biotope classification. In extreme cases, turbidity and suspended sediment (see changes in suspended sediment) may attenuate light and be detrimental. Hence, resistance is assessed as ‘Medium’ to represent the potential loss of the associated community. Therefore, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Holt et al., (1995) suggest that Laminaria hyperborea may be tolerant of organic enrichment since healthy populations are found at ends of sublittoral untreated sewage outfalls in the Isle of Man. Increased nutrient levels e.g. from sewage outfalls, has been associated with increases in abundance, primary biomass and Laminaria hyperborea stipe production, but with concomitant decreases in species numbers and diversity (Fletcher, 1996). Increase organic enrichment has also been found to increase the abundance and dominance of suspension feeding fauna within Laminaria hyperborea holdfasts (Sheppard et al., 1980). Increase in ephemeral and opportunistic algae are associated with reduced numbers of perennial macrophytes (Fletcher, 1996). Increased nutrients may also result in phytoplankton blooms that increase turbidity. Therefore, although nutrients may not affect kelps directly, indirect effects such as turbidity and the increased abundance of suspension feeding fauna may affect the structure of Laminaria hyperborea biotopes (see water clarity above).

Sensitivity assessment. While organic enrichment may not have any direct effects on Laminaria hyperborea, increased turbidity and abundance of suspension feeding fauna may have significant effects on the biotope structure. Resistance to the pressure has therefore been considered ‘Medium’, and resilience ‘High’. The sensitivity of this biotope to organic enrichment is assessed as ‘Low’.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

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. Further detail

Evidence

If rock substrata were replaced with sedimentary substrata this would represent a fundamental change in habitat type, which Laminaria hyperborea would not be able to tolerate (Birket et al., 1998). The biotope would be lost.

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

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Not relevant

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). Further detail

Evidence

IR.MIR.KR.LhypTX plus associated sub biotopes, are found on tide swept boulders, cobbles, pebbles and gravel. Extraction of the substratum would likely result in high mortality of Laminaria hyperborea plus the associated community.

Sensitivity assessment. Resistance has been assessed as ‘None’, resilience as ‘Medium’. Sensitivity has been assessed as ‘Medium’

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Kelp harvesting can cause significant changes to this biotope through the removal of Laminaria hyperborea and the habitat space that it provides for its associated communities. Removing 26% of the canopy led to a 67% reduction in epiphytes and an 89% reduction of invertebrates (Norderhaug et al., 2020).

Christie et al. (1998) observed Laminaria hyperborea habitat regeneration following commercial Laminaria hyperborea trawling in south Norway. Trawling removed all large canopy-forming adult Laminaria hyperborea, however, sub-canopy recruits were largely unaffected. Within 2 to 6 years of harvesting, a new canopy had formed 1 m off the seabed. The associated holdfast communities recovered in six years. However, the epiphytic stipe community did not fully recover within the same period. Christie et al. (1998) suggested that kelp habitats were relatively resistant to direct disturbance/removal of Laminaria hyperborea canopy.

In Nord-Trøndelag, Norway, Laminaria hyperborea was harvested for the first time in 2010 (Steen et al., 2016). Video surveys and plant sampling conducted two days prior to the trawling and in each year for the following four years, showed that Laminaria hyperborea coverage had returned to pre-harvest levels (around 94%). However, the new canopy, was significantly lower in density, average plant age, length, weight, and epiphyte biomass. In addition, the density of understorey recruits had only recovered by one-third by the end of the study period. It was suggested that 80% of the new canopy consisted of understorey plants that had survived the harvesting, and that the resilience of this biotope was dependent on the frequency of harvesting (Steen et al., 2016).

Recurrent disturbance on a timescale shorter than the 2 to 6-year recovery period could prolong the recovery. Kain (1975) cleared sublittoral blocks of Laminaria hyperborea at different times of the year for several years. The first colonizers and succession communities differed between blocks and the time of year they were cleared. However, within two years of clearance, the blocks were dominated by Laminaria hyperborea. Leinaas & Christie (1996) also observed Laminaria hyperborea recolonizing urchin barrens following urchin removal. The substratum was initially colonized by filamentous macroalgae and Saccharina latissima. However, Laminaria hyperborea dominated the community after 2 to 4 years.

Laminaria hyperborea forests subjected to regular harvesting support different associated communities compared to unharvested, preserved forests (Leclerc et al., 2015). Macroalgal species richness was consistently higher at the harvested site across all parts of the kelp and on the surrounding rock. Sessile fauna richness was slightly higher on the stipes and surrounding rock at the harvested site, but lower on the holdfast compared to the preserved site. In contrast, mobile fauna richness and density were generally greater on all parts of the kelp in the preserved site, although both were higher on the surrounding rock at the harvested site. Following disturbance, or in areas experiencing frequent disturbance, Laminaria hyperborea recruitment may be affected by interspecific competition with Non-Indigenous Invasive Species (INIS) or ephemeral algae (Brodie et al., 2014; Smale et al., 2013) (see INIS below).

Sensitivity assessment. Resistance to the pressure is considered ‘Low’, and resilience ‘Medium’. The sensitivity of this biotope to damage to seabed surface features is assessed as ‘Medium’.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Not relevant, please refer to pressure 'Abrasion/ disturbance of the substratum on the surface of the seabed'.

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. Further detail

Evidence

Suspended Particle Matter (SPM) concentration has a linear relationship with sub-surface light attenuation (Kd) (Devlin et al., 2008). An increase in SPM results in a decrease in sub-surface light attenuation. Light availability and water turbidity are principal factors in determining kelp depth range (Birkett et al., 1998). Light penetration influences the maximum depth at which kelp species can grow and it has been reported that laminarians grow down to 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 Laminaria hyperborea 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 (Birkett et al. 1998b; Lüning, 1990).

Ecklonia radiata 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 hyperborea and Laminaria ochroleuca and decrease Laminaria hyperborea abundance and density (see sub-biotope IR.MIR.KR.LhypT.Pk). Kain (1964) suggested that early Laminaria hyperborea gametophyte development could occur in the absence of light. Furthermore, observations from south Norway found that a pool of Laminaria hyperborea recruits could persist growing beneath Laminaria hyperborea canopies for several years, indicating that sporophyte growth can occur in light-limited environments (Christe et al., 1998). However in habitats exposed to high levels of suspended silts Laminaria hyperborea is out-competed by Saccharina latissima, a silt tolerant species, and thus, a decrease in water clarity is likely to decrease the abundance of Laminaria hyperborea in the affected area (Norton, 1978).

Sensitivity Assessment. Changes in water clarity are likely to affect photosynthetic rates and enable Saccharina latissima to compete more successfully with Laminaria hyperborea.  A decrease in turbidity is likely to support enhanced growth (and possible habitat expansion) and is therefore not considered in this assessment.  An increase in water clarity from clear to intermediate (10-100 mg/l) represents a change in light attenuation of ca 0.67-6.7 Kd/m, and is likely to result in a greater than 50% reduction in photosynthesis of Laminaria spp. Therefore, the dominant kelp species will probably suffer a significant decline and resistance to this pressure is assessed as ‘Low’. Resilience to this pressure is probably ‘Medium’ at the benchmark.  Hence, this biotope is assessed as having a sensitivity of ‘Medium ‘to this pressure.

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. Further detail

Evidence

Smothering by sediment e.g. 5 cm material during a discrete event, is unlikely to damage Laminaria hyperborea plants but is likely to affect gametophyte survival as well as holdfast fauna, and interfere with zoospore settlement.  Given the microscopic size of the gametophyte, 5 cm of sediment could be expected to significantly inhibit growth. However, laboratory studies showed that gametophytes can survive in darkness for between 6 - 16 months at 8 °C and would probably survive smothering by a discrete event.  Once returned to normal conditions the gametophytes resumed growth or maturation within 1 month (Dieck, 1993). Intolerance to this factor is likely to be higher during the peak periods of sporulation and/or spore settlement.

If inundation is long lasting then the understorey epifauna/flora may be adversely affected, e.g. suspension or filter feeding fauna and/or algal species. If clearance of deposited sediment occurs rapidly then understorey communities are expected to recover quickly. IR.MIR.KR.LhypT/TX (and their associated sub-biotopes) occur in high to moderate energy habitats (due to water flow or wave action) so deposited sediment is unlikely to remain for more than a few tidal cycles, except in the deepest of rock-pools.

Sensitivity assessment. Due to the strong tidal flows that characterize IR.MIR.KR.LhypT, IR.MIR.KR.LhypTX, deposited sediments are likely to be rapidly dispersed from the affected site.  Resistance to the pressure is therefore considered ‘High’, and resilience ‘High’. The sensitivity of this biotope to light deposition of up to 5cm of fine material added to the seabed in a single discreet event is assessed as ‘Not Sensitive’.

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. Further detail

Evidence

Smothering by sediment e.g. 30 cm material during a discrete event, is unlikely to damage Laminaria hyperborea plants but is likely to affect gametophyte survival, holdfast communities, the epiphytic community at the base of the stype and interfere with zoospore settlement. Given the microscopic size of the gametophyte, 30 cm of sediment could be expected to significantly inhibit growth. However, laboratory studies showed that gametophytes can survive in darkness for between 6 and 16 months at 8°C and would probably survive smothering within a discrete event. Once returned to normal conditions the gametophytes resumed growth or maturation within 1 month (Dieck, 1993). Intolerance to this factor is likely to be higher during the peak periods of sporulation and/or spore settlement.

If inundation is long lasting then the understorey epifauna/flora may be adversely affected, e.g. suspension or filter feeding fauna and/or algal species. If clearance of deposited sediment occurs rapidly then understorey communities are expected to recover quickly.  IR.MIR.KR.LhypT/TX (and their associated sub-biotopes) occur in high to moderate energy habitats (due to water flow or wave action) so deposited sediment is unlikely to remain for more than a few tidal cycles, except in the deepest of rock-pools.

Sensitivity assessment. Resistance to the pressure is considered ‘High’, and resilience ‘High’. The sensitivity of this biotope to heavy deposition of up to 30cm of fine material added to the seabed in a single discreet event is assessed as ‘Not Sensitive’.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

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

No evidence

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

Light availability is a key environmental factor influencing the distribution, morphology, and productivity of Laminaria hyperborea. Several studies have demonstrated that biomass accumulation, canopy density, and morphological traits are positively correlated with light levels. For example, Smale et al. (2016) found that summer daytime light had a strong positive effect on canopy biomass and standing stock of carbon. Similarly, Smith et al. (2022) reported that percentage surface irradiance significantly predicted total and canopy density, canopy standing biomass, total fresh weight, blade fresh weight, and blade length in Laminaria hyperborea populations across the UK. In southern regions, blade width, total length, and age were also positively affected.

Stahl et al. (2024) conducted a study on the potential for Laminaria hyperborea afforestation in the German Bight, with a specific focus on light requirements and habitat suitability. Their study identified a minimum compensation irradiance of approximately 30 µmol photons /m²/s¹ under summer conditions. Monteiro et al. (2015) supported these findings, showing that over 75% of observed kelp species, including Laminaria hyperborea, occurred in areas where more than 3.65% of surface light reached the seafloor. These studies show that Laminaria hyperborea occurs in areas with moderate to high levels of light.

The effects of light on Laminaria hyperborea physiology can vary depending on temperature. For instance, at 10ºC, three different photoperiods: polar day (24:0 light:dark), long day (16:8 light:dark) and polar night (0:24 light:dark) had very little effect on photosynthetic efficiency, while 5ºC and 0ºC treatments had varied photosynthetic responses (Diehl et al., 2024). Photosynthetic efficiency declined significantly over 12 weeks in the polar day and long day photoperiods at 0ºC, but increased slightly with the polar night treatment. In the same time frame, photosynthetic efficiency declined significantly at 5ºC with the polar day treatment, but was relatively unchanged with the other two photoperiods. Other measured responses – such as dry weight, pigments, phlorotannins, and storage carbohydrates – all varied by light and temperature treatments. Cold and long light conditions significantly decreased chlorophyll a, accessory pigments and VAZ pigments, which indicates a photoprotective stress response. In the 10°C treatment, these pigments either decreased or showed no change, suggesting that the relatively higher temperature mitigated light stress. Dry weight increased significantly, despite no measurable change in surface area, when the highest temperature (10°C) treatment was combined with moderate (16:8 h) and long (24:0 h) photoperiods. This increase in dry weight was not detrimental to the kelp and was likely due to the accumulation of storage carbohydrates rather than growth. No significant responses were observed in phlorotannin (compounds that protect against light stress) levels. Mannitol (a storage carbohydrate) decreased under the long night treatment, but this effect is expected and not detrimental to the kelp. Laminarin (the other storage carbohydrate that was measured) increased significantly under both light treatments and the two warmer treatments (5°C and 10°C), which is a positive metabolic response.

Shading of the biotope (e.g. by coastal development) could adversely affect the biotope in areas already low in water clarity. This may shift the balance toward shade-tolerant species, leading to loss of the biotope within shaded zones or a reduction in Laminaria hyperborea abundance, shifting from forest to park-type biotopes. Ecklonia radiata show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient = 0.1 to 0.2/m; Staehr & Wernberg, 2009). Therefore, any activity that decreases incident light (e.g. shading) may be detrimental.

While incident light has an overall positive effect on Laminaria hyperborea at optimal temperatures, the effects of artificial light on kelp are not yet fully understood. There is now a growing body of evidence to show that artificial light at night (ALAN) is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50m (Davies et al., 2020; Smyth et al., 2021).

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

Sensitivity assessment. An increase in incident light is likely to increase plant productivity and increase the density of the kelps. Constant artificial light may affect the reproductive cues and recruitment in macroalgae, but no evidence was found specifically for Laminaria hyperborea. However, shading, especially from permanent structures (e.g. pontoons, jetties) are likely to reduce incident light and will probably result in the reduction in kelp density, or even its exclusion from the affected area. Therefore, a resistance of 'Low' is suggested. Resilience is probably 'High' if the shading is temporary but 'Very low' if permanent. Therefore, a precautionary sensitivity of 'High' is suggested.

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. Further detail

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. Further detail

Evidence

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

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

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 that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

No evidence regarding the genetic modification or effects of translocation of native populations was found.

Introduction of microbial pathogens

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

Evidence

Galls on the blade of Laminaria hyperborea and spot disease are associated with the endophyte Streblonema sp. although the causal agent is unknown (bacteria, virus or endophyte). Resultant damage to the blade and stipe may increase losses in storms. The endophyte inhibits spore production and therefore recruitment and recoverability (Lein et al., 1991).

Sensitivity assessment. Resistance to the pressure is considered ‘Medium’, and resilience ‘High’. The sensitivity of this biotope to introduction of microbial pathogens is assessed as ‘Low’.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Kelp harvesting can cause significant changes to this biotope through the removal of Laminaria hyperborea and the habitat space that it provides for its associated communities. Removing 26% of the canopy led to a 67% reduction in epiphytes and an 89% reduction of invertebrates (Norderhaug et al., 2020).

Christie et al. (1998) observed Laminaria hyperborea habitat regeneration following commercial Laminaria hyperborea trawling in south Norway. Trawling removed all large canopy-forming adult Laminaria hyperborea, however, sub-canopy recruits were largely unaffected. Within 2 – 6 years of harvesting, a new canopy had formed 1 m off the seabed. The associated holdfast communities recovered in 6 years. However, the epiphytic stipe community did not fully recover in the same time. Christie et al. (1998) suggested that kelp habitats were relatively resistant to direct disturbance/removal of Laminaria hyperborea canopy.

In Nord-Trøndelag, Norway, Laminaria hyperborea was harvested for the first time in 2010 (Steen et al., 2016). Video surveys and plant sampling, conducted 2 days prior to the trawling and in each year for the following 4 years, showed that Laminaria hyperborea coverage had returned to pre-harvest levels (around 94%). However, the new canopy was significantly lower in density, average plant age, length, weight, and epiphyte biomass. In addition, the density of understorey recruits had only recovered by one-third by the end of the study period. It was suggested that 80% of the new canopy consisted of understorey plants that had survived the harvesting, and that the resilience of this biotope is dependent on the rate of harvesting (Steen et al., 2016).

Recurrent disturbance on a timescale shorter than the 2 to 6-year recovery period could prolong the recovery. Kain (1975) cleared sublittoral blocks of Laminaria hyperborea at different times of the year for several years. The first colonizers and succession communities differed between blocks and the time of year they were cleared. However, within 2 years of clearance, the blocks were dominated by Laminaria hyperborea. Leinaas & Christie (1996) also observed Laminaria hyperborea recolonizing urchin barrens following urchin removal. The substratum was initially colonized by filamentous macroalgae and Saccharina latissima. However, Laminaria hyperborea dominated the community after 2 to 4 years.

Laminaria hyperborea forests subjected to regular harvesting support different associated communities compared to unharvested, preserved forests (Leclerc et al., 2015). Macroalgal species richness was consistently higher at the harvested site across all parts of the kelp and on the surrounding rock. Sessile fauna richness was slightly higher on the stipes and surrounding rock at the harvested site, but lower on the holdfast compared to the preserved site. In contrast, mobile fauna richness and density were generally greater on all parts of the kelp in the preserved site, although both were higher on the surrounding rock at the harvested site.

Following disturbance, or in areas experiencing frequent disturbance occurs, Laminaria hyperborea recruitment may be affected by interspecific competition with Non-Indigenous Invasive Species or ephemeral algae (Brodie et al., 2014; Smale et al., 2013). However, evidence for this is limited and thus not included in this assessment.

Sensitivity assessment. Resistance to the pressure is considered ‘Low’, and resilience ‘Medium’. The sensitivity of this biotope to damage to seabed surface features is assessed as ‘Medium’.

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. Further detail

Evidence

Incidental/accidental removal of Laminaria hyperborea from other fisheries or extraction processes are likely to cause similar effects to that of direct harvesting; as such the same evidence has been used for both pressure assessments.

Christie et al., (1998) observed Laminaria hyperborea habitat regeneration following commercial Laminaria hyperborea trawling in south Norway. Within the study area trawling removed all large canopy-forming adult Laminaria hyperborea. Within 2-3 years of harvesting a new canopy had formed 1 m off the seabed. The associated holdfast communities recovered in six years however the epiphytic stipe community did not fully recover within the same time period. Christie et al. (1998) suggested that kelp habitats were relatively resistant to direct disturbance of Laminaria hyperborea canopy.

Recurrent disturbance occurring at a smaller time scale than the recovery period of 2-6 years (stated above) could extend recovery time. Kain (1975a) cleared sublittoral blocks of Laminaria hyperborea at different times of the year for several years. The first colonizers and succession community differed between blocks and at what time of year the blocks were cleared however within two years of clearance the blocks were dominated by Laminaria hyperborea. Leinaas & Christie (1996) also observed Laminaria hyperborea re-colonization of ‘urchin barrens’, following removal of urchins. The substratum was initially colinized by filamentous macroalgae and Saccharina latissima however after 2-4 years Laminaria hyperborea dominated the community.

Following disturbance or in areas were recurrent rapid disturbance occurs Laminaria hyperborea recruitment could also be affected by interspecifc competitive interactions with Non Indigenous Invasive Species or ephemeral algal species (Brodie et al., 2014; Smale et al., 2013), however evidence for this is limited and thus not included within this assessment.

Sensitivity assessment. Resistance to the pressure is considered ‘Low’, and resilience ‘Medium’.  The sensitivity of this biotope to damage to seabed surface features 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 ppt) although it prefers around 30 ppt (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. It has also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020).

In the eastern Solent harbours of Portsmouth, Langstone, and Chichester, 75% to 98% of Crepidula larvae settled on dead Crepidula shells, while ~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 ~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 ~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 ~0.7 m) and was absent from high tidal level (~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 hyperborea 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 has led to the widespread misidentification of Didemnum vexillum, and it is often recorded as Didemnum spp. Hence, the native range of the species is not known conclusively (Lambert, 2009; Stefaniak et al., 2012; Mckenzie et al., 2017; Holt, 2024). However, molecular data and limited historical evidence have suggested that the species may be native to Japan, with its native range possibly extending into continental Asia and north-western Pacific (Stefaniak et al., 2012; Tillin et al., 2020; Holt, 2024). Previously unrecorded populations of a colonial ascidian have been recently identified as Didemnum vexillum (Tillin et al., 2020).

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

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

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

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

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

Didemnum vexillum tolerates a wide range of environmental conditions, including temperature and salinity (Herborg et al., 2009; Tillin et al., 2020). Didemnum vexillum can withstand a wide range of salinities from 20 to 44 ppt, is commonly found in marine waters around 33 ppt but is unable to survive in salinities below 20 ppt (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 ppt (Bullard & Whitlatch, 2009). However, in the Wadden Sea, colonies of Didemnum vexillum were abundant in salinities between 17.91 to 25.97 ppt (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 (2015) from Norway to Morocco and into the Mediterranean in the eastern Atlantic and from Alaska to Baja California in the eastern Pacific and from southern Russia to southern China in the western Pacific (Engelen et al., 2015). It colonizes a variety of habitats and can tolerate -1°C to 30°C and survive salinities below 10 ppt. Although fertilization does not occur below 15 ppt 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 out-compete leathery, canopy-forming macroalgae such as Saccharina latissima, Halidrys siliquosa, and Fucus spp. and, to a lesser degree, understorey species such as Codium fragile, Chondrus crispus and Dictyota dichotoma in Limfjorden, Denmark between 1984 and 1997 (Staehr et al., 2000; Engelen et al., 2015; de Bettignies et al., 2021). The invasion in Limfjorden had stabilized by 2005 although many of the native macroalgal species continued to decline (Engelen et al., 2015). In Limfjorden, the distribution of Sargassum muticum was limited to areas with hard substratum, in particular stones >10 cm in diameter, while smaller stones, gravel and sand were unsuitable. It was most abundant between 1 and 4 m in depth but had low cover at 0 to 0.5 m and 4 to 6 m, in the turbid waters of the Limfjorden. Limfjorden is wave sheltered but wave exposure has been reported to restrict the growth and survival of Sargassum muticum (Staehr et al., 2000). Viejo et al. (1995) reported that Sargassum muticum transplanted to wave exposed shores in Spain experienced >80% breakages within a month and that the growth of undamaged plants was significantly lower than that of plants on sheltered shores. Similarly, Andrew & Viejo (1998) noted that Sargassum muticum was restricted to intertidal rockpools in wave exposed sites in the Bay of Biscay. 

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

Sensitivity Assessment. This Laminaria hyperborea dominated biotope (IR.MIR.KR.LhypT.Ft) is found within the infralittoral with extreme to moderate wave exposure and strong tidal streams. The evidence above suggests that Sargassum muticum prefers wave sheltered, shallow sites in the sublittoral fringe. No evidence of the effects of Sargassum on Laminaria hyperborea beds was found. Therefore, 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 and tidal streams that characterize this biotope. 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 out-compete native UK kelp species (see Farrell & Fletcher, 2006; Thompson & Schiel, 2012; Brodie et al., 2014; Heiser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017; Epstein & Smale, 2018; Kraan, 2017; Epstein et al., 2019a,b; Tidbury, 2020). Undaria pinnatifida originates from Japan but is established currently on the coastlines of New Zealand, Australia, Northern France, Spain, Italy, the UK, Portugal, Belgium, Holland, Argentina, Mexico, and the USA (De Leij et al., 2017). Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016) and has since proliferated along UK coastlines. One year after its discovery at the Queen Anne Battery marina, Plymouth, it became a major fouling plant on pontoons (Minchin & Nunn, 2014). Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound. 

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

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

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

In St Malo, France, there was evidence that Undaria pinnatifida 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 ca 0.5 Laminaria hyperborea individuals per m² present compared to ca 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. 

In Plymouth Sound (UK), 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, such as Laminaria hyperborea had more resistance to Undaria pinnatifida invasion than disturbed or sparse canopies, due to limited space and light availability for Undaria recruits. However, the dense canopies will not prevent the invasion of Undaria, as sporophytes were still recorded within dense Laminaria canopies, and this suggests that canopy disturbance is not always required. 

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

Sensitivity Assessment. Undaria pinnatifida has the potential to colonize and co-exist in refugia within Laminaria sp. dominated habitats, especially in shallow examples of their biotopes that are within its depth range (1 to 4m) and sheltered from wave action. A dense kelp canopy may restrict or slow the proliferation of Undaria pinnatifida, however, there is mixed evidence of its colonization with Laminaria hyperborea beds and in some areas, a lower abundance of Laminaria hyperborea may result in increased Undaria pinnatifida growth. In addition, it is highly unlikely that Undaria pinnatifida will be able to colonize or survive the degree of wave exposure and tidal streams 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

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

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

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

Another potential advantage of Laminaria ochroleuca is its greater average stipe length compared to Laminaria hyperborea, potentially reducing light availability for Laminaria hyperborea recruits in mixed-population forests (Smale et al., 2014). This shading effect may exaggerate the impacts of marine heatwaves on Laminaria hyperborea, as elevated temperatures increase metabolic demands that cannot be met under light-limited conditions (Bass et al., 2023).

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

Sensitivity Assessment. The evidence for Laminaria hyperborea poleward range contraction (Assis et al., 2016; Casado-Amezúa et al., 2019), alongside the expansion of Laminaria ochroleuca into higher latitudes (Franco et al., 2018), suggests that Laminaria ochroleuca could displace existing kelp biotopes in the southern UK. In Plymouth Sound, Laminaria ochroleuca is already rivalling Laminaria hyperborea, which used to be the dominant kelp in the area (Smale et al., 2014; Taylor-Robinson et al., 2024). Its greater stipe length could reduce light availability for smaller kelps, and when combined with elevated temperatures, could create unfavourable conditions for the persistence and recovery of native species. Laminaria ochroleuca, however, does form mixed forests with Laminaria hyperborea in moderately sheltered to exposed shores, and has physiological and morphological advantages that could allow it to proliferate if Laminaria hyperborea density was reduced.

Resistance to Laminaria ochroleuca is assessed as ‘Low’ based on the evidence of Laminaria ochroleuca rivalling Laminaria hyperborea in Plymouth Sound, southwest UK. Hence, resilience is assessed as ‘Very Low’, and sensitivity as ‘High’. While the quality and applicability of the evidence are high, there is contrasting evidence regarding both species’ resistance and resilience to storm damage. Therefore, confidence in this sensitivity assessment is ‘Medium’.