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). 

Saccorhiza polyschides is a fast-growing, short-lived opportunist species that rapidly colonizes cleared areas of the substratum. Saccorhiza polyschides has a wide geographic distribution from Norway to Morocco and can tolerate a wide range of temperatures (Hawkins & Harkin 1985), with an upper survival temperature tolerance of 24°C (Dieck, 1993). Sporophyte growth can occur from 3-24°C and gametophyte development from 5-25°C (Norton, 1977). Fernández (2011) however suggested that summer temperatures of >20°C sustained for longer than a period of 30 days may inhibit development and recruitment.

Saccorhiza polyschides abundance has increased rapidly in the English Channel displacing cold water laminarian species in the intertidal (Birchenough et al., 2013). The abundance of Saccorhiza polyschides is predicted to increase in the UK as a response to ocean warming (Smale et al., 2013). However, studies conducted in Spain and Portugal have documented declines in abundance, range constrictions and local extinctions of Saccorhiza polyschides (Smale et al., 2020). 

Saccharina latissima is a polar to temperate macroalgae distributed from Greenland to the coast of Portugal, and in the NW Atlantic, is found as far south as New York State, USA. In the UK, sea surface temperatures are range between 6-19°C (Huthnance, 2010), and Saccharina latissima is in the middle of its biogeographic range. At its southern distribution in New York, temperatures can regularly reach ≥20°C for six weeks or more during summer months (Gerard & Du Bois, 1988).

Saccharina latissima has an optimal growth temperature between 10- 15°C, with growth reducing by 50-70% at 20°C, and all experimental specimens disintegrating after 7 days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19-20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23-25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. 

In the field, Saccharina latissima has shown significant regional variation in its acclimation response to changing environmental conditions.  For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima that were regularly exposed to ≥20°C tolerated these high temperatures, whereas sporophytes from other populations which rarely experience ≥17°C showed 100% mortality after 3 weeks of exposure to 20°C.

Saccharina latissima has suffered a dramatic decline in the Skagerrak region, Norway. Where community structure has shifted from Saccharina latissima forests to communities dominated by filamentous macroalgae (Moy & Christie, 2012). In 2006, Andersen et al. (2011) transplanted Saccharina latissima into areas from where this species had been lost previously to determine whether the kelp could grow and mature. High mortality occurred from August-November each year. In 2008, only six of the seventeen original transplanted Saccharina latissima sporophytes survived (approx. 65% mortality rate). All surviving sporophytes were heavily fouled by epiphytic organisms (estimated cover of 80 & 100%). Between 1960 and 2009, sea surface temperatures in the region had regularly exceeded 20°C and so had the duration at which temperatures remain above 20°C. High sea temperatures have been linked to the slow growth of Saccharina latissima which is likely due to a decrease in the photosynthetic ability of Saccharina latissima, and an increase in vulnerability to epiphytic loading, bacterial and viral attacks (Anderson et al., 2011).

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima and Saccorhiza polyschides would move northwards, retreating from their southernmost locations, and predicted the loss of Saccharina latissima from the south-west coast of the UK.  

Many of the red algae species associated with the understory turf can tolerate warm water temperatures. Corallina officinalis may tolerate between -4 to 28°C (Lüning, 1990), although when Colthart & Johansen (1973) exposed this species to a number of different temperatures, they found that growth was maintained at 18°C and ceased at 25°C. Abrupt temperature changes (10°C in California, Seapy & Littler 1984; 4.8 to 8.5°C, Hawkins & Hartnoll, 1985) resulted in dramatic declines. However, in both cases recovery was rapid, suggesting that the crustose bases survived. 

Sensitivity Assessment. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988), and maybe crucial in the persistence of this species around the UK, as seawater temperatures rise. In the UK, Saccorhiza polyschides is in the middle of its geographic range, so is unlikely to be affected by an increase of 3°C by the end of the century. However, a change in 4-5°C may put the species outside its lethal limits. 

With sea surface temperature around the UK of between 6-19°C (Huthnance, 2010), populations of Saccorhiza polyschides, Saccharina latissima, and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C (middle emission scenario) by the end of this century, leading to maximum summer high temperatures in the south of the UK of 22°C. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming in the middle emission scenario.

For the high emission scenario and extreme scenario, whereby sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century Saccorhiza polyschides and Saccharina latissima are likely to be lost from southern England. This assessment corresponds with the results of ecological niche modelling by Assis et al. (2018) who predicted that Saccharina latissima would be lost from the southwest coast of the UK, because of climate change.  Therefore, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very low’. This biotope SlatSac is assessed as having ‘High’ sensitivity to ocean warming in the high and extreme emission scenarios.

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). 

Saccorhiza polyschides is a fast-growing, short-lived opportunist species that rapidly colonizes cleared areas of the substratum. Saccorhiza polyschides has a wide geographic distribution from Norway to Morocco and can tolerate a wide range of temperatures (Hawkins & Harkin 1985), with an upper survival temperature tolerance of 24°C (Dieck, 1993). Sporophyte growth can occur from 3-24°C and gametophyte development from 5-25°C (Norton, 1977). Fernández (2011) however suggested that summer temperatures of >20°C sustained for longer than a period of 30 days may inhibit development and recruitment.

Saccorhiza polyschides abundance has increased rapidly in the English Channel displacing cold water laminarian species in the intertidal (Birchenough et al., 2013). The abundance of Saccorhiza polyschides is predicted to increase in the UK as a response to ocean warming (Smale et al., 2013). However, studies conducted in Spain and Portugal have documented declines in abundance, range constrictions and local extinctions of Saccorhiza polyschides (Smale et al., 2020). 

Saccharina latissima is a polar to temperate macroalgae distributed from Greenland to the coast of Portugal, and in the NW Atlantic, is found as far south as New York State, USA. In the UK, sea surface temperatures are range between 6-19°C (Huthnance, 2010), and Saccharina latissima is in the middle of its biogeographic range. At its southern distribution in New York, temperatures can regularly reach ≥20°C for six weeks or more during summer months (Gerard & Du Bois, 1988).

Saccharina latissima has an optimal growth temperature between 10- 15°C, with growth reducing by 50-70% at 20°C, and all experimental specimens disintegrating after 7 days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19-20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23-25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. 

In the field, Saccharina latissima has shown significant regional variation in its acclimation response to changing environmental conditions.  For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima that were regularly exposed to ≥20°C tolerated these high temperatures, whereas sporophytes from other populations which rarely experience ≥17°C showed 100% mortality after 3 weeks of exposure to 20°C.

Saccharina latissima has suffered a dramatic decline in the Skagerrak region, Norway. Where community structure has shifted from Saccharina latissima forests to communities dominated by filamentous macroalgae (Moy & Christie, 2012). In 2006, Andersen et al. (2011) transplanted Saccharina latissima into areas from where this species had been lost previously to determine whether the kelp could grow and mature. High mortality occurred from August-November each year. In 2008, only six of the seventeen original transplanted Saccharina latissima sporophytes survived (approx. 65% mortality rate). All surviving sporophytes were heavily fouled by epiphytic organisms (estimated cover of 80 & 100%). Between 1960 and 2009, sea surface temperatures in the region had regularly exceeded 20°C and so had the duration at which temperatures remain above 20°C. High sea temperatures have been linked to the slow growth of Saccharina latissima which is likely due to a decrease in the photosynthetic ability of Saccharina latissima, and an increase in vulnerability to epiphytic loading, bacterial and viral attacks (Anderson et al., 2011).

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima and Saccorhiza polyschides would move northwards, retreating from their southernmost locations, and predicted the loss of Saccharina latissima from the south-west coast of the UK.  

Many of the red algae species associated with the understory turf can tolerate warm water temperatures. Corallina officinalis may tolerate between -4 to 28°C (Lüning, 1990), although when Colthart & Johansen (1973) exposed this species to a number of different temperatures, they found that growth was maintained at 18°C and ceased at 25°C. Abrupt temperature changes (10°C in California, Seapy & Littler 1984; 4.8 to 8.5°C, Hawkins & Hartnoll, 1985) resulted in dramatic declines. However, in both cases recovery was rapid, suggesting that the crustose bases survived. 

Sensitivity Assessment. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988), and maybe crucial in the persistence of this species around the UK, as seawater temperatures rise. In the UK, Saccorhiza polyschides is in the middle of its geographic range, so is unlikely to be affected by an increase of 3°C by the end of the century. However, a change in 4-5°C may put the species outside its lethal limits. 

With sea surface temperature around the UK of between 6-19°C (Huthnance, 2010), populations of Saccorhiza polyschides, Saccharina latissima, and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C (middle emission scenario) by the end of this century, leading to maximum summer high temperatures in the south of the UK of 22°C. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming in the middle emission scenario.

For the high emission scenario and extreme scenario, whereby sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century Saccorhiza polyschides and Saccharina latissima are likely to be lost from southern England. This assessment corresponds with the results of ecological niche modelling by Assis et al. (2018) who predicted that Saccharina latissima would be lost from the southwest coast of the UK, because of climate change.  Therefore, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very low’. This biotope SlatSac is assessed as having ‘High’ sensitivity to ocean warming in the high and extreme emission scenarios.

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). 

Saccorhiza polyschides is a fast-growing, short-lived opportunist species that rapidly colonizes cleared areas of the substratum. Saccorhiza polyschides has a wide geographic distribution from Norway to Morocco and can tolerate a wide range of temperatures (Hawkins & Harkin 1985), with an upper survival temperature tolerance of 24°C (Dieck, 1993). Sporophyte growth can occur from 3-24°C and gametophyte development from 5-25°C (Norton, 1977). Fernández (2011) however suggested that summer temperatures of >20°C sustained for longer than a period of 30 days may inhibit development and recruitment.

Saccorhiza polyschides abundance has increased rapidly in the English Channel displacing cold water laminarian species in the intertidal (Birchenough et al., 2013). The abundance of Saccorhiza polyschides is predicted to increase in the UK as a response to ocean warming (Smale et al., 2013). However, studies conducted in Spain and Portugal have documented declines in abundance, range constrictions and local extinctions of Saccorhiza polyschides (Smale et al., 2020). 

Saccharina latissima is a polar to temperate macroalgae distributed from Greenland to the coast of Portugal, and in the NW Atlantic, is found as far south as New York State, USA. In the UK, sea surface temperatures are range between 6-19°C (Huthnance, 2010), and Saccharina latissima is in the middle of its biogeographic range. At its southern distribution in New York, temperatures can regularly reach ≥20°C for six weeks or more during summer months (Gerard & Du Bois, 1988).

Saccharina latissima has an optimal growth temperature between 10- 15°C, with growth reducing by 50-70% at 20°C, and all experimental specimens disintegrating after 7 days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19-20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23-25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. 

In the field, Saccharina latissima has shown significant regional variation in its acclimation response to changing environmental conditions.  For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima that were regularly exposed to ≥20°C tolerated these high temperatures, whereas sporophytes from other populations which rarely experience ≥17°C showed 100% mortality after 3 weeks of exposure to 20°C.

Saccharina latissima has suffered a dramatic decline in the Skagerrak region, Norway. Where community structure has shifted from Saccharina latissima forests to communities dominated by filamentous macroalgae (Moy & Christie, 2012). In 2006, Andersen et al. (2011) transplanted Saccharina latissima into areas from where this species had been lost previously to determine whether the kelp could grow and mature. High mortality occurred from August-November each year. In 2008, only six of the seventeen original transplanted Saccharina latissima sporophytes survived (approx. 65% mortality rate). All surviving sporophytes were heavily fouled by epiphytic organisms (estimated cover of 80 & 100%). Between 1960 and 2009, sea surface temperatures in the region had regularly exceeded 20°C and so had the duration at which temperatures remain above 20°C. High sea temperatures have been linked to the slow growth of Saccharina latissima which is likely due to a decrease in the photosynthetic ability of Saccharina latissima, and an increase in vulnerability to epiphytic loading, bacterial and viral attacks (Anderson et al., 2011).

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima and Saccorhiza polyschides would move northwards, retreating from their southernmost locations, and predicted the loss of Saccharina latissima from the south-west coast of the UK.  

Many of the red algae species associated with the understory turf can tolerate warm water temperatures. Corallina officinalis may tolerate between -4 to 28°C (Lüning, 1990), although when Colthart & Johansen (1973) exposed this species to a number of different temperatures, they found that growth was maintained at 18°C and ceased at 25°C. Abrupt temperature changes (10°C in California, Seapy & Littler 1984; 4.8 to 8.5°C, Hawkins & Hartnoll, 1985) resulted in dramatic declines. However, in both cases recovery was rapid, suggesting that the crustose bases survived. 

Sensitivity Assessment. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988), and maybe crucial in the persistence of this species around the UK, as seawater temperatures rise. In the UK, Saccorhiza polyschides is in the middle of its geographic range, so is unlikely to be affected by an increase of 3°C by the end of the century. However, a change in 4-5°C may put the species outside its lethal limits. 

With sea surface temperature around the UK of between 6-19°C (Huthnance, 2010), populations of Saccorhiza polyschides, Saccharina latissima, and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C (middle emission scenario) by the end of this century, leading to maximum summer high temperatures in the south of the UK of 22°C. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming in the middle emission scenario.

For the high emission scenario and extreme scenario, whereby sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century Saccorhiza polyschides and Saccharina latissima are likely to be lost from southern England. This assessment corresponds with the results of ecological niche modelling by Assis et al. (2018) who predicted that Saccharina latissima would be lost from the southwest coast of the UK, because of climate change.  Therefore, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very low’. This biotope SlatSac is assessed as having ‘High’ sensitivity to ocean warming in the high and extreme emission scenarios.

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). 

Saccharina latissima has disappeared almost completely from the Danish estuary Limfjorden, where maximum surface temperatures in summer have increased by 0.7°C per decade over the last 40 years while the number of days with temperatures above 20°C has increased dramatically from 1-2 days year to >25 days year (Pedersen, 2015). Similarly, Saccharina latissima has been lost from the Skagerrak coast of Norway, which is thought to be due to an increase in summer temperatures, coupled with eutrophication (Moy & Christie, 2012).

Under experimental conditions, Nepper-Davidson et al. (2019) exposed a northern (Denmark) population of Saccharina latissima 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). These results suggest that this species is unlikely to survive heatwaves of the length and magnitude predicted by the end of this century for both the middle and high emission scenarios.

Simonson et al. (2015) investigated the impacts of four temperature treatments (11°C, 14°C, 18°C & 21°C) on Saccharina latissima tissue over three weeks. Histological analysis showed temperature mediated tissue damage, including holes, splitting of the medulla, damage to the meristoderm and loss of differentiation between tissue layers at temperatures between 14-21°C. 

Studies conducted in Spain and Portugal have documented declines in abundance, range constrictions and local extinctions of Saccorhiza polyschides (Smale et al., 2020). A long-term study has shown a significant decrease in Saccorhiza polyschides populations with decreases in the number of reproductive plants and recruitment, which suggested that the long warm summer periods of temperatures above 20°C sustained for over 30 days could be altering the kelps survival (Fernàndez, 2011). 

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. A heatwave of this magnitude is likely to cause mass mortality of Saccharina latissima and severe mortality of Saccorhiza polyschides. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ This biotope IR.HIR.KSed.Sac is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. Under this scenario, Saccorhiza polyschides and Saccharina latissima. are likely to be already lost from this biotope as a result of rising temperatures (see Global warming), although mortality of any surviving specimens would occur as a result of this projected heatwave. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope IR.HIR.KSed.Sac 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). 

Saccharina latissima has disappeared almost completely from the Danish estuary Limfjorden, where maximum surface temperatures in summer have increased by 0.7°C per decade over the last 40 years while the number of days with temperatures above 20°C has increased dramatically from 1-2 days year to >25 days year (Pedersen, 2015). Similarly, Saccharina latissima has been lost from the Skagerrak coast of Norway, which is thought to be due to an increase in summer temperatures, coupled with eutrophication (Moy & Christie, 2012).

Under experimental conditions, Nepper-Davidson et al. (2019) exposed a northern (Denmark) population of Saccharina latissima 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). These results suggest that this species is unlikely to survive heatwaves of the length and magnitude predicted by the end of this century for both the middle and high emission scenarios.

Simonson et al. (2015) investigated the impacts of four temperature treatments (11°C, 14°C, 18°C & 21°C) on Saccharina latissima tissue over three weeks. Histological analysis showed temperature mediated tissue damage, including holes, splitting of the medulla, damage to the meristoderm and loss of differentiation between tissue layers at temperatures between 14-21°C. 

Studies conducted in Spain and Portugal have documented declines in abundance, range constrictions and local extinctions of Saccorhiza polyschides (Smale et al., 2020). A long-term study has shown a significant decrease in Saccorhiza polyschides populations with decreases in the number of reproductive plants and recruitment, which suggested that the long warm summer periods of temperatures above 20°C sustained for over 30 days could be altering the kelps survival (Fernàndez, 2011). 

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. A heatwave of this magnitude is likely to cause mass mortality of Saccharina latissima and severe mortality of Saccorhiza polyschides. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ This biotope IR.HIR.KSed.Sac is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. Under this scenario, Saccorhiza polyschides and Saccharina latissima. are likely to be already lost from this biotope as a result of rising temperatures (see Global warming), although mortality of any surviving specimens would occur as a result of this projected heatwave. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope IR.HIR.KSed.Sac 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). 

Research on most 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, that found ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Under experimental CO₂ enrichment at levels expected by the end of this century, germination rates in Saccharina latissima were the same as control samples but gametophyte size increased, suggesting a benefit for juvenile stages of this species (Roleda et al., 2012). Nunes et al. (2015) found that experimental exposure of adult Saccharina latissima to enhanced CO₂ led to an increase in net primary production, whilst Gordillo et al. (2015) found that enhanced CO₂ led to increased photosynthesis and growth. In contrast, Iñiguez et al. (2016) found no increase in carbon fixation under elevated CO₂ conditions. Whilst contrasting in findings, these studies suggest that ocean acidification will not negatively impact Saccharina latissima. No evidence of the effect of ocean acidification on Saccorhiza polyschides was found. 

Corallina officinalis is a highly calcified, erect, red algae. Results of experimental CO₂ enrichment suggest that this species could be significantly negatively affected by future ocean acidification. Hofmann et al. (2012) found that growth and photosynthesis decreased as a result of a 0.3 unit decrease in pH. Further investigation showed that skeletal CaCO₃ decreased with increasing CO₂ at levels expected for both the middle emission and high emission scenarios, although this decrease was small (< 2%) (Hofmann et al., 2013). Yildiz et al. (2013) showed that although CaCO₃ decreased in Corallina officinalis as a result of ocean acidification, photosynthesis increased. When ocean acidification was combined with an increase in UV radiation, which led to an increase in growth rate. They summarised that a decrease in CaCO₃ content may not be negative but may lead to this species absorbing and using light differently. Brodie et al. (2014) reported that Corallina species were more resilient to ocean acidification than other calcified algae species, although competition from flesh algal species that benefit from high CO₂ may indirectly cause the loss of calcified species from biotopes. Similarly, observations have indicated Corallinales to be adversely affected at locations where CO₂ gradients occur naturally, with evidence of Corallinales being outcompeted by heterokont algae at Mediterranean CO₂ seeps (Martin & Hall-Spencer, 2017).  

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). Saccharina latissima is not expected to exhibit negative effects from ocean acidification at levels expected for the end of this century, and Saccorhiza polyschides may not be affected. Due to the disturbed nature of the biotope, the understorey community can vary locally, therefore impacts on the understory community have not been included in the assessment. Under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ so that sensitivity is assessed as ‘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). 

Research on most 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, that found ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Under experimental CO₂ enrichment at levels expected by the end of this century, germination rates in Saccharina latissima were the same as control samples but gametophyte size increased, suggesting a benefit for juvenile stages of this species (Roleda et al., 2012). Nunes et al. (2015) found that experimental exposure of adult Saccharina latissima to enhanced CO₂ led to an increase in net primary production, whilst Gordillo et al. (2015) found that enhanced CO₂ led to increased photosynthesis and growth. In contrast, Iñiguez et al. (2016) found no increase in carbon fixation under elevated CO₂ conditions. Whilst contrasting in findings, these studies suggest that ocean acidification will not negatively impact Saccharina latissima. No evidence of the effect of ocean acidification on Saccorhiza polyschides was found. 

Corallina officinalis is a highly calcified, erect, red algae. Results of experimental CO₂ enrichment suggest that this species could be significantly negatively affected by future ocean acidification. Hofmann et al. (2012) found that growth and photosynthesis decreased as a result of a 0.3 unit decrease in pH. Further investigation showed that skeletal CaCO₃ decreased with increasing CO₂ at levels expected for both the middle emission and high emission scenarios, although this decrease was small (< 2%) (Hofmann et al., 2013). Yildiz et al. (2013) showed that although CaCO₃ decreased in Corallina officinalis as a result of ocean acidification, photosynthesis increased. When ocean acidification was combined with an increase in UV radiation, which led to an increase in growth rate. They summarised that a decrease in CaCO₃ content may not be negative but may lead to this species absorbing and using light differently. Brodie et al. (2014) reported that Corallina species were more resilient to ocean acidification than other calcified algae species, although competition from flesh algal species that benefit from high CO₂ may indirectly cause the loss of calcified species from biotopes. Similarly, observations have indicated Corallinales to be adversely affected at locations where CO₂ gradients occur naturally, with evidence of Corallinales being outcompeted by heterokont algae at Mediterranean CO₂ seeps (Martin & Hall-Spencer, 2017).  

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). Saccharina latissima is not expected to exhibit negative effects from ocean acidification at levels expected for the end of this century, and Saccorhiza polyschides may not be affected. Due to the disturbed nature of the biotope, the understorey community can vary locally, therefore impacts on the understory community have not been included in the assessment. Under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ so that sensitivity is assessed as ‘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). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal 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. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and has adapted to low-light conditions (Gerard, 1990).

This biotope IR.HIR.KSed.Sac occurs on very exposed, exposed and moderately exposed infralittoral and sublittoral bedrock and boulders (JNCC, 2015). Both Saccharina latissima and Saccorhiza polyschides occur in a wide range of water flow rates, from strong tidal currents to areas with low wave exposure (Norton, 1978; Birkett et al., 1998b). Therefore, these species are unlikely to be affected by a change in water flow. 

Understanding how sea-level rise will affect 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.

Sensitivity assessment. The IR.HIR.KSed.Sac biotope occurs in disturbed areas on seasonally unstable boulders and/or sand/pebble scour (JNCC, 2015). Disturbance, in particular, due to winter storms prevents long-lived kelps (e.g., Laminaria hyperborea) from becoming established so that the biotope is dominated by opportunistic kelps and other fast-growing brown macroalgae.  

This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is expected that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. More importantly, in areas where wave action is the greatest contributor to water movement and, hence, scour, especially due to storms, an increase in sea-level is likely to reduce the depth of this biotope as the substratum becomes more stable and opportunistic kelps are outcompeted by other macroalgae. This biotope will probably be replaced by another disturbed kelp dominated biotope such as IR.HIR.KSed.SlatSac, which occurs at greater depths than KSed.Sac.

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, this biotope (KSed.Sac) is recorded from the sublittoral fringe to 5 m in depth but ‘generally above 5 m’ (JNCC, 2015). Therefore, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) may result in a change of biotope, but that a 107cm rise (the extreme emission scenario) might result in a significant loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘Medium’ under the middle and high emission scenarios so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’. However, resistance may be ‘Low’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘High’, albeit with ‘Low’ confidence as the assessments are based on ‘expert judgement’. 

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). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal 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. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and has adapted to low-light conditions (Gerard, 1990).

This biotope IR.HIR.KSed.Sac occurs on very exposed, exposed and moderately exposed infralittoral and sublittoral bedrock and boulders (JNCC, 2015). Both Saccharina latissima and Saccorhiza polyschides occur in a wide range of water flow rates, from strong tidal currents to areas with low wave exposure (Norton, 1978; Birkett et al., 1998b). Therefore, these species are unlikely to be affected by a change in water flow. 

Understanding how sea-level rise will affect 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.

Sensitivity assessment. The IR.HIR.KSed.Sac biotope occurs in disturbed areas on seasonally unstable boulders and/or sand/pebble scour (JNCC, 2015). Disturbance, in particular, due to winter storms prevents long-lived kelps (e.g., Laminaria hyperborea) from becoming established so that the biotope is dominated by opportunistic kelps and other fast-growing brown macroalgae.  

This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is expected that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. More importantly, in areas where wave action is the greatest contributor to water movement and, hence, scour, especially due to storms, an increase in sea-level is likely to reduce the depth of this biotope as the substratum becomes more stable and opportunistic kelps are outcompeted by other macroalgae. This biotope will probably be replaced by another disturbed kelp dominated biotope such as IR.HIR.KSed.SlatSac, which occurs at greater depths than KSed.Sac.

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, this biotope (KSed.Sac) is recorded from the sublittoral fringe to 5 m in depth but ‘generally above 5 m’ (JNCC, 2015). Therefore, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) may result in a change of biotope, but that a 107cm rise (the extreme emission scenario) might result in a significant loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘Medium’ under the middle and high emission scenarios so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’. However, resistance may be ‘Low’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘High’, albeit with ‘Low’ confidence as the assessments are based on ‘expert judgement’. 

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). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal 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. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and has adapted to low-light conditions (Gerard, 1990).

This biotope IR.HIR.KSed.Sac occurs on very exposed, exposed and moderately exposed infralittoral and sublittoral bedrock and boulders (JNCC, 2015). Both Saccharina latissima and Saccorhiza polyschides occur in a wide range of water flow rates, from strong tidal currents to areas with low wave exposure (Norton, 1978; Birkett et al., 1998b). Therefore, these species are unlikely to be affected by a change in water flow. 

Understanding how sea-level rise will affect 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.

Sensitivity assessment. The IR.HIR.KSed.Sac biotope occurs in disturbed areas on seasonally unstable boulders and/or sand/pebble scour (JNCC, 2015). Disturbance, in particular, due to winter storms prevents long-lived kelps (e.g., Laminaria hyperborea) from becoming established so that the biotope is dominated by opportunistic kelps and other fast-growing brown macroalgae.  

This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is expected that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. More importantly, in areas where wave action is the greatest contributor to water movement and, hence, scour, especially due to storms, an increase in sea-level is likely to reduce the depth of this biotope as the substratum becomes more stable and opportunistic kelps are outcompeted by other macroalgae. This biotope will probably be replaced by another disturbed kelp dominated biotope such as IR.HIR.KSed.SlatSac, which occurs at greater depths than KSed.Sac.

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, this biotope (KSed.Sac) is recorded from the sublittoral fringe to 5 m in depth but ‘generally above 5 m’ (JNCC, 2015). Therefore, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) may result in a change of biotope, but that a 107cm rise (the extreme emission scenario) might result in a significant loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘Medium’ under the middle and high emission scenarios so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’. However, resistance may be ‘Low’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘High’, albeit with ‘Low’ confidence as the assessments are based on ‘expert judgement’. 

Temperature increase (local)

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

Evidence

Saccorhiza polyschides has a wide geographic distribution and can tolerate a wide range of temperatures. Sporophyte growth can occur from 3-24°C and gametophyte development from 5-25°C (Norton, 1977). Fernández (2011) however suggested that summer temperatures of >20°C sustained for longer than a period of 30 days may inhibit development and recruitment.

Desmarestiales are unusual in that they produce and accumulate sulphuric acid within intracellular vacuoles (McClintock et al., 1982; Connor et al., 2004; Gagnon et al., 2013). Seasonal increases in temperatures limit the ability of the storage vacuoles to contain the acid and release it into the surrounding environment. The continued release of acid results in progressive decolourisation, tissue degradation and mortality of Desmarestia sporophytes (Gagnon et al., 2013). Gagnon et al. (2013) exposed Desmarestia viridis samples during 30 hour salinity and temperature treatments. At 29 and 32psu (MNCR: Full Salinity scale), Desmarestia viridis was able to tolerant to changes in temperature from 5-12°C, but exposure to 18°C was lethal to sporophytes.

The geographic distribution of Saccharina latissima is determined by the 19-20°C isotherm (Müller et al., 2009). Gametophytes can develop in ≤23°C (Lüning, 1990), but the optimal temperature range for sporophyte growth is 10-15°C (Bolton & Lüning, 1982).  Bolton & Lüning (1982) experimentally observed that sporophyte growth was inhibited by 50-70% at 20°C and following 7 days at 23°C all specimens completely disintegrated. In the field, Saccharina latissima has significant regional variation in its acclimation to temperature change. For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima which were regularly exposed to ≥20°C could tolerate these temperatures whereas sporophytes from other populations which rarely experience ≥17°C showed 100% mortality after 3 weeks of exposure to 20°C. Therefore, the response of Saccharina latissima to a change in temperatures is likely to be locally variable.

Anderson et al. (2011) transplanted Saccharina latissima from the Skagerrak region, Norway an area which has experienced a 50.7-83% Saccharina latissima decline since 2002 (Moy & Christie, 2012). Since 1960-2009 sea surface temperatures in the region have regularly exceeded 20°C (the temperature at which Saccharina latissima growth is severely inhibited) and so has the number of days which remain above 20°C. Anderson et al. (2011) hypothesised that high sea temperatures were indirectly linked to Saccharina latissima deforestation in the region, causing high epiphytic loading of sporophyte fronds (estimated to cover 80 & 100% of transplanted sporophytes). High sea temperatures have been linked to the slow growth of Saccharina latissima which is likely to decrease the photosynthetic ability of, and increase the vulnerability of Saccharina latissima to epiphytic loading, bacterial and viral attacks (Anderson et al., 2011).

Desmarestiales are sensitive to high temperatures and low salinities (Gagnon et al., 2013). Desmarestiales are unusual in that they produce and accumulate sulphuric acid (H₂SO₄) within intracellular vacuoles (McClintock et al., 1982; Connor et al., 2004; Gagnon et al., 2013). Increases in temperatures and low salinities limit the ability of the storage vacuoles to contain the acid and release it into the surrounding environment. The continued release of acid results in progressive decolourisation, tissue degradation and mortality of Desmarestia sporophytes (Gagnon et al., 2013). Gagnon et al. (2013) exposed Desmarestia viridis samples during 30 hour salinity and temperature treatments, observing at 29 and 32 psu (MNCR: Full Salinity scale) Desmarestia viridis was tolerant to changes in temperature from 5-12°C, exposure to 18°C was lethal to sporophytes. Furthermore, sporophytes that had already begun the senescence phase were exposed to 10.8 ± 0.3°C and completely shed all tissue from the stipe and laterals within ca15 days, whereas those exposed to lower temperatures of 2.5± 0.1°C lasted ca30 days. Gagnon et al. (2013) also observed Desmarestia spp. degraded progressively in low salinity treatments of 26, 23 & 20psu (<20psu was not tested). Therefore indicating Desmarestia spp. are highly sensitive to both high temperature (12-18°C) and low salinities (<26 psu).

IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR are distributed throughout the UK (Connor et al., 2004). Northern to southern Sea Surface Temperature (SST) ranges from 8-16°C in summer and 6-13°C in winter (Beszczynska-Möller & Dye, 2013)

Sensitivity assessment. Acute 5°C increases in temperature for a period of 1 month combined with high summer temperatures may exceed the threshold temperature of 18-20°C in biotopes within the south of the UK, which would likely cause mortality of Desmarestia spp., and severely limit Saccorhiza polyschides recruitment & Saccharina latissima sporophyte growth. A 2°C increase in temperature for a period of 1 year would likely result in the exceeding an 18°C temperature threshold in the south of the UK. This temperature threshold would likely result in high mortality of Desmarestia spp. Saccharina latissima that is not acclimated to similar temperatures may also experience high and rapid mortality. Resistance has been assessed as ‘None’, Resilience as ‘High’. Sensitivity 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

Demarestia spp. are a cold water adapted genus with a polar distribution, which can grow abundantly in water temperatures of ca 0.5°C (Gagnon et al., 2013).  Saccharina latissima is also widely spread throughout the Arctic and has a lower temperature threshold for sporophyte growth at 0°C (Lüning, 1990). Subtidal red algae can survive at -2°C (Lüning, 1990; Kain & Norton, 1990). These temperatures are well below that considered within this pressure benchmark. Demarestia spp. Saccharina latissima are therefore unlikely to be adversely affected by a decrease in temperature at the benchmark level.

Saccorhiza polyschides sporophyte growth can occur within a range from 3-24°C and gametophyte development within a range of 5-25°C (Norton, 1977). Norton (1977) experimentally observed that at 3-5°C gametophytes failed to develop into viable sporophytes. This temperature range also corresponds with Saccorhiza polyschides northern range edge (ca 65° 35’N, mid-Norway), above which the average winter temperature is ≤4°C (U.S. Navy, 1958).

IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, and IR.HIR.KSed.XKScrR are distributed throughout the UK (Connor et al., 2004). Northern to southern Sea Surface Temperature (SST) ranges from 8-16°C in summer and 6-13°C in winter (Beszczynska-Möller & Dye, 2013)

Sensitivity assessment. Both long-term and acute temperature decrease 2-5°C combined with low winter temperatures could negatively affect Saccorhiza polyschides recruitment in biotopes located in the north of the UK. Resistance has been assessed as ‘Low’, resilience as ‘High’. Sensitivity has been assessed as ‘Low’.

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 occurring 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 kelp growth and may result in loss of affected kelp, and, therefore, loss of the biotope.

Karsten (2007) tested the photosynthetic ability of Saccharina latissima under acute 2 & 5 day exposure to salinity treatments ranging from 5-60 psu. A control experiment was also carried at 34 PSU (Salinity under normal marine conditions). Between 25-55 PSU Saccharina latissima had a high photosynthetic ability at >80% of the control levels.

Sensitivity assessment. The evidence suggests that Saccharina latissima can tolerate exposure to hypersaline conditions of ≥40‰ (MNCR full salinity range=30-40‰). Optimal salinities for other kelps are assumed to be 30-35 PSU. Hypersaline tolerances for Desmarestia spp. are unknown. Resistance has been assessed as ‘Low’, resilience as ‘High’. The sensitivity of this biotope to an increase in salinity has been assessed as ‘Low’.

Salinity decrease (local)

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

Evidence

Saccorhiza polyschides is affected by low salinities. Norton & South (1969) observed at ≤9‰ zoospores often burst due to internal osmotic pressure and none developed. At ≤25‰,  only 25% of gametophytes germinated and at ≤20‰ sporophyte growth was often retarded. At ≤35‰, 76% of gametophytes germinated. These results demonstrate that at ≤25‰ recruitment may be inhibited and sporophyte growth retarded.

Karsten (2007) tested the photosynthetic ability of Saccharina latissima under acute 2 & 5 day exposure to salinity treatments ranging from 5-60 psu. A control experiment was also carried at 34 psu (Salinity under normal marine conditions). Between 25-55 psu Saccharina latissima had a high photosynthetic ability at >80% of the control levels. Hyposaline treatment of 10-20 psu led to a gradual decline of photosynthetic ability. After 2 days at 5 psu Saccharina latissima s photosynthetic ability was ~30% of control. After 5 days at 5 psu Saccharina latissima specimens became bleached and showed signs of severe damage. The experiment was conducted on Saccharina latissima from the Arctic, and the authors suggest that at extremely low water temperatures (1-5°C) macroalgae acclimation to rapid salinity changes could be slower than at temperate latitudes. It is, therefore, possible that Saccharina latissima of the UK may be able to acclimate to salinity changes more effectively and quicker.

Gagnon et al. (2013) observed Desmarestia spp. sporophytes degraded progressively in low salinity treatments of 26, 23 & 20 psu (<20 psu was not tested). Desmarestia spp. accumulate sulphuric acid throughout the growth season which is released when the sporophyte becomes stressed under high temperatures or low salinities. Acid release causes progressive degradation of the sporophyte and mortality. Gagnon et al. (2013) exposed Desmarestia viridis samples during 30 hour salinity and temperature treatments, observing Desmarestia spp. degraded progressively in low salinity treatments of 26, 23 & 20psu (<20psu was not tested).

Sensitivity assessment. A decrease in one MNCR salinity scale from “Full Salinity” (30-40psu) to “Reduced Salinity” (18-30 psu) may result in mortality of Desmarestia spp. inhibit Saccorhiza polyschides recruitment and inhibit Saccharina latissima photosynthesis. Resistance has been assessed as ‘Low’ resilience as ‘High’. The sensitivity of this biotope to a decrease in salinity has been assessed as ‘Low’.

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

Peteiro & Freire (2013) measured Saccharina latissima growth from 2 sites, the 1^st had maximal water velocities of 0.3 m/sec and the 2^nd 0.1 m/sec. At site 1 Saccharina latissima had significantly larger biomass than at site 2 (16kg /m to 12 kg /m respectively). Peteiro & Freire (2013) suggested that faster water velocities were beneficial to Saccharina latissima growth. However, Gerard & Mann (1979) measured Saccharina latissima productivity at greater water velocities and found Saccharina latissima productivity is reduced in moderately strong tidal streams (≤1m/sec) when compared to weak tidal streams (<0.5m/sec).

IR.HIR.KSed.SlatSac can be found from very strong (>3m/sec) to very weak tidal streams. IR.HIR.KSed.Sac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR can be found from moderately strong (0.5-1.5m/sec) to weak tidal streams (0.5m/sec). An increase in tidal flow may increase local sediment mobility and scour, potentially increasing dislodgment of kelps (Birket et al., 1998) and Desmarestia spp.

Sensitivity assessment. Due to the range of tidal streams which these biotopes can be found a change of 0.1m/s to 0.2m/s, is not likely to dramatically affect biotope structure. Resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’.

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

IR.HIR.KSed.Sac and IR.HIR.KSed.XKScr can be found on the sublittoral fringe rock so that the characterizing species would be exposed during low spring tides. IR.HIR.KSed.SlatSac and IR.HIR.KSed.DesFilR are found in the infralittoral and as such would only be exposed on extreme low tides (Connor et al., 2004).

An increase in emergence will result in an increased risk of desiccation and mortality of the dominant seaweeds (Desmarestia spp., Saccorhiza polyschides, Saccharina latissima). Removal of canopy forming seaweeds has also been shown to increase desiccation and mortality of the understorey macroalgae (Hawkins & Harkin, 1985). Providing that suitable substrata are present, the biotope is likely to re-establish further down the shore within a similar emergence regime to that which existed previously.  A decrease in emergence could, however, result in  the extension of the biotope further up the shore, although its lower limit is still probably controlled by light penetration, competition and grazing so that entire extent of the biotope may  move.

Sensitivity assessment. Resistance has been assessed as ‘Low’. Resilience as ‘High’. The sensitivity of this biotope to a change in emergence is considered as ‘Low’.

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

IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR are recorded from extremely exposed to wave sheltered sites and characterized by rapidly colonizing macro-algae (Connor et al., 2004). As a result, of rapid recovery, the community is relatively resistant to disturbance when compared to other kelp biotopes (e.g. IR.HIR.Kfar.LhypR). Birkett et al., (1998) suggest that Saccharina latissima and Saccorhiza polyschides are rarely present in areas of wave exposure, where they may be spatially out-competed by Laminaria hyperborea.  However, the seasonally unstable nature of the substrata or periodic sediment scour within theses biotopes is likely to inhibit the growth of long lived species, such as Laminaria hyperborea and allow opportunistic species such as Demarestia spp., Saccharina latissima and Saccorhiza polyschides to proliferate.  An increase in local wave height may increase local sediment mobility and scour, potentially increasing dislodgment of kelps (Birket et al., 1998) and Desmarestia spp.. The biotopes may appear sparse after winter storms but the biotope recovers again due to rapid colonization and growth by the dominant kelps and Desmarestia spp.

Sensitivity assessment. The biotope is dominated by rapid colonizing species that tolerate or rapidly recover from scour, siltation and burial.  They occur across a broad wave exposure range, and, therefore, a change in nearshore significant wave height of 3-5% is not likely to have a significant effect on biotope structure. Resistance has been assessed as ’High’, resilience as ‘High’. Therefore, sensitivity has been assessed as ‘Not Sensitive’ at the benchmark level.

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

Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes. Similarly, Hopkin & Kain (1978) demonstrated sub-lethal effects of heavy metals on kelp gametophytes and sporophytes, including reduced growth and respiration. Sheppard et al. (1980) noted that increasing levels of heavy metal contamination along the west coast of Britain reduced species number and richness in holdfast fauna, except for suspension feeders which became increasingly dominant. Gastropods may be relatively tolerant of heavy metal pollution (Bryan, 1984). Although macroalgae species may not be killed, except by high levels of contamination, reduced growth rates may impair the ability of the biotope to recover from other environmental disturbances. Thompson & Burrows (1984) observed the growth of Saccharina latissima sporophyte growth was significantly inhibited at 50 µg Cu /l, 1000 µg Zn/l and 50 µg Hg/l. Zoospores were found to be more intolerant and significant reductions in survival rates were observed at 25 µg Cu/l, 1000 µg Zn/l and 5 µg/l.

However, this pressure is Not assessed.

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

IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR, are predominantly recorded in the sub-tidal (<0 m). These habitats are therefore not likely to 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 showed no noticeable impacts of the Sea Empress oil spill and clean up (Rostron & Bunker, 1997). 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.

However, this pressure is Not assessed.

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

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

O'Brian & Dixon (1976) suggested that red algae were the most sensitive group of macrophytes to oil and dispersant contamination (see Smith, 1968). Saccharina latissima has also been found to be sensitive to antifouling compounds. Johansson (2009) exposed samples of Saccharina latissima to several antifouing compounds, observing chlorothalonil, DCOIT, dichlofluanid and tolylfluanid inhibited photosynthesis. Exposure to Chlorothalonil and tolylfluanid, was also found to continue inhibiting oxygen evolution after exposure had finished, and may cause irreversible damage.

Smith (1968) observed that epiphytic and benthic red algae were intolerant of dispersant or oil contamination during the Torrey Canyon oil spill; only the epiphytes Crytopleura ramosa and Spermothamnion repens and some tufts of Jania rubens survived together with Osmundea pinnatifida, Gigartina pistillata and Phyllophora crispa from the sublittoral fringe. Delesseria sanguinea was probably the most intolerant since it was damaged at depths of 6m (Smith, 1968). Holt et al.(1995) suggested that Delesseria sanguinea is probably generally sensitive of chemical contamination.

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 can 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). 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).  However, in wave exposed, highly mixed, areas, the hypoxic conditions are likely to be transient.

Sensitivity Assessment. Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly. Resistance has been assessed as ‘High’, Resilience as ‘High’. Sensitivity has been assessed as ‘Not sensitive’ at the benchmark level.

Nutrient enrichment

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

Evidence

Conolly & Drew (1985) found that Saccharina latissima sporophytes plants at the most eutrophic site in a study on the east coast of Scotland where nutrient levels were 25% higher than ambient levels exhibited a high growth rate. However, Read et al. (1983) reported after removal of a major sewage pollution in the Firth of Forth, Saccharina latissima became abundant where previously it had been absent. Bokn et al. (2003) conducted a nutrient loading experiment on intertidal fucoids. Within 3 years of the experiment no significant effect was observed in the communities, however, 4-5 years into the experiment a shift occurred from perennials to ephemeral algae occurred. Although Bokn et al. (2003) focussed on fucoids the results could indicate that long-term (>4 years) nutrient loading can result in community shift to ephemeral algae species, and explain the disparity between the findings of Conolly & Drew (1985) & Read et al. (1983).

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

Sensitivity assessment. However, the biotope is assessed as ‘Not sensitive’ at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Organic enrichment

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

Evidence

Conolly & Drew (1985) found Saccharina latissima sporophytes had relatively higher growth rates close to a sewage outlet in St Andrews, UK when compared to other sites along the east coast of Scotland. At St Andrews, nitrate levels were 20.22µM, which represents an approx 25% increase when compared to other comparable sites (approx 15.87 µM). Handå et al. (2013) also reported Saccharina latissima sporophytes grew approx 1% faster per day when in close proximity to Norwegian Salmon farms, where elevated ammonium can be readily absorbed.  Read et al. (1983) reported after the installation of new sewage treatment works, which reduced the suspended solid content of liquid effluent by 60% in the Firth of Forth, Saccharina latissima became abundant where previously it had been absent. Bokn et al. (2003) conducted a nutrient loading experiment on intertidal fucoids. Within 3 years of the experiment no significant effect was observed in the communities, however, 4-5 years into the experiment a shift occurred from perennials to ephemeral algae occurred. Although Bokn et al. (2003) focussed on fucoids the results could indicate that long-term (>4 years) nutrient loading can result in community shift to ephemeral algae species. Differences between the findings of the aforementioned studies are likely to be related to the level of organic enrichment, however, could also be time dependent.

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

Sensitivity assessment. Although short-term exposure (<4 years) to organic enrichment may not affect seaweeds directly, indirect effects such as turbidity may significantly affect photosynthesis.  Therefore, resistance has been assessed as ‘Medium’, resilience as ‘High’ and sensitivity 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 Saccharina latissima, Saccorhiza polyschides and Desmarestia spp. would not be able to tolerate. The biotope would be lost.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very Low’. 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

Not relevant on hard rock substrata.

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

Abrasion of the substratum e.g. from bottom or pot fishing gear, cable laying etc. may cause localised mobility of the substrata and mortality of the resident community. The effect would be situation dependent however if bottom fishing gear were towed over a site it may mobilise a high proportion of the rock substrata and cause high mortality in the resident community.

However, the characteristic species within IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR have rapid growth rates and are distinctive of 'disturbed' areas. Information on from experimental clearances is summarised under resilience above.

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘High’. Sensitivity has been 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 subsurface light attenuation (Kd) (Devlin et al., 1998). Light penetration influences the maximum depth at which kelp species can grow and it has been reported that laminarians grow at depths at which the light levels are reduced to 1 percent of incident light at the surface. Maximal depth distribution of laminarians, therefore, varies from 100 m in the Mediterranean to only 6-7m in the silt laden German Bight.  In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters the depth at which kelp is found may be reduced, or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Lüning, 1990; Birkett et al., 1998b). Limited information is available on which to assess the effect of a decrease in water clarity on Laminariales 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). Demarestia spp. recruitment has also been found highly affected by light attenuation and frequency, typically rare (ca <10% coverage (Edwards, 1998) beneath kelp canopies where light levels can be 1-5% of surface irradiance (Kitching, 1941).

A decrease in water clarity as a result of mobilised sediments may also increase sediment scour of biotopes within close proximity. However, the characterizing species within IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR have rapid growth and colonisation rates are as such relatively resilient to sediment scouring.

Sensitivity assessment. An increase in water clarity from clear to intermediate (10-100 mg/l) represent 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. Resistance to this pressure is defined as ‘Low’ as the biotope is typical of sediment affected habitats. Resilience to this pressure is defined as ‘High’ and this biotope is regarded as having a sensitivity of ‘Low‘.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Smothering by sediment e.g. 5 cm material during a discrete event is unlikely to damage Saccharina latissima, Saccorhiza polyschides, and Desmarestia spp. sporophytes but may provide a physical barrier to zoospore settlement and, therefore, negatively impact on recruitment processes (Moy & Christie, 2012). Laboratory studies showed that kelp and gametophytes can survive in darkness for between 6-16 months at 8°C and would probably survive smothering by a discrete event and once returned to normal conditions gametophytes resumed growth or maturation within 1 month (Dieck, 1993). Saccorhiza polyschides zoospores successfully developed after 180 days of darkness (Norton, 1977). Intolerance to this factor is likely to be higher during the peak periods of sporulation and/or spore settlement.

IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR are found from extreme wave exposed-sheltered sites (Connor et al., 2004). In wave exposed biotopes deposited sediment is unlikely to remain for more than a few tidal cycles (due to water flow or wave action). In sheltered biotopes deposited sediment could remain and the effects of deposition could be longer lasting.

Sensitivity assessment. Resistance has been assessed as ‘High’, resilience as ‘High’ and sensitivity 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 Saccharina latissima, Saccorhiza polyschides, and Desmarestia spp. sporophytes but may provide a physical barrier to zoospore settlement and therefore negatively impact on recruitment processes (Moy & Christie, 2012). The volume of sediment may also inundate juvenile sporophytes. Given the short life expectancy of Saccharina latissima (2-4 years; Parke, 1948), self-sustaining populations are likely to be dependent on annual recruitment (Moy & Christie, 2012). Given the microscopic size of the gametophyte, 30 cm of sediment could be expected to significantly inhibit growth.  Laboratory studies showed that kelp and gametophytes can survive in darkness for between 6-16 months at 8°C and would probably survive smothering by a discrete event and once returned to normal conditions gametophytes resumed growth or maturation within 1 month (Dieck, 1993).  Saccorhiza polyschides zoospores, specifically, successfully developed after 180 days of darkness (Norton, 1977). Intolerance to this factor is likely to be higher during the peak periods of sporulation and/or spore settlement.

IR.HIR.KSed.XKScrR is subject to periodic burial from surrounding sediments, however, IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR are recorded from extreme wave exposed-sheltered sites (Connor et al., 2004) and, therefore, the effects of burial are likely to be mediated. In highly wave exposed biotopes, deposited sediment is unlikely to remain for more than a few tidal cycles (due to water flow or wave action). In sheltered biotopes, the high volume of deposited sediment could remain and the effects could be longer lasting. However, these biotopes are periodically disturbed by winter storms or sediment scour therefore deposited sediments are unlikely to remain for a full season.

Sensitivity assessment. Resistance has been assessed as ‘Medium’, resilience as ‘High’. Sensitivity has been assessed as ‘Low’.

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

No evidence

Underwater noise changes

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

Evidence

Not relevant

Introduction of light or shading

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

Evidence

There is no evidence to suggest that anthropogenic light sources would affect Saccharina latissima, Saccorhiza polyschides, and Desmarestia spp. Shading of the biotope (e.g. by the construction of a pontoon, pier etc) could adversely affect the biotope in areas where the water clarity is also low, and tip the balance to shade tolerant species, resulting in the loss of the biotope directly within the shaded area, or a reduction in seaweed abundance.

Sensitivity assessment. Resistance is probably 'Low', with a 'Medium' resilience and a sensitivity of 'Medium', albeit with 'low' confidence due to the lack of direct evidence.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. 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 kelp populations was found.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

Competition with invasive macroalgae may be a potential threat to kelp biotopes.  Potential invasives include Undaria pinnatifida and Sargassum muticum. 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 (Staeher 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., 20125).

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 0-0.5 m or 4-6 m, in the turbid waters of the Limfjorden.  Limfjorden is wave sheltered although 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 (1-2 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 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, 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 completion to light, rather than changes in nutrient availability, sedimentation or water flow (Britton-Simmons, 2004; Engelen et al., 2015).   

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; Hieser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017; Kraan, 2017; Epstein & Smale, 2018; 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. Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound. One year after its discovery at the Queen Anne Battery marina, Plymouth, it had become a major fouling plant on pontoons (Minchin & Nunn, 2014).

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). De Leij et al. (2017) suggested that in natural substrata, Undaria pinnatifida can be inhibited by the presence of native competitors, such as large perennial species. The species 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).

Epstein & Smale (2018) reported clear similarities between Undaria pinnatifida and Saccorhiza polyschides, that is they both are annual kelp species, occupy similar niches, and are opportunistic, which suggested that they could be in direct competition. In Plymouth Sound (UK), under certain environmental conditions, Undaria pinnatifida can out-compete and subsequently suppress, displace but not completely exclude Saccorhiza polyschides (Epstein & Smale, 2018; Epstein et al., 2019a). For example, Undaria pinnatifida is less tolerant of wave action than Saccorhiza polyschides, so competitive exclusion and displacement of Saccorhiza polyschides is more likely to occur in wave-sheltered areas (Epstein et al., 2019a). Epstein et al. (2019a) reported that Saccorhiza polyschides did not increase significantly after the removal of 50% of Undaria pinnatifida, which suggested that Undaria pinnatifida exerted a relatively strong suppressive effect on this native species even at relatively low densities and cover. In contrast, the removal of 100% of Undaria pinnatifida had a statistically significant increase in abundance and biomass of Saccorhiza polyschides (5.3- and 3.6-fold respectively) (Epstein et al., 2019a). Although not conclusive, Epstein et al. (2019b) suggested that Undaria pinnatifida may have higher fitness than native canopy-formers on artificial substrata (i.e. man-made structures such as marina pontoons, pilings and port walls), where it can proliferate with or without disturbance to native macroalgae and become the dominant canopy-former.

Undaria pinnatifida was found to co-exist with Saccorhiza polyschides in St Malo, France (Castric-Fey et al., 1993). Epstein & Smale (2018) also found that a higher coverage of Saccorhiza polyschides was positively associated with the occurrence of Undaria pinnatifida and there was a significant pattern of co-occurrence of the two species. This evidence suggested that the presence of one species indicated that the other species could also occur, due to their similarities in habitat preferences (Epstein & Smale, 2018). Also, Epstein et al. (2019b) observed a positive relationship between Saccorhiza polyschides and Undaria pinnatifida in intertidal and subtidal habitats in Plymouth Sound (UK), although it was not statistically significant.

Arnold et al. (2016) suggested that Saccorhiza polyschides and Undaria pinnatifida attract similar epifaunal and epifloral assemblages, which implied that community-level impacts may be minimal. By contrast, Salland & Smale (2021) found that the large, hollow structure of Saccorhiza polyschides holdfasts supported a distinct and diverse assemblage compared to that found in claw-like Laminarian holdfasts, the latter of which are more similar in structure to those of Undaria pinnatifida. Undaria pinnatifida was successfully eradicated on a sunken ship in Clatham 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. The above evidence suggests that Undaria pinnatifida can out-compete but not exclude Saccorhiza polyschides, especially in artificial habitats and/or wave sheltered habitats. However, this biotope IR.HIR.KSed.Sac is structured by high-energy conditions due to wave exposure and tidal streams, and resultant disturbance due to storm damage and scour. As a result, the biotope is dominated by opportunistic kelps and seaweeds, although patches of perennial kelps may also be present. Therefore, while Undaria pinnatifida might be able to colonize this biotope after disturbance it will probably not be able to compete with Sacchorhiza polyschides under the conditions that characterize this biotope. Similarly, Sargassum muticum does not grow well under wave exposed conditions and is unlikely to out-compete the native species in this biotope. 

Therefore, resistance is assessed as ‘Medium’ to represent the potential Undaria or Sargassum to colonize the biotope, probably at low abundance.  Resilience is assessed as ‘High’ as the biotope is disturbed and both the native and non-native species would recolonize and/or regrow annually. Hence, sensitivity is assessed as ‘Low’.  Overall, confidence is assessed as ‘Low’ due to evidence of variation and site-specific nature of competition between native kelps and both Undaria pinnatifida and Sargassum muticum.

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

Laminarians may be infected by the microscopic brown alga Streblonema aecidioides. Infected algae show symptoms of Streblonema disease, i.e. alterations of the blade and stipe ranging from dark spots to heavy deformations and completely crippled thalli Infection can reduce growth rates of host algae (Peters & Scaffelke, 1996). The marine fungi Eurychasma spp can also infect early life stages of laminarians and Desmarestia viridis, however, the effects of infection are unknown (Müller et al., 1999).

Sensitivity assessment. Due to a lack of conclusive evidence the sensitivity of this biotope cannot be assessed against this pressure.

Removal of target species

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

Evidence

There has been recent commercial interest in Saccharina latissima as a consumable called “sea vegetables” (Birkett et al., 1998b). However, Saccharina latissima sporophytes are typically matured on ropes (Handå et al., 2013) and not directly extracted from the seabed, as with Laminaria hyperborea (Christie et al., 1998). No evidence has been found for commercial extraction of Saccorhiza polyschides or Desmarestia spp. However, if the biotopes were subject to harvest, then a large proportion of the resident kelp population could be removed.  Thus evidence to assess the resistance of IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR to direct harvesting is limited. It has been assumed that if targeted harvesting were in operation it would remove >75% of sporophytes.

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘High’. Sensitivity has been 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

IR.HIR.KSed.Sac, IR.HIR.KSed.SlatSac, IR.HIR.KSed.DesFilR, IR.HIR.KSed.XKScrR Are characterised by a canopy of Saccorhiza polyschides, Saccharina latissima and Desmarestia spp. If the canopy were removed the red seaweeds understorey community may become bleached, and/or perish (Hawkins & Harkin, 1985), leading to further reductions in biodiversity. The biotope is however naturally periodically disturbed and as such would recover rapidly.

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