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, while southern limits are set by high lethal summer temperatures or winter temperatures too high for the induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which make them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Laminaria digitata is a boreal species of kelp, distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight & Thompson, 2008). Laminaria digitata distribution suggests that the species would tolerate chronic temperature change (e.g. by 2°C for a year). However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. In addition, the distribution of this species suggests that Laminaria digitata is a northern species and, as such, may be vulnerable to increases in temperature and could be outcompeted at its southern limits by other kelp species.

Laminaria digitata can grow over a range of temperatures with a thermal optimum between 10-15°C (Bolton & Lüning 1982; Dieck 1992; Arzel, 1998). With optimum reproductivity between 5-10°C, and reproductive ability impaired to 20% at 18°C (Arzel, 1998; Bartsch et al., 2013). Therefore, while the current population may not be affected, recruitment may be reduced.  Spore production only occurs between 5-10°C and is the most temperature sensitive stage of reproduction in Laminaria digitata. A minimum of ten weeks a year between 5-18°C is needed in order to ensure spore formation and hence reproduction (Bartsch et al., 2013). Outside this temperature range, reproduction is severely reduced, and the species is at risk from local extinction in the long-term. Studies have shown temperatures of 18-20°C to cause reduced growth (Hargrave et al., 2017), tissue loss (Simonson et al., 2015) and increased mortality (Wilson et al., 2015) of Laminaria digitata. Therefore, the sensitivity of this species relies on the current sea temperatures of the specific location (Bartsch et al., 2013). 

Merzouk & Johnson (2011) combined predicted sea surface temperate over the next century with the current distribution of Laminaria digitata and predicted an expansion of its northern limits and localised extinctions across its southern range (mid Bay of Biscay, Northern France and southern England; Birkett et al., 1998b). These results suggest that local extinction of the biotope may occur at sites where sea temperature is artificially increased as a result of anthropogenic activity (e.g. effluent output) (Raybaud et al., 2013), especially if combined with high UK summer sea temperatures in southern examples of this biotope (Bartsch et al., 2013). In southern examples of IR.MIR.KT.LdigT, Laminaria digitata may also be out-competed by its Lusitanian competitor Laminaria ochroleuca, which is regionally abundant across the south UK coastline (Smale et al., 2014). 

Sensitivity assessment. As temperatures rise to up to 20°C, it is predicted that Laminaria digitata could be lost and that more, warm adapted, algae will take its place. Sea surface temperatures around the UK are currently between 6-19°C (Huthnance, 2010). Under the middle emission scenario, Laminaria digitata is likely to be lost in southern parts of the UK, where summer temperatures would exceed 20°C, but is likely to remain present around the rest of the UK. Therefore, resistance is assessed as ‘Low’ under the middle emission scenario. 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 IR.MIR.KT.LdigT is assessed as having ‘High’ sensitivity to ocean warming under a middle emission scenario.

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

Global warming (high)

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

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

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

• A 3°C rise in surface air temperature in intertidal habitats exclusive to Scotland. 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, while southern limits are set by high lethal summer temperatures or winter temperatures too high for the induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which make them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Laminaria digitata is a boreal species of kelp, distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight & Thompson, 2008). Laminaria digitata distribution suggests that the species would tolerate chronic temperature change (e.g. by 2°C for a year). However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. In addition, the distribution of this species suggests that Laminaria digitata is a northern species and, as such, may be vulnerable to increases in temperature and could be outcompeted at its southern limits by other kelp species.

Laminaria digitata can grow over a range of temperatures with a thermal optimum between 10-15°C (Bolton & Lüning 1982; Dieck 1992; Arzel, 1998). With optimum reproductivity between 5-10°C, and reproductive ability impaired to 20% at 18°C (Arzel, 1998; Bartsch et al., 2013). Therefore, while the current population may not be affected, recruitment may be reduced.  Spore production only occurs between 5-10°C and is the most temperature sensitive stage of reproduction in Laminaria digitata. A minimum of ten weeks a year between 5-18°C is needed in order to ensure spore formation and hence reproduction (Bartsch et al., 2013). Outside this temperature range, reproduction is severely reduced, and the species is at risk from local extinction in the long-term. Studies have shown temperatures of 18-20°C to cause reduced growth (Hargrave et al., 2017), tissue loss (Simonson et al., 2015) and increased mortality (Wilson et al., 2015) of Laminaria digitata. Therefore, the sensitivity of this species relies on the current sea temperatures of the specific location (Bartsch et al., 2013). 

Merzouk & Johnson (2011) combined predicted sea surface temperate over the next century with the current distribution of Laminaria digitata and predicted an expansion of its northern limits and localised extinctions across its southern range (mid Bay of Biscay, Northern France and southern England; Birkett et al., 1998b). These results suggest that local extinction of the biotope may occur at sites where sea temperature is artificially increased as a result of anthropogenic activity (e.g. effluent output) (Raybaud et al., 2013), especially if combined with high UK summer sea temperatures in southern examples of this biotope (Bartsch et al., 2013). In southern examples of IR.MIR.KT.LdigT, Laminaria digitata may also be out-competed by its Lusitanian competitor Laminaria ochroleuca, which is regionally abundant across the south UK coastline (Smale et al., 2014). 

Sensitivity assessment. As temperatures rise to up to 20°C, it is predicted that Laminaria digitata could be lost and that more, warm adapted, algae will take its place. Sea surface temperatures around the UK are currently between 6-19°C (Huthnance, 2010). Under the middle emission scenario, Laminaria digitata is likely to be lost in southern parts of the UK, where summer temperatures would exceed 20°C, but is likely to remain present around the rest of the UK. Therefore, resistance is assessed as ‘Low’ under the middle emission scenario. 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 IR.MIR.KT.LdigT is assessed as having ‘High’ sensitivity to ocean warming under a middle emission scenario.

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

Global warming (middle)

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

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

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

• A 2°C rise in surface air temperature in intertidal habitats exclusive to Scotland. 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, while southern limits are set by high lethal summer temperatures or winter temperatures too high for the induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which make them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Laminaria digitata is a boreal species of kelp, distributed from Brittany to the coast of Norway, while its UK distribution encompasses the whole of the UK coast (Blight & Thompson, 2008). Laminaria digitata distribution suggests that the species would tolerate chronic temperature change (e.g. by 2°C for a year). However, local populations may have acclimatized to local physical conditions meaning that populations at the extremes of the species’ range are less comparable than those populations in the middle of its range. In addition, the distribution of this species suggests that Laminaria digitata is a northern species and, as such, may be vulnerable to increases in temperature and could be outcompeted at its southern limits by other kelp species.

Laminaria digitata can grow over a range of temperatures with a thermal optimum between 10-15°C (Bolton & Lüning 1982; Dieck 1992; Arzel, 1998). With optimum reproductivity between 5-10°C, and reproductive ability impaired to 20% at 18°C (Arzel, 1998; Bartsch et al., 2013). Therefore, while the current population may not be affected, recruitment may be reduced.  Spore production only occurs between 5-10°C and is the most temperature sensitive stage of reproduction in Laminaria digitata. A minimum of ten weeks a year between 5-18°C is needed in order to ensure spore formation and hence reproduction (Bartsch et al., 2013). Outside this temperature range, reproduction is severely reduced, and the species is at risk from local extinction in the long-term. Studies have shown temperatures of 18-20°C to cause reduced growth (Hargrave et al., 2017), tissue loss (Simonson et al., 2015) and increased mortality (Wilson et al., 2015) of Laminaria digitata. Therefore, the sensitivity of this species relies on the current sea temperatures of the specific location (Bartsch et al., 2013). 

Merzouk & Johnson (2011) combined predicted sea surface temperate over the next century with the current distribution of Laminaria digitata and predicted an expansion of its northern limits and localised extinctions across its southern range (mid Bay of Biscay, Northern France and southern England; Birkett et al., 1998b). These results suggest that local extinction of the biotope may occur at sites where sea temperature is artificially increased as a result of anthropogenic activity (e.g. effluent output) (Raybaud et al., 2013), especially if combined with high UK summer sea temperatures in southern examples of this biotope (Bartsch et al., 2013). In southern examples of IR.MIR.KT.LdigT, Laminaria digitata may also be out-competed by its Lusitanian competitor Laminaria ochroleuca, which is regionally abundant across the south UK coastline (Smale et al., 2014). 

Sensitivity assessment. As temperatures rise to up to 20°C, it is predicted that Laminaria digitata could be lost and that more, warm adapted, algae will take its place. Sea surface temperatures around the UK are currently between 6-19°C (Huthnance, 2010). Under the middle emission scenario, Laminaria digitata is likely to be lost in southern parts of the UK, where summer temperatures would exceed 20°C, but is likely to remain present around the rest of the UK. Therefore, resistance is assessed as ‘Low’ under the middle emission scenario. 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 IR.MIR.KT.LdigT is assessed as having ‘High’ sensitivity to ocean warming under a middle emission scenario.

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

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C. 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). Changes in primary productivity, community composition, and biogeography have been associated with marine heatwaves (Smale et al., 2019).

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

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

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

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England and 19°C in Scotland. Under the middle emission scenario, Laminaria digitata is likely to be lost from the southern parts of the UK (see Global warming pressure). In Scotland, a significant proportion of Laminaria digitata is likely to survive in areas where temperatures do not exceed 20°C but will suffer mortality elsewhere. Therefore, resistance has been assessed as ‘None’. As a further heatwave is likely to affect this habitat before full recovery, resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England 21.5°C in Scotland. Laminaria digitata may be lost from large parts of the UK as its range contracts (see Global warming pressure). Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Marine heatwaves (middle)

Middle emission scenario benchmark:  A marine heatwave occurring every three years, with a mean duration of 80 days, with a maximum intensity of 2°C. 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). Changes in primary productivity, community composition, and biogeography have been associated with marine heatwaves (Smale et al., 2019).

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

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

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

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England and 19°C in Scotland. Under the middle emission scenario, Laminaria digitata is likely to be lost from the southern parts of the UK (see Global warming pressure). In Scotland, a significant proportion of Laminaria digitata is likely to survive in areas where temperatures do not exceed 20°C but will suffer mortality elsewhere. Therefore, resistance has been assessed as ‘None’. As a further heatwave is likely to affect this habitat before full recovery, resilience has been assessed as ‘Low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C in southern England 21.5°C in Scotland. Laminaria digitata may be lost from large parts of the UK as its range contracts (see Global warming pressure). Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Ocean acidification (high)

High emission scenario benchmark: a further decrease in pH of 0.35 (annual mean) and corresponding 120% increase in H+ ions , seasonal aragonite saturation of 20% of UK coastal waters and North Sea bottom waters, and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, occurring at a depth of 400 m by the end of this century 2081-2100. 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 the emission scenario. Marine autotrophs will generally benefit from ocean acidification, through an increase in the availability of aqueous CO₂ for photosynthesis (Koch et al., 2013). Most species of kelp appear to be under-saturated in respect to carbon dioxide, although they can generally utilise HCO₃ and have external carbonic anhydrase for extracellular dehydration of HCO₃ to CO₂ (Koch et al., 2013).

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

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

Sensitivity assessment. Kelp forests occur in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Laminaria digitata is not predicted to suffer negative impacts from future acidification. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’.

Ocean acidification (middle)

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

Evidence

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

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

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

Sensitivity assessment. Kelp forests occur in a naturally variable pH habitat, with diel fluctuations of 0.3 - 0.45 pH units (Krause-Jensen et al., 2015, Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Laminaria digitata is not predicted to suffer negative impacts from future acidification. Therefore, under both the middle and high emission scenario resistance is assessed as ‘High’, and resilience is assessed as ‘High’ leading to a score of ‘Not sensitive’.

Sea level rise (extreme)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). 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. An increase in depth due to sea-level rise is likely to impact both Laminaria digitata and any understory algae, negatively impacting this biotope. 

Understanding how sea-level rise will affect exposure and tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Sensitivity assessment. This biotope IR.MIR.KT.LdigT is recorded from the lower shore to 5 m in depth.  As wave surge diminishes with increased depth, sea-level rise is likely to lead to the density of faunal turf reducing at the deeper reaches of this biotope and transitioning into a biotope characterised by kelp and dense red seaweeds. Tidal streams are also likely to be reduced so that LdigT may transition into Ldig with a lower faunal diversity). This biotope LdigT may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of suitable substratum or human modified shorelines.

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

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. An increase in depth due to sea-level rise is likely to impact both Laminaria digitata and any understory algae, negatively impacting this biotope. 

Understanding how sea-level rise will affect exposure and tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Sensitivity assessment. This biotope IR.MIR.KT.LdigT is recorded from the lower shore to 5 m in depth.  As wave surge diminishes with increased depth, sea-level rise is likely to lead to the density of faunal turf reducing at the deeper reaches of this biotope and transitioning into a biotope characterised by kelp and dense red seaweeds. Tidal streams are also likely to be reduced so that LdigT may transition into Ldig with a lower faunal diversity). This biotope LdigT may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of suitable substratum or human modified shorelines.

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

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. An increase in depth due to sea-level rise is likely to impact both Laminaria digitata and any understory algae, negatively impacting this biotope. 

Understanding how sea-level rise will affect exposure and tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Sensitivity assessment. This biotope IR.MIR.KT.LdigT is recorded from the lower shore to 5 m in depth.  As wave surge diminishes with increased depth, sea-level rise is likely to lead to the density of faunal turf reducing at the deeper reaches of this biotope and transitioning into a biotope characterised by kelp and dense red seaweeds. Tidal streams are also likely to be reduced so that LdigT may transition into Ldig with a lower faunal diversity). This biotope LdigT may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of suitable substratum or human modified shorelines.

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

Temperature increase (local)

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

Evidence

Laminaria digitata is distributed from Brittany to the Spitzbergen (Birkett et al., 1998; Blight & Thompson, 2008). The Northern/Boreal distribution of Laminaria digitata suggests it may be slightly vulnerable to temperature increases in southern examples of IR.MIR.KT.LdigT.

The thermal optimum of Laminaria digitata is between 10-15°C, with reproductive ability impaired to 20% at 18°C (Arzel, 1998). Spore production only occurs between 5-10°C and is the most temperature sensitive stage of reproduction in Laminaria digitata. Outside this temperature range, reproduction is severely reduced and the species is at risk from local extinction in the short-term. A temperature increase to 22-23 °C causes cell damage and death (Sudene, 1964; Bolton & Lüning, 1982). During an exceptionally warm summer in Norway Sundene (1964) reported the destruction of Laminaria digitata plants exposed to temperatures of 22-23 °C. The sensitivity of this species therefore relies on the current sea temperatures of the specific location (Bartsch, 2013). A minimum of 10 weeks a year between 5-18 °C is needed in order to ensure spore formation and hence reproduction and recruitment (Bartsch, 2013).

Combining predicted sea surface temperate over the next century with the current distribution of Laminaria digitata, Merzouk & Johnson (2011) predict an expansion of it’s northern limits and localised extinctions across it’s southern range edge (Mid Bay of Biscay, Northern France and southern England; Birkett et al, 1998). Suggesting at sites where sea temperature is artificially increased as a result of anthropogenic activity (e.g. effluent output) local extinction of the biotope may occur (Raybaud et al., 2013) especially if combined with high summer sea temperature (Bartsch et al. 2013). In southern examples of IR.MIR.KT.LdigT, Laminaria digitata may also be out-competed by it’s Lusitanian competitor Laminaria ochroleuca which is regionally abundant across the south UK coastline (Smale et al., 2014).

The star ascidian Botryllus schlosseri and the breadcrumb sponge Halichondria panicea have large geographical ranges in which the UK is almost central. At the benchmark level these species are therefore likely to be tolerant of chronic temperature changes.

IR.MIR.KT.LdigT is 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. Northern examples of this biotope are unlikely to be affected at the benchmark level, however biotopes within the south of the UK where high summer temperatures combined with an increase of 2 & 5 °C would be above the temperature optimum of Laminaria digitata and may therefore cause declines in growth and abundance. Resistance has been assessed as ‘Medium’, Resilience as ‘High’. Sensitivity has been assessed as ‘Low’.

Temperature decrease (local)

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

Evidence

Laminaria digitata is distributed from Brittany to the Spitzbergen (Birkett et al., 1998; Blight & Thompson, 2008). The Northern/Boreal distribution of Laminaria digitata suggests it would tolerate a decrease in temperature at the benchmark level.

Sensitivity assessment. Resistance has been assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not sensitive’.

Salinity increase (local)

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

Evidence

Kelps are tolerant to short-term daily fluctuation in salinity; however they are much less tolerant to long-term changes with growth rates declining typically either side of 20-45 psu (Karsten, 2007).  Laminaria digitata tolerates a large salinity range (5-60 psu; Karsen, 2007) at the extremes of this range; decreases in photosynthetic rates were evident, (Gordillo, 2002). Laminaria digitata is considered to be a stenohaline species, therefore this biotope is only found in conditions of full salinity (Connor et al 1997, Connor et al 2004) Axelsson & Axelsson (1987) indicated damage of the plants’ plasma membranes occurs when salinity is below 20 or above 50 psu.

Sensitivity assessment. Laminaria digitata is unlikely to tolerate an increase to >40‰ for a year. Resistance to this pressure is considered ‘Low’, and resilience as ‘High’. This biotope is considered ‘Low’ to this pressure.

Salinity decrease (local)

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

Evidence

Birkett et al. (1998) suggested that kelps are stenohaline, in that they do not tolerate wide fluctuations in salinity. Growth rate may be adversely affected if the kelp plant is subjected to periodic salinity stress. The lower salinity limit for Laminaria digitata lies between 10 and 15 psu. On the Norwegian coast, Sundene (1964) found healthy Laminaria digitata plants growing between 15 and 25 psu. Axelsson & Axelsson (1987) indicated damage of the plants’ plasma membranes occurs when salinity is below 20 or above 50 psu. Localized, long-term reductions in salinity, to below 20 psu, may result in the loss of kelp beds in affected areas (Birkett et al., 1998).

In laboratory experiments maximum rates of photosynthesis and respiration in Palmaria palmata were observed at a salinity 32 psu (Robbins, 1978) although photosynthetic rates were high down to a salinity of 21 psu. Palmaria palmata is likely to be tolerant of small changes in salinity because as an intertidal species it is regularly exposed to precipitation. Corallina officinalis inhabits rock pools and gullies from mid to low water. Therefore, it is likely to be exposed to short-term hyposaline (freshwater runoff and rainfall) and hypersaline (evaporation) events. However, its distribution in the Baltic is restricted to increasingly deep water as the surface salinity decreases, suggesting that it requires full salinity in the long-term (Kinne, 1971).

Some of the fauna, including Halichondria panicea are tolerant of wide variety of salinity habitats from reduced to full salinity and are therefore unlikely to be affected by a drop in salinity at the benchmark level.

Sensitivity assessment. The evidence suggests that a decrease in one MNCR salinity scale from ‘Full Salinity’ (30-40 psu) to ‘Reduced Salinity’ (18-30 psu) would still be within Laminaria digtata’s salinity tolerance. Furthermore IR.MIR.KT.LdigT is recorded within low salinity (albeit at low occurrence), indicating many of the characterizing species can tolerate <30‰. Resistance has been assessed as ‘High’ and resilience as ‘High’. Therefore, sensitivity of this biotope to a decrease in salinity has been assessed as ‘Not Sensitive’.

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

IR.MIR.KT.LdigT is recorded from very strong-very weak tidal streams (Negligible->3 m/s) (Connor et al., 2004). The filter feeding community within understorey community is likely dependent upon high water flow. However, the distribution of IR.MIR.KT.LdigT across a wide range of tidal streams indicates a change in water flow from 0.1-0.2 m/s would not significantly affect IR.MIR.KT.LdigT.

Sensitivity assessment. Resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’ at the benchmark level.

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

Laminaria digitata biotopes are predominantly sublittoral but extend into the lower eulittoral and therefore have some ability to resist desiccation. At the sub-littoral fringe Laminaria digitata regularly becomes exposed to air at low water. Dring & Brown (1982) found that plants that lost up 40-50% of their initial water content were still able to return to their original photosynthetic rate on re-immersion. Many species living beneath the kelp canopy, such as Halichondria panicea and Botryllus schlosseri are also found further up the shore and are therefore likely to be tolerant to a certain degree of desiccation. Furthermore, the kelp canopy is likely to protect the algal understorey and benthic fauna from the worst effects of desiccation by the kelp canopy. However, at the benchmark level, some Laminaria digitata plants at the upper extent of the biotope may perish from the effects of desiccation. In turn, flora and fauna in the understorey may die since the canopy offers protection from desiccation, wind and insolation. The upper extent of the biotope may be reduced although this may be counteracted by an extension of the biotope at the lower limit.

Sensitivity assessment. Resilience has been assessed as ‘Low’. Resistance 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.MIR.KT.LdigT is predominantly recorded from wave sheltered sites, however is also recorded up to moderate wave exposure (Connor et al., 2004). The greatest wet weight of Laminaria digitata occurs at low wave exposure (mean significant wave height <0.4 m) decreasing by a mean of 83% in medium to high wave exposures (mean significant wave height >0.4 m; Gorman et al., 2013).  At medium to high levels of wave exposure, Laminaria digitata biomass has been shown to decrease by 83% in the field (Wernberg and Thomsen, 2005). A flexible stipe and low profile holdfast allows Laminaria digitata to flourish in moderately to strongly wave exposed areas. In areas of high wave exposure Laminaria digitata may extend its upper limits into the lower eulittoral zone. However, IR.HIR.KFaR.Ala.Ldig typically replaces this biotope under conditions of extreme wave exposure, while in predominantly wave sheltered and lower water flow conditions IR.LIR.K.Slat.Ldig becomes prevalent.

The physiology of seaweeds grown at exposed sites differs morphologically to those at sheltered sites with those exposed to greater wave action. A transplant experiment of Laminaria digitata, from exposed to sheltered sites resulted in a changed morphology with the frond widening, while individuals transplanted from sheltered to exposed sites became thinner more streamlined (Sundene, 1964; Gerard, 1987). This morphological plasticity is evident during the spore stage; because of this it is suggested that if wave height is increased or decreased the kelp with adapt morphologically over time to optimise its survival in the new environment.

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

Sensitivity assessment. Wave exposure is one of the principal defining features of kelp biotopes, and large changes in wave exposure are likely to alter the relative abundance of the kelp species, grazing and understorey community, and hence, the biotope. However a change in near shore significant wave height of 3-5% is unlikely to have any significant effect on IR.MIR.KT.LdigT. Resistance has been assessed as ’High’, resilience as ‘High’ and sensitivity 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

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

No information was found concerning the specific effects of transitional elements on the characterizing or important functional flora and/or fauna of IR.MIR.KT.LdigT.

The tolerance of Laminaria digitata to heavy metals is highly variable depending the on the metal concerned. Zinc was found to inhibit growth in Laminaria digitata at a concentration of 100 µg/L and at 515 µg/L, growth had almost completely ceased (Bryan, 1969). Axelsson & Axelsson (1987) investigated the effect of exposure to mercury (Hg), lead (Pb) and nickel (Ni) for 24 hours by measuring ion leakage to indicate plasma membrane damage. Inorganic and organic Hg concentrations of 1 mg/l resulted in the loss of ions equivalent to ion loss in seaweed that had been boiled for 5 minutes. Laminaria digitata was unaffected when subjected to Pb and Ni at concentrations up to 10 mg/l. The results also indicated that the species was intolerant of the tin compounds butyl-Sn and phenyl-Sn. 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 and Boney (1971) reported that the red algae Plumaria elegans experienced 100% growth inhibition at 1 ppm Hg. However, no information was found concerning the specific effects of heavy metals on either Palmaria palmata or Corallina officinalis or any of the important faunal components of this biotope.

Hydrocarbon & PAH contamination

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

Evidence

This pressure is Not assessed but evidence is presented where available

The brown algae are thought to be largely protected from oil penetration damage by the presence of a mucilaginous coating (O'Brian & Dixon, 1976). In addition, effects of oil accumulation on the thalli are mitigated by the perennial growth of kelps. Laminaria digitata is less susceptible to coating than some other seaweeds because of its preference for exposed locations where wave action will rapidly dissipate oil. The strong tidal flow in this biotope may provide some protection to all seaweeds within this community. No significant effects of the Amoco Cadiz spill were observed for Laminaria populations and the World Prodigy spill of 922 tons of oil in Narragansett Bay had no discernible effects on Laminaria digitata (Peckol et al., 1990). Furthermore, the upper limit of distribution for Laminaria digitata moved up wave exposed shores by as much as 2 m during the first few years after the Torrey Canyon oil spill due to the death of animals that graze the plants (Southward & Southward, 1978). Mesocosm studies in Norwegian waters showed that chronic low level oil pollution (25 µg/L) reduced growth rates in Laminaria digitata but only in the second and third years of growth (Bokn, 1985).

O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil contamination. Where exposed to direct contact with fresh hydrocarbons, encrusting calcareous algae have a high intolerance. Crump et al. (1999) noted a dramatic bleaching on encrusting corallines and signs of bleaching in Corallina officinalis, Chondrus crispus and Mastocarpus stellatus at West Angle Bay, Pembrokeshire after the Sea Empress oil spill. However, encrusting corallines recovered quickly and Corallina officinalis was not killed. Laboratory studies of the effects of oil and dispersants on several red algae species, including Palmaria palmata (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. It is possible that Corallina officinalis and other algae are more intolerant of the dispersants used during oil spills than the oil itself. Where exposed to direct contact with fresh hydrocarbons, encrusting coralline algae appear to have a high intolerance. Crump et al. (1999) describe "dramatic and extensive bleaching" of 'Lithothamnia' following the Sea Empress oil spill. Observations following the Don Marika oil spill (K. Hiscock, own observations) were of rockpools with completely bleached coralline algae. However, Chamberlain (1996) observed that although Lithophyllum incrustans was quickly affected by oil during the Sea Empress spill, recovery occurred within about a year. The oil was found to have destroyed about one third of the thallus thickness but regeneration occurred from thallus filaments below the damaged area.

No information was found concerning the specific effects of hydrocarbon contamination on the important faunal component of this biotope. However, the intolerance of the sponges, ascidians and bryozoans is likely to be related to depth of the oil / tar deposition and the strong tidal flow associated with this biotope is likely to reduce this. Nevertheless, analysis of kelp holdfast fauna after the Sea Empress oil spill in Milford Haven illustrated decreases in number of species, diversity and abundance at sites nearest the spill (SEEEC, 1998).

Synthetic compound contamination

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

Evidence

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

Laminaria digitata along with almost all red algal species and many animal species were found to be absent from sites close to acidified, halogenated effluent from a bromide extraction plant (Hoare & Hiscock, 1974). Axelsson & Axelsson (1987) investigated the effect on Laminaria digitata of exposure to various chemicals for 24 hours by measuring ion leakage as an indication of plasma membrane damage. However, only limited ion loss was seen on exposure to two detergents, nonylphenol ethoxylate (NP-10) and linear alkylbenzene sulfonate (LAS). Cole et al. (1999) suggested that herbicides such as Simazina and Atrazine were very toxic to macrophytic algae. Laboratory studies of the effects of oil and dispersants on several red algae species, including Palmaria palmata (Grandy, 1984, cited in Holt et al., 1995) concluded that they were all intolerant of oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. Smith (1968) reported that, in areas of heavy spraying of oil and detergent dispersants, Corallina officinalis was killed, and was affected down to a depth of 6 m in one site, presumably due to wave action and mixing. However, re-growth of fronds had begun within 2 months after spraying ceased.

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). A rapid recovery from a state of low oxygen is expected if the environmental conditions are transient. If levels do drop below 4 mg/l negative effects on these organisms can be expected with adverse effects occurring below 2mg/l (Cole et al., 1999). The understorey faunal community may be affected by de-oxygenation, however IR.MIR.KT.LdigT is a tide swept biotope (Connor et al., 2004) and therefore the effects of de-oxygenation are likely to be short lived.

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

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 macro-algal communities are relative 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).

High ambient levels of phosphate and nitrogen enhance spore formation in a number of Laminaria spp. (Nimura et al., 2002), but will eventually inhibit spore production, particularly at the extremes of temperature tolerances as seen in Saccharina latissima (syn. Laminaria saccharina; Yarish et al., 1990). Laminaria digitata seems to follow this trend with a growth peak occurring in conjunction with nutrient generation from deeper waters in Norway (Gévaert et al., 2001). Despite this, enhancement of coastal nutrients is likely to favour those species with more rapid growth rates including turf forming algae (Gorgula & Connell, 2004).

Sensitivity assessment. Although nutrients may not affect kelps directly, indirect effects such as turbidity may significantly affect photosynthesis. Furthermore organic enrichment may denude the associated community. However, the biotope is probably ‘Not sensitive’ (resistance is ‘High’ and resilience is ‘High) at the benchmark levels (i.e. compliance with WFD criteria).

Organic enrichment

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

Evidence

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 macro-algal communities are relative 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).

Macroalgal growth is generally nitrogen-limited in the summer, as illustrated by increased growth rates of Laminaira digitata in a eutrophic site when compared to an oligotrophic in Abroath, Scotland (Davison et al., 1984). The deposition of sewage effluent into coastal environments resulted in the absence of Laminaria digitata from the coastline of the Firth of Forth (Read et al., 1983) although this was probably coupled with the decrease in water clarity also observed at the time.

The use of some kelp species in conjunction with fish aquaculture (buffering the effects of organic enrichment in the local area) suggests that many commercial kelps (including Laminaria digitata) are tolerant to increased levels of ammonia and faecal matter, dependent on fish species and aquaculture design (Troell et al., 2003).

Sensitivity assessment. Although organic enrichment may not affect kelps directly, indirect effects such as turbidity may significantly affect photosynthesis. Furthermore organic enrichment may denude the associated community. Resistance has therefore been assessed as ‘Medium’, resilience as ‘High’. Sensitivity has been assessed as ’Low’.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

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

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

Physical change (to another sediment type)

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

Evidence

Not relevant

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Not relevant

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 localized mortality of the resident community. The effect would be situation dependant however if bottom fishing gear were towed over a site it may cause high mortality in the resident community and potentially remove areas of the kelp.

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

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

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

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

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

Smothering and siltation rate changes (light)

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

Evidence

Laminaria digitata is more sensitive to this pressure than other subtidal brown algae (e.g. Sargassum muticum; Morrell & Furnhan, 1982). A layer of fine grained sediment (0.1-0.2cm thick) caused rotting of the plant and 25% mortality after 4 weeks of coverage in a laboratory experiment suggesting that in locations of low wave and current mediated water flow, sedimentation is a threat to this biotope (Lyngby & Mortensen, 1996). The abundance of filter feeders may experience some short lived interference with their feeding apparatus and respiratory flows. Furthermore, the holdfasts and lower end of the stipes of Laminaria digitata may experience some mild sand scour. IR.MIR.KT.LdigT is however a tide swept biotope, recorded from very strong to very weak tidal streams (negligible->3m/s). High water movement is likely to remove sediment within a couple of tidal cycles.

Sensitivity assessment. Resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not sensitive’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Laminaria digitata is more sensitive to this pressure than other subtidal brown algae (e.g. Sargassum muticum; Morrell & Furnhan, 1982). A layer of fine grained sediment (0.1-0.2cm thick) caused rotting of the plant and 25% mortality after 4 weeks of coverage in a laboratory experiment suggesting that in locations of low wave and current mediated water flow, sedimentation is a threat to this biotope (Lyngby & Mortensen, 1996). The abundance of filter feeders may experience some short lived interference with their feeding apparatus and respiratory flows. Furthermore, the holdfasts and lower end of the stipes of Laminaria digitata may experience some mild sand scour. IR.MIR.KT.LdigT is however a tide swept biotope, recorded from very strong to very weak tidal streams (negligible->3m/s). High water movement is likely to remove sediment within a few of tidal cycles.

Sensitivity assessment. Due to the volume of deposited sediment within this pressure it is anticipated the sediment will be retained within the host habitat longer than with “light” deposition. Sediment retention may therefore inundate filter feeding fauna and cause Laminaria digitata mortality. Resistance has been assessed as ‘Low’, 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 Laminaria digitata. Shading of the biotope (e.g. by 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 laminarian abundance from forest to park type biotopes.

Sensitivity assessment. Resistance is 'Low', with a 'High' resilience and a sensitivity of 'Low', 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

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 of Laminaria digitata was found. Although Laminaria digitata is harvested, cultivation appears to be done on wild kelp stands in a sustainable 5 year cycle (Vea and Ask, 2011), therefore, translocation of Laminaria digitata is unlikely.  Additionally if translocation of populations does occur, a loss in genetic diversity is not regarded as an issue for Laminaria digitata, unless additionally pressures result in the isolation and fragmentation of wild coastal populations (Valero et al., 2011).

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 this biotope. Potential invasives include Undaria pinnatifida, Sargassum muticum and Codium fragile spp. tormentosoides. In Nova Scotia, Codium fragile spp. tormentosoides competes successfully with native kelps for space including Laminaria digitata, exploiting gaps within the kelp beds. Once established the algal mat created by Codium fragile spp. tormentosoides prevents re-colonization by other macro-algae (Scheibling & Gagnon, 2006).

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., 2015).

Sargassum muticum was shown to replace and out-compete leathery, canopy-forming macroalgae such as Saccharina latissima, Halidrys siliquosa, and Fucus spp. and, to a lesser degree, understorey species such as Codium fragile, Chondrus crispus and Dictyota dichotoma in Limfjorden, Denmark between 1984 and 1997 (Staehr et al., 2000; Engelen et al., 2015; de Bettignies et al., 2021).  The invasion in Limfjorden had stabilized by 2005 although many of the native macroalgal species continued to decline (Engelen et al., 2015).  In Limfjorden, the distribution of Sargassum muticum was limited to areas with hard substratum, in particular stones >10 cm in diameter, while smaller stones, gravel and sand were unsuitable.  It was most abundant between 1 and 4 m in depth but had low cover at 0 to 0.5 m or 4 to 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, the experimental removal of Sargassum resulted in the recovery of native species within about 1 year (Britton-Simmons, 2004; Engelen et al., 2015).  The negative effects of Sargassum muticum on native macroalgae are mainly due to competition for light, rather than changes in nutrient availability, sedimentation or water flow (Britton-Simmons, 2004; Engelen et al., 2015).   

Cosson (1999) reported a significant decline in Laminaria digitata at two sites between 1983 and 1997 on the coast of Normandy, France, due to an increase in Sargassum muticum abundance in the same areas.  For example, on the Grandcamp rocks, Laminaria digitata has almost disappeared while Sargassum muticum had covered 80% of the lower intertidal and subtidal zone in summer.

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; Epstein & Smale, 2018; Kraan, 2017; Epstein et al., 2019a,b; Tidbury, 2020).  Undaria pinnatifida originates from Japan but is established currently on the coastlines of New Zealand, Australia, Northern France, Spain, Italy, the UK, Portugal, Belgium, Holland, Argentina, Mexico, and the USA (De Leij et al., 2017). Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016).  It has since proliferated along UK coastlines.  One year after its discovery at the Queen Anne Battery marina, Plymouth, it became a major fouling plant on pontoons (Minchin & Nunn, 2014).  Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound.

Undaria pinnatifida seems to settle better on artificial substrata (e.g. floats, marinas or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014).  It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018).  James (2017) suggested that Undaria pinnatifida could out-compete native species on artificial substrata (such as marinas and wharf structures).  In Plymouth, UK, De Leij et al. (2017) found that natural habitats with dense native macroalgal canopies, such as Laminaria hyperborea, Laminaria ochroleuca, Laminaria digitata and Saccharina latissima had more resistance to Undaria pinnatifida invasion than disturbed or sparse canopies, due to limited space and light availability for Undaria pinnatifida recruits.

However, the dense canopies did not always prevent invasion of Undaria pinnatifida as sporophytes were still recorded within dense Laminaria canopies, so that canopy disturbance was not always required (De Leij et al., 2017; Epstein & Smale, 2018.

Undaria pinnatifida 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).

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

In St Malo, France, there was evidence that Undaria pinnatifida could co-exist with Laminaria digitata under certain conditions (Castric-Fey et al., 1993).  Epstein et al. (2019b) observed that, in Plymouth Sound, UK, Undaria pinnatifida co-existed with seven species of canopy-forming brown macroalgae within its depth range (+1 to –4 m), including Laminaria digitata; which suggested that they could occupy an overlapping niche.  Epstein & Smale (2018) also observed that Undaria pinnatifida was relatively common (abundance of >70 individuals per 25 m transect) at three sites in Devon, UK (Jennycliff, Bovisand and Beacon Cove) where Laminaria spp. were abundant (40-79%) or superabundant (>80%), which suggested that Undaria pinnatifida could co-exist within refugia amongst areas with dense Laminaria spp..

In many cases, Undaria pinnatifida seems to have minimal impacts on native communities (e.g. Forrest & Taylor, 2002; Valentine & Johnson, 2003; South et al., 2016; Epstein & Smale, 2017; Epstein & Smale 2018). Laminaria digitata forms dense monospecific canopies and its thick, extensive laminae are likely to restrict light penetration to the underlying substratum, including Undaria pinnatifida (De Leij et al., 2017).  Disturbance to the native kelp canopy can facilitate the spread of INIS by increasing the availability of resources such as light and space. Experimental full removal of the existing kelp canopy in Plymouth Sound allowed the mean cover of Undaria pinnatifida to increase from ca 10% to ca 50% within three months (De Leij et al.,2017).  Their experiment showed that the density of Laminaria digitata was important to Undaria pinnatifida invasion (De Leij et al., 2017).

Similarly, a primary succession experiment by Epstein et al. (2019b) in Plymouth Sound (UK) showed that clearance of Laminaria digitata in 2016 resulted in an increase in Undaria pinnatifida abundance.  However, this was quickly followed by the recovery of Laminaria digitata in 2017 and the concurrent decline in Undaria pinnatifida, which suggested that Laminaria digitata had a higher fitness (Epstein et al., 2019b).  Within the same study, Epstein et al. (2019b) observed that Undaria pinnatifida exhibited a significant negative relationship with Laminaria digitata on intertidal rocky reef substrata, which suggested that Laminaria digitata negatively affected Undaria pinnatifida abundance.  It was also suggested that Undaria pinnatifida has a lower resistance to desiccation than Laminaria digitata.  As a result, Epstein & Smale (2019b) concluded that due to its lower fitness, Undaria found within natural habitats in the northeast Atlantic has low ecological and community level impacts and was competitively inferior to Laminaria spp.  However, Heiser et al. (2014) found that in Plymouth, UK, Laminaria digitata was significantly less abundant at sites with the presence of Undaria pinnatifida, with only ca 1.5 Laminaria digitata individuals per m² with Undaria pinnatifida, compared to ca 7 individuals per m² at sites without 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 co-exist with native kelp species within its depth range (-1 to 4 m) e.g. Laminaria digitata as shown in Plymouth Sound, UK.  The dense kelp canopy may restrict or slow the proliferation of Undaria pinnatifida.  But, Laminaria digitata has been shown to out-compete Undaria pinnatifida.  For example, removal of the Laminaria digitata canopy resulted in the increase of Undaria pinnatifida abundance but recovery of Laminaria digitata in the following year resulted in a concurrent decline in Undaria pinnatifida.

This Laminaria digitata dominated biotope (IR.MIR.KT.LdigT) is found in the sublittoral fringe sheltered from wave action but structured by strong tidal streams.  The evidence above suggests that Undaria prefers sheltered conditions, with low tidal flow, and is less tolerant of desiccation than Laminaria digitata.  It is unlikely to out-compete or replace Laminaria digitata under the physical conditions that characterize this biotope.  However, Sargassum muticum prefers wave sheltered, shallow sites in the sublittoral fringe, and has been reported to invade and out-compete Laminaria digitata beds in France (Cosson, 1999; de Bettignies et al., 2021).

Therefore, resistance is assessed as ‘Low’ to represent the possibility of colonization by Sargassum.  While Laminaria digitata may out-compete Undaria the recovery after invasion by Sargassum, although rapid, would require direct intervention (removal).  Hence, resilience is probably ‘Very low’ so sensitivity is assessed as ‘High’.  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 and the limited direct evidence of the effect of Sargassum on Laminaria digitata.

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

Laminaria digitata 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 (Peters & Scaffelke, 1996). Infection can reduce growth rates of host algae.

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

Removal of target species

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

Evidence

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

Canopy removal of Laminaria digitata has been shown to reduce shading, resulting in the bleaching of sub canopy algae (Hawkins & Harkins, 1985). Harvesting may also result in habitat fragmentation, a major threat to this biotope’s ecosystem functioning (Valero et al., 2011). Maintaining a sustainable crop of Laminaria digitata has been suggested as possible if the industry continues employing small vessels evenly dispersed along the coastline. This would protect against habitat fragmentation and buffer over exploitation (Davoult et al., 2011). A fallow period of 18-24 months has been suggested for Laminaria digitata in France, where competition between the juvenile sporophytes of Laminaria digitata and Saccorhiza polyschides has been indicated as a threat to the continued harvesting effort of Laminaria digitata (Engelen et al., 2011).

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

Removal of non-target species

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

Evidence

Incidental/accidental removal of Laminaria digitata as a result of non-targeted fisheries practices is likely to have a significant effect on the ecology of IR.MIR.KT.LdigT. Targeted Laminaria hyperborea trawling in southern Norway is reported to remove all canopy forming sporophytes (Christie et al. 1998).

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