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

Sea surface temperatures around the UK currently fall between 6 to 19°C (Huthnance, 2010). Under the middle emission, high emission and extreme scenarios, sea surface temperatures are expected to increase by 3, 4 and 5°C respectively, leading to temperatures increasing to between 22 to 24°C by the end of this century, although northern UK temperatures will be up to 5°C lower. Davenport & Davenport (2005) demonstrated that the limits of thermal tolerance to high and low temperatures reflect distribution of intertidal macroinvertebrate species. Species that occur highest on the shore are more tolerant of a wider range of temperatures than species that occurred low on the shore or subtidally. As subtidal biotopes are less exposed to temperature fluctuations, the characterizing species may be less able to tolerate temperature fluctuations.

The characterizing species Spirobranchus triqueter is found in both warmer and colder waters than experienced in the UK. Spirobranchus triqueter occurs from the Arctic, the eastern North Atlantic up to the Mediterranean, Adriatic, Black and Red Sea, the English Channel, the whole North Sea, Skagerrak, Kattegat, the Belts and Öresund up to Bay of Kiel (de Kluijver et al., 2016). Klöckner, (1978, summary only) found adult Spirobranchus triqueter (as Pomatoceros triqueter) tolerated temperatures from -3 to 30°C. Oxygen consumption and regeneration of calcareous tube in Spirobranchus triqueter is temperature dependent, and increased metabolic rates in polychaetes have been found at high temperatures (Klöckner, 1978). OBIS (2024) lists records of Spirobranchus triqueter from sea surface temperatures of 5 to 30°C although the majority of records were from 10 to 15°C. Castric-Fey (1983) found that animals settling during spring showed the best growth rate and the best larval settlement occurred in the summer months. Therefore, it is assumed that Spirobranchus triqueter has some tolerance to increased temperatures.

Balanus crenatus and Balanus balanus are described as boreal species (Newman & Ross, 1976, NBN atlas). Balanus crenatus is found throughout the North East Atlantic, from the Arctic to the west coast of France, as far south as Bordeaux, and the east and west coasts of North America and Japan. In Queens Dock, Swansea, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite where the water was on average 10°C higher than average due to the effects of a condenser effluent, after the water temperature cooled Balanus crenatus returned (Naylor, 1965). Balanus crenatus has a peak rate of cirral beating at 20°C and all spontaneous activity ceases at about 25 °C (Southward, 1955). The tolerance of Balanus crenatus, collected in the summer (and thus acclimated to higher temperatures) to increased temperatures was tested in the laboratory. The median upper lethal temperature tolerance was 25.2°C (Davenport & Davenport, 2005) confirming the observations of Southward (1955). Balanus balanus has boero-arctic distribution ranging from Long Island Sound in the western Atlantic to around 80°N. Balanus balanus has a similar distribution to Balanus crenatus, and been recorded from the North Pacific including Alaska, British Columbia and the Okhotsk Sea and is widespread around the United Kingdom and Irish coastal waters. OBIS (2024) lists records of Balanus balanus and Balanus cernatus from sea surface temperatures of -5 to 30°C although the majority of records were from 10 to 15°C. The predominantly northern distribution of the Balanus species could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species.

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Most of the encrusting bryozoan species occurring in the biotope are distributed to the north and south of Britain and Ireland. For example, the bryozoans Electra pilosa and Parasmittina trispinosa are considered unlikely to be affected by long-term changes in temperature. Acclimation to temperatures is possible. Menon (1972), for example, reported that the upper lethal temperature and median lethal temperature of Electra pilosa varied significantly with acclimation temperature, e.g. the 24-hour upper lethal temperature was ca 25°C in colonies acclimated to 5°C but ca 29°C when acclimated to 22°C (Menon, 1972). An acute temperature change may affect growth, feeding and hence reproduction in bryozoans.

Corallina officinalis may tolerate from -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 (1982); 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. The distribution and available evidence on temperature tolerance of characterizing species Spirobranchus triqueter suggest it may be able to withstand predicted global warming temperatures. For example, all of the characteristic polychaetes occur in the Mediterranean Sea where sea surface temperatures can reach 28°C in summer months (www.seatemperature.org). However, evidence on Balanus balanus and Balanus crenatus suggest both boreal species may not be able as tolerate to high temperatures out of its preferred range, suggesting the species may be vulnerable to global warming. Therefore, for all three scenarios (middle and high emission and extreme scenarios) resistance is assessed as ‘Medium’ to represent the potential loss or movement of some characteristic species and resilience is assessed as ‘Very Low’, as loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Hence, sensitivity is assessed as ‘Medium’ to ocean warming under all three scenarios, albeit with ‘Low’ confidence. It should be noted that the physical habitat is likely to remain, and become colonized by other more warm water tolerant opportunists over time, although the biotope, as defined, may be degraded. 

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

Sea surface temperatures around the UK currently fall between 6 to 19°C (Huthnance, 2010). Under the middle emission, high emission and extreme scenarios, sea surface temperatures are expected to increase by 3, 4 and 5°C respectively, leading to temperatures increasing to between 22 to 24°C by the end of this century, although northern UK temperatures will be up to 5°C lower. Davenport & Davenport (2005) demonstrated that the limits of thermal tolerance to high and low temperatures reflect distribution of intertidal macroinvertebrate species. Species that occur highest on the shore are more tolerant of a wider range of temperatures than species that occurred low on the shore or subtidally. As subtidal biotopes are less exposed to temperature fluctuations, the characterizing species may be less able to tolerate temperature fluctuations.

The characterizing species Spirobranchus triqueter is found in both warmer and colder waters than experienced in the UK. Spirobranchus triqueter occurs from the Arctic, the eastern North Atlantic up to the Mediterranean, Adriatic, Black and Red Sea, the English Channel, the whole North Sea, Skagerrak, Kattegat, the Belts and Öresund up to Bay of Kiel (de Kluijver et al., 2016). Klöckner, (1978, summary only) found adult Spirobranchus triqueter (as Pomatoceros triqueter) tolerated temperatures from -3 to 30°C. Oxygen consumption and regeneration of calcareous tube in Spirobranchus triqueter is temperature dependent, and increased metabolic rates in polychaetes have been found at high temperatures (Klöckner, 1978). OBIS (2024) lists records of Spirobranchus triqueter from sea surface temperatures of 5 to 30°C although the majority of records were from 10 to 15°C. Castric-Fey (1983) found that animals settling during spring showed the best growth rate and the best larval settlement occurred in the summer months. Therefore, it is assumed that Spirobranchus triqueter has some tolerance to increased temperatures.

Balanus crenatus and Balanus balanus are described as boreal species (Newman & Ross, 1976, NBN atlas). Balanus crenatus is found throughout the North East Atlantic, from the Arctic to the west coast of France, as far south as Bordeaux, and the east and west coasts of North America and Japan. In Queens Dock, Swansea, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite where the water was on average 10°C higher than average due to the effects of a condenser effluent, after the water temperature cooled Balanus crenatus returned (Naylor, 1965). Balanus crenatus has a peak rate of cirral beating at 20°C and all spontaneous activity ceases at about 25 °C (Southward, 1955). The tolerance of Balanus crenatus, collected in the summer (and thus acclimated to higher temperatures) to increased temperatures was tested in the laboratory. The median upper lethal temperature tolerance was 25.2°C (Davenport & Davenport, 2005) confirming the observations of Southward (1955). Balanus balanus has boero-arctic distribution ranging from Long Island Sound in the western Atlantic to around 80°N. Balanus balanus has a similar distribution to Balanus crenatus, and been recorded from the North Pacific including Alaska, British Columbia and the Okhotsk Sea and is widespread around the United Kingdom and Irish coastal waters. OBIS (2024) lists records of Balanus balanus and Balanus cernatus from sea surface temperatures of -5 to 30°C although the majority of records were from 10 to 15°C. The predominantly northern distribution of the Balanus species could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species.

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Most of the encrusting bryozoan species occurring in the biotope are distributed to the north and south of Britain and Ireland. For example, the bryozoans Electra pilosa and Parasmittina trispinosa are considered unlikely to be affected by long-term changes in temperature. Acclimation to temperatures is possible. Menon (1972), for example, reported that the upper lethal temperature and median lethal temperature of Electra pilosa varied significantly with acclimation temperature, e.g. the 24-hour upper lethal temperature was ca 25°C in colonies acclimated to 5°C but ca 29°C when acclimated to 22°C (Menon, 1972). An acute temperature change may affect growth, feeding and hence reproduction in bryozoans.

Corallina officinalis may tolerate from -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 (1982); 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. The distribution and available evidence on temperature tolerance of characterizing species Spirobranchus triqueter suggest it may be able to withstand predicted global warming temperatures. For example, all of the characteristic polychaetes occur in the Mediterranean Sea where sea surface temperatures can reach 28°C in summer months (www.seatemperature.org). However, evidence on Balanus balanus and Balanus crenatus suggest both boreal species may not be able as tolerate to high temperatures out of its preferred range, suggesting the species may be vulnerable to global warming. Therefore, for all three scenarios (middle and high emission and extreme scenarios) resistance is assessed as ‘Medium’ to represent the potential loss or movement of some characteristic species and resilience is assessed as ‘Very Low’, as loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Hence, sensitivity is assessed as ‘Medium’ to ocean warming under all three scenarios, albeit with ‘Low’ confidence. It should be noted that the physical habitat is likely to remain, and become colonized by other more warm water tolerant opportunists over time, although the biotope, as defined, may be degraded. 

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

Sea surface temperatures around the UK currently fall between 6 to 19°C (Huthnance, 2010). Under the middle emission, high emission and extreme scenarios, sea surface temperatures are expected to increase by 3, 4 and 5°C respectively, leading to temperatures increasing to between 22 to 24°C by the end of this century, although northern UK temperatures will be up to 5°C lower. Davenport & Davenport (2005) demonstrated that the limits of thermal tolerance to high and low temperatures reflect distribution of intertidal macroinvertebrate species. Species that occur highest on the shore are more tolerant of a wider range of temperatures than species that occurred low on the shore or subtidally. As subtidal biotopes are less exposed to temperature fluctuations, the characterizing species may be less able to tolerate temperature fluctuations.

The characterizing species Spirobranchus triqueter is found in both warmer and colder waters than experienced in the UK. Spirobranchus triqueter occurs from the Arctic, the eastern North Atlantic up to the Mediterranean, Adriatic, Black and Red Sea, the English Channel, the whole North Sea, Skagerrak, Kattegat, the Belts and Öresund up to Bay of Kiel (de Kluijver et al., 2016). Klöckner, (1978, summary only) found adult Spirobranchus triqueter (as Pomatoceros triqueter) tolerated temperatures from -3 to 30°C. Oxygen consumption and regeneration of calcareous tube in Spirobranchus triqueter is temperature dependent, and increased metabolic rates in polychaetes have been found at high temperatures (Klöckner, 1978). OBIS (2024) lists records of Spirobranchus triqueter from sea surface temperatures of 5 to 30°C although the majority of records were from 10 to 15°C. Castric-Fey (1983) found that animals settling during spring showed the best growth rate and the best larval settlement occurred in the summer months. Therefore, it is assumed that Spirobranchus triqueter has some tolerance to increased temperatures.

Balanus crenatus and Balanus balanus are described as boreal species (Newman & Ross, 1976, NBN atlas). Balanus crenatus is found throughout the North East Atlantic, from the Arctic to the west coast of France, as far south as Bordeaux, and the east and west coasts of North America and Japan. In Queens Dock, Swansea, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite where the water was on average 10°C higher than average due to the effects of a condenser effluent, after the water temperature cooled Balanus crenatus returned (Naylor, 1965). Balanus crenatus has a peak rate of cirral beating at 20°C and all spontaneous activity ceases at about 25 °C (Southward, 1955). The tolerance of Balanus crenatus, collected in the summer (and thus acclimated to higher temperatures) to increased temperatures was tested in the laboratory. The median upper lethal temperature tolerance was 25.2°C (Davenport & Davenport, 2005) confirming the observations of Southward (1955). Balanus balanus has boero-arctic distribution ranging from Long Island Sound in the western Atlantic to around 80°N. Balanus balanus has a similar distribution to Balanus crenatus, and been recorded from the North Pacific including Alaska, British Columbia and the Okhotsk Sea and is widespread around the United Kingdom and Irish coastal waters. OBIS (2024) lists records of Balanus balanus and Balanus cernatus from sea surface temperatures of -5 to 30°C although the majority of records were from 10 to 15°C. The predominantly northern distribution of the Balanus species could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species.

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Most of the encrusting bryozoan species occurring in the biotope are distributed to the north and south of Britain and Ireland. For example, the bryozoans Electra pilosa and Parasmittina trispinosa are considered unlikely to be affected by long-term changes in temperature. Acclimation to temperatures is possible. Menon (1972), for example, reported that the upper lethal temperature and median lethal temperature of Electra pilosa varied significantly with acclimation temperature, e.g. the 24-hour upper lethal temperature was ca 25°C in colonies acclimated to 5°C but ca 29°C when acclimated to 22°C (Menon, 1972). An acute temperature change may affect growth, feeding and hence reproduction in bryozoans.

Corallina officinalis may tolerate from -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 (1982); 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. The distribution and available evidence on temperature tolerance of characterizing species Spirobranchus triqueter suggest it may be able to withstand predicted global warming temperatures. For example, all of the characteristic polychaetes occur in the Mediterranean Sea where sea surface temperatures can reach 28°C in summer months (www.seatemperature.org). However, evidence on Balanus balanus and Balanus crenatus suggest both boreal species may not be able as tolerate to high temperatures out of its preferred range, suggesting the species may be vulnerable to global warming. Therefore, for all three scenarios (middle and high emission and extreme scenarios) resistance is assessed as ‘Medium’ to represent the potential loss or movement of some characteristic species and resilience is assessed as ‘Very Low’, as loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Hence, sensitivity is assessed as ‘Medium’ to ocean warming under all three scenarios, albeit with ‘Low’ confidence. It should be noted that the physical habitat is likely to remain, and become colonized by other more warm water tolerant opportunists over time, although the biotope, as defined, may be degraded. 

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 due to increased air-sea heat flux 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). Climate change will not only shift mean sea surface temperatures but will also increase the intensity of extreme events, exerting additional stress on ecosystems. There was no direct evidence on the effect of heat waves on the characterizing species in this biotope but evidence is available on the thermal limits of the species.

The characterizing species Spirobranchus triqueter is found in both warmer and colder waters than experienced in the UK and its global distribution suggests the species is tolerant to a wide range of temperatures (see global warming). Klöckner, (1978, summary only) found adult Spirobranchus triqueter (as Pomatoceros triqueter) tolerated temperatures from -3 to 30°C. OBIS (2024) lists records of Spirobranchus triqueter from sea surface temperatures of 5 to 30°C although the majority of records were from 10 to 15°C. 

Balanus crenatus and Balanus balanus are described as boreal species (Newman & Ross, 1976, NBN atlas) and have a low thermal tolerance (see global warming above). OBIS (2024) lists records of Balanus balanus and Balanus cernatus from sea surface temperatures of -5 to 30°C although the majority of records were from 10 to 15°C. The predominantly northern distribution of the Balanus species could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species.

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Although Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, there are records of this species between 25 and 30°C (www.obis.org). Therefore, Parasmittina trispinosa is considered unlikely to be affected by long-term changes in temperature in the UK, as it is likely to acclimate to temperatures with time. However, the occurrence of marine heatwaves could cause mass mortality to populations that have not been acclimated to warmer temperatures.

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, summer sea temperatures could reach up to 24°C in southern England. 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. As a precautionary approach, resistance is assessed as ‘Medium’. The resilience of the characteristic opportunistic species is likely to be ‘High’ (<2 years),  Therefore, sensitivity is assessed as ‘Low’ but with low confidence due to lack of direct evidence.

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 due to increased air-sea heat flux 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). Climate change will not only shift mean sea surface temperatures but will also increase the intensity of extreme events, exerting additional stress on ecosystems. There was no direct evidence on the effect of heat waves on the characterizing species in this biotope but evidence is available on the thermal limits of the species.

The characterizing species Spirobranchus triqueter is found in both warmer and colder waters than experienced in the UK and its global distribution suggests the species is tolerant to a wide range of temperatures (see global warming). Klöckner, (1978, summary only) found adult Spirobranchus triqueter (as Pomatoceros triqueter) tolerated temperatures from -3 to 30°C. OBIS (2024) lists records of Spirobranchus triqueter from sea surface temperatures of 5 to 30°C although the majority of records were from 10 to 15°C. 

Balanus crenatus and Balanus balanus are described as boreal species (Newman & Ross, 1976, NBN atlas) and have a low thermal tolerance (see global warming above). OBIS (2024) lists records of Balanus balanus and Balanus cernatus from sea surface temperatures of -5 to 30°C although the majority of records were from 10 to 15°C. The predominantly northern distribution of the Balanus species could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species.

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Although Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, there are records of this species between 25 and 30°C (www.obis.org). Therefore, Parasmittina trispinosa is considered unlikely to be affected by long-term changes in temperature in the UK, as it is likely to acclimate to temperatures with time. However, the occurrence of marine heatwaves could cause mass mortality to populations that have not been acclimated to warmer temperatures.

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, summer sea temperatures could reach up to 24°C in southern England. 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. As a precautionary approach, resistance is assessed as ‘Medium’. The resilience of the characteristic opportunistic species is likely to be ‘High’ (<2 years),  Therefore, sensitivity is assessed as ‘Low’ but with low confidence due to lack of direct evidence.

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). There is limited evidence on the effects of ocean acidification on the characterizing species.

Ocean acidification is expected to negatively affect reproduction (fecundity and sperm mobility) of annelids and molluscs (David, 2021). Calcifying polychaetes (Serpuild polychaetes) are vulnerable to more acidic oceans due to dissolution of the calcite and aragonite, which impact the structural integrity of the polychaetes’ tubes (David, 2021). Studies on free-living benthic polychaetes found an increase in metabolic rate, oxidative stress and reduction in sperm motility in hypercapnic conditions (David, 2021). Polychaete species that live in CO₂ vents are physiologically more tolerant to elevated pCO₂ levels.

Characterizing polychaete Spirobranchus triqueter is a serpulid calcifying polychaete and may, therefore, be vulnerable to ocean acidification. A study on the impacts of ocean acidification to the calcified tube growth of Spirobranchus triqueter showed a moderate decrease in the tube elongation rates at pH 7.7 and severe reductions at pH 7.4 (Dìaz-Castañeda et al., 2019). Dìaz-Castañeda et al. (2019) also reported visual evidence of dissolution and found that the tubes has thinner calcareous layers and were more fragile in habitats at lower pH (also cited by Byrne & Fitzer, 2019 and David, 2021) Serpulids are highly vulnerable to dissolution due to its mineral composition and it is suggested that the early stages of Spirobranchus triqueter are less aragonitic compared to other Serpuilds. Therefore, they are more tolerant to seawater chemistry changes, as there were no specimens observed which were unable to build tubes (Dìaz-Castañeda et al., 2019). In the early life stage of Spirobranchus triqueter, larval growth was most affected in juvenile tubes at pH 7.7 and 7.4, which were half the size of ones at pH 8.1. However, despite observations of reduced density and size of larvae there was no impact on metabolic rate at low pH 7.4 and the settlement success was similar at all pH levels (8.1, 7.7, 7.4). In addition, another study suggested Spirobranchus larmarcki ‘s sperm mortality reduced significantly during increased pCO₂ conditions over a short period (David, 2021). Li et al. (2014) observed changes to the structure, volume and density of serpulid polychaete tubes, with reductions in hardiness and elasticity, in addition to a 64% reduction in tube crushing force. The reduction in tube size and hardiness could impact the survival of the serpulidae polychaetes with increased predation and reduction in the ability to withstand wave force (Chan et al., 2012; Li et al., 2014), especially as this biotope is structured by scour and wave action.

Adult barnacles have a hard, calcareous shell (Iglikowska et al., 2018). Evidence suggests barnacles appear to be resistant to ocean acidification effects (Brown et al., 2016), but not all species show high resistance. For example, Semibalanus balanoides exhibits decreased calcification rates, decreased growth and development rate and dissolution in response to a decrease in pH levels (Findlay et al., 2010a, Findlay et al., 2010c cited by Wicks & Roberts, 2012). On the other hand, ocean acidification conditions over a nine year period resulted an increase in acorn barnacles (Balanus glandula and Semibalanus cariosus) (Wootton et al., 2008 cited by Wicks & Roberts, 2012). Therefore, the effects of ocean acidification depends on the species. Brown et al. (2016) found that ocean acidification had no effect on the percentage cover or maximum size of the characterizing barnacle Balanus crenatus, over a 10-week experiment and no significant trend in mortality.

Bryozoans are invertebrate calcifiers, therefore, they are potentially highly sensitive to ocean acidification (Smith, 2009). The decrease in water pH from global climate change could cause corrosion, changes in mineralogy and decrease the survival of bryozoans (Smith, 2014). Swezey et al. (2017) observed that populations of bryozoans raised under high CO₂ (1254 μatm; pH 7.60) conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO₂ values (400 μatm; pH 8.04). In addition, the bryozoans under high CO₂ altered Mg/Ca ratio of skeletal calcite, which could be a protective mechanism against acidification (Swezey et al., 2017). 

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. There is limited evidence on the effects of ocean acidification on all characterizing species within this biotope. It is suggested that calcifying polychaetes such as the characterizing Spirobranchus triqueter are vulnerable to effects of ocean acidification. However, despite variation in effects of acidification on barnacle species, evidence suggests the characterizing species Balanus crenatus is likely to be tolerant of ocean acidification. Therefore, under both the middle and high emission scenarios the biotope is assessed as having ‘Medium’ resistance to ocean acidification to represent the possible loss of serpulidae polychaete species. Resilience is assessed as ‘Very low’, and sensitivity as of ‘Medium’ at the benchmark level with low confidence due to lack of evidence.

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). There is limited evidence on the effects of ocean acidification on the characterizing species.

Ocean acidification is expected to negatively affect reproduction (fecundity and sperm mobility) of annelids and molluscs (David, 2021). Calcifying polychaetes (Serpuild polychaetes) are vulnerable to more acidic oceans due to dissolution of the calcite and aragonite, which impact the structural integrity of the polychaetes’ tubes (David, 2021). Studies on free-living benthic polychaetes found an increase in metabolic rate, oxidative stress and reduction in sperm motility in hypercapnic conditions (David, 2021). Polychaete species that live in CO₂ vents are physiologically more tolerant to elevated pCO₂ levels.

Characterizing polychaete Spirobranchus triqueter is a serpulid calcifying polychaete and may, therefore, be vulnerable to ocean acidification. A study on the impacts of ocean acidification to the calcified tube growth of Spirobranchus triqueter showed a moderate decrease in the tube elongation rates at pH 7.7 and severe reductions at pH 7.4 (Dìaz-Castañeda et al., 2019). Dìaz-Castañeda et al. (2019) also reported visual evidence of dissolution and found that the tubes has thinner calcareous layers and were more fragile in habitats at lower pH (also cited by Byrne & Fitzer, 2019 and David, 2021) Serpulids are highly vulnerable to dissolution due to its mineral composition and it is suggested that the early stages of Spirobranchus triqueter are less aragonitic compared to other Serpuilds. Therefore, they are more tolerant to seawater chemistry changes, as there were no specimens observed which were unable to build tubes (Dìaz-Castañeda et al., 2019). In the early life stage of Spirobranchus triqueter, larval growth was most affected in juvenile tubes at pH 7.7 and 7.4, which were half the size of ones at pH 8.1. However, despite observations of reduced density and size of larvae there was no impact on metabolic rate at low pH 7.4 and the settlement success was similar at all pH levels (8.1, 7.7, 7.4). In addition, another study suggested Spirobranchus larmarcki ‘s sperm mortality reduced significantly during increased pCO₂ conditions over a short period (David, 2021). Li et al. (2014) observed changes to the structure, volume and density of serpulid polychaete tubes, with reductions in hardiness and elasticity, in addition to a 64% reduction in tube crushing force. The reduction in tube size and hardiness could impact the survival of the serpulidae polychaetes with increased predation and reduction in the ability to withstand wave force (Chan et al., 2012; Li et al., 2014), especially as this biotope is structured by scour and wave action.

Adult barnacles have a hard, calcareous shell (Iglikowska et al., 2018). Evidence suggests barnacles appear to be resistant to ocean acidification effects (Brown et al., 2016), but not all species show high resistance. For example, Semibalanus balanoides exhibits decreased calcification rates, decreased growth and development rate and dissolution in response to a decrease in pH levels (Findlay et al., 2010a, Findlay et al., 2010c cited by Wicks & Roberts, 2012). On the other hand, ocean acidification conditions over a nine year period resulted an increase in acorn barnacles (Balanus glandula and Semibalanus cariosus) (Wootton et al., 2008 cited by Wicks & Roberts, 2012). Therefore, the effects of ocean acidification depends on the species. Brown et al. (2016) found that ocean acidification had no effect on the percentage cover or maximum size of the characterizing barnacle Balanus crenatus, over a 10-week experiment and no significant trend in mortality.

Bryozoans are invertebrate calcifiers, therefore, they are potentially highly sensitive to ocean acidification (Smith, 2009). The decrease in water pH from global climate change could cause corrosion, changes in mineralogy and decrease the survival of bryozoans (Smith, 2014). Swezey et al. (2017) observed that populations of bryozoans raised under high CO₂ (1254 μatm; pH 7.60) conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO₂ values (400 μatm; pH 8.04). In addition, the bryozoans under high CO₂ altered Mg/Ca ratio of skeletal calcite, which could be a protective mechanism against acidification (Swezey et al., 2017). 

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. There is limited evidence on the effects of ocean acidification on all characterizing species within this biotope. It is suggested that calcifying polychaetes such as the characterizing Spirobranchus triqueter are vulnerable to effects of ocean acidification. However, despite variation in effects of acidification on barnacle species, evidence suggests the characterizing species Balanus crenatus is likely to be tolerant of ocean acidification. Therefore, under both the middle and high emission scenarios the biotope is assessed as having ‘Medium’ resistance to ocean acidification to represent the possible loss of serpulidae polychaete species. Resilience is assessed as ‘Very low’, and sensitivity as of ‘Medium’ at the benchmark level with low confidence due to lack of evidence.

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). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario. 

Understanding how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, 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. The effects of sea-level rise and increased wave action may be increased further due to storms and storm surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Spirobranchus triqueter is found predominantly sublittoral down to 70 m. Balanus balanus is found attached to hard substrates at low water to 150 m, but prefers depths between 20 to 30 m (Iglikowska et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 100 m and the characterizing barnacle species are abundant at depths more than 100 m. This biotope is structured by scour due to water flow and storm action. Increase sea-level could reduce the depth penetration of wave action and reduce scour allowing a more diverse community to colonize the biotope, and result in a change in the biotope. However, this is only likely in shallow examples of the biotope where water movement is dominated by wave action rather than tidal flow. Deeper examples of the biotope are already probably structured by tidal flow rather than wave action. Therefore, an increase in sea-level rise might impact on this biotope depending on local hydrography. Therefore, resistance to sea-level rise has been assessed as ‘High’ for the middle (50 cm), and high (70 cm), which is a minor change compared to the depth range of the biotope (5-100 m). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks. However, resistance is assessed as ‘Medium’ for the extreme scenario (107 cm), as a precaution on a site-by-site basis. Resilience is assessed as ‘Very low’, and sensitivity as of ‘Medium’ at the benchmark level with low confidence due to lack of evidence.

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). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario. 

Understanding how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, 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. The effects of sea-level rise and increased wave action may be increased further due to storms and storm surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Spirobranchus triqueter is found predominantly sublittoral down to 70 m. Balanus balanus is found attached to hard substrates at low water to 150 m, but prefers depths between 20 to 30 m (Iglikowska et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 100 m and the characterizing barnacle species are abundant at depths more than 100 m. This biotope is structured by scour due to water flow and storm action. Increase sea-level could reduce the depth penetration of wave action and reduce scour allowing a more diverse community to colonize the biotope, and result in a change in the biotope. However, this is only likely in shallow examples of the biotope where water movement is dominated by wave action rather than tidal flow. Deeper examples of the biotope are already probably structured by tidal flow rather than wave action. Therefore, an increase in sea-level rise might impact on this biotope depending on local hydrography. Therefore, resistance to sea-level rise has been assessed as ‘High’ for the middle (50 cm), and high (70 cm), which is a minor change compared to the depth range of the biotope (5-100 m). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks. However, resistance is assessed as ‘Medium’ for the extreme scenario (107 cm), as a precaution on a site-by-site basis. Resilience is assessed as ‘Very low’, and sensitivity as of ‘Medium’ at the benchmark level with low confidence due to lack of evidence.

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). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario. 

Understanding how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, 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. The effects of sea-level rise and increased wave action may be increased further due to storms and storm surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Spirobranchus triqueter is found predominantly sublittoral down to 70 m. Balanus balanus is found attached to hard substrates at low water to 150 m, but prefers depths between 20 to 30 m (Iglikowska et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 100 m and the characterizing barnacle species are abundant at depths more than 100 m. This biotope is structured by scour due to water flow and storm action. Increase sea-level could reduce the depth penetration of wave action and reduce scour allowing a more diverse community to colonize the biotope, and result in a change in the biotope. However, this is only likely in shallow examples of the biotope where water movement is dominated by wave action rather than tidal flow. Deeper examples of the biotope are already probably structured by tidal flow rather than wave action. Therefore, an increase in sea-level rise might impact on this biotope depending on local hydrography. Therefore, resistance to sea-level rise has been assessed as ‘High’ for the middle (50 cm), and high (70 cm), which is a minor change compared to the depth range of the biotope (5-100 m). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks. However, resistance is assessed as ‘Medium’ for the extreme scenario (107 cm), as a precaution on a site-by-site basis. Resilience is assessed as ‘Very low’, and sensitivity as of ‘Medium’ at the benchmark level with low confidence due to lack of evidence.

Temperature increase (local)

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

Evidence

This biotope occurs in the subtidal and is therefore protected from exposure to air so that the thermal regime is more stable and desiccation is not a factor.  Examples of distribution and thermal tolerances tested in laboratory experiments are provided as evidence to support the sensitivity assessment. In general, populations can acclimate to prevailing conditions which can alter tolerance thresholds and care should, therefore, be used when interpreting reported tolerances.

Balanus crenatus is described as a boreal species (Newman & Ross, 1976), it is found throughout the northeast Atlantic, from the Arctic to the west coast of France, as far south as Bordeaux, and the east and west coasts of North America and Japan. In Queens Dock, Swansea where the water was on average 10 °C higher than average due to the effects of a condenser effluent, Balanus crenatus was replaced by the subtropical barnacle Balanus amphitrite.  After the water temperature cooled Balanus crenatus returned (Naylor, 1965).  The increased water temperature in Queens Dock is greater than an increase at the pressure benchmark (2-5 °C).  Balanus crenatus has a peak rate of cirral beating at 20 °C and all spontaneous activity ceases at about 25 °C (Southward, 1955). The tolerance of Balanus crenatus, collected in the summer (and thus acclimated to higher temperatures) to increased temperatures was tested in the laboratory. The median upper lethal temperature tolerance was 25.2 ^oC (Davenport & Davenport, 2005) confirming the observations of Southward (1955).

The characterizing species Spirobranchus triqueter is found in both warmer and colder waters than experienced in the UK.  Spirobranchus triqueter occurs from the Arctic, the eastern North Atlantic up to the Mediterranean, Adriatic, Black and Red Sea, the English Channel, the whole North Sea, Skagerrak, Kattegat, the Belts and Öresund up to Bay of Kiel (de Kluijver et al., 2016).

Most of the encrusting bryozoan species occurring in the biotope are distributed to the north and south of Britain and Ireland. E.g. the bryozoans Electra pilosa and Parasmittina trispinosa are considered unlikely to be affected by long-term changes in temperature. Acclimation to temperatures is possible. Menon (1972), for example,  reported that the upper lethal temperature and median lethal temperature of Electra pilosa varied significantly with acclimation temperature, e.g. 24 hr upper lethal temperature was ca 25 °C in colonies acclimated to 5 °C but ca 29 °C when acclimated to 22 °C (Menon, 1972).  An acute temperature change may affect growth, feeding and hence reproduction in bryozoans.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4-19^oC (Huthnance, 2010). The biotope is considered to tolerate a 2^oC increase in temperature for a year. An acute increase at the pressure benchmark may be tolerated in winter, but a sudden return to typical temperatures could lead to mortalities among acclimated animals. No evidence was found to support this assessment. However, an acute increase of 5^oC in summer would be close to the lethal thermal temperature for Balanus crenatus and loss of this species would alter the character of the biotope. Therefore,  resistance is assessed as ‘Medium’ and resilience as ‘High’ and sensitivity as therefore ‘Low’.

Temperature decrease (local)

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

Evidence

This biotope occurs in the subtidal and is therefore protected from exposure to air so that the thermal regime is more stable and desiccation is not a factor.  Examples of distribution and thermal tolerances tested in laboratory experiments are provided as evidence to support the sensitivity assessment. In general, populations can acclimate to prevailing conditions which can alter tolerance thresholds and care should, therefore, be used when interpreting reported tolerances.

Within the biotope, the key characterizing barnacles Balanus crenatus have a more northern distribution and are absent from warmer Mediterranean and equatorial waters.  Balanus crenatus is described as a boreal species (Newman & Ross, 1976), it is found throughout the northeast Atlantic from the Arctic to the west coast of France, as far south as Bordeaux; east and west coasts of North America and Japan.

Balanus crenatus was unaffected during the severe winter of 1962-63 when average temperatures were 5 to 6°C below normal for the British Isles and much of Europe (Crisp, 1964a). Meadows (1969) noted decreased temperatures in Newcastle (England) during the severe winter of 1962-63. Balanus crenatus were among the fauna on the settlement panels that were deployed in the area and not affected.   The temperature tolerance of Balanus crenatus collected from the lower intertidal in the winter (and thus acclimated to lower temperatures) was tested in the laboratory. The median lower lethal temperature tolerance was -1.4^oC (Davenport & Davenport, 2005). An acute or chronic decrease in temperature, at the pressure benchmark, is therefore unlikely to negatively affect this species. 

The characterizing Spirobranchus triqueter is found in both warmer and colder waters than experienced in the UK.  Spirobranchus triqueter occurs from the Arctic, the eastern North Atlantic up to the Mediterranean, Adriatic, Black and Red Sea, the English Channel, the whole North Sea, Skagerrak, Kattegat, the Belts and Öresund up to Bay of Kiel (de Kluijver et al., 2016). Thomas (1940) noted that Spirobranchus (as Pomatoceros) triqueter could not form tubes below 7°C, however, this effect is not considered to lead to mortality in adults at the duration of the acute pressure benchmark. Intertidal populations of Spirobranchus triqueter were reported to suffer 50% mortality at Mumbles, on the Gower after the extreme winter of 1962-63 (Crisp, 1964b), however, the decrease in temperature exceeds the pressure benchmark.

The bryozoan Electra pilosa is widely distributed in temperate seas, occurring as far north as the Barents Sea within the Arctic Circle (Gontar & Denisenko, 1989). Menon (1972) reported that individual zooids on the growing rim of colonies survived when kept at -4°C for 14 days, although the inner zooids died. Menon (1972) demonstrated that all the zooids on the rim of colonies acclimated to 6 °C for 6 months before being kept in ice at -4°C for 14 days, although apparently killed regenerated when returned to 6 °C. Therefore, Electra pilosa is unlikely to be adversely affected by long-term temperature changes in British waters. Hyman (1959) reported that a reduction in the temperature of only 3 °C was enough to interrupt feeding and that Electra pilosa colonies became unresponsive at 4°C. Therefore, acute short-term decreases in temperature may interfere with the feeding and reproduction of bryozoans.

Sensitivity assessment. Overall, a long-term chronic change in temperature at the pressure benchmark is considered likely to fall within natural variation and to be tolerated by the characterizing and associated species although, Lithothyllum incrustans may experience reduced growth (as it is primarily a southern species). An acute change at the pressure benchmark is considered unlikely to adversely affect the biotope as the characterizing species can potentially adapt to a wide range of temperatures experienced in both northern and southern waters (Spirobranchus triqueter) or are found primarily in colder, more northern waters (Balanus crenatus).  Therefore, resistance is assessed as ‘High’ and resilience as ‘High’, so sensitivity is assessed as 'Not sensitive'.

Salinity increase (local)

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

Evidence

This biotope is recorded in full salinity (30-35 ppt) habitats (JNCC, 2022) and the sensitivity assessment considers an increase from full to >40 ppt (the pressure benchmark).

Balanus crenatus occurs in estuarine areas and is therefore adapted to variable salinity (Davenport, 1976). When subjected to sudden changes in salinity Balanus crenatus closes its opercular valves so that the blood is maintained temporarily at a constant osmotic concentration (Davenport, 1976).  Early stages may be more sensitive than adults. Experimental culturing of Balanus crenatus eggs, found that viable nauplii larvae were obtained between 25-40‰ but eggs did not develop into viable larvae when held at salinities above 40‰ and only a small proportion (7%) of eggs exposed at later stages developed into viable nauplii and these were not vigorous swimmers (Barnes & Barnes, 1974). When eggs were exposed to salinities of 50‰, and 60‰ at an early developmental stage, viable larvae were not produced and, again, only a small proportion (7% and 1%, respectively) of eggs exposed at a later developmental stage produced nauplii; these were deformed and probably non-viable.  There was no development at 70‰ (Barnes & Barnes, 1974).

Sensitivity assessment.  Some increases in salinity may be tolerated by the characterizing species. However, the biotope is considered to be sensitive to a persistent increase in salinity to >40ppt (based on species distribution, Barnes & Barnes, 1974). Therefore, resistance is assessed as ‘Low’ and recovery as ‘High’ (following the restoration of usual salinity). Sensitivity is therefore 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

This biotope is recorded in full salinity (30-35 ppt) (JNCC, 2022). A change from full to reduced salinity (18-30 ppt) is assessed at the pressure benchmark. The characterizing species are found in a similar biotope, CR.MCR.EcCr.UrtScr., which is present in reduced salinities (JNCC, 2022). It is therefore likely that the characterizing species will tolerate a reduction in salinity from full to reduced.

Balanus crenatus occurs in estuarine areas and is therefore adapted to variable salinity (Davenport, 1976). When subjected to sudden changes in salinity Balanus crenatus closes its opercular valves so that the blood is maintained temporarily at a constant osmotic concentration (Davenport, 1976).  Acclimation to different salinity regimes alters the point at which opercular closure and resumption of activity occur (Davenport, 1976). Balanus crenatus can tolerate salinities down to 14 psu if given time to acclimate (Foster, 1970).  At salinities below 6 psu motor activity ceases, respiration falls and the animal falls into a "salt sleep" (Barnes & Barnes, 1974).  In this state the animals may survive in freshwater for three weeks, enabling them to withstand changes in salinity over moderately long periods (Barnes & Powell, 1953). Larvae are more sensitive than adults. In culture experiments, eggs maintained below 10‰ rupture, due to osmotic stress (Barnes & Barnes, 1974).  At 15-17‰  there was either no development of early stages, or the nauplii larvae are deformed and “probably not viable” (Barnes & Barnes, 1974) Similarly at 20‰ development occurs, but about half of the larvae are deformed and not viable. (Barnes & Barnes, 1974). Normal development resulting in viable larvae occurred between salinities of 25 and 40‰ (Barnes & Barnes, 1974).

Spirobranchus triqueter has not been recorded from brackish or estuarine waters.  Therefore, it is likely that the species will be very intolerant of a decrease in salinity.  However, Dixon (1985, cited in Riley & Ballerstedt, 2005), views the species as able to withstand significant reductions in salinity. The degree of reduction in salinity and time that the species could tolerate those levels were not recorded.  Therefore, there is insufficient information available to assess the intolerance of Spirobrannchus triqueter to a reduction in salinity and the assessment is based on its presence in the biotope CR.MCR.EcCr.UrtScr in reduced and full salinity habitats (JNCC, 2022). 

Ryland (1970) stated that, with a few exceptions, bryozoans were fairly stenohaline and restricted to full salinity (ca 35 psu) and noted that reduced salinities result in an impoverished bryozoan fauna.

Sensitivity assessment.  As the characterizing species are found in biotopes in both full and reduced salinity habitats, the biotope is considered ‘Not sensitive’ to a decrease in salinity from full to reduced. Therefore, resistance is assessed as ‘High’ and resilience is assessed as ‘High’ (by default) and the biotope is assessed as ‘Not sensitive’. Some losses of sensitive species such as Electra pilosa and other bryozoans may occur but over the course of a year, this is not considered to significantly alter the biotope assemblage from the description.

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

This biotope occurs across a range of flow speeds, from strong to moderately strong (0.5-1.5 m/s) (JNCC, 2022). The suspension feeders within the biotope benefit from high water flow supplying food and the mobility of the sediment that prevents other species from colonizing and out-competing the characterizing species present. Scour from sediment mobility is a key factor in structuring this biotope (JNCC, 2022); changes in flow exceeding the pressure benchmark may increase or decrease the movement of cobbles, pebbles and slates resulting in indirect changes in the character of the biotope.

Spirobranchus triqueter is found in biotopes exposed to flow speeds varying from very weak to moderately strong (negligible - >1.5m/s) and was considered ‘Not sensitive’ at the pressure benchmark (Tillin & Tyler-Walters, 2014).  Balanus crenatus is found in a very wide range of water flows (Tillin & Tyler-Walters, 2014), although it usually occurs in sites sheltered from wave action (Eckman & Duggins, 1993) and can adapt feeding behaviour according to flow rates. In the absence of any current, the barnacle rhythmically beats its cirri to create a current to collect zooplankton. Growth of Balanus crenatus (measured as an increase in basal area), maintained for 69 days at constant flow speeds in laboratory experiments was greatest at intermediate flow speeds (0.08 m/s) and decreased at higher speeds (Eckman & Duggins, 1993). Over the entire range of flow speeds measured (0.02 m/s – 0.25 m/s), Balanus crenatus, was able to control the cirrus, with little or no deformation by flow observed (Eckman, & Duggins, 1993). The bryozoans characterizing this biotope are securely attached and as these are flat they are subject to little or no drag compared to upright growth forms of algae. 

Sensitivity assessment.  The biotope is recorded in strong to moderately strong (0.5-1.5 m/s) water flow (JNCC, 2022).  Therefore a change in the water flow of 0.1 to 0.2 m/s (the benchmark) is unlikely to be significant. Therefore, the resistance of the biotope to changes in water flow is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is 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

Not relevant to subtidal biotopes. NB. 100% mortality could be expected in adult Spirobranchus triqueter after 24.1 h and 35.4 h when exposed to air at 7 °C and 13 °C respectively (Campbell & Kelly, 2002).

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

This biotope is recorded from locations that range from very exposed to moderately exposed (JNCC, 2022).  

Balanus crenatus and Spirobranchus triqueter and other characterizing species are firmly attached to the substratum and are unlikely to be dislodged by an increase in wave action at the pressure benchmark. Balanus crenatus and Spirobranchus triqueter are found in biotopes from a range of wave exposures from extremely sheltered to very exposed and were therefore considered ‘Not sensitive’ to this pressure (at the pressure benchmark), by a previous review (Tillin & Tyler-Walters, 2014). 

Sensitivity assessment. This biotope (SS.SCS.CCS.SpiB) is structured by the scour and physical disturbance caused by wave action. Therefore, a reduction in wave action would stabilise the substratum and result in the loss of this biotope and a change to another, probably to SS.SMx.CMx.FluHyd or more stable biotope (JNCC, 2022).  However, a 3-5% change in significant wave height (the benchmark) is probably insignificant considering the level of wave exposure that characterizes this biotope.  Therefore, resistance is assessed as 'High' at the benchmark level. Hence, resilience is assessed as ‘High’, by default, and the biotope is considered ‘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

Barnacles accumulate heavy metals and store them as insoluble granules as a detoxification pathway (Rainbow, 1987). Pyefinch & Mott (1948) recorded a median lethal concentration (LC50) of 0.19 mg/l copper and 1.35 mg/l mercury, for Balanus crenatus over 24 hours. Barnacles may tolerate a fairly high level of heavy metals in nature. For example, they are found in Dulas Bay, Anglesey, where copper reaches concentrations of 24.5 µg/l due to acid mine waste (Foster et al., 1978).

Bryozoans are common members of fouling communities and amongst those organisms most resistant to antifouling measures, such as copper-containing anti-fouling paints. Bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1979; Holt et al., 1995).

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 any evidence is presented where available

No information is available on the intolerance of Balanus crenatus to hydrocarbons. However, other littoral barnacles generally have a high tolerance to oil (Holt et al., 1995) and were little impacted by the Torrey Canyon oil spill (Smith, 1968) so Balanus crenatus is probably fairly resistant to oil.

Where exposed to direct contact with fresh hydrocarbons, encrusting coralline algae appear to have a high intolerance. Crump et al. (1999) described 'dramatic and extensive bleaching' of 'Lithothamnia' following the Sea Empress oil spill. Observations following the Don Marika oil spill (K. Hiscock, pers. comm.) were of rockpools with completely bleached coralline algae. However, Chamberlain (1996) observed that although Lithophyllum incrustans was affected in a short period of time 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.

But bryozoans may be highly intolerant of the effects of oil spills and possibly hydrocarbons).

Little information on the effects of hydrocarbons on bryozoans was found. Ryland & de Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. or other sessile fauna in Milford Haven or St. Catherine's Island, south Pembrokeshire. Houghton et al. (1996) reported a reduction in the abundance of intertidal encrusting bryozoa (no species given) at oiled sites after the Exxon Valdez oil spill. Soule and Soule (1979) reported that the encrusting bryozoan Membranipora villosa was not found in the impacted area for 7 months after the December 1976 Bunker C oil spill in Los Angeles Harbour. Additionally, Soule and Soule (1979) reported that Bugula neritina was lost from breakwater rocks in the vicinity (in December 1979) of the Bunker C oil spill and had not recovered within a year. However, Bugula neritina had returned to a nearby area within 5 months (May 1977) even though the area was still affected by sheens of oil. Furthermore, only three of eight recorded species two weeks after the incident were present in April within the affected breakwater area. By June all the species had been replaced by dense growths of the erect bryozoan Scrupocellaria diegensis.

Mohammad (1974) reported that Bugula spp. and Membranipora spp. were excluded from settlement panels near a Kuwait Oil terminal subject to minor but frequent oil spills. Encrusting bryozoans are also probably intolerant of the smothering effects of acute hydrocarbon contamination and pollution, resulting in suffocation of colonies and communities may be lost or damaged. Circalittoral communities are likely to be protected from the direct effects of oil spills by their depth. However, the biotope may be exposed to emulsified oil treated with dispersants, especially in areas of turbulence, or may be exposed to water soluble fractions of oils, PAHs or oil adsorbed onto particulates (Tyler-Walters, 2002).

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.

Barnacles have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved (Holt et al., 1995). They are less intolerant than some species (e.g. Patella vulgata) to dispersants (Southward & Southward, 1978) and Balanus crenatus was the dominant species on pier pilings at a site subject to urban sewage pollution (Jakola & Gulliksen, 1987). Hoare & Hiscock (1974) found that Balanus crenatus survived near to an acidified halogenated effluent discharge where many other species were killed, suggesting a high tolerance to chemical contamination. Little information is available on the impact of endocrine disrupters on adult barnacles. Holt et al. (1995) concluded that barnacles are fairly sensitive to chemical pollution.

Cole et al. (1999) suggested that herbicides were (not surprisingly) very toxic to algae and macrophytes. Hoare & Hiscock (1974) noted that with the exception of Phyllophora species, all red algae including encrusting coralline forms were excluded from the vicinity of an acidified halogenated effluent discharge in Amlwch Bay, Anglesey. Furthermore, intertidal populations of Corallina officinalis occurred in significant abundance only 600m east of the effluent. 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.

Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in Bryozoans with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100ng/l TBT. Hoare & Hiscock (1974) suggested that the Bryozoa, were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey, e.g. Electra pilosa occurred at low abundance on laminarian holdfasts within the bay, compared to sites outside the affected area. Rees et al. (2001) reported that the abundance of epifauna (including bryozoans) had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that bryozoans may be at least inhibited by the presence of TBT.

Most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods isopods, mysids, shrimp and crabs) and fish (Cole et al., 1999).

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

Specific information concerning oxygen consumption and reduced oxygen tolerances were not found for the key characterizing species within the biotope. This pressure is not assessed for the biotope due to a lack of evidence. It should be noted that Balanus crenatus respires anaerobically so it can withstand some decrease in oxygen levels. When placed in wet nitrogen, where oxygen stress is maximal and desiccation stress is minimal, Balanus crenatus has a mean survival time of 3.2 days (Barnes et al., 1963) and this species is considered to be ‘Not sensitive’ to this pressure. 

Nutrient enrichment

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

Evidence

Nutrient enrichment at the pressure benchmark is unlikely to affect the fauna within this biotope. In general, at the pressure benchmark, the characterizing invertebrates are unlikely to be affected by changes in plant nutrient levels. A slight increase in nutrient levels could be beneficial for barnacles and other suspension feeders by promoting growth of phytoplankton and therefore increasing food supplies. Balanus crenatus was the dominant species on pier pilings, which were subject to urban pollution (Jakola & Gulliksen, 1987).  If nutrient enrichment promoted the growth of green algae, they would be removed by the scour that characterizes this biotope. Similarly, the high-energy environment (wave exposure and tidal stream) would probably flush nutrients through the system quickly. 

Sensitivity assessment. The biotope probably has a ‘High’ resistance and ‘High' resilience (by default) to nutrient enrichment at the benchmark level and is judged to be ‘Not sensitive’, albeit with 'Low' confidence due to the lack of direct evidence. 

Organic enrichment

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

Evidence

As the biotope occurs in tide swept or wave exposed areas (Connor et al., 2004; JNCC, 2022), water movements will disperse organic matter reducing the level of exposure. The animals found within the biotope may be able to utilise the input of organic matter as food, or are likely to be tolerant of inputs at the benchmark level. In a recent review, assigning species to ecological groups based on tolerances to organic pollution (the AMBI index), the characterizing animal species; Balanus crenatus and Spirobranchus triqueter and bryozoans (Electra pilosa) were assigned to AMBI Group II described as 'species indifferent to enrichment, always present in low densities with non-significant variations with time, from initial state, to slight unbalance' (Gittenberger & Van Loon, 2011). 

Sensitivity assessment. It is not clear whether the pressure benchmark would lead to enrichment effects in this dynamic habitat.  High water movements would disperse organic matter particles, mitigating the effect of this pressure. Based on the AMBI categorisation (Borja et al., 2000, Gittenberger & Van Loon, 2011), the characterizing species are assessed as ‘Not Sensitive’ to this pressure.

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

This biotope is characterized by the hard rock substratum to which the characterizing and associated species can firmly attach to (Connor et al., 2004; JNCC, 2022). Changes to a sedimentary habitat or an artificial substratum would significantly alter the character of the biotope through the loss of habitat.

Tillin & Tyler-Walters (2014) used records from the MNCR database as a proxy indicator of the resistance to physical change by Balanus crenatus and Spirobranchus triqueter. These species were reported from a variety of substratum types including fine (muddy sand, sandy mud and fine sands) and coarse sediments, where some hard surfaces (such as pebbles or shells) are present for the attached species. Balanus crenatus and Spirobranchus triqueter are fouling organisms and occur on a wide variety of substrata (Harms & Anger, 1983; Andersson et al., 2009). As well as artificial and natural hard substrata Balanus crenatus and Spirobranchus triqueter also encrust a range of invertebrates. For example, Spirobranchus triqueter has been recorded on the hermit crab, Pagurus bernhardus (Fernandez-Leborans & Gabilondo, 2006) among other species.  Similarly, Balanus crenatus has been reported to encrust empty shells of the invasive non-indigenous species Ensis americanus (Donovan, 2011) and Carcinus maenas (Heath, 1976).

Sensitivity assessment It should be noted that the basis of the sensitivity assessment for this pressure is the sensitivity of the biotope to changes in substratum type, rather than the sensitivity of the species. A permanent change in substratum type to artificial or sedimentary would lead to re-classification of the biotope. Biotope resistance to this pressure is therefore assessed as ‘None’, as the change at the benchmark is permanent, and resilience is assessed as ‘Very low’. Sensitivity, based on combined resistance and resilience is therefore 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

This biotope is characterized by the hard substratum provided by the pebbles and cobbles to which the key characterizing species barnacles, tube worms and encrusting corallines can firmly attach to (Connor et al., 2004; JNCC, 2022). A change to a mobile gravel or soft sedimentary substratum would significantly alter the character of the biotope. The biotope is considered to have 'None' resistance to this pressure based on a change to a soft sediment substratum.  Recovery of the biological assemblage (following habitat restoration) is considered to be 'High'. However, the pressure benchmark is considered to refer to a permanent change and recovery is therefore ‘Very low’.  Sensitivity is therefore assessed as 'High'.

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

The species characterizing this biotope are epifauna occurring on the cobbles and pebbles that characterize this biotope (Connor et al., 2004, JNCC, 2022).  Removal of the substratum would remove both the habitat (cobbles and pebbles) and the characterizing, attached species.  In areas where large amounts of gravel have been extracted, Balanus crenatus has been observed to rapidly recolonize within months (Kenny & Rees, 1996).

Sensitivity assessment. Biotope resistance is assessed as ‘None’ (in the extraction footprint), and resilience (following habitat restoration, or where the underlying substratum remains the same) is assessed as ‘High’. Sensitivity is therefore assessed as ‘Medium’. Recovery will be prolonged (and sensitivity greater) where all the habitat is removed and restoration (artificial or natural)  to the previous state does not occur.

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

The species characterizing this biotope occur on the rock surface and therefore have no protection from surface abrasion. High levels of abrasion from scouring by mobile sands and gravels is an important structuring factor in this biotope (Connor et al., 2004, JNCC, 2022) and prevents replacement by less scour-tolerant species, such as red algae. Where individuals are attached to mobile pebbles, cobbles and boulders rather than bedrock, surfaces can be displaced and turned over leading to the smothering of attached algae and animals or at least reducing photosynthesis, respiration, feeding efficiency and fertilization of gametes in the water column.

Hiscock (1983) noted that a community, under conditions of scour and abrasion from stones and boulders moved by storms, developed into a community consisting of fast-growing species such as Spirobranchus (as Pomatoceros) triqueter.  Off Chesil Bank, the epifaunal community dominated by Spirobranchus (as Pomatoceros) triqueter, Balanus crenatus decreased in cover in October as it was scoured away in winter storms, but recolonized in May to June (Gorzula, 1977). Warner (1985) reported that the community did not contain any persistent individuals but that recruitment was sufficiently predictable to result in dynamic stability and a similar community, dominated by Spirobranchus (as Pomatoceros triqueter), Balanus crenatus and Electra pilosa, (an encrusting bryozoan), was present in 1979, 1980 and 1983 (Riley and Ballerstedt, 2005). 

Re-sampling of fishing grounds that were historically studied (from the 1930s) indicated that some encrusting species including serpulid worms and several species of barnacles had decreased in abundance in gravel substrata subject to long-term scallop fishing (Bradshaw et al., 2002). These may have been adversely affected by the disturbance of the stones and dead shells to which they attach (Bradshaw et al. 2002). Where individuals are attached to mobile pebbles, cobbles and boulders rather than bedrock, surfaces can be displaced and turned over; preventing feeding and leading to smothering.  This observation is supported by experimental trawling, carried out in shallow, wave-disturbed areas using a toothed, clam dredge, which found that Spirobranchus spp. decreased in intensively dredged areas over the monitoring period (Constantino et al., 2009).  In contrast, a study of Spirobranchus spp. aggregations found that the tube heads formed were not significantly affected by biannual beam trawling in the eastern Irish Sea (Kaiser et al., 1999).  No changes in the number or size of serpulid tube heads were apparent throughout the course of the study, and no significant changes were detectable in the composition of the tube head fauna that could be attributed to fishing disturbance (Kaiser et al., 1999). Subsequent laboratory experiments on collected tube heads found that these were unlikely to resettle on the seabed in an orientation similar to that prior to disturbance (Kaiser et al., 1999).  This may lead to the death of the resident serpulids and sessile associated fauna.

Sensitivity assessment. The impact of surface abrasion will depend on the footprint, duration and magnitude of the pressure. High levels of abrasion from scouring by mobile cobbles and pebbles is an important structuring factor in this biotope (Connor et al., 2004; JNCC, 2022) but the persistence of the assemblage may depend on rapid recovery rather than high resistance (Gorzula, 1977). Evidence for the effects of severe scour and trawling on Balanus crenatus and Spirobranchus triqueter, suggest that resistance, to a single abrasion event is ‘Low’ and resilience is ‘High’, so sensitivity is assessed as ‘Low’. 

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

This biotope is characterized by mobile pebbles and cobbles, pressures that lead to penetration and disturbance could damage associated species through abrasion and by overturning surfaces could result in the smothering of attached algae and animals or reductions in photosynthesis, respiration, feeding efficiency or fertilization of gametes in the water column. The biotope is, however, likely to be exposed to at least seasonal movement of substrata and this movement and scour maintains this biotope by preventing species that require more stable habitats from colonizing and developing stable populations (Connor et al., 2004; JNCC, 2022). Evidence presented above for surface abrasion is considered equally relevant to this pressure as abrasion in this biotope is likely to lead to movement and displacement of mobile substrata.

Sensitivity assessment. The impact of pressures that disturb and penetrate the mobile substrata will depend on the footprint, duration and magnitude of the pressure. High levels of abrasion from scouring by mobile cobbles and pebbles is an important structuring factor in this biotope but the persistence of the assemblage may depend on rapid recovery rather than high resistance (Gorzula, 1977). Evidence for the effects of severe scour and trawling on Balanus crenatus and Spirobranchus triqueter, suggest that resistance, to a single abrasion event is ‘Low’ and resilience is ‘High’, so sensitivity is assessed as ‘Low’.

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

This biotope occurs in scoured habitats and it is likely, depending on local sediment supply, that the biotope is exposed to chronic or intermittent episodes of high-levels of suspended solids as local sediments are re-mobilised and transported. A significant increase in suspended solids may result in smothering (see siltation pressures) where these are deposited. Based on Cole et al. (1999) and Devlin et al. (2008) this biotope is considered to experience intermediate turbidity (10-100 mg/l) based on UK TAG (2014). An increase at the pressure benchmark refers to a change to medium turbidity (100-300 mg/l) and a decrease is assessed as a change to clear (<10 mg/l) based on UK TAG (2014).

An increase in turbidity could be beneficial if the suspended particles are composed of organic matter, however high levels of suspended solids with increased inorganic particles may reduce filter-feeding efficiencies. A reduction in suspended solids will reduce food availability for filter-feeding species in the biotope (where the solids are organic), although the effects are not likely to be lethal over the course of a year. A reduction in light penetration could also reduce the growth rate of phytoplankton and so limit zooplankton levels and food supply to filter feeders such as Balanus crenatus. However, light penetration itself is unlikely to be an important factor as both Balanus crenatus and Spirobranchus triqueter are recorded from the lower eulittoral or the lower circalittoral. The biotope occurs in shallow waters where light attenuation due to increases in turbidity is probably low and the characterizing animals are unlikely to be affected by increased or decreased clarity. Red algae and encrusting coralline algae especially, are known to be shade tolerant and are common components of the understorey on seaweed-dominated shores. Therefore, an increase or decrease in light intensity is unlikely to adversely affect the crustose corallines as plants can acclimate to different light levels.

Available evidence indicates that Spirobranchus triqueter is tolerant of a wide range of suspended sediment concentrations (Riley and Ballerstedt, 2005).  Stubbings and Houghton (1964) recorded Spirobranchus (as Pomatoceros) triqueter in Chichester harbour, which is a muddy environment.  However, Spirobranchus (as Pomatoceros) triqueter has been noted to also occur in areas where there is little or no silt present (Price et al., 1980). Encrusting bryozoans may be more intolerant, although Electra pilosa is relatively tolerant of suspended sediment, for example, Moore (1973c; 1977a) regarded Electra pilosa to be ubiquitous with respect to turbidity in subtidal kelp holdfasts in northeast England

Barnes & Bagenal (1951) found that the growth rate of Balanus crenatus epizoic on Nephrops norvegicus was considerably slower than animals on raft-exposed panels. This was attributed to reduced currents and increased silt loading of water in the immediate vicinity of Nephrops norvegicus. In dredge disposal areas in the Weser estuary, Germany, where turbidity is 35% above the natural rate of 10-100 mg/l, the abundance of Balanus crenatus was lower than in reference areas (Witt et al., 2004). Separating the effect of increased suspended solids from increased sedimentation and changes in sediment from sediment dumping in this example is, however, problematic (Witt et al., 2004). Balanids may stop filtration after silt layers of a few millimetres have been discharged (Witt et al., 2004), as the feeding apparatus is very close to the sediment surface.

Sensitivity assessment. Overall biotope resistance is assessed as ‘High’ to an increase or decrease in suspended solids. Resilience is categorised as ‘High’ (by default). The biotope is considered to be ’Not sensitive’ to decreased suspended solids.

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

As small, sessile species that are attached to the substratum, siltation at the pressure benchmark would bury Balanus crenatus and Spirobranchus triqueter and encrusting bryozoans and corallines.  Holme and Wilson (1985) described a Pomatoceros-Balanus assemblage on ‘hard surfaces subjected to periodic severe scour and ‘deep submergence by sand or gravel’ in the English Channel. They inferred that the Pomatoceros-Balanus assemblage was restricted to fast-growing settlers that were able to establish in short periods of stability during summer months, as all fauna were removed in the winter months (Holme & Wilson, 1985). Barnacles may stop filtration after silt layers of a few millimetres have been discharged as the feeding apparatus is very close to the sediment surface (Witt et al., 2004). In dredge disposal areas in the Weser estuary, Germany, where the modelled exposure to sedimentation was 10 mm for 25 days, with the centre of the disposal ground exposed to 65 mm for several hours before dispersal, Balanus crenatus declined in abundance compared to reference areas (Witt et al., 2004). However, separating the effect of sedimentation from increased suspended solids and changes in sediment as a result of sediment dumping was problematic (Witt et al., 2004).

Sensitivity assessment. Based on the presence of the characterizing and associated species in biotopes subject to sedimentation and scour (such as CR.MCR.EcCr.UrtScr), biotope resistance to this pressure, at the benchmark, is assessed as 'High', resilience is assessed as 'High' (by default) and the biotope is considered to be 'Not sensitive'. The assessment considers that sediments are rapidly removed from the biotope and that the scour tolerance of the characterizing animal species and encrusting corallines would prevent significant mortalities, although some damage and abrasion may occur. However, if the deposit remained in place; i.e. due to the scale of the pressure or where biotopes were sheltered, or only seasonally subject to water movements or where water flows and wave action were reduced e.g. by the presence of tidal barrages, then resistance would be lower and sensitivity would be greater.

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

The characterizing species occur in biotopes subject to sedimentation and scour (such as CR.MCR.EcCr.UrtScr) and are therefore likely to tolerate intermittent episodes of sediment movement and deposition.  At the pressure benchmark ‘heavy deposition’ represents a considerable thickness of deposit and complete burial of the characterizing species would occur. Removal of the sediments by wave action and tidal currents would result in considerable scour. The effect of this pressure will be mediated by the length of exposure to the deposit and the nature of the deposit. 

As small, sessile species attached to the substratum, siltation at the pressure benchmark would bury Balanus crenatus and Spirobranchus triqueter.  Holme and Wilson (1985) described a Pomatoceros-Balanus assemblage on ‘hard surfaces subjected to periodic severe scour and ‘deep submergence by sand or gravel’ in the English Channel. They inferred that the Pomatoceros-Balanus assemblage was restricted to fast-growing settlers able to establish themselves in short periods of stability during the summer months (Holme and Wilson 1985), as all fauna were removed in the winter months. Barnacles may stop filtration after silt layers of a few millimetres have been discharged as the feeding apparatus is very close to the sediment surface (Witt et al., 2004). In dredge disposal areas in the Weser estuary, Germany, where the modelled exposure to sedimentation was 10 mm for 25 days, with the centre of the disposal ground exposed to 65 mm for several hours before dispersal, Balanus crenatus declined in abundance compared to reference areas (Witt et al., 2004). However, separating the effect of sedimentation from increased suspended solids and changes in sediment from sediment dumping was problematic (Witt et al., 2004).

Sensitivity assessment. Resistance is assessed as ‘Medium’ as the biotope is exposed to frequent abrasion and scouring (the impact may therefore be mitigated by rapid removal of the deposit) but some of the characterizing species may die. Resilience is assessed as ‘High’ based on re-growth from the scour tolerant, surviving bases of the encrusting corallines and larval recolonization by Balanus crenatus and Spirobranchus triqueter. Biotope sensitivity is therefore 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

Spirobranchus triqueter and Balanus crenatus are also found in a variety of light environments from shallow sublittoral biotopes where light levels are relatively high, to deeper sites that are aphotic (De Kluijver, 1993).  Balanus crenatus possesses a rudimentary eye and can detect and respond to sudden shading which may be an anti-predator defence (Forbes et al., 1971). Balanus crenatus tend to orient themselves when settling, with the least light sensitive area directed towards the light (Forbes et al., 1971). So that the more sensitive area can detect shading from predator movements in the area where light availability is lower (Forbes et al., 1971).

Sensitivity assessment. As the key characterizing species colonize a broad range of light environments, from intertidal to deeper subtidal and shaded understorey habitats; the biotope is considered to have ‘High’ resistance and, by default, ‘High’ resilience and therefore is ‘Not sensitive’ to this pressure.

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

Barriers that reduce the degree of tidal excursion may alter larval supply to suitable habitats from source populations. Conversely, the presence of barriers may enhance local population supply by preventing the loss of larvae from enclosed habitats.  No direct evidence was found to assess this pressure. As the larvae of Balanus crenatus and Spirobranchus triqueter are planktonic and are transported by water movements, barriers that reduce the degree of tidal excursion may alter larval supply to suitable habitats from source populations. However, the presence of barriers may enhance local population supply by preventing the loss of larvae from enclosed habitats.  As both species are widely distributed and have larvae capable of long-distance transport, resistance to this pressure is assessed as 'High' and resilience as 'High' by default. This biotope is therefore considered to be 'Not sensitive'. 

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

Visual disturbance

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

Evidence

Balanus crenatus possesses a rudimentary eye and can detect and respond to sudden shading which may be an anti-predator defence (Forbes et al., 1971). However, this species and others within the biotope are not considered sensitive to visual disturbance from passing ships or other disturbances and this pressure is considered to be '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

Key characterizing species within this biotope are not cultivated or translocated. This pressure is therefore considered ‘Not relevant’ to this biotope group.

Introduction or spread of invasive non-indigenous species

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

Evidence

The high levels of scour in this biotope will limit the establishment of all but the most scour-resistant invasive non-indigenous species (INIS) from this biotope and no direct evidence was found for the effects of INIS on this biotope.  Increased warming has allowed the Australian barnacle Austrominius (formerly, Elminius) modestus, to dominate sites previously occupied by Semibalanus balanoides and Balanus crenatus (Witte et al, 2010). However, on test settlement panels deployed in southwest Ireland, Austrominius modestus initially dominated panels in the lower subtidal. Yet post-recruitment mortality observed over a year allowed Balanus crenatus to become the dominant barnacle (Watson et al., 2005). Balanus crenatus and Austrominus modestus recruit at different times of the year in some sites and this alters seasonal dominance patterns (Witte et al., 2010).  Crepidula fornicata was reported from 'rough sediments' subject to strong tidal streams in the English Channel (Hinz et al., 2011). However, this biotope is characterized by unstable, mobile, cobbles and pebbles whose scour due to wave action and in winter storms removes most of the resident fauna on a seasonal basis, so Crepidula is unlikely to become established. 

Sensitivity assessment. As scouring of this biotope by mobile coarse sediments limits the establishment of all but robust species, resistance to INIS is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is considered to be ‘Not sensitive’.

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

No evidence was found that microbial pathogens cause high levels of disease or mortality in this biotope.

The commensal ciliate Trichodina pediculus was observed in "fair numbers" moving over the branchial crown of Spirobranchus (syn. Pomatoceros) triqueter (Thomas, 1940). Parasites found in the worm include gregarines and ciliated protozoa, as well as parasites that had the appearance of sporozoan cysts. However, no information was found about the effects of these parasites on Spirobranchus triqueter.

Removal of target species

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

Evidence

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of target species on this biotope. No commercial application or harvesting of characterizing or associated species was described in the literature, this pressure is therefore considered to be 'Not relevant'.

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 removal of the key characterizing species would alter the character of the biotope, resulting in reclassification and the loss of species richness. The ecological services such as primary and secondary production, provided by species, would also be lost.

Sensitivity assessment.  Removal of a large percentage of the characterizing species resulting in bare rock would alter the character of the biotope, species richness and ecosystem function. Resistance is therefore assessed as ‘Low’ and recovery as ‘High’, so biotope sensitivity is assessed as 'Low’.