The Marine Life Information Network

Temperature increase (local)

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

Evidence

Flustra foliacea is perennial Stebbing (1971a) and widespread throughout the British Isles (NBN, 2015). It is distributed across north-west Europe (Fish& Fish, 1996). Stebbing (1971a) noted that Flustra foliacea on the Gower peninsula, South Wales, had an annual growth season between March and November. Flustra foliacea found in the German North Sea (Helgoländer Steingrund) experience seasonal water temperatures from 2 to 19°C (Becker et al., 2020).

Polyclinum aurantium is distributed across the British Isles and from Norway to the Mediterranean (Picton & Morrow, 2004b).

Clavelina lepadiformis is native to the Mediterranean Sea, Atlantic subarctic regions (i.e., Norwegian Sea and Greenland), and the Bay of Biscay, and has successfully spread to the coastline of South Africa, the coasts of North and Central America, and Brazil (Van Name, 1945, Turon et al., 2003, Pyo & Shin, 2011, and Reinhardt et al., 2010 cited in Hiebert et al., 2022). Reproduction of Clavelina lepadiformis is temperature-dependent (Berrill, 1975; Millar, 1970). A change in temperature could affect the time and duration of spawning, however, the distribution of Clavelina lepadiformis extends to the north and south of the British Isles from Norway to the Adriatic (Hayward & Ryland, 1996).

In lab experiments involving Clavelina lepadiformis collected from the harbour of La Spezia, Italy, Hiebert et al. (2022) noted that the production of their dormant form, the ‘winter buds’, occurred at 18°C. Adult zooids of Clavelina lepadiformis were also exposed to abrupt shifts in temperature (−20°C to 37°C) and salinity (10 to 44 ppt) for 24 hours, then transferred to 24°C, and the viability of zooids and the germination of dormant forms were observed after one week. Clavelina lepadiformis zooids were more resistant to lower temperatures (between 0 and 28°C) and higher salinity (30 to 45 ppt), and winter buds resisted a wider range of conditions compared to zooids (temperatures between 0 and 32°C and salinity between 15 and 45 ppt) (Hiebert et al., 2022). Clavelina lepadiformis have also been found to exist in the absence of zooids, and winter buds without zooids have been documented in the North Sea in the winter months (Orton, 1914 and Orton, 1921 and cited in Hiebert et al., 2022), in the summer months in the eastern Mediterranean, and Clavelina regress during summer months (called aestivation) in the western Mediterranean (Caralt et al., 2002; Mukai, 1977 cited in Hiebert et al., 2022). Hiebert et al. (2022) concludes that their results show that the dormant stages of Clavelina lepadiformis is resistant to extremes in temperature and salinity; thus, this linked capacity for dormancy and regeneration is likely an important adaptation that allows these species to survive drastic seasonal changes.

Casso et al. (2018) observed Clavelina lepadiformis growing in Fangar Bay, at the northern side of the Ebro Delta in the NE Iberian Coast and noted how Clavelina lepadiformis showed a slightly higher occurrence during colder seasons but was present throughout the year. Picton & Morrow (2004c) reported regression of Atlantic colonies of Clavelina lepadiformis in winter, with re-growth occurring in spring, with reproduction occurring in winter and spring (Caralt et al., 2002).

Stachowicz et al. (2002) and Dean and Hurd (1980), cited in Uzun (2019), showed a positive relationship between growth rate and seawater temperature for ascidians.

Sensitivity assessment

All characterizing species are not at their southerly limit in the British Isles. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is ‘Not sensitive’ at the benchmark level.

Temperature decrease (local)

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

Evidence

Flustra foliacea is perennial (Stebbing, 1971a) and widespread throughout the British Isles (NBN, 2015). It is distributed across north-west Europe (Fish& Fish, 1996). Stebbing (1974) noted that Flustra foliacea on the Gower peninsula, South Wales, had an annual growth season between March and November, with a dormant winter period, when no growth occurred. Growth resumed in spring, leading to a line forming across the fronds, which can be used to age specimens (Stebbing, 1971a). Flustra foliacea found in the German North Sea (Helgoländer Steingrund) experience seasonal water temperatures from 2 to 19°C (Becker et al., 2020).

Polyclinum aurantium regresses into an overwintering phase when it divides, reproducing asexually before beginning to feed again in the spring and is distributed across the British Isles and from Norway to the Mediterranean (Picton & Morrow, 2004). 

Clavelina lepadiformis is native to the Mediterranean Sea, Atlantic subarctic regions (i.e., Norwegian Sea and Greenland), and the Bay of Biscay, and has successfully spread to the coastline of South Africa, the coasts of North and Central America, and Brazil (Van Name, 1945, Turon et al., 2003, Pyo & Shin, 2011, and Reinhardt et al., 2010 cited in Hiebert et al., 2022). Reproduction of Clavelina lepadiformis is temperature-dependent (Berrill, 1975; Millar, 1970). A change in temperature could affect the time and duration of spawning, however, the distribution of Clavelina lepadiformis extends to the north and south of the British Isles from Norway to the Adriatic (Hayward & Ryland, 1996).

Clavelina lepadiformis undergoes stolonic asexual budding. At the end of the sexual breeding season, towards the end of the summer, zooids disappear or are resorbed. Over winter, the colony survives as 'winter buds' from which new zooids develop in spring (Berrill, 1950; Fish & Fish, 1996; Picton, 1997). In the winter months, when the zooids undergo de-differentiation, the resulting cylindrical bodies of many species of Clavelinidae are often found on rocky shores (Millar, 1970). During the severe winter of 1962-63, although no significant mortality of Clavelina lepadiformis was noted, Crisp (1964) found that many compound ascidians were retarded in renewal of the colony after 'winter budding', and some individuals may have been killed.

In lab experiments involving Clavelina lepadiformis collected from the harbour of La Spezia, Italy, Hiebert et al. (2022) noted that the production of their dormant form, the ‘winter buds’, occurred at 18°C. Adult zooids of Clavelina lepadiformis were also exposed to abrupt shifts in temperature (−20°C to 37°C) and salinity (10 to 44 ppt) for 24 hours, then transferred to 24°C, and the viability of zooids and the germination of dormant forms were observed after one week. Clavelina lepadiformis zooids were more resistant to lower temperatures (between 0 and 28°C) and higher salinity (30 to 45 ppt), and winter buds resisted a wider range of conditions compared to zooids (temperatures between 0 and 32°C and salinity between 15 and 45 ppt) (Hiebert et al., 2022). Clavelina lepadiformis have also been found to exist in the absence of zooids, and winter buds without zooids have been documented in the North Sea in the winter months (Orton, 1914 and Orton, 1921 and cited in Hiebert et al., 2022), in the summer months in the eastern Mediterranean, and Clavelina regress during summer months (called aestivation) in the western Mediterranean (Caralt et al., 2002; Mukai, 1977 cited in Hiebert et al., 2022). Hiebert et al. (2022) concludes that their results show that the dormant stages of Clavelina lepadiformis is resistant to extremes in temperature and salinity, thus, this linked capacity for dormancy and regeneration is likely an important adaptation that allows these species to survive drastic seasonal changes.

Casso et al. (2018) observed Clavelina lepadiformis growing in Fangar Bay, at the northern side of the Ebro Delta in the NE Iberian Coast and noted how Clavelina lepadiformis showed a slightly higher occurrence during colder seasons but was present throughout the year. Picton & Morrow (2004c) reported regression of Atlantic colonies of Clavelina lepadiformis in winter, with re-growth occurring in spring, with reproduction occurring in winter and spring (Caralt et al., 2002).

Stachowicz et al. (2002) and Dean and Hurd (1980), cited in Uzun (2019), showed a positive relationship between growth rate and seawater temperature for ascidians.

It should be noted that Crisp’s (1964b) general observations on all marine life stated that damage decreased the deeper the habitat and that the extremely cold temperatures (sea temperatures between 4 and 6°C colder than the five-year mean over a period of two months) is more extreme than the benchmark level for assessment.

Sensitivity assessment

Flustra foliacea is not at its northerly limit in the British Isles, and growth, which is halted in the winter, resumes in spring (Stebbing, 1971a). The characterizing species have mechanisms in place to cope with cold conditions, and no evidence of mortality of the characteristic species due to low temperature in the British Isles was found. Resistance is therefore assessed as ‘High’, resilience as ‘High’, and the biotope is ‘Not Sensitive’ at the benchmark level.

Salinity increase (local)

Benchmark. An increase in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition).

Evidence

This biotope occurs in full salinity, and an increase in salinity would result in hypersaline conditions. 

Flustra foliacea found in the German North Sea (Helgoländer Steingrund) experiences salinities ranging from 30 to 33 psu, which is influenced by the fluctuating estuarine water inflow from the river Elbe (Becker et al., 2020). Soule & Soule (1979) cite Hastings (1927), who described the presence of five bryozoans in the Suez Canal at salinities of up to 49‰. No other evidence for bryozoans, including Flustra foliacea, in hypersaline conditions was found.

In lab experiments involving Clavelina lepadiformis collected from the harbour of La Spezia, Italy, Hiebert et al. (2022) noted that the production of their dormant form, the ‘winter buds’, occurred at 18°C. Adult zooids of Clavelina lepadiformis were also exposed to abrupt shifts in temperature (−20°C to 37°C) and salinity (10 to 44 ppt) for 24 hours, then transferred to 24°C, and the viability of zooids and the germination of dormant forms were observed after one week. Clavelina lepadiformis zooids were more resistant to lower temperatures (between 0 and 28°C), and higher salinity (30 to 45 ppt), and winter buds resisted a wider range of conditions compared to zooids (temperatures between 0 and 32°C and salinity between 15 and 45 ppt) (Hiebert et al., 2022). Clavelina lepadiformis have also been found to exist in the absence of zooids, and winter buds without zooids have been documented in the North Sea in the winter months (Orton, 1914 and Orton, 1921 and cited in Hiebert et al., 2022), in the summer months in the eastern Mediterranean, and Clavelina regress during summer months (called aestivation) in the western Mediterranean (Caralt et al., 2002; Mukai, 1977 cited in Hiebert et al., 2022). Hiebert et al. (2022) concludes that their results show that the dormant stages of Clavelina lepadiformis is resistant to extremes in temperature and salinity, thus, this linked capacity for dormancy and regeneration is likely an important adaptation that allows these species to survive drastic seasonal changes.

Sensitivity assessment

CR.HCR.XFa.FluCoAs and associated biotopes occur in the circalittoral and are recorded at full salinity. No evidence was found to assess the effects of hypersaline conditions on Flustra foliacea or other characterizing species.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition detail).

Evidence

Ryland (1970) reported that, with a few exceptions, the Gymnolaemata were stenohaline and restricted to full salinity (30 to 35 ppt), noting that reduced salinities resulted in an impoverished bryozoan fauna.

Flustra foliacea appears to be restricted to areas with high salinity (Tyler-Walters & Ballerstedt 2007; Budd 2008). Flustra foliacea found in the German North Sea (Helgoländer Steingrund) experiences salinities ranging from 30 to 33 psu, which is influenced by the fluctuating estuarine water inflow from the river Elbe (Becker et al., 2020). Dyrynda (1994) noted that Flustra foliacea and Alcyonidium diaphanum were probably restricted to the vicinity of the Poole Harbour entrance by their intolerance to reduced salinity. Although protected from extreme changes in salinity due to their subtidal habitat, severe hyposaline conditions could adversely affect Flustra foliacea colonies.

In lab experiments involving Clavelina lepadiformis collected from the harbour of La Spezia, Italy, Hiebert et al. (2022) noted that the production of their dormant form, the ‘winter buds’, occurred at 18°C. Adult zooids of Clavelina lepadiformis were also exposed to abrupt shifts in temperature (−20°C to 37°C) and salinity (10 to 44 ppt) for 24 hours, then transferred to 24°C, and the viability of zooids and the germination of dormant forms were observed after one week. Clavelina lepadiformis zooids were more resistant to lower temperatures (between 0 and 28°C) and higher salinity (30 to 45 ppt), and winter buds resisted a wider range of conditions compared to zooids (temperatures between 0 and 32°C and salinity between 15 and 45 ppt) (Hiebert et al., 2022). Clavelina lepadiformis have also been found to exist in the absence of zooids, and winter buds without zooids have been documented in the North Sea in the winter months (Orton, 1914 and Orton, 1921 and cited in Hiebert et al., 2022), in the summer months in the eastern Mediterranean, and Clavelina regress during summer months (called aestivation) in the western Mediterranean (Caralt et al., 2002; Mukai, 1977 cited in Hiebert et al., 2022). Hiebert et al. (2022) concluded that the dormant stages of Clavelina lepadiformis were resistant to extremes in temperature and salinity. Thus, this linked capacity for dormancy and regeneration is likely an important adaptation that allows these species to survive drastic seasonal changes.

Sensitivity assessment

Although protected from extreme changes in salinity due to their subtidal habitat, distribution suggests that hyposaline conditions would adversely affect the abundance of Flustra foliacea and the associated characteristic bryozoan turf. Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’, and sensitivity as ‘Medium’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s and 0.2 m/s for more than one year (Water flow pressure definition). 

Evidence

Water flow has been shown to be important for the development of bryozoan communities and the provision of suitable hard substrata for colonization (Eggleston, 1972b; Ryland, 1976). In addition, areas subject to high mass transport of water, such as the Menai Strait and tidal rapids, generally support large numbers of bryozoan species (Moore, 1977a). While the pumping activity of the bryozoan lophophores provides the greatest proportion of the colonies' food requirements (McKinney, 1986; Hayward & Ryland, 1998), the current generated is probably very localized and the colonies are likely to be dependent on water currents for food supply. A significant decrease in water flow is likely to result in a decrease in the abundance of bryozoans  

Flustra foliacea colonies are flexible, robust and reach high abundances in areas subject to strong currents and tidal streams (Stebbing, 1971; Eggleston, 1972; Knight-Jones & Nelson-Smith, 1977; Hiscock, 1983, 1985; Holme & Wilson, 1985). Dyrynda (1994) suggested that mature fronded colonies do not occur on unstable substrata due to the drag caused by their fronds, resulting in rafting of colonies on shells or the rolling of pebbles and cobbles, resulting in destruction of the colony. Dyrynda (1994) reported that the distribution of Flustra foliacea in the current swept entrance to Poole Harbour was restricted to circalittoral boulders, on which it dominated as nearly mono-specific stands. Flustra foliacea found in the German North Sea (Helgoländer Steingrund between 9 and 18 m deep) experiences tidal currents with a mean velocity of 0.6 knots, which is strong enough to saturate bottom waters with 8 to 10 mg/l of oxygen (Kühne and Rachor, 1996 cited in Becker et al., 2020)..

Clavelina lepadiformis thrives in areas where there is little water movement (Hiscock & Hoare, 1975; De Caralt et al., 2002). Naranjo et al. (1996) found that the species was dominant in a low rate of water renewal, excess silting and high suspended solid concentrations. High water flow rates may be detrimental to feeding ability and posture but are unlikely to cause detachment.Clavelina lepadiformis was observed growing in the shallow coastal lagoon of Ria de Aveiro, Portugal, where it experiences tidal current velocities of 1 m/s, which progressively get weaker in the many innermost lagoon canals (Martins et al., 2010 cited in Marques et al., 2022).

Sensitivity assessment

The CR.HCR.XFa.FluCoAs biotope complex occurs high energy environments in wave exposed conditions and a range of water flow conditions, from weak (<1 knots; <0.5 m/sec) to strong (3 to 6 knots; 1.5-3 m/sec), although most records occur in moderately strong 1 to 3 knots (0.5-1.5 m/sec.) flow (Connor et al., 2004; JNCC, 2022). Flustra foliacea has been reported in areas subject to high water flow, with greater abundance in stronger water flow (Stebbing, 1971a). A significant decrease in water flow would likely result in a reduction in the abundance of Flustra foliacea, although areas of weak flow probably occur in areas of high wave exposure. Therefore, a change in water flow at the benchmark level (0.1 to 0.2 m/s) is unlikely to be significant. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is ‘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. (Emergence regime change pressure definition).

Evidence

Water flow has been shown to be important for the development of bryozoan communities and the provision of suitable hard substrata for colonization (Eggleston, 1972b; Ryland, 1976). In addition, areas subject to high mass transport of water such as the Menai Strait and tidal rapids generally support large numbers of bryozoan species (Moore, 1977a). Although active suspension feeders, their feeding currents are probably fairly localized and they are dependent on water flow to bring adequate food supplies within reach (McKinney, 1986). A substantial decrease in water flow will probably result in impaired growth due to a reduction in food availability, and an increased risk of siltation (Tyler-Walters, 2005).

Okamura (1984) reported that an increase in water flow from slow flow (1-2cm/s) to fast flow (10-12cm/s) reduced feeding efficiency in small colonies but not in large colonies of Bugula stolonifera. 

Flustra foliacea colonies are flexible, robust and reach high abundances in areas subject to strong currents and tidal streams (Stebbing, 1971a; Eggleston, 1972b; Knight-Jones & Nelson-Smith, 1977; Hiscock, 1983, 1985; Holme & Wilson, 1985). Dyrynda (1994) suggested that mature fronded colonies do not occur on unstable substrata due to the drag caused by their fronds, resulting in rafting of colonies on shells or the rolling of pebbles and cobbles, resulting in destruction of the colony. Dyrynda (1994) reported that the distribution of Flustra foliacea in the current swept entrance to Poole Harbour was restricted to circalittoral boulders, on which it dominated as nearly mono-specific stands. While the pumping activity of the lophophores provides the greatest proportion of the colonies' food requirements (Hayward & Ryland, 1998), the current generated is probably localized and the colonies are likely to be dependent on water currents for food supply.  A significant decrease in water flow is likely to result in a decrease in the abundance of bryozoans.

Flustra foliacea abundance is lower in weak currents (Stebbing, 1971a). While the pumping activity of the lophophores provides the greatest proportion of the colonies' food requirements (Hayward & Ryland, 1998), the current generated is probably very localized and the colonies are dependent on water currents to carry food particles to them.  Increased competition and sedimentation could also affect the community.  Riisgard et al. (1993) discussed the low energy cost of filtration for sponges and concluded that passive current-induced filtration may be insignificant for sponges. Pumping and filtering occur in choanocyte cells that generate water currents in sponges using flagella (De Vos et al., 1991). 

Clavelina lepadiformis thrives in areas where there is little water movement (Hiscock & Hoare, 1975; De Caralt et al., 2002). Naranjo et al. (1996) found that the species was dominant in a low rate of water renewal, excess silting and high suspended solid concentrations. High water flow rates may be detrimental to feeding ability and posture but are unlikely to cause detachment.

Sensitivity assessment

The CR.HCR.XFa.FluCoAs biotope complex occurs in a range of water flow conditions, from weak (>1kn) to strong (3-6kn) (Connor et al., 2004).  Flustra foliacea has been reported in areas subject to high water flow, with greater abundance in stronger water flow (Stebbing, 1971a).  Significant decrease in water flow would likely result in a reduction in the abundance of Flustra foliacea, but changes at the benchmark level are unlikely to result in mortality.  Resistance is therefore assessed as ‘High’, resilience as ‘High’, and the biotope is ‘Not sensitive’ at the benchmark level.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year (Wave action pressure definition). 

Evidence

Flustra foliacea occurs from very wave-exposed to sheltered waters, although probably limited to deeper waters in very wave-exposed conditions (Tyler-Walters & Ballerstedt, 2007). The oscillatory water flow generated by wave action may be more damaging than constant strong currents, e.g. strong wave action may generate an oscillatory flow of 2 m/sec at 20 m (Hiscock, 1983, 1985). Flustra foliacea is a common member of the flotsam, having been removed from its substratum by storms. Whilst the biotope is circalittoral, a severe increase in wave exposure (e.g. storms) could affect bryozoan colonies, especially on mobile substrata such as cobbles and pebbles. Cocito et al. (1998) described a severe winter storm of 1993 that had devastating effects on a Flustra foliacea population, sweeping away most of the colonies down to 11 m. Dead colonies of Flustra foliacea are commonly found washed up, having been removed from their substratum by storms (Hayward & Ryland, 1995a). Whilst the biotope is circalittoral, a severe increase in wave exposure (e.g. storms) could affect bryozoan colonies.

Clavelina lepadiformis is tolerant of a wide range of exposure but is most abundant in moderately exposed sites (Picton, 1997).

Sensitivity assessment

The CR.HCR.XFa.FluCoAs biotope complex occurs in high energy environments in range of water flow conditions, from weak (<1 knots; <0.5 m/sec) to strong (3 to 6 knots; 1.5 to 3 m/sec), although most records occur in moderately strong 1 to 3 knots (0.5 to 1.5 m/sec) flow and from moderately wave exposed to extremely exposed conditions (Connor et al., 2004; JNCC, 2022). An increase in wave exposure to extremely exposed is within the normal range of the biotope. The storm damage that significantly affected Flustra foliacea populations reported by Cocito et al. (1998b) was described as an exceptionally severe, once in 100-year event, probably in excess of the benchmark. Flustra foliacea is a regular part of the flotsam, washed up after stormy weather, although loss of a few individuals may be minor and part of the natural dynamics of the populations, especially on unstable substrata (e.g. boulders and cobbles), in shallow examples of the biotope. Hence, an increase in wave exposure is unlikely to be significant, except on unstable substrata. However, a decrease in wave exposure from moderately exposed to sheltered would probably result in a change in those examples of the biotope, especially where tidal streams were weak. The dominant species, Flustra foliacea, and several of the characteristic species (e.g. Clavelina, Alcyonium digitatum) are long-lived, so a reduction in wave exposure for a year (the benchmark) may result in a minor change in abundance of the shorter-lived species while the biotope remains recognisable. Therefore, resistance is assessed as ‘Medium’, with resilience as ‘High’, and sensitivity as ‘Low’ at the benchmark level.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant Transitional metal or organometal (e.g. TBT) contaminants via uncontrolled releases or incidental spills (Transitional metals and organometals pressure definition).

Evidence

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

Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1979; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoa with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100ng/l TBT. Rees et al. (2001) reported that the abundance of epifauna (including bryozoans) had increased in the Crouch estuary in the 5 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. Bryozoans were shown to bioaccumulate heavy metals to a certain extent (Holt et al., 1995). For example, Bowerbankia gracialis and Nolella pusilla accumulated Cd, exhibiting sublethal effects (reduced sexual reproduction and inhibited resting spore formation) between 10-100 µg Cd /l and fatality above 500 µg Cd/l (Kayser, 1990).

De Caralt et al. (2002) reported that Clavelina lepadiformis accumulated copper, lead and vanadium (vanadium is used in ascidian metabolism). A harbour population contained significantly more copper and lead than an open littoral population despite its abundance being an order of magnitude higher in the harbour (De Caralt et al., 2002). Neither reproduction nor growth were affected in this harbour population compared with a population outside of the harbour.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant hydrocarbon or polyaromatic hydrocarbon (PAH) contaminants via uncontrolled releases or incidental spills (Hydrocarbon & PAH pressure definition).

Evidence

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

CR.HCR.XFa.FluCoAs is a sub-tidal biotope complex (Connor et al., 2004). Oil pollution is mainly a surface phenomenon its impact upon circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sub-littoral habitats (Castège et al., 2014).

Filter feeders are highly sensitive to oil pollution, particularly those inhabiting the tidal zones which experience high exposure and show correspondingly high mortality, as are bottom dwelling organisms in areas where oil components are deposited by sedimentation (Zahn et al., 1981). There is little information on the effects of hydrocarbons on bryozoans. Ryland & De Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. in Milford Haven or St. Catherine's Island, south Pembrokeshire, although it did alter the breeding period. Banks & Brown (2002) found that exposure to crude oil significantly impacted recruitment in the bryozoan Membranipora savartii.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more synthetic compound contaminants via uncontrolled releases or incidental spills (Synthetic compound contamination pressure definition).

Evidence

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

Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1979; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoa with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100ng/l TBT. Rees et al. (2001) reported that the abundance of epifauna (including bryozoans) had increased in the Crouch estuary in the 5 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. Hoare & Hiscock (1974) suggested that polyzoa (bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Flustra foliacea did not occur within 165m of the effluent source.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels (Radionuclides contamination pressure definition).

Evidence

No evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant "other" substances (solid, liquid or gas) contaminants via uncontrolled releases or incidental spills (Introduction of other substances pressure definition). 

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status) (deoxygenation pressure definition).

Evidence

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates this concentration is about 2 ml/l (ca 2.66 mg/l) (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l.

Little information on the effects of oxygenation on bryozoans was found. Sagasti et al. (2000) reported that epifaunal communities, including dominant species such as bryozoans (Membranipora tenuis and Conopeum tenuissimum), were unaffected by periods of moderate hypoxia (ca 0.35 to 1.4 ml/l) and short periods of anoxia (<0.35 ml/l) in the York River, Chesapeake Bay, although bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation. An anoxic event in the northern Adriatic (1989–1990) exterminated the Pentapora fascialis population (McKinney & Jaklin, 2000; Hayward and McKinney, 2002). Colonies of Pentapora fascialis established after that anoxic event exceeded 15 cm in diameter by the summer of 1998 (Hayward & McKinney, 2002).

Sensitivity assessment

There is evidence that bryozoans may tolerate short periods of hypoxia, although moderate to long-term events or anoxia is likely to cause significant mortality. Also, the sand-scoured nature of the biotope would likely result in occasional burial, and an event at the benchmark level would likely result in quite significant mortality. No evidence of deoxygenation on the colonial ascidians could be found. Therefore, resistance at the benchmark level is assessed as ‘Medium’, resilience as ‘High’, and sensitivity as ‘Low’, albeit with ‘Low’ confidence due to the lack of direct evidence.

Nutrient enrichment

Benchmark. Increased levels of the elements nitrogen, phosphorus, silicon, and iron in the marine environment compared to background concentrations (Nutrient enrichment pressure definition).

Evidence

Hartikainen et al. (2009) reported that increased nutrient concentrations resulted in freshwater bryozoans achieving higher biomass. O’Dea & Okamura (2000) found that the annual growth of Flustra foliacea in western Europe has substantially increased since 1970. They suggested that this was due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998).

Clavelina lepadiformis was found to dominate Spanish harbours and nearby zones with highly transformed substrata, low rate of water renewal and excess silting and suspended matter and was described as biofouling and opportunistic (Naranjo et al., 1996).

Sensitivity assessment

Limited evidence on the effects of nutrient enrichment on the characteristic species was found. The evidence suggests that the characteristic species vary in their response to nutrients. However, the evidence is ‘Insufficient’ to form the basis of an assessment.

Organic enrichment

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

Evidence

O’Dea & Okamura (2000) found that annual growth of Flustra foliacea in western Europe has substantially increased since 1970.  They suggested that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998).  

Clavelina lepadiformis was found to dominate Spanish harbours and nearby zones with highly transformed substrata, low rate of water renewal, excess silting and suspended matter.  The species was described as biofouling and opportunistic (Naranjo et al., 1996).

Sensitivity assessment

Therefore, the important characteristic species could probably resist organic enrichment at the benchmark level. Resistance to this pressure is assessed as 'High', but with Low confidence, and resilience as 'High'. This biotope is therefore considered to be 'Not sensitive'. 

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site (Physical loss pressure definition). 

Evidence

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

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata, or vice versa (Physical change in subtratum type pressure definition).

Evidence

Becker et al. (2020) stated that Flustra foliacea dominated the surface structure of bottom waters (meaning coverage of nearly 100%), being significantly positively correlated with hard ground (25%) and shell <2 mm more than 0.05 weight% (Becker et al., 2020); in the same region, Michaelis et al. (2019) noted how Flustra foliacea largely colonized boulder-sized stones. Furthermore, Flustra foliacea found in the Loch Linnhe Artificial Reef complex, on the west coast of Scotland, had higher rates of productivity (2.4 times higher) growing on ‘complex’ reef structures compared to ‘simple’ ones, which had a smaller surface area. (Rouse, Porter & Wilding, 2020). Productivity rates were highest on external areas of reefs and decreased by 1.56% per cm distance into the reef on complex reefs and 2.93% per cm into the reef on simple block reefs (Rouse, Porter & Wilding, 2020). The differences in productivity rates between reefs constructed from simple and complex blocks were assumed to reflect different current regimes and food supply between the external and internal reef areas, according to reef type (Rouse, Porter & Wilding, 2020). More complex habitats could encourage the growth and recovery of Flustra foliacea.

Clavelina lepadiformis was observed growing in the shallow coastal lagoon of Ria de Aveiro, Portugal, on oyster shells and was the dominant species present (Marques et al., 2022), as well as being abundant on PVC plates in Fangar Bay, at the northern side of the Ebro Delta in the NE Iberian Coast (Casso et al., 2018).

Sensitivity assessment

This biotope, and child biotopes, are characterized by the hard substratum provided by the rock, boulders, cobbles, or pebbles to which the key characterizing species can firmly attach (Connor et al., 2004; JNCC, 2022). A change to a mobile gravel or soft sedimentary substratum would result in loss of the biotope. Hence, resistance is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification) (Physical change in sediment type pressure definition). 

Evidence

‘Not relevant’ to biotopes occurring on bedrock.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

The species characterizing this biotope are epifauna occurring on the more stable cobbles or boulders present in CR.HCR.XFa.FluCoAs.X (Connor et al., 2004). Extraction of 30 cm of substrata (the benchmark level) could result in removal of all epifauna and resistance is therfore assessed as 'None'. Assuming recovery of the habitat, resilience is assessed as 'Medium' and sensitivity as 'Medium'.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat) (Surface abrasion/disturbance pressure definition).

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 are an important structuring factor in this biotope (Connor et al., 2004) and may prevent succession. 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.

Clavelina lepadiformis is permanently attached to the substratum and is unable to move out of the way of abrasive objects. The body of the species is soft and delicate, so abrasion is likely to cause physical damage and possibly death.

Flustra foliacea is tolerant of sediment abrasion (Stebbing, 1971a; Knight-Jones & Nelson-Smith, 1977; Holme & Wilson, 1985), and although Flustra foliacea is flexible, physical disturbance by a passing scallop dredge is likely to damage fronds and remove some colonies. Colonies on hard substrata are probably less vulnerable to fishing activity but would probably be damaged or partially removed. Colonies of Flustra foliacea are capable of regenerating areas of the frond which have been removed by grazing, which can result in new branches (Stebbing, 1971a). Silén (1981) reported that Flustra foliacea could repair physical damage (a notch) to its fronds within five to ten days and regenerated at ca four to five zooid lengths per month. As long as the holdfast remains intact, Flustra foliacea would survive and grow back.

Staniforth et al. (2023) studied the decadal-scale temporal change in epibenthic megafaunal assemblages on scallop fishing grounds in the Bay of Fundy. Previous data from 1967 to 1997 showed that the epibenthic assemblage in the Digby scallop ground had become more homogeneous over the 30-year interval, with attached, fragile, epifaunal taxa declining, whereas scavengers and robust burrowing filter-feeders increased in frequency of occurrence (Staniforth et al., 2023). Staniforth et al. (2023) resampled the Digby ground along with the commercial scallop grounds around Grand Manan Island and on Lurcher Shoal in 2007 and 2008 to determine whether the changes off Digby had continued and if they also occurred in other parts of the Bay. They observed significant spatial differences among the three scallop grounds, including the leafy bryozoan, Flustra foliacea, which was expanding its local range through time and was the leading contributor to the spatial dissimilarity in the Bay of Fundy (Staniforth et al., 2023). For example, Flustra foliacea was the greatest contributor to the dissimilarity in assemblage composition, which was found at 97% of the stations off Digby but not at all around Grand Manan, nor on Lurcher Shoal in 1997, although it was present at a minority of the stations there ten years later, such as making up to 14% of the observed taxa in 2007 and 2008 at Lurcher Shoal (Staniforth et al., 2023). In addition, significantly more taxa were recorded per station in 1997 than in 2007 and 2008, while assemblage compositions were significantly different between the two periods (Staniforth et al., 2023). Staniforth et al. (2023) concluded that decadal-scale declines in the frequencies of attached, epibenthic filter-feeders were observed, continuing the trend previously reported and consistent with ongoing impacts of bottom-contact fishing gears, despite decades of trawling and dredging. Therefore, although fishing in the Bay of Fundy has negatively impacted some epibenthic taxa, Flustra foliacea seemed to benefit from the regular abrasion occurring in the area.

Physical disturbance by fishing gear has been shown to adversely affect sessile benthic and emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas and increase when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018; Long et al., 2021; Langton, Stirling & Boulcott, 2023). Also, heavy mobile gears could also result in the movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998). Picton & Goodwin (2007) noted that an area of boulders with a rich fauna of sponges and hydroids on the east coast of Rathlin Island, Northern Ireland, was significantly altered since the 1980s. Scallop dredging had begun in 1989, and boulders were observed to have been turned and the gravel harrowed. In addition, many of the boulders had disappeared, and rare hydroid communities were greatly reduced (Picton & Goodwin, 2007). Prior records also indicated the presence of large sponges, mainly Axinella infundibuliformis (Picton & Goodwin, 2007).

Sensitivity assessment

Given the sessile, erect nature of the sponges, ascidians, and bryozoans, damage and mortality following a physical disturbance event is likely to be significant, however, some studies have brought into question the extent of damage to the faunal turf. Whilst disturbance would damage the sessile Flustra foliacea, the flexibility and ability to regenerate damaged fronds (if the holdfast is undamaged) would allow a significant proportion of the colonies to survive disturbance. Therefore, resistance is assessed as ‘Medium’, resilience as ‘High’, and 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) (Sub-surface penetration pressure definition).

Evidence

CR.HCR.XFa.FluCoAs.X is characterized by the epifauna growing on more stable boulders and cobbles which overlie coarse muddy sand and gravel. Whilst the direct effects to the characterizing species is likely to be similar to the surface abrasion pressure assessed above, penetration events could result in greater movement of the boulders and cobbles.  This could lead to an increase in crushing incidents, and resistance is therefore assessed as 'Low', resilience as 'Medium' and sensitivity as 'Medium'.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale, e.g. from clear to intermediate for one year (Suspended sediment pressure definition).

Evidence

Bryozoans are suspension feeders that may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus. However, Tyler-Walters & Ballerstedt (2007) reported Flustra foliacea as tolerant to suspended sediment based on its occurrence in areas of high suspended sediment, e.g. abundant in turbid, fast-flowing waters of the Menai Straits (Moore 1977). Communities dominated by Flustra foliacea were described on tide-swept seabed, exposed to high levels of suspended sediment and sediment scour in the English Channel, subject to sediment transport (mainly sand) and periodic, temporary submergence by thin layers of sand (ca <5 cm) (Holme & Wilson 1985). Flustra foliacea is also characteristic of sediment-scoured, silty rock communities CR.HCR.XFa.FluCoAs and CR.MCR.EcCr.UrtScr (Connor et al., 2004), and is also found at greater depths, where organic particulates (detritus) are probably more important.

Increased siltation can cause clogging of ascidian respiratory organs (Bakus, 1968). Clavelina lepadiformis has relatively wide apertures that help prevent clogging from particles (Naranjo et al., 1996). The simplistic structure of its branchial sac (Fiala-Medioni, 1978) may be less efficient in expelling particles and more likely to suffer from clogging of feeding apparatus than other forms of sea squirts, such as Ciona intestinalis. Clavelina lepadiformis was found to dominate Spanish harbours and nearby zones with a low rate of water renewal, excess silting and suspended matter and the species was described as biofouling and opportunistic (Naranjo et al., 1996).

Sensitivity assessment

Sediment scour within CR.HCR.XFa.FluCoAs and associated biotopes is an important factor in the dominance of the scour-tolerant Flustra foliacea (Connor et al., 2004). Whilst an increase is unlikely to have an effect, a reduction in suspended sediment could reduce scour and allow other species to colonize the biotope. On return to the original sediment levels, it is probable that Flustra foliacea would again dominate the biotope. Resistance is assessed as ‘High’, resilience as ‘High’, and the biotope is ‘Not Sensitive’ at the benchmark level.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Smothering by 5 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005). Communities dominated by Flustra foliacea were described on tide-swept seabed, exposed to high levels of suspended sediment and sediment scour in the English Channel, subject to sediment transport (mainly sand) and periodic, temporary submergence by thin layers of sand (ca <5 cm) (Holme & Wilson 1985).

Clavelina lepadiformis reaches up to 2 cm in height and often colonizes vertical surfaces and overhangs (Fish & Fish, 1992). Smothering by a 5 cm depth of sediment would completely cover most of the population, with only those colonizing overhangs and vertical surfaces protected. The complete disappearance of the sea squirt Ascidiella aspera biocoenosis in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev, 2008 cited in Tillin & Tyler-Walters, 2014).

Sensitivity assessment

A deposit of 5 cm of fine sediment could smother and damage many of the smaller individuals of the faunal community. For example, Flustra foliacea is probably resistant while Clavelina lepadiformis is probably not resistant. However, in the high-energy environment where the biotope occurs, deposited sediment would probably be removed quickly. Therefore, resistance is ‘Medium’, resilience is ‘High’, and the sensitivity is ‘Low’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Smothering by 30 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c). While studying coralligenous assemblages of the Apulian continental shelf in the Mediterranean Sea, Piazzi et al. (2019) found that sedimentation was higher on deep outcrops and suggested that it was the main driver of differences between shallow and deep assemblages.

Holme & Wilson (1985) examined the bottom fauna in a tide-swept region of the central English Channel. Flustra foliacea-dominated communities were reported to form in areas subject to sediment transport (mainly sand) and periodic, temporary submergence by thin layers of sand (ca <5 cm). If inundated by 30 cm of sediment, respiration and larval settlement are likely to be blocked until the deposited sediment is removed.  

Clavelina lepadiformis reaches up to 2 cm in height and often colonizes vertical surfaces and overhangs (Fish & Fish, 1992). Smothering by 30 cm of sediment would completely cover most of the population, with only those colonizing overhangs and vertical surfaces protected. The complete disappearance of the sea squirt Ascidiella aspera biocoenosis in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev, 2008 cited in Tillin & Tyler-Walters, 2014). 

It should also be noted that all the characterizing species are likely to be buried by 30 cm of sediment deposition.

Sensitivity assessment

A deposit of 30 cm of fine sediment would smother and damage most of the faunal community. In the high-energy environment in which the biotope occurs, deposited sediment would probably be removed quickly and mitigate the impacts. Resistance is therefore assessed as ‘Medium’, resilience as ‘Medium’ and sensitivity as ‘Medium’. 

Litter

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

Evidence

All characterizing species for this biotope are sessile epifauna, being either encrusting or branching. Physical disturbance by fishing gear has been shown to adversely affect sessile benthic and emergent epifaunal communities; bryozoan and sponge matrices are reported to be greatly reduced in fished areas and increased when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018).

There are no records of ghost fishing affecting the characterizing species for this biotope. However, epifaunal communities are vulnerable to damage from fishing gear and are likely vulnerable to being dislodged or damaged through lost fishing gear, and possibly certain types of marine litter. At present, there is 'Insufficient evidence' to complete a sensitivity assessment on the effect of litter on this biotope.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT (Electromagnetic pressure definition).

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms. 

No studies have examined the effect of EMFs on the characterizing species. However, one study was performed on the reef-forming annelid, Ficopomatus enigmaticus (Oliva et al., 2023). Sperm cells from this species were exposed to 0.5 and 1.0 mT of a static magnetic field. After only three hours of exposure, sperm fertilization rate was reduced, and significant increases in DNA damage and mitochondrial activity, indicative of a stress response, were reported. However, there is ‘Insufficient evidence’ on which to base an assessment of the likely sensitivity of this biotope to EMFs.

Underwater noise changes

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

Evidence

Stanley et al. (2014) studied the effects of vessel noise on fouling communities and found that the bryozoans Bugula neritina, Watersipora arcuate and Watersipora subtorquata responded positively.  More than twice as many bryozoans settled and established on surfaces with vessel noise (128 dB in the 30–10,000 Hz range) compared to those in silent conditions.  Growth was also significantly higher in bryozoans exposed to noise, with a 20% higher growth rate in encrusting and a 35% higher growth rate in branching species.  No evidence could be found for the effects of noise on sponges, but they are unlikely to be sensitive.

Sensitivity assessment

Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is therefore considered to be 'Not sensitive' at the benchmark level.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means (Introduced light or shade pressure definition).

Evidence

Flustra foliacea larvae are positively phototactic on release, swimming for only short periods (Hayward & Ryland, 1998). However, at the depths where Flustra foliacea can occur, light may not be important, especially in circalittoral habitats.

Since 2016, research on artificial light at night (ALAN) has expanded considerably in the marine and coastal environment. Light was previously assumed to be of low ecological significance in subtidal and intertidal habitats, but there is now evidence that ALAN is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50m (Davies et al., 2020; Smyth et al., 2021). ALAN can alter biological processes across taxa and at multiple levels of organisation. Documented responses include disruption of diel and circalunar rhythms, changes in activity and foraging, altered predator–prey interactions, shifts in community composition, and impacts on algal growth and phenology (Davies et al., 2014, 2015b; Gaston et al., 2017; Tidau et al., 2021; Lynn et al., 2022; Marangoni et al., 2022; Miller & Rice, 2023; Ferretti et al., 2025). Evidence for benthic habitats and assemblages specifically is beginning to emerge (e.g. Trethewy et al., 2023; Schaefer et al., 2025), but remains limited and fragmented, often focusing on single taxa or short-term experiments. Mortality thresholds, long-term consequences, and responses at the biotope scale are rarely addressed, and there are major gaps around indirect effects such as trophic cascades or habitat modification.

Sensitivity assessment

These Flustra foliacea-dominated biotopes occur in the circalittoral where light is excluded due to the turbidity (from silt and suspended sediment) rather than depth alone. Hence, an increase in shading would probably benefit the community, and an increase in light may not penetrate. However, given the rapid expansion of the evidence base on the effect of artificial light and the continuing lack of data at the level of individual biotopes, resistance and resilience cannot be robustly assessed. Therefore, sensitivity is recorded as ‘Insufficient evidence’.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion (Barrier to species movement pressure definition).

Evidence

Barriers and changes in tidal excursion are 'Not relevant' to biotopes restricted to open waters.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure (Death for collision pressure definition).

Evidence

Not relevant to seabed habitats. NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature (Visual disturbance pressure definition). 

Evidence

Not relevant

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species may result in changes in the genetic structure of local populations, hybridization, or a change in community structure (Translocation pressure definition).

Evidence

There is currently ‘No evidence’ on which to assess this pressure.

Introduction of microbial pathogens

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

Evidence

Pukall et al. (2001) studied the microbial community associated with Flustra foliacea and reported colonization of surfaces by bacteria typical of the marine environment and which may have been transferred into this environment from terrestrial sites.  No information on diseases was found. Stebbing (1971a) reported that encrusting epizoites reduced the growth rate of Flustra foliacea by ca 50%.  The bryozoan Bugula flabellata produces stolons that grow in and through the zooids of Flustra foliacea, causing "irreversible degeneration of the enclosed polypide" (Stebbing, 1971b).

Gochfeld et al. (2012) found that diseased sponges hosted significantly different bacterial assemblages compared to healthy sponges, with diseased sponges also exhibiting significant decline in sponge mass and protein content.  Sponge disease epidemics can have serious long-term effects on sponge populations, especially in long-lived, slow-growing species (Webster, 2007).  Numerous sponge populations have been brought to the brink of extinction including cases in the Caribbean with 70-95% disappearance of sponge specimens (Galstoff, 1942) and the Mediterranean (Vacelet,1994; Gaino et al.,1992).  Decaying patches and white bacterial film were reported in Haliclona oculata and Halichondria panicea in North Wales, 1988-89 (Webster, 2007).  Specimens of Cliona spp. exhibited blackened damage since 2013 in Skomer. Preliminary results have shown that clean, fouled and blackened Cliona all have very different bacterial communities. The blackened Cliona were effectively dead and had a bacterial community similar to marine sediments. The fouled Cliona had a very distinct bacterial community that may suggest a specific pathogen caused the effect (Burton, pers comm; Preston & Burton, 2015). 

There appears to be little research into ascidian diseases particularly in the Atlantic.  The parasite Lankesteria ascidiae targets the digestive tubes and can cause ‘long faeces syndrome’ in Ciona intestinalis (although it has also been recorded in other species).  Mortality occurs in severely affected individuals within about a week following first symptoms. (Mita et al., 2012). Ooishi (2010) reported the copepod Enterocola hessei parasitizing Clavelina lepadiformis, however no evidence for microbial infection was found.

Sensitivity assessment

Current research on disease indicates that Flustra foliacea and some sponges are susceptible to disease, although the extent and long term implications are still being researched.  There is no evidence to suggest mortality of sponges in the British Isles, although mass mortality and even extinction have been reported further afield.  Resistance has been assessed as ‘Medium' with a resilience of ‘Medium’ and sensitivity is therefore ‘Medium’.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale (targeted removal pressure definition).

Evidence

Flustra foliacea is not presently known to be subject to extraction. However, many bryozoans have been recently found to contain pharmacologically active substances (Hayward & Ryland, 1998; Lysek et al., 2002; Peters et al., 2003). Spongia officinalis (a Mediterranean species) has been targeted as a commercial species for use as bath sponges, although this species does not occur in the British Isles and no record of commercial exploitation of sponges in the British Isles could be found.  Many different bioactive compounds have been found in the Haliclona order, such as lectins, peptides, ketosteroids, and sterol esters (Pajic et al. 2002; Aoki et al. 2003; Santalova et al. 2003; Koopmans & Wijffels, 2008), and it is possible that these sponges may be subject to future harvesting.

Sensitivity assessment

Whilst not presently harvested, it is possible that Flustra foliacea and Haliclonids may be subject to harvesting in the future.  Flustra foliacea and the sponges are sessile epifauna and would therefore have no defence against targeted extraction. Therefore a precautionary resistance of ‘None’ is suggested with Low confidence. Resilience is ‘Medium’ and overall sensitivity is therefore ‘Medium’.

Removal of non-target species

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

Evidence

The characteristic species probably compete for space within the biotope, so that loss of one species would probably have little if any effect on the other members of the community. However, removal of the characteristic epifauna due to by -catch is likely to remove a proportion of the biotope and change the biological character of the biotope.

Whilst disturbance would damage the sessile Flustra foliacea, the flexibility and ability to regenerate damaged fronds (as long as the holdfast is undamaged) would result in survival of a significant proportion of the colonies. Resistance is therefore ‘Medium’, resilience is ‘High’ and sensitivity is ‘Low’.

The American slipper limpet, Crepidula fornicata

Evidence

Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, the shells of other Crepidula, or other species, e.g., oysters and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds at 60 metres in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on a gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders. Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’, it preferred shell, especially that of conspecifics. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1-0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders, so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space, and many are fast-growing and opportunistic, so they may out-compete Crepidula for space even if it gained a foothold in the community. 

Sensitivity assessment

The moderately wave-exposed circalittoral rock characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata. Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011), and has been recorded from the lower intertidal to ca 160 m in depth, but it is most common in the shallow subtidal above 50 m (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020). However, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or circalittoral habitats. At present, there is 'Insufficient evidence' to suggest that the circalittoral biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; further evidence is required.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024). In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Minchin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localized populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks, transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

While natural fragmentation of tendrils is thought to allow Didemnum vexillum to invade longer distances and increase its dispersal potential, Stefaniak & Whitlatch (2014) found that only one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight-hour) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction, and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-mediated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffith et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009). For example, in oyster farms in British Columbia, large fragments of Didemnum sp. come off oyster strings when they are pulled out of water, and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables, and dredging (Holt, 2024). Recent evidence from genetic material suggests that human-mediated dispersal between marinas and shellfish culture sites is the most likely pathway for connectivity of Didemnum vexillum populations throughout Ireland and Britain (Prentice et al., 2021; Holt, 2024). Didemnum vexillum can disperse away from artificial substrata, invading and colonizing natural substrata in surrounding areas (Tillin et al., 2020). Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. The current evidence of Didemnum vexillum’s ability to spread on natural habitats in this area is sparse and often conflicting, complicated by genetics, and its apparent variable habitat preferences and tolerances and its variable ability to adapt to ‘new’ conditions (Holt 2024).

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July), colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September, sometimes continuing into December. Colonies begin to decline in health and ‘die-off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009; Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in Sandwich tide pool (temperature range from -1 °C to 24 °C, with daily fluctuations), probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4 °C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a). Larval release and recruitment typically occur between 14 and 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; McKenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C), and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures, such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020).

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

In contrast, Didemnum vexillum’s preference for sheltered conditions, established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al. (2007b) describe the substratum as immobile, presumably consolidated, gravel, cobbles, and pebbles. Kleeman (2009) stated that the presence of a consistent mild wave action or ‘swash zone’ appears to favour Didemnum sp. establishment in the intertidal. Although some evidence suggests that waves and currents can facilitate the fragmentation and spread of Didemnum vexillum (Mckenzie et al., 2017), the tidal current velocities at some sites where Didemnum vexillum has been reported (for example, New England, where current velocities reach up to around 3 m/s) is lower than the current velocity required for the dislodgement of Didemnum vexillum fragments (around 7.6 m/s) (Reinhardt et al., 2012). This suggests that not all tidal currents are likely to dislodge Didemnum vexillum fragments. When on boat hulls, the species can experience higher current velocities, which are enough to cause dislodgement (Reinhardt et al., 2012).

However, there are few observations of Didemnum vexillum on soft-bottom habitats, as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found established in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; McKenzie et al., 2017; Tillin et al., 2020). For example, at Georges Bank, USA, the Didemnum vexillum mats were limited to gravelly areas and unable to colonize the sand ridges that bounded the site, which have a mobile surface that is moved daily by the strong tidal currents (Valentine et al., 2007b). In addition, the species cannot survive being buried or smothered by coarse or fine-grained sediment. Furthermore, in Holyhead marina, Didemnum vexillum colonies were contained in the harbour and established on artificial pontoons, and they were not present on the natural seabed under the pontoon, which is composed of silty mud or on deeper sections of mooring chains that are immersed in mud at low spring tides (Griffiths et al., 2009).

Sensitivity assessment

Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope, and child biotopes, occurs from bedrock to mixed substrata (boulder, cobble, pebble, and mud), which could provide a suitable hard substratum for colonization by Didemnum sp. Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). This biotope, and child biotopes, experiences very weak to strong water flow (1 to 6 m/s) and moderately exposed to extremely exposed wave exposure. However, the effect of wave action reduces with depth, so it is possible that only the most wave-exposed examples of the biotope could be unsuitable for Didemnum. However, the FluCoAS group of biotopes are exposed to varying amounts of scour (due to nearby patches of sediment) and is dominated by dense Flustra folicaea, colonial ascidians and a variety of other scour/silt-tolerant species. Therefore, Didemnum’s exclusion from areas of mobile sediment and smothering (Valentine et al., 2007b; Griffiths et al., 2009) suggests these biotopes would not be suitable for Didemnum. Thus, for the FluCoAS group of biotopes, a resistance of ‘High’ is suggested. Resilience is assessed as ‘High’ and sensitivity as ‘Not sensitive’, but with 'Low' confidence due to the lack of direct evidence.

The Pacific oyster, Magallana gigas

Evidence

Most of the evidence indicates that circalittoral rock and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). Therefore, this biotope is probably 'Not sensitive to this INIS.

Wireweed, Sargassum muticum

Evidence

The depth and sedimentation exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wakame, Undaria pinnatifida

Evidence

The depth and sedimentation exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria. Therefore, this biotope is probably 'Not sensitive’ to this INIS.

Other INIS

Evidence

This biotope is classified as circalittoral and therefore no algal species have been considered. Several invasive bryozoans are of concern, including Bugula neritina (Porter et al., 2017), Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013b). However, evidence of potential effects is sparse. Limited evidence exists for Bugula neritina in the UK; however, it was found in Northern Ireland for the first time in 2006, then seen again in both 2008 and 2013 in abundance and attached to buoys and kelp stipes throughout Carrickfergus Marina (Porter et al., 2017). Tricellaria inopinata has been reported to colonize the byssal threads of the mussel Mytilus galloprovincialis, Hymeniacidon perleve and the ascidian Styela plicata (Dyrynda et al., 2000). Tricellaria inopinata dominated the fouling community in the Lagoon of Venice, within seven years of being introduced (Ambrogi, 2000). At present, there is 'Insufficient evidence' to suggest that the circalittoral biotopes are sensitive to colonization by algal or other invasive species; further evidence is required.