The Marine Life Information Network

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.

Tubularia indivisa is recorded from the Arctic Ocean to the Mediterranean (WORMS, 2015). Alcyonium digitatum is described as a northern species by Hiscock et al. (2004) and is distributed from Portugal (41°N) to Northern Norway (70°N) (Hartnoll, 1975).  Across this latitudinal gradient,  both species are likely to experience a range of temperatures from approx. 5-18°C (Seatemperature, 2015).  Zintzen et al. (2008a) that the Tubularia indivisa dominated communities on shipwrecks in the North Sea experienced a range of water temperature from 4.2°C in March to 20.3°C in August. The seasonal changes in Tubularia abundance were, however, probably correlated with fluctuations in predation and competition, coupled with lack of recruitment and dormancy in Tubularia indivisa in summer (Zintzen et al., 2008a).

A Metridium senile population at Barnstable, Massachusetts is reported to persist within a temperature range of 1.8 to 21.8°C (Sassaman & Mangum, 1970; Walsh & Somero, 1981). There is evidence to suggest Metridium senile temperature response can vary with latitude and acclimation (Walsh & Somero, 1981; Chomsky et al., 2004). For example, Sassaman & Magnum (1970) demonstrated Metridium senile “maintained” within 10°C prior to experimentation incurred 100% mortality when exposed to 27.5°C for 120 to 240 minutes, whereas Metridium senile “maintained” within 20°C prior to experimentation all survived (Sassaman & Mangum, 1970; Walsh & Somero, 1981). It is, therefore, possible that Metridium senile would be susceptible to acute temperature changes at the pressure benchmark. Actinothoe sphyrodeta has been recorded from Cabo Pino, Mediterranean coast of Spain (Williams, 1997), where sea temperature range from 14 to 24°C (Seatemperature, 2015), and is, therefore, unlikely to be affected at the benchmark level. Urticina felina is distributed from the Arctic Ocean (Ofwegen et al., 2001) to Portugal (Ramos, 2010). Gosse (1860) observed that Urticina felina (as Actinia crassicornis) was "one of the most difficult (anemones) to keep in an aquarium" and that "the heat of the summer is generally fatal to our captive specimens". It is, therefore, likely that local warming may adversely affect individuals, especially in southern examples of this biotope, and some mortality might occur.

CR.HCR.FaT.CTub is recorded from North Scotland to Pembrokeshire, Wales. Sea surface temperature across this distribution ranges from northern to southern Sea Surface Temperature (SST) ranges of 8-16°C in summer and 6-13°C in winter (Beszczynska-Möller & Dye, 2013). In addition, Goodwin et al. (2013) found little evidence to suggest that rising seawater temperatures (ca 1-2°C) had an effect on subtidal benthic assemblages in Northern Ireland between pre-1986 and post-2006 surveys. However, significant effects were noted in rarer species at the edge of the biogeographic ranges (Goodwin et al., 2013).

Sensitivity assessment.  An increase in sea surface temperature of 2°C for a period of one year with high temperatures within the south of the UK may approach the upper temperature threshold of Alcyonium digitatum and Urticina felina, which may, therefore, cause minor declines in abundance in the CTub.Adig, while CTub.CuSp may not be adversely affected. An increase in temperature may affect reproduction and growth in Tubularia indivisa as it becomes dormant in summer months (Zintzen et al., 2008a). Biotopes in the north of the UK are unlikely to be affected at the benchmark level. A short-term increase in temperature may cause mortality of Urticina felina, however, the effect on the rest of the community is not known. Therefore, a precautionary resistance is assessed as ‘Medium’, resilience ‘High’ and sensitivity assessed as ‘Low’.

Temperature decrease (local)

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

Evidence

CR.HCR.FaT.CTub is recorded from North Scotland to Pembrokeshire, Wales. Sea surface temperature across this distribution ranges from Northern to southern Sea Surface Temperature (SST) ranges from 8-16 °C in summer and 6-13 °C in winter (Beszczynska-Möller & Dye, 2013). Alcyonium digitatum is described as a northern species by Hiscock et al. (2004), however, is distributed from Portugal (41°N) to Northern Norway (70°N) (Hartnoll, 1975). Similarly, Tubularia indivisa is recorded from the Arctic Ocean to the Mediterranean (WORMS, 2015). Across this latitudinal gradient, both species are likely to experience a range of temperatures from approx. 5-18°C. Alcyonium digitatum was also reported to be apparently unaffected by the severe winter of 1962-1963 where air temperature reached -5.8°C (Crisp, 1964). Zintzen et al. (2008a) that the Tubularia indivisa dominated communities on shipwrecks in the North Sea experienced a range of water temperature from 4.2°C in March to 20.3°C in August. The seasonal changes in Tubularia abundance were, however, probably correlated with fluctuations in predation and competition, coupled with lack of recruitment and dormancy in Tubularia indivisa in summer (Zintzen et al., 2008a). Palerud et al. (2004) described the presence of the characterizing hydroids Halecium halecinum and Nemertesia sp. in Svalbard, suggesting that these hydroids are probably tolerant of the lowest temperatures they are likely to encounter in Britain and Ireland of ca 4°C (Beszczynska-Möller & Dye, 2013).

The characteristic sponges (Pachymastia johnstonia and Halichondria panacea) are widely distributed across the coasts of the British Isles and are all found from the Channel Isles to Northern Scotland (NBN, 2015).  Some sponges exhibit morphological strategies to cope with winter temperatures e.g.  Halichondria bowerbanki goes into a dormant state below 4°C, characterized by major disintegration and loss of choanocyte chambers with many sponges surviving mild winters in more protected areas from where it can recolonize (Vethaak et al., 1992). Crisp et al. (1964) observed the effects of an unusually cold winter (1962-3) on the marine life in Britain, including Porifera in North Wales.  Whilst difficulty in distinguishing between mortality and delayed development was noted, Crisp et al. found that Pachymastia johnstonia and Halichondria panicea were wholly or partly killed by frost and several species appeared to be missing including Amphilectus fucorum. Barthel (1986) reported that Halichondria panicea in the Kiel Bight degenerated and disintegrated after reproduction before winter, however, young colonies were observed from September.

Metridium senile and Urticina felina are recorded within the Arctic circle (Stephenson, 1935; Walsh & Somero, 1981) and are therefore unlikely to be affected at the benchmark level. Actinothoe sphyrodeta most northerly record is the Shetland Isles (Williams, 1997). Indicating Actinothoe sphyrodeta could be sensitive to a temperature decrease within the UK. However, due to a general lack of information concerning the resilience of Actinothoe sphyrodeta, it has not been included in this assessment.

Sensitivity assessment.  Tubularia indivisa, Alcyonium digitatum, Metridium senile, Urticina felina and other anthozoans are recorded to the north of the British Isles and are considered unlikely to be affected at the benchmark level for either acute or chronic decrease in temperature. However, some of the characteristic sponges may be lost or regress due to a decrease in temperature at the benchmark level. Therefore, a cautious resistance assessment of ‘Medium’ is applied, albeit with a ‘Low’ confidence due to the lack of evidence.  Resilience is probably ‘High’ and sensitivity is assessed as 'Low'.

Salinity increase (local)

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

Evidence

Tubularia indivisa and Alcyonium digitatum are recorded from a few variable salinity (18-40) biotopes but probably at lower abundance than in the full salinity biotopes in which the majority of their records occur. Metridium senile is recorded from the intertidal (Bucklin, 1987), and Shumway (1978) found in low salinity Metridium senile could retract to a ¼ of its body size, which may be a behaviour adaptation to variable salinity. Therefore, Metridium senile may be tolerant of short-term variation in salinity, however at the time of writing there was no available evidence to assess the effect of long-term hypersaline environments on Metridium senile. Although Urticina felina occurs in rockpools where some increases in salinity from evaporation may occur, it is typically found in those on the low shore, where fluctuations in salinity are limited by the short emergence time.

An increase in salinity at the benchmark level would result in a salinity of >40 psu for one year.  Hypersaline water is likely to sink to the seabed and the biotope could be affected by hypersaline effluents (brines).  Ruso et al. (2007) reported that changes in the community structure of soft sediment communities due to desalinisation plant effluent in Alicante, Spain. In particular, in close vicinity to the effluent, where the salinity reached 39 psu, the community of polychaetes, crustaceans and molluscs was lost and replaced by one dominated by nematodes. Roberts et al. (2010b) suggested that hypersaline effluent dispersed quickly but was more of a concern at the seabed and in areas of low energy where widespread alternations in the community of soft sediments were observed. In several studies, echinoderms and ascidians were amongst the most sensitive groups examined (Roberts et al., 2010b).

Sensitivity assessment. The majority of species present are probably sensitive to an increase in salinity of >40 psu, especially if persistent. However, hypersaline effluents are likely to be dispersed quickly in the high energy environments that this biotope and its sub-biotopes occur.  Therefore, resistance is assessed as 'High', resilience as 'High' and the biotope is probably 'Not sensitive' to this pressure. The confidence in the assessment is 'Low' due to the lack of direct evidence.

Salinity decrease (local)

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

Evidence

The effects of decreased salinity on Tubularia indivisa are unclear. Tubularia indivisa is recorded as abundant at a number of locations within the Mersey estuary (Bassindale, 1938). However, the majority of hydroids are subtidal and, although some brackish water species exist (Gili & Hughes, 1995) they are probably intolerant of prolonged decreases in salinity. Alcyonium digitatum does inhabit situations such as the entrances to sea lochs (Connor et al., 2004) or the entrances to estuaries (Braber & Borghouts, 1977) where salinity may vary occasionally. However, its distribution and the depth at which it occurs suggest that Alcyonium digitatum is unlikely to survive significant reductions in salinity. Tubularia indivisa and Alcyonium digitatum are recorded from a few variable salinity (18-40) biotopes but probably at lower abundance than in the full salinity biotopes in which the majority of their records occur.

Metridium senile is predominantly marine, however, does penetrate into estuaries and can be found in the intertidal (Shumway, 1978). Braber & Borghouts (1977) found that Metridium senile occurred in about 10ppt Chlorinity (about 19 psu) in the Delta Region of the Netherlands suggesting that it would be tolerant of reduced salinity conditions. Shumway (1978) found that, during exposure to 50% seawater, animals retracted their tentacles whilst animals exposed to fluctuating salinity, contracted their body wall and produced copious mucus. Therefore, the species seems to have a high tolerance to a reduction in salinity but may have to retract tentacles, suffer reduced opportunity to feed and expend energy to produce mucus.

Zintzen et al. (2008a&b) examined Tubularia spp. dominated epifaunal communities on ship wrecks in the Southern Bight of the North Sea. While Tubularia indivisa dominated offshore and intermediate sites, coastal sites were dominated by Metridium senile and had a lower biomass of turfs of Tubularia spp. The coastal sites were characterized by periodic salinity decreases, large seasonal temperature fluctuations, high total suspended sediment and reduced current velocity when compared to offshore and intermediate sites (Zintzen et al., 2008b). Salinity alone does not explain the differences in abundance but is a factor.

Although Urticina felina is predominantly marine, the species does penetrate into estuaries (e.g. the Thames estuary at Mucking; NMMP, 2001) and the River Blackwater estuary (Davis, 1967). Braber & Borghouts (1977) found that Urticina (as Tealia) felina penetrated to about the 11 ppt Chlorinity (about 20 psu) isohaline at mid-tide during average water discharge in the Westerschelde estuary suggesting that, during high river flow, it would be tolerant of reduced salinity conditions. Intertidal and rock pool individuals will also be subject to variations in salinity because of precipitation on the shore; albeit for short periods on the lower shore.

Sensitivity review. CR.HCR.FaT.CTub is recorded exclusively in full marine conditions (30-40 psu) (Connor et al., 2004). There is also little available evidence which suggests that the characterizing fauna of CR.HCR.FaT.CTub are abundant at sites of reduced salinity (18-40 psu).  Therefore the many of the species that characterize the biotope, and especially the Tubularia turf may not tolerate decreases in salinity at the benchmark level.  The community may begin to change to one typical of variable or reduced salinity conditions. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Medium’ and sensitivity assessed as ‘Medium’.

Water flow (tidal current) changes (local)

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

Evidence

CR.HCR.FaT.CTub and its sub-biotopes CTub.Adig and CTub.CuSp are recorded from very strong (>3 m/s) and strong (1.5-3 m/s) tidal streams (Connor et al., 2004). Alcyonium digitatum, Tubularia indivisa and Actinarians are suspension feeders relying on water currents to supply food. These taxa, therefore, thrive in conditions of vigorous water flow e.g. around Orkney and St Abbs, Scotland, where the community may experience tidal currents of 3 and 4 knots during spring tides (Kluijver, 1993). Tubularia indivisa is also a dominant species on the wreck of Kilmore, Belgium, where tidal velocities can vary between 0.86-0.5 m/s (Zintzen et al., 2008).

Sensitivity assessment. The available evidence suggests an increase in tidal velocity may benefit the characterizing species of CR.HCR.FaT.CTub and its sub-biotopes. A significant decrease in water movement due to a reduction in tidal streams is likely to result in loss of the biotope and its replacement by an epifaunal community typical of lower energy habitats. However, a decrease in tidal velocity of 0.1-0.2 m/s is probably not significant where the tidal streams are in excess of 1.5 m/s. A decrease may favour the development of CTub.Adig over CTub.CuSp.  Biotope resistance has, therefore, been assessed as ‘High’, resilience has been assessed as ‘High’ (by default) and the biotope is, therefore, considered to be '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

Changes in emergence are not relevant to CR.HCR.FaT.CTub.Adig, which is restricted to fully subtidal/circalittoral conditions. The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

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

CR.HCR.FaT.CTub. and its sub-biotopes are recorded from extremely exposed to moderately exposed sites (Connor et al., 2004). Alcyonium digitatum, Tubularia indivisa and Actinarians are suspension feeders relying on water currents to supply food. These taxa, therefore, thrive in conditions of vigorous water flow. CTub.Adig is recorded from 5-30 m, however, the majority of core records occur from 10-20 m and CTub.CuSp is typically recorded deeper at 10-30 m (Connor et al., 2004). The depth at which the biotope is recorded may therefore also negate the effects of a localised change in wave height; wave attenuation is directly related to water depth (Hiscock, 1985).

Metridium senile occurs in greatest abundance in wave sheltered situations with significant tidal flow. Those wave sheltered situations may, from time-to-time, be subject to strong wave action when non–prevailing winds blow. For instance, on the open east coast of Lundy, Metridium senile occurs on shallow jetty piles and on wrecks at 15 m depth where they persist despite occasional strong wave action during easterly winds. It seems most likely that individuals close and shrink-down during strong wave action but survive. Metridium senile is strongly adherent and when closed probably produces little resistance to water flow. Therefore significant increases in wave height will most likely decrease feeding opportunities and perhaps a loss of condition but recovery will be rapid. Bucklin (1987) found that the small size of intertidal populations was imposed, most likely, by limited food and feeding time and damage from wave action, which stimulates budding. Urticina felina favours areas with strong wave action (Manuel, 1988) and strong tidal currents (Migné & Davoult, 1997).

Sensitivity assessment. CTub and its sub-biotopes are high energy habitats so that a significant decrease in water movement (either tidal streams or wave action) will result in loss of the biotope. However, due to the high wave exposure levels at which CR.HCR.FaT.CTub.CuSp is recorded, an increase or a decrease in nearshore significant wave height of 3-5% is not likely to have a significant effect. Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and the biotope (and its sub-biotopes) is assessed as ‘Not sensitive’ at the benchmark level.

Transition elements & organo-metal contamination

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

Evidence

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

No information on the direct biological effects of heavy metal contamination on Alcyonium digitatum. Possible sub-lethal effects of exposure to heavy metals, may result in a change in morphology, growth rate or disruption of reproductive cycle. The vulnerability of this species to concentrations of pollutants may also depend on variations in other factors e.g. temperature and salinity conditions outside the normal range. French & Evans (1986) conducted a colonization experiment on panels coated in copper and zinc based anti-fouling paints and compared the community to panels which weren’t covered in anti-fouling paint. Tubularia indivisa was an abundant species on the panels not coated in anti-fouling paint, indicating Tubularia indivisa is highly sensitive to anti-fouling chemicals.

Hydrocarbon & PAH contamination

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

Evidence

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

CR.HCR.FaT.CTub are a sub-tidal biotopes (Connor et al., 2004) and, because oil pollution is mainly a surface phenomenon its impact upon circalittoral turf communities is likely to be limited (Hartnoll, 1998). However, Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16 m in the locality of Sennen Cove, Cornwall resulting from the offshore spread and toxic effect of detergents sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill. No additional information concerning the direct biological effects of hydrocarbon and PAH contamination on Tubularia indivisa was found.

One month after the Torrey Canyon oil spill Urticina felina was thought to be one of the most resistant animals on the rocky shore, being commonly found alive in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968).

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.

Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16 m in the locality of Sennen Cove, Cornwall resulting from the offshore spread and toxic effect of detergents (a mixture of a surfactant and an organic solvent) e.g. BP 1002 sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill. Possible sub-lethal effects of exposure to synthetic chemicals may result in a change in morphology, growth rate or disruption of the reproductive cycle. No additional information concerning the direct biological effects of synthetic compound contamination on Tubularia indivisa was found.

Very little information has been found. Hoare & Hiscock (1974) observed that Urticina felina survived near to an acidified halogenated effluent discharge in a 'transition' zone where many other species were unable to survive, suggesting a tolerance to chemical contamination. However, Urticina felina was absent from stations closest to the effluent which were dominated by pollution tolerant species particularly polychaetes. Those specimens closest to the effluent discharge appeared generally unhealthy.

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

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 mg/l or even less (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Species such as Metridium senile are thought of as resistant to severe hypoxia. Most epifauna are suspension feeders adapted to water flow, although their tolerance or requirement for water flow varies with species or taxonomic group. For example, Gray et al. (2002) concluded that fish were more sensitive to hypoxia that crustacean and echinoderms, which in turn were more sensitive than annelids with molluscs the most tolerant.  Riedel et al. (2012) noted that infauna were generally more tolerant than epifauna.  They also noted that decapods, echinoderms, and polychaetes showed lower tolerance while ascidians and anthozoans showed higher tolerance to hypoxia and anoxia (Riedel et al., 2012). Mobile species, such as fish, would probably move away from the affected area.

Hydroids mainly inhabit environments in which the oxygen concentration exceeds 5 ml/l (ca 7 mg/l) (Gili & Hughes, 1995). Although no information was found on oxygen consumption for the characterizing hydroids, Sagasti et al. (2000) reported that epifaunal species, including bryozoans and several hydroids and Obelia bidentata (as bicuspidata) in the York River, Chesapeake Bay, tolerated summer hypoxic episodes of between 0.5 and 2 mg O₂/l (0.36 and 1.4 ml/l) for 5-7 days at a time, with few changes in abundance or species composition, although bryozoans were more abundant in the area with generally higher oxygen. Diaz & Rosenberg (1995) reported that the abundance of crustaceans and echinoderms decreased in hypoxic conditions, and suggested that the amphipods Gammarus tigrinus and Ampelisca agassizi were intolerant of hypoxia. Echinoderms are probably intolerant of hypoxia (see reviews). Mobile fauna are also likely to begin to leave the habitat once the oxygen fall below ca 2.8 mg/l (Diaz & Rosenberg, 1995).

Alcyonium digitatum mainly inhabits environments in which the oxygen concentration usually exceeds 5 ml/l and respiration is aerobic. In August 1978, a dense bloom of a dinoflagellate, Gyrodinium aureolum occurred surrounding Geer reef in Penzance Bay, Cornwall and persisted until September that year. Observations by local divers indicated a decrease in underwater visibility (<1 m) from below 8m BSL (Griffiths et al., 1979). It was also noted that many of the faunal species appeared to be affected, e.g. no live Echinus esculentus were observed whereas on surveys prior to August were abundant, Alcyonium sp. were also in an impoverished state. During follow-up surveys conducted in early September Alcyonium sp. were noted to be much healthier and feeding. It was suggested the decay of Gyrodinium aureolum either reduced oxygen levels or physically clogged faunal feeding mechanisms.  Adjacent reefs were also surveyed during the same time period and the effects of the Gyrodinium aureolum bloom were less apparent. It was suggested that shallow water on reefs more exposed to wave action were less affected by the phytoplankton bloom (Griffiths et al., 1979). As no mass mortality occurred. Indicating Alcyonium sp. can potentially survive short-term hypoxic events.

Hiscock & Hoare (1975) reported an oxycline forming in the summer months (Jun-Sep) in a quarry lake (Abereiddy, Pembrokeshire) from close to full oxygen saturation at the surface to <5% saturation below ca 10 m.  No Tethya citrina, Kirchenpaueria pinnata, Hymeniacidon pereleve, Polymastia boletiformis or Ascidia mentula were recorded at depths below 10 to 11 m.  Demosponges maintained under laboratory conditions can tolerate hypoxic conditions for brief periods. Gunda & Janapala (2009) investigated the effects of variable oxygen levels on the survival of the marine sponge, Haliclona pigmentifera. Under hypoxic conditions (1.5-2.0 ppm O₂), Haliclona pigmentifera with intact ectodermal layers and subtle oscula survived for 42 ± 3 days.  Sponges with prominent oscula, foreign material, and damaged pinacoderm exhibited poor survival (of 1-9 days) under similar conditions. Complete mortality of the sponges occurred within 2 days under anoxic conditions (<0.3 ppm O₂).

Sensitivity assessment.  This biotope (CTub) and its sub-biotopes occur in areas of high energy, that is moderate to strong water flow and wave exposure. The epifauna are probably dependent on water movement to supply oxygen and nutrients and remove waste products. However, the high energy of the habitat suggests that any low oxygen episodes would be very transient due to the rapid mixing of the water column by wave action and/or its replacement due to mass transport of water by the strong tidal flow.  Therefore, the biotope and its sub-biotopes are probably resistant of deoxygenation and resistance is assessed as 'High'. Resilience is assessed as  'High' (by default) and the biotope is considered to be 'Not sensitive' at the benchmark level.

Nutrient enrichment

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

Evidence

Alcyonium digitatum and Tubularia indivisa are passive suspension feeders on phytoplankton and zooplankton. Nutrient enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to Alcyonium digitatum and Tubularia indivisa in terms of an increased food supply but the effects are uncertain (Hartnoll, 1998). However, the survival of Alcyonium digitatum and Tubularia indivisa may be influenced indirectly. High primary productivity in the water column combined with high summer temperature and the development of thermal stratification (which prevents mixing of the water column) can lead to hypoxia of the bottom waters which faunal species are likely to be highly intolerant of periods of hypoxia (see de-oxygenation pressure).

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including subtidal reefs). A 30-50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types.

Sensitivity assessment. The pressure benchmark is relatively protective and the biotope is considered to be 'Not sensitive' at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Organic enrichment

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

Evidence

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, Alcyonium digitatum were described as 'very sensitive to organic enrichment and present under unpolluted conditions' while. the associated animal species; Urticina felina; Balanus crenatus and Spirobranchus triqueter were 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. Although species within the biotope may be sensitive to gross organic pollution resulting from sewage disposal and aquaculture they are considered to have ‘High’ resistance to the pressure benchmark which represents organic enrichment. Therefore, resilience is assessed as ‘High’ and the biotope is considered to be ‘Not Sensitive’ at the benchmark level.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

If rock were replaced with sediment, this would represent a fundamental change to the physical character of the biotope and the species would be unlikely to recover. The biotope would be lost. Therefore, resistance to the pressure is considered ‘None’, and resilience ‘Very low’ (as the change at the pressure benchmark is permanent) and sensitivity is assessed as ‘High’.

Physical change (to another sediment type)

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

Evidence

Not relevant

Habitat structure changes - removal of substratum (extraction)

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

Evidence

The species characterizing this biotope are epifauna or epiflora occurring on rock and would be sensitive to the removal of the habitat. Extraction of bedrock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats. However, 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 indicated the presence of large sponges, mainly Axinella infundibuliformis (Picton & Goodwin, 2007).  Freese et al. (1999) also noted that trawling could remove important substratum such as boulders. Therefore, where this biotope occurs on boulders that could be subject to removal or extraction, resistance is likely to be 'Low'. Hence, as resilience is probably 'Medium' (assuming suitable substratum remains) and sensitivity is assessed as 'Medium'. 

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Alcyonium digitatum, Tubularia indivisa plus the anthozoan community are sedentary species that would likely suffer from the effects of abrasion. Boulcott & Howell (2011) conducted experimental Newhaven scallop trawling (a source of abrasion) over a circalittoral rock habitat in the sound of Jura, Scotland and recorded the damage to the resident community. The results indicated that the sponge Pachymatisma johnstoni was highly damaged by the experimental trawl, however, Alcyonium digitatum showed comparatively little damage; 13% of photographic samples showed visible damage to Alcyonium digitatum. Where Alcyonium digitatum damage was evident it tended to be small colonies that were ripped off the rock. The authors highlight that assessing physical damage to faunal turfs (erect bryozoans and hydroids) was difficult to quantify. However, the faunal turf communities did not show large signs of damage and were only damaged by the scallop dredge teeth, an impact which was often limited in extent (approx. 2cm wide tracts). The authors indicated that species such as Alcyonium digitatum and faunal turf communities were not as vulnerable to damage through trawling as sedimentary fauna and whilst damage to circalittoral rock fauna did occur it was of an incremental nature, with loss of species such as Alcyonium digitatum and faunal turf communities increasing with repeated trawls.

Sensitivity assessment. The evidence indicates that CR.HCR.FaT.CTub and its sub-biotopes, especially on vertical and steep faces, are relatively resistant to single abrasion events. Therefore, resistance is assessed as ‘Medium’ based on Boulcott & Howell (2011), resilience assessed as ‘High’ and sensitivity assessed as ‘Low’. Please note Boulcott & Howell (2011) did not mention the abrasion caused by fully loaded collection bags on the new haven dredges. A fully loaded Newhaven dredge may cause higher damage to the community as indicated in their study.

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

The species characterizing this biotope group are epifauna or epiflora occurring on rock, which is resistant to subsurface penetration.  The assessment for abrasion at the surface only is therefore considered to equally represent sensitivity to this pressure.

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 tide-swept 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. 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 effects are not likely to be lethal over the course of a year. A reduction in light penetration could also reduce growth rate of phytoplankton and so limit zooplankton levels.

Little information could be found for the tolerance of Tubularia indivisa to changes in water clarity, other than survey reports of abundant Tubularia indivisa in Isle of Thanet, which is well known as an area characterized by poor underwater visibility (Howson et al., 2005). Zintzen et al. (2008a&b) examined Tubularia spp. dominated epifaunal communities on ship wrecks in the Southern Bight of the North Sea. While Tubularia indivisa dominated offshore and intermediate sites, coastal sites were dominated by Metridium senile and had a lower biomass of turfs of Tubularia spp. The coastal sites were characterized by periodic salinity decreases, large seasonal temperature fluctuations, high total suspended sediment and reduced current velocity when compared to offshore and intermediate sites (Zintzen et al., 2008b). Turbidity alone does not explain the differences in abundance but is a factor.

Alcyonium digitatum has been shown to be tolerant of high levels of suspended sediment. Hill et al. (1997) demonstrated that Alcyonium digitatum sloughed off settled particles with a large amount of mucous. Alcyonium digitatum is also known to inhabit the entrances to sea lochs (Budd, 2008) or the entrances to estuaries (Braber & Borghouts, 1977) where water clarity is likely to be highly variable. Repeated energetic expenditure in cleaning off silt particles may cause sub-lethal effects.

Metridium senile may benefit from increase in turbidity as algal growth on hard substrata will be reduced. For Instance, Svane & Groendahl (1988) found that, in comparison with records from 1926-29, Metridium senile had colonized areas in the Gullmar Fjord, Finland where it had not previously been recorded and suggested the reason was because of increases in turbidity (and tolerance of pollution).

Urticina felina occurs in clear to highly turbid waters and occurs down to depths of at least 100 m (Manuel, 1988) where light levels are low. The anemone is not known to contain symbiotic algae and is unlikely to be sensitive to changes in turbidity. Urticina felina is found in highly turbid areas associated with biotopes such as CR.MCR.SfR.Pol (Connor et al., 2004) and is therefore considered to be unaffected by an increase in turbidity at the benchmark. Increases in siltation may begin to cover the anemone or interfere with feeding. An energetic cost will result from efforts to clean off the silt particles, e.g. through mucus production and sloughing. Repeated energetic expenditure in cleaning off silt particles may cause sub-lethal effects.

Whilst many sponges are disadvantaged by sedimentation (as would be expected, being sessile filter feeders), many examples exist of sponges adapting to sediment presence (Bell et al., 2015; Schönberg, 2015) and many encrusting sponges appear to be able to survive in highly sedimented conditions, and, in fact, many species prefer such habitats (Bell & Barnes 2001; Bell & Smith 2004). Castric-Fey & Chassé (1991) conducted a factorial analysis of the subtidal rocky ecology near Brest, France and rated the distribution of species in varying turbidity (corroborated by the depth at which laminarians disappeared). Cliona celata and Stelligera rigida were classed as indifferent to turbidity, Tethya citrina, Pachymatisma johnstonia and Polymastia boletiformis (as Polymastia robusta) had a slight preference for clearer water, while Dysidea fragilis, Polymastia mamillaris, and Raspailia ramosa had a strong preference for turbid water. 

Sensitivity assessment. Connor et al. (2004) noted that the sub-biotope CTub.CuSp was more prevalent in areas (of strong water movement) where turbidity was high for much of the year. Therefore, the above evidence suggests that, where water movement remains strong, an increase in turbidity could change the relative abundance of the Tubularia and bryozoan turf and sponge cover and favour CTub.CuSp over CTub or CTub.Adig.  Overall biotope resistance is assessed as ‘High’ to an increase in suspended solids at the benchmark level. Resilience is assessed as ‘High’ (by default) and the biotope is considered to be 'Not sensitive' at the benchmark level. The biotope is considered to be ’Not sensitive’ to decreased suspended solids where periodic scour and abrasion are unaffected.

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

The anthozoan community of CR.HCR.FaT.CTub. (Alcyonium digitatum, Tubularia indivisa, hydroids and anemones) is largely sessile and thus would be unable to avoid the deposition of a smothering layer of material up to a depth of 5 cm. Some Alcyonium digitatum colonies can attain a height of up to 20 cm and mature Tubularia indivisa can attain a height of 10-15 cm (Edwards, 2008). Both would still be able to expand tentacles and columns of the polyps to filter feed, and materials may be sloughed off with a large amount of mucous. However, Cylista elegans, Urticina felina, Metridium senile, Actinothoe sphyrodeta, Corynactis viridis and smaller / younger Alcyonium digitatum colonies (that initially form encrustation's between 5 and 10 mm thick) are likely to be smothered and respiration is likely to be hindered. However, CR.HCR.FaT.CTub and its sub-biotopes are recorded from very strong to moderately strong tidal streams (0.5->3 m/sec). A layer of deposited sediment (at the pressure benchmark) is likely to be removed from the biotope within a few tidal cycles.

Schönberg (2015) reviewed and observed the interactions between sediments and marine sponges in Australia and described the lack of research on Porifera.  Bell et al. (2015) reviewed the effects and interactions of sponges with sediment in suspension and after deposition. Whilst many sponges are disadvantaged by sedimentation (as would be expected, being sessile filter feeders), many examples exist of sponges adapting to sediment presence (Bell et al., 2015; Schönberg, 2015). Adaptations included sediment incorporation, sediment encrusting, structural modification (such as reduction in numbers of oscula, or repositioning of inhalant and exhalant openings), soft sediment anchoring using spicules, modification of spicules to shield the body from sediment, backwashing, mucus production, morphology (e.g. upright forms intercept less settling sediment) and living, at least partially, embedded within the sediment (Bell et al., 2015; Schönberg, 2015). Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig 1979), many encrusting sponges appear to be able to survive in highly sedimented conditions, and, in fact, many species prefer such habitats (Bell & Barnes 2001; Bell & Smith 2004). 

Urticina felina anemones adhere strongly to the substratum and would be entirely covered by smothering material. However, Urticina felina lives in situations where it may be covered from time-to-time by sediment, especially coarser substrata which suggests some ability to survive. For example, Holme & Wilson (1985) observed Urticina felina attached to pebbles, cobbles or rock subject to sand scour or periodic smothering by sand at 50-55 m depth, offshore, in the western English Channel. The tidal streams in the central parts of the Channel may reach 125 cm/s during neaps and 166 cm/s on springs. Therefore, they suggested that Urticina felina was tolerant of sand scour or periodic smothering by

Sensitivity assessment. Based on biotope exposure to high levels of water flow that will remobilise and remove sediments, biotope resistance is assessed as ‘High’ and resilience as ‘High’ (by default) and the biotope is considered to be ‘Not Sensitive’ at the benchmark level.

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 epifaunal community of CR.HCR.FaT.CTub (Alcyonium digitatum and anemones, Tubularia indivisa and other hydroids, bryozoan turf and sponges) is sessile and thus would be unable to avoid the deposition of a smothering layer of material up to a depth of 30 cm. Alcyonium digitatum colonies can attain a height of up to 20 cm, Tubularia indivisa can attain a height of 10-15 cm (Edwards, 2008), and will, therefore, be inundated along with other important species within CR.HCR.FaT.CTub e.g. Sagartia elegens, Urticina felina, Metridium senile, Actinothoe sphyrodeta, Corynactis virdis.

Schönberg (2015) reviewed and observed the interactions between sediments and marine sponges in Australia and described the lack of research on Porifera.  Bell et al. (2015) reviewed the effects and interactions of sponges with sediment in suspension and after deposition. Whilst many sponges are disadvantaged by sedimentation (as would be expected, being sessile filter feeders), many examples exist of sponges adapting to sediment presence (Bell et al., 2015; Schönberg, 2015). Adaptations included sediment incorporation, sediment encrusting, structural modification (such as reduction in numbers of oscula, or repositioning of inhalant and exhalant openings), soft sediment anchoring using spicules, modification of spicules to shield the body from sediment, backwashing, mucus production, morphology (e.g. upright forms intercept less settling sediment) and living, at least partially, embedded within the sediment (Bell et al., 2015; Schönberg, 2015). Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig 1979), many encrusting sponges appear to be able to survive in highly sedimented conditions, and, in fact, many species prefer such habitats (Bell & Barnes 2001; Bell & Smith 2004). 

The community forms on vertical and upper faces of bedrock and boulders, so that the deposit is only likely to affect a proportion of the biotope, that is the upper faces. However, CR.HCR.FaT.CTub and its sub-biotopes are recorded from very strong to moderately strong tidal streams (0.5->3 m/sec). A layer of deposited sediment (at the pressure benchmark) is likely to be removed from the biotope within a few tidal cycles and only remain at the base of boulders, crevices, or areas in the lee of the prevailing currents a the time of deposition. Therefore, a precautionary resistance of 'Medium' is suggested to represent the damage to some of the more sensitive species in the assemblage. Nevertheless, recovery will probably be rapid and resilience is assessed as 'High' and sensitivity 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

At the time of writing there is no evidence on which to assess this pressure. 

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

CR.HCR.FaT.CTub and its sub-biotopes are circalittoral biotopes (Connor et al., 2004) and the community is, therefore, not dependent on direct sunlight.  Increased shading (e.g. by construction of a pontoon, pier etc) could benefit the characterizing species of this biotope.

Sensitivity assessment. Resistance is probably 'High', with a 'High' resilience and a sensitivity of 'Not Sensitive'. 

Barrier to species movement

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

Evidence

Not Relevant.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

Not Relevant.

Visual disturbance

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

Evidence

Not Relevant.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

No evidence for translocation or genetic modification of Alcyonium digitatum, Tubularia indivisa or other important epifaunal species within this biotope was found.

Introduction or spread of invasive non-indigenous species

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

Evidence

Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. 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 in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’ it preferred shell, especially that of conspecifics. 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 may out-compete Crepidula for space even if it gained a foothold in the community. 

Species tolerant of tide-swept conditions that can occupy rock surfaces and out-compete native species pose the greatest threat to this biotope. The tunicates Didemnum vexillum and Asterocarpa humilis, the hydroid Schizoporella japonica and the bryozoan Watersipora subatra (Bishop, 2012c, Bishop, 2015a and b; Wood, 2015) are currently only recorded from artificial hard substratum in the UK and it is not clear what their established range and impacts in the UK would be.

Didemnum vexillum is an invasive colonial sea squirt native to Asia which was first recorded in the UK in Darthaven Marina, Dartmouth in 2005. Didemnum vexillum can form extensive mats over the substrata it colonizes; binding boulders, and cobbles and altering the host habitat (Griffith et al., 2009). Didemnum vexillum can also grow over and smother the resident biological community. Recent surveys within Holyhead Marina, North Wales have found Didemnum vexillum growing on and smothering native tunicate communities (Griffith et al., 2009). Due to the rapid-re-colonization of Didemnum vexillum eradication attempts have to date failed.  Presently Didemnum vexillum is isolated to several sheltered locations in the UK (NBN, 2015), however, Didemnum vexillum has successfully colonized the offshore location of the Georges Bank, USA (Lengyel et al., 2009) which is more exposed than the locations which Didemnum vexillum have colonized in the UK. Therefore, it is possible that Didemnum vexillum could colonize more exposed locations within the UK and could pose a threat to CR.HCR.FaT.CTub.Adig.  The tunicate Styela clava appears to prefer more sheltered conditions than this biotope (Bishop, 2012d).

Sensitivity assessment. The circalittoral rock characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata due to the wave exposed to moderately wave exposed conditions, in which wave action and storms may mitigate or prevent the colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or infralittoral or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the circalittoral rock biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; further evidence is required. 

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

Alcyonium digitatum acts as the host for the endoparasitic species Enalcyonium forbesiand and Enalcyonium rubicundum (Stock, 1988). Parasitisation may reduce the viability of a colony but not to the extent of killing them but no further evidence was found to substantiate this suggestion. Tubularia indivisa and Cylista elegans can host an array of potentially pathogenic bacteria, however there is insufficient evidence to suggest significant population wide mortality (Schuett & Doepke, 2010). 

Sensitivity assessment. Based on the lack of reported mortalities of the characterizing and associated species, the biotope is judged to have ‘High’ resistance to this pressure. Resilience is assessed as ‘High’ and the biotope is classed as ‘Not sensitive’ at the pressure benchmark. 

Removal of target species

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

Evidence

None of the characterizing species within CR.HCR.FaT.CTub and its sub-biotopes are commercially exploited. This pressure is, therefore, considered ‘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

Alcyonium digitatum and faunal turf communities (which include hydroids such as Tubularia indivisa) have been found to be resistant to abrasion through bottom fishing (see abrasion pressure) and only biological effects are considered here.

Zintzen et al. (2008a&b) examined Tubularia spp. dominated epifaunal communities on ship wrecks in the Southern Bight of the North Sea. They noted that the Tubularia spp. turf supported a high diversity and biomass of epifaunal, including species that grew attached to Tubularia spp. (e.g. Jassa spp. and caprellids).  Although not exactly equivalent, the Tubularia spp. and associated hydroid and bryozoan turf probably support numerous epifaunal and epiphytic species, together with faunal grazing gastropods and nudibranchs. Zintzen et al. (2008a) noted that during the summer months the Tubularia biomass deceased as it became dormant and was predated or out-competed by other species.  Zintzen et al. (2008a) also noted that species richness and the density of 23% of the species in the community were positively correlated with Tubularia biomass. Similarly, Alcyonium digitatum goes through an annual cycle, from February to July all Alcyonium digitatum colonies are feeding, from July to November, an increasing number of colonies stop feeding. During this period a large number of polyps can retract and a variety of filamentous algae, hydroids and amphipods can colonize the surface of colonies epiphytically. From December-February the epiphytic community is, however, sloughed off (Hartnoll, 1975).

Metridium senile can aggressively out-compete other spatial competitors (Whomersley & Picken, 2003; Nelson & Craig 2011). Nelson & Craig (2011) found mature Metridium senile can have significant negative effects on invertebrate recruitment. If Alcyonium digitatum and Tubularia indivisa were removed from the biotope Metridium senile and other anthozoans could spread rapidly and dominate the biotope in the short-term. However, high water flow within this biotope may preclude the long-term dominance of Metridium senile.

Sensitivity assessment. If Alcyonium digitatum and Tubularia indivisa were removed the other epifaunal species would likely colonize rock surfaces as they are not dependent on the characterizing species. However, if Tubularia indivisa, Alcyonium digitatum or the characteristic sponges were removed or damaged this would alter the character of the biotope and potentially decrease species richness. Therefore, resistance has been assessed as ‘Low’, resilience assessed as ’Medium’ and sensitivity assessed as ‘Medium’.