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

Little direct evidence was found to support the assessment of this pressure. Few laboratory studies have been carried out and the sensitivity assessment is based on studies monitoring settlement and recruitment and records of species distribution. 

Kröncke et al. (1998) examined long-term changes in the macrofauna in the subtidal zone off Norderney, one of the East Frisian barrier islands. The analysis suggested that macrofauna were severely affected by cold winters whereas storms and hot summers have no impact on the benthos. A long-term increase in temperature might cause a shift in species composition. Long‐term analysis of the North Sea pelagic system has identified yearly variations in larval abundance of Echinodermata, Arthropoda, and Mollusca larvae that correlate with sea surface temperatures. Larvae of benthic echinoderms and decapod crustaceans increased after the mid‐1980s, coincident with a rise in North Sea sea surface temperature, whereas bivalve larvae underwent a reduction (Kirby et al., 2008). An increase in temperature may alter larval supply and, in the long-term and over large spatial scales, may result in changes in community composition.

Mediomastus fragilis is recorded from northern Norway, south to the Iberian Peninsula and into the Mediterranean, and across the North Sea into the Baltic (OBIS, 2022). OBIS records reported a sea surface temperature range of -5°C to 25°C but the majority of records were in the range of 10-15°C (OBIS, 2022). Chaetozone gibber has a similar distribution from Norway, south to the Iberian Peninsula and into the Mediterranean and is recorded from a sea surface temperature of 5-20°C, although most records are at 10-15°C.  Nucula nucleus is recorded from northern Norway, south to the Iberian Peninsula and into the Mediterranean but is also recorded in South Africa (OBIS, 2022). OBIS records reported a sea surface temperature range of 0°C to 25°C but the majority of records were in the range of 10-15°C (OBIS, 2022).  The cirratulid Aphelochaeta marioni (studied as Tharyx marioni) has been recorded from the Baltic to the Indian Ocean and so it probably has some degree of adaptation or tolerance to a range of temperatures (Hartmann-Schroder, 1974; Rogall, 1977, cited in Farke, 1979).  However, acute rises in temperature may have a more deleterious effect. 

George (1964a) reported that a rapid rise or fall in temperature of 3°C was sufficient to induce spawning in 25% of mature Cirriformia tentaculata. If this occurred at a time of year that was not suitable for larval survival then larval mortality could be high.  The upper lethal limits for Cirriformia tentaculata from the Hamble were reported to be 32°C and 29°C for 5-6 day old and adult Cirriformia tentaculata respectively (George, 1964b). Cirriformia tentaculata is reported to be near its northern limit in the British Isles (George, 1968) and an increase in temperature may lead to the extension of its upper distribution range. An increase in temperature could also serve to decrease the length of time spent in the larval phase and so reduce the risk of predation. The rate of larval growth in Cirriformia tentaculata was found to be twice as fast at 20°C as at 8°C.

Sensitivity assessment. Little direct evidence was available to assess this pressure.  Most of the characteristic species occur to the north or south of UK waters so it is likely that they would be resistant to an increase in temperature at the benchmark level.  An acute change may lead to spawning or other sublethal biological effects. Therefore, resistance is assessed as 'High' at the benchmark level.  Hence, resilience is 'High' and sensitivity is assessed as '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. Further detail

Evidence

Little direct evidence was found to support the assessment of this pressure. Few laboratory studies have been carried out and the sensitivity assessment is based on studies monitoring settlement and recruitment and records of species distribution.  Kröncke et al. (1998) examined long-term changes in the macrofauna in the subtidal zone off Norderney, one of the East Frisian barrier islands. The analysis suggested that macrofauna were severely affected by cold winters whereas storms and hot summers have no impact on the benthos. A long-term increase in temperature might cause a shift in species composition.

Mediomastus fragilis is recorded from northern Norway, south to the Iberian Peninsula and into the Mediterranean, and across the North Sea into the Baltic (OBIS, 2022). OBIS records reported a sea surface temperature range of -5°C to 25°C but the majority of records were in the range of 10-15°C (OBIS, 2022). Chaetozone gibber has a similar distribution from Norway, south to the Iberian Peninsula and into the Mediterranean and is recorded from a sea surface temperature of 5-20°C, although most records are at 10-15°C.  Nucula nucleus is recorded from northern Norway, south to the Iberian Peninsula and into the Mediterranean but is also recorded in South Africa (OBIS, 2022). OBIS records reported a sea surface temperature range of 0°C to 25°C but the majority of records were in the range of 10-15°C (OBIS, 2022).  The cirratulid Aphelochaeta marioni (studied as Tharyx marioni) has been recorded from the Baltic to the Indian Ocean and so it probably has some degree of adaptation or tolerance to a range of temperatures (Hartmann-Schroder, 1974; Rogall, 1977, cited in Farke, 1979).  However, acute rises in temperature may have a more deleterious effect. 

George (1964a) reported that a rapid rise or fall in temperature of 3°C was sufficient to induce spawning in 25% of mature Cirriformia tentaculata. If this occurred at a time of year that was not suitable for larval survival then larval mortality could be high. However, George (1964b) noted that although in Southampton the incoming tide incurred a drop of 6 °C in five minutes, such rapid changes in temperature had no significant effect on the mortality of either juvenile or adult Cirriformia tentaculata in the laboratory. The larvae of this species grow twice as slow at 8°C as they do at 20°C (George, 1964a). Any increase in the length of time spent in the larval phase will increase the risk of predation. In adults, field data suggests that growth ceases at 6°C (George, 1964a). On the Hamble, lower lethal limits of -6°C (by extrapolation) and 2°C have been reported for 5-6 day old and adult Cirriformia tentaculata respectively (George, 1964b). These are temperatures that can reasonably be expected in winter in this intertidal biotope and so some mortality is likely. Furthermore, Cirriformia tentaculata is reported to be near its northern limit in the British Isles (George, 1968) and a long-term chronic decrease in temperature could serve to exclude this species from the northern extent of its distribution. George (1968) reported several major changes and a major reduction in the distribution range of Cirriformia tentaculata following the severe winter of 1962/3. In temperature tolerance experiments, no Cirriformia tentaculata survived even a brief exposure to -2°C or 96 hours at 0°C.

The cirratulid Cirratulus cirratus was found to tolerate lower temperatures and it is possible that this species will become more prevalent in this biotope if the temperature falls. George (1968) reported that the ciliary feeding mechanisms of Cirriformia tentaculata became so inefficient at low temperatures that, over long periods, the animal may die of starvation. George (1968) also mentioned that the animal does not withdraw its branchiae in cold weather. Due to their delicate nature, the branchiae may subsequently freeze on the surface. In such a case, the animal would be living under anaerobic conditions and so emerges from the burrow to enable them to respire through their body surface.

Long‐term analysis of the North Sea pelagic system has identified yearly variations in larval abundance of Echinodermata, Arthropoda, and Mollusca larvae that correlate with sea surface temperatures. Larvae of benthic echinoderms and decapod crustaceans increased after the mid‐1980s, coincident with a rise in North Sea sea surface temperature, whereas bivalve larvae underwent a reduction (Kirby et al., 2008). A decrease in temperature may alter larval supply and in the long-term, over large spatial scales, may result in changes in community composition.

Sensitivity assessment. Little direct evidence was available to assess this pressure.  Most of the characteristic species occur to the north or south of UK waters so it is likely that they would be resistant to an increase in temperature at the benchmark level. An acute change may exceed thermal tolerances or lead to spawning or other sublethal effects. The exception is Cirriformia tentaculata, whose population may be severely reduced by acute changes typical of severe winter temperatures in the UK. . Therefore, resistance is assessed as ‘Medium’. Hence, resilience is assessed as ‘High’ and sensitivity as ‘Low’ at the benchmark level. 

Salinity increase (local)

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

Evidence

No directly relevant evidence was found to assess this pressure. A study from the Canary Islands indicated that exposure to high salinity effluents (47- 50 psu) from desalination plants altered the structure of biological assemblages, reducing species richness and abundance (Riera et al., 2012). Bivalves and amphipods appeared to be less tolerant of increased salinity than polychaetes and were largely absent at the point of discharge. Polychaetes, including species or genera that occur in this biotope, such as Lumbrineris spp. were present at the discharge point (Riera et al., 2012).  However, Russo et al. (2006, 2007; cited in Roberts et al., 2010b) reported that polychaete abundance and diversity decreased in proximity to the hypersaline discharge, and noted that sensitivity varied between polychaetes families with Ampharetidae being the most sensitive.  Roberts et al. (2010b) noted that the effects of desalination plants effluents varied from none to significant impacts in seagrass, coral reef and soft-sediment communities in poorly flushed environments. They also noted that in most other cases the effects were limited to within 10s of metres of the outfalls. 

Sensitivity assessment. High saline effluents have the potential to alter the structure of biological assemblages in close proximity to outfalls and/or in poorly flushed locations. Polychaete species may be more tolerant than bivalves but an increase in salinity is likely to result in declines in species richness and abundance based on Roberts et al. (2010b) and Riera et al. (2012). This biotope (SS.SMx.IMx.MedCirr) is found in full to variable salinity waters, moderately exposed to sheltered from wave action and in strong to weak tidal streams. It is probably well flushed so that exposure to hypersaline conditions may be limited to the close proximity to any outfall.  Therefore, resistance is assessed as ‘Medium’. Hence, resilience is assessed as ‘High’ and sensitivity is assessed as 'Low', albeit with 'Low' confidence 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 polychaetes Mediomastus fragilis, Chaetozone zetlandica, Aphelochaeta marioni, Caulleriella alata, Cirriformia tentaculata and the bivalve Nucula nucleus and Melinna palmata were recorded in areas of 30-35 psu, with few records at lower salinities (OBIS, 2022). Ranchor (1976) successfully reared specimens of Nucula nitidosa in the laboratory at a salinity of 27 to 32 ppt. However, very little information on its salinity tolerance was found. 

Populations of Aphelochaeta marioni inhabit the open coast where seawater is at full salinity. Farke (1979) studied the effects of changing salinity on Aphelochaeta marioni (studied as Tharyx marioni) in a microsystem in the laboratory. Over several weeks, the salinity in the microsystem was increased from 25-40 psu and no adverse reaction was noted. However, when individuals were removed from the sediment and displaced to a new habitat, they only dug into their new substratum if the salinities in the two habitats were similar. If the salinities differed by 3-5 psu, the worms carried out random digging movements, failed to penetrate the sediment and died at the substratum surface after a few hours. This would suggest that Aphelochaeta marioni can tolerate salinity changes when living infaunally but is less tolerant when removed from its habitat.

Sensitivity assessment.  This biotope (SS.SMx.IMx.MedCirr) is found in full to variable (18-35) salinity waters, which suggests some tolerance to a reduction in salinity. A reduction in salinity may result in changes in biotope composition as some sensitive species are lost and replaced by typical estuarine species more tolerant of the changed conditions. Therefore, a reduction in salinity regime from full to reduced (18-35) for a year (the benchmark) may result in a reduction in the abundance of some of the characteristic species in the short term and resistance is assessed as ‘Low’. Hence, resilience is assessed as ‘High’ and sensitivity as ‘Low' at the benchmark level.

Water flow (tidal current) changes (local)

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

Evidence

This biotope (SS.SMx.IMx.MedCirr) is found in strong (1.5 -3 m/s) and weak (>0.5 m/s) tidal streams and in moderately exposed, sheltered and very sheltered wave conditions (JNCC, 2022). Sands are less cohesive than mud sediments and coarse sediments dominate areas of higher water flow. Hjulström (1939) concluded that fine sand (particle diameter of 0.3-0.6 mm) was easiest to erode and required a mean velocity of 0.2 m/s. Erosion and deposition of particles greater than 0.5 mm require a velocity >0.2 m/s to alter the habitat and gravel (ca 10 mm dia.) may require >1 m/s to erode. The presence of mud may help to consolidate the coarse sediment in areas of strong water flow. 

Many of the species occur in a range of sediment types, which, given the link between hydrodynamics and sediment type, suggests that these species are not sensitive to changes in water flow at the pressure benchmark. For example, Mediomastus fragilis, Chaetozone gibba, Cirriformia tentaculata, Aphelochaeta marioni, Caulleriella zetlandica and Nucula nucleus are found in coarse sediments, muddy sands and sandy muds as well as mixed sediments (JNCC, 2015)

Sensitivity assessment. Changes in water flow may alter the topography of the habitat and may cause some shifts in abundance. An increase to very strong tidal streams may winnow away the biotope while a decrease to weak tidal streams in the absence of wave action may result in the deposition of muddier sediments. However, a change at the pressure benchmark (increase or decrease) is unlikely to affect biotopes that occur in strong to weak flows. Therefore, resistance is assessed as ‘High’, resilience as ‘High’, and sensitivity assessed as ‘Not sensitive’ at the benchmark level.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Changes in emergence are 'Not relevant' to this biotope which is restricted to fully subtidal habitats. 

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

This biotope (SS.SMx.IMx.MedCirr) is found in strong (1.5 -3 m/s) and weak (>0.5 m/s) tidal streams and in moderately exposed, sheltered and very sheltered wave conditions at a depth of 5-10 metres (JNCC, 2022).  It is not directly exposed to the action of breaking waves but to the resultant oscillatory flow at the surface of the sediment.  No specific evidence was found to assess this pressure.  An increase in wave exposure (however unlikely) to 'exposed' or higher may remove or re-sort the substratum, resulting in coarser or mobile sediments and a potential change in the biotope. A decrease to extremely wave-sheltered conditions is unlikely to result in a significant change in the water flow that characterizes the biotope. In particular, a 3-5% change in significant wave height (the benchmark) is unlikely to result in a significant change. Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and sensitivity assessed as ‘Not sensitive’ at the benchmark level.

Transition elements & organo-metal contamination

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

Evidence

Bryan (1984) suggested that polychaetes were "fairly resistant" to heavy metals. In the short-term, mercury, copper and silver were the most toxic, aluminium, chromium, zinc and lead less toxic and cadmium, nickel, cobalt and selenium the least toxic.  The most rapidly absorbed metals (mercury, copper, silver) are generally the most toxic but for others, toxicity was variable and species-specific (Bryan, 1984).

Owenia fusiformis from the south coast of England were found to have loadings of 1335 µg Cu per gram body weight and 784 µg Zn per gram body weight. The metals were bound in spherules within the cells of the gut (Gibbs et al., 2000). No mention was made of any ill effects of these concentrations of metal within the body and it is presumed that Owenia fusiformis is tolerant of heavy metal contamination. Rygg (1985) classified Lumbrineris spp. as non-tolerant of Cu because the species was only occasionally found at stations in Norwegian fjords where Cu concentrations were >200 ppm (mg/kg). 

In Restronguet Creek, the sediments contained levels of arsenic, copper and tin two orders of magnitude higher than in unpolluted estuaries, while levels of silver and zinc were approximately forty times higher (Bryan & Gibbs, 1983). The presence of Aphelochaeta marioni in this area (Bryan & Gibbs, 1983) suggested that the species was tolerant of heavy metal contamination. Furthermore, Aphelochaeta marioni was shown to accumulate arsenic (Gibbs et al., 1983). Aphelochaeta marioni (studied as Tharyx marioni) was found to have whole body concentrations of arsenic greater than 2000 µg/g dry weight (even when living under low ambient arsenic conditions). For reference, other Cirratulids, e.g. Cirriformia tentaculata, from the same habitat contained arsenic at concentrations lower than 100 µg/g dry weight. The purpose of arsenic accumulation was unclear. Trials with gobies failed to confirm that it was a predator deterrent mechanism and it is probably not a detoxification mechanism as arsenic accumulations were similar for worms living in widely varying arsenic concentrations. Hence, there is no evidence to suggest that Aphelochaeta marioni is intolerant of heavy metal contamination. However, other annelids have been shown to be intolerant of heavy metal contamination (e.g. see review by Crompton, 1997) and therefore an intolerance of low is recorded. Due to their tolerance, a recoverability of very high is recorded.

Sensitivity assessment. The review by Bryan (1984) suggested that polychaetes were "fairly resistant" to heavy metal pollution. The information on accumulation suggests that the species mentioned were resistant, while the evidence on copper in Lumbrineris suggests sensitivity. Overall, polychaetes are probably resistant but the level of resistance (or toxicity) varies between species, habitats and metals. Therefore, resistance is assessed as 'Medium' as a precaution to represent a possible reduction in the abundance of some species within the community. Hence, resilience is assessed as 'High' and sensitivity as 'Low' but with 'Low' confidence due to the lack of direct evidence on the effects on the important characteristic species within this biotope. 

Hydrocarbon & PAH contamination

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

Evidence

Suchanek (1993) suggested that cirratulids were mostly immune to oil spills because their feeding tentacles are protected by a heavy secretion of mucus. This immunity is supported by observations of Aphelochaeta marioni following the Amoco Cadiz oil spill in March 1978 (Dauvin, 1982, 2000). Prior to the spill, Aphelochaeta marioni (studied as Tharyx marioni) was present in very low numbers in the Bay of Morlaix, western English Channel. Following the spill, the level of hydrocarbons in the sediment increased from 10 mg/kg dry sediment to 1443 mg/kg dry sediment six months afterwards. In the same period, Aphelochaeta marioni increased in abundance to a mean of 76 individuals per m², which placed it among the top five dominant species in the faunal assemblage. It was suggested that the population explosion occurred due to the increased food availability because of the accumulation of organic matter resulting from the high mortality of browsers. Six years later, an abundance of Aphelochaeta marioni began to fall away again, accompanied by gradual decontamination of the sediments. Mediomastus fragilis also increased in abundance (Dauvin, 2000).

George (1971) reported that the spawning, growth and mortality of Cirriformia tentaculata and Cirratulus cirratus were unaffected by a fuel oil spill on mudflats at the mouth of the River Hamble, England. However, the dispersants (Essolvene and BP1002) killed both species at low concentrations, although Cirratulus cirratus was the most tolerant. George (1971) reported 24-hour LC50 of 30 ppm BP1002 and 63 ppm Essolvene in Cirriformia tentaculata and 129 ppm and 162 ppm respectively in Cirratulus cirratus. Populations showed recovery two years after the pollution event.

Conan (1982) investigated the long-term effects of the Amoco Cadiz oil spill at St Efflam beach in France. Fabulina fabula (studied as Tellina fabula) started to disappear from the intertidal zone a few months after the spill and from then on was restricted to subtidal levels. In the following 2 years, recruitment of Fabulina fabula was very much reduced. The author commented that, in the long-term, the biotas most severely affected by oil spills are low-energy sandy and muddy shores, bays and estuaries. In such places, populations of species with long and short-term life expectancies (e.g. Fabulina fabula, Echinocardium cordatum and Ampelisca sp.) either vanished or displayed long-term decline following the Amoco Cadiz oil spill. However, polychaetes including Nephtys hombergii, cirratulids and capitellids were largely unaffected. Other studies support the conclusion that polychaetes are generally tolerant taxa. Hiscock et al. (2004; 2005a, from Levell et al., 1989) described Glycera spp. as a very tolerant taxon, found in enhanced abundances in the transitional zone along hydrocarbon contamination gradients surrounding oil platforms.

The amphipods, Ampelisca sp. are also very intolerant of oil contamination and the recovery of the Ampelisca populations in the fine sand community in the Bay of Morlaix took up to 15 years following the Amoco Cadiz oil spill (Poggiale & Dauvin, 2001).

Sensitivity assessment. Overall, the characteristic polychaetes are probably resistant to exposure to oil spills and some species may increase in abundance due to the indirect effects of oil and resistance is assessed as 'High'.  Hence, resilience is 'High' and sensitivity is assessed as 'Not sensitive' to oil spills. However, dispersants have the potential to be more toxic. Therefore, resistance to dispersants is assessed as 'Low' but with 'Low' confidence since it is based on a single study. Hence, resilience is 'High' and sensitivity to dispersants is assessed as 'Low'.

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

The anti-parasite compound ivermectin is highly toxic to benthic polychaetes and crustaceans (Black et al., 1997; Collier & Pinn, 1998; Grant & Briggs, 1998, cited in Wildling & Hughes, 2010). Data from a farm in Galway, Ireland indicated that ivermectin was detectable in sediments adjacent to the farm at concentrations up to 6.8 μm/kg and to a depth of 9 cm (reported in OSPAR, 2000). Infaunal polychaetes have been affected by deposition rates of 78-780 mg ivermectin/m².

However, Mediomastus californiensis and other polychaetes (inc. Nephtys sp.) were little affected by the application of the pesticide carbaryl (at 5.6 kg/ha)) used to control mud shrimp on oyster beds in Willapa Bay, Washington, USA (Dumbauld et al., 2001). However, the amphipods Corophium acherusicum and Eohaustorius estuarius suffered the highest short-term mortalities but recruited back within three weeks and were often more abundant on treated than in control sites a year after treatment (Dumbauld et al., 2001). 

Pridmore et al. (1992) reported that bivalves appeared to be the most affected by the application of the organochlorine pesticide Chlordane to intertidal sandflats. The three most abundant bivalves Chione stutchburyi, Tellina liliana and Nucula hartvigiana declined in number by 31, 40 and 56% respectively. The burrowing capitellid Heteromastus filiformis, also declined in abundance by ca 48% while other polychaetes showed no significant change in abundance (Pridmore et al., 1992). 

Sensitivity assessment. The evidence is limited to the effects of a few chemicals on a few species that are congeners of species that occur in this biotope. The effects are likely to vary between species, habitat, and chemical as well as the concentration of chemical used and its dosing rate.  There is evidence that ivermectin has affected polychaetes and non-target crustaceans adversely. However, polychaetes may be resistant to other pesticides while amphipods are not likely to be resistant to insecticides.  Therefore, resistance is assessed as 'Medium' to represent the potential for synthetic contaminants to affect some of the characteristic species.  Hence, resilience is assessed as 'High' and sensitivity as 'Low' but with 'Low' confidence in the absence of more evidence. 

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence was found to support an assessment at the pressure benchmark. Following the Fukushima Dai-ichi nuclear power plant accident in August 2013, radioactive cesium concentrations in invertebrates collected from the seabed were assessed. Concentrations in bivalves and gastropods were lower than in polychaetes (Sohtome et al., 2014). The data does not indicate that there were mortalities.

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

No evidence was found

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

Riedel et al. (2012) assessed the response of benthic macrofauna to hypoxia advancing to anoxia in the Mediterranean. The hypoxic and anoxic conditions were created for 3-4 days in a box that enclosed in-situ sediments. In general, molluscs were more resistant than polychaetes, with 90% surviving hypoxia and anoxia, whereas only 10% of polychaetes survived. Exposed individual Timoclea ovata and Tellina serrata survived the experiment but the exposed Glycera spp. died. In general, epifauna were more sensitive than infauna, mobile species more sensitive than sedentary species and predatory species more sensitive than suspension and deposit feeders. The test conditions did not lead to the production of hydrogen sulphide which may have reduced mortalities compared to some observations. In their review,  Vaquer-Sunyer & Duarte (2008) concluded that crustaceans were more sensitive to hypoxia than polychaetes which were more sensitive than echinoderms while molluscs were amongst the most resistant. 

Further evidence of sensitivity was available for some of the polychaete species associated with this biotope. Rabalais et al. (2001) observed that hypoxic conditions on the north Coast of the Gulf of Mexico (oxygen concentrations from 1.5 to 1 mg/l (1 to 0.7 ml/l) led to the emergence of  Lumbrineris sp. from the substratum that then laid motionless on the surface. Glycera alba was found to be able to tolerate periods of anoxia resulting from inputs of organic-rich material from a wood pulp and paper mill in Loch Eil (Scotland) (Blackstock & Barnes, 1982). Nierman et al. (1990) reported changes in a fine sand community for the German Bight in an area with regular seasonal hypoxia. In 1983, oxygen levels were exceptionally low (<3 mg O₂/l) in large areas and <1 mg O₂/l in some areas. Species richness decreased by 30-50% and overall biomass fell. Owenia fusiformis were reduced in abundance significantly by the hypoxia Spiophanes bombyx was found in small numbers at some, but not all areas, during the period of hypoxia. Once oxygen levels returned to normal Spiophanes bombyx increased in abundance; the evidence suggests that at least some individuals would survive hypoxic conditions.

Diaz & Rosenberg (1995) suggested that Mediomastus ambiseta and Lumbrinereis verrilli were resistant to moderate hypoxia. They reported that the effects of hypoxia in the Rappahannock River on communities dominated by opportunistic polychaetes were species-specific. For example, Streblospio benedicti and Mediomastus ambiseta became extinct locally after severe hypoxia. Similarly, a hypoxic event in the Gulf of Mexico in 1981 significantly reduced the abundance of otherwise dominant species such as Mediomastus californiensis and Cirratulus filiformis. Diaz & Rosenberg (1995) also suggested that Ampelisca agassizi and Ampharete grubei were sensitive to hypoxia. 

Sensitivity assessment. Riedel et al. (2012) and Vaquer-Sunyer & Duarte (2008) provide evidence on general sensitivity trends. As the biotope is characterized by polychaetes, and in the absence of direct evidence, resistance is assessed as ‘Low’ and resilience as ‘High’ so that sensitivity is assessed as ‘Low’.

Nutrient enrichment

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

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. The polychaetes and other associated invertebrate species are unlikely to be directly affected by changes in nutrient enrichment. The resultant additional growth of benthic microalgae may increase food for infaunal deposit-feeders. If an algal bloom was triggered by resultant eutrophication then it may result in organic enrichment of the sediment (see below) or a hypoxic event (see deoxygenation above). However, there is inadequate evidence to assess the direct effects of changes in nutrient levels at the benchmark level.  

Organic enrichment

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

Evidence

Borja et al. (2000) and Gittenberger & Van Loon (2011) assigned Aphelochaeta marioni to AMBI Group IV: "second-order opportunistic species (slight to pronounced unbalanced situations); mainly small sized polychaetes: subsurface deposit feeders, such as cirratulids.  Mediomastus fragilis, Glycera alba, Glycera lapidum and Spiophanes bombyx were characterized as AMBI Group III, defined as "species tolerant to excess organic matter enrichment; these species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)". Lumbrineris latreilli was characterized as AMBI Group II: "species indifferent to enrichment, always present in low densities with non-significant variations with time (from the initial state, to slight unbalance)". However, Nucula nitidosa was characterized as Group I: "species very sensitive to organic enrichment and present under unpolluted conditions (initial state)" (Borja et al., 2000, Gittenberger & Van Loon, 2011). 

Sensitivity assessment.  At the pressure benchmark, organic inputs are likely to represent a food subsidy for the associated deposit-feeding species. Most of the characteristic polychaetes are unlikely to be significantly affected, while some species, e.g. Nucula sp. may be impacted. Therefore, resistance is assessed as 'Medium', resilience as 'High' and sensitivity assessed as 'Low'. 

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

The biotope is characterized by sediment (JNCC, 2022), so a change to an artificial or rock substratum would alter the character of the biotope leading to reclassification and the loss of the sedimentary community including the characterizing polychaetes and bivalves that live buried within the sediment.

Sensitivity assessment. Based on the loss of the biotope, resistance is assessed as ‘None’, resilience as ‘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

Sediment type is a key factor structuring the biological assemblage present in the biotope. Surveys over sediment gradients and before-and-after impact studies from aggregate extraction sites where sediments have been altered indicate patterns in change. The biotope classification (JNCC, 2015) provides information on the sediment types where biotopes are found and indicate likely patterns in change if the sediment were to alter. 

Long-term alteration of sediment type to finer more unstable sediments was observed six years after aggregate dredging at moderate energy sites (Boyd et al., 2005). The ongoing sediment instability was reflected in a biological assemblage composed largely of juveniles (Boyd et al., 2005). Differences in biotope assemblages in areas of different sediment types are likely to be driven by pre and post-recruitment processes. Sediment selectivity by larvae will influence levels of settlement and distribution patterns. Snelgrove et al. (1999) demonstrated that Spisula solidissima, selected coarse sand over muddy sand, and capitellid polychaetes selected muddy sand over coarse sand, regardless of site. Both larvae selected sediments typical of adult habitats, however, some species were nonselective (Snelgrove et al., 1999) and presumably in unfavourable habitats post recruitment, mortality will result for species that occur in a restricted range of habitats. Holme (1966) observed that Glycymeris glycymeris was absent from areas of the English Channel with finer sediments but was abundant in tidally-swept coarse areas. Some species may, however, be present in a range of sediments. Post-settlement migration and selectivity also occurred on small scales (Snelgrove et al., 1999). Desprez (2000) found that a change of habitat to fine sands, from coarse sands and gravels (from deposition of screened sand following aggregate extraction), changed the biological communities present. Tellina pygmaea and Nephtys cirrosa dominated the fine sand community. Dominant species of coarse sands, Echinocyamus pusillus and Amphipholis squamata, were poorly represented and the characteristic species of gravels and shingles were absent (Desprez, 2000). Cooper et al. (2011) found that characterizing species from sand-dominated sediments were equally likely to be found in gravel-dominated sediments.

Sensitivity assessment.  This biotope (SS.SMx.IMx.MedCirr) is found in mixed sediments, gravelly muddy sand, and muddy sandy gravel (JNCC, 2022). The change referred to in the pressure benchmark is a change in sediment classification (based on Long, 2006). For mixed sediments, resistance is assessed based on a change to either coarse (gravel-dominated) sediments or muds and sandy muds.  A change in sediment type may not result in the loss of all the characterizing species but will affect the community composition and diversity and result in re-classification and, hence, loss of the biotope.  A change to coarser sediment will probably result in a biotope similar to SS.SCS.CCS.MedLumVen, SS.SCS.ICS.MoeVen or SS.SCS.CCS.Pkef.  A change to finer sediments may result in a biotope similar to SS.SMu.ISaMu.MysAbr or SS.SMu.ISaMu.MelMagThy.  Therefore, resistance is assessed as ‘Low’ as some species may remain. Hence, biotope resilience is assessed as ‘Very low’ (the pressure is a permanent change) and sensitivity as ‘High’.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

A number of studies assess the impacts of aggregate extraction on sand and gravel habitats. Recovery of sediments will be site-specific and will be influenced by currents, wave action and sediment availability (Desprez, 2000). Except in areas of mobile sands, the process tends to be slow (Kenny & Rees, 1996; Desprez, 2000).  Boyd et al. (2005) found that in a site subject to long-term extraction (25 years), extraction scars were still visible after six years and sediment characteristics were still altered in comparison with reference areas with ongoing effects on the biota. The strongest currents are unable to transport gravel. A further implication of the formation of these depressions is a local drop in current strength associated with the increased water depth, resulting in the deposition of finer sediments than those of the surrounding substrata (Desprez et al., 2000). 

Sensitivity assessment. Resistance is assessed as ‘None’ as the extraction of the sediment will remove the characterizing and associated species present, within the affected area. Resilience is assessed as ‘Medium’ as some species may require longer than two years to re-establish (see resilience section) and sediments may need to recover (where exposed layers are different). Hence, 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

Comparative studies between disturbed and undisturbed areas indicate that abrasion and disturbance from bottom trawling on coarse gravels, sands and muds reduce the abundance of organisms, biomass and species diversity (Collie et al., 1997). For example, Aphelochaeta marioni, and Lumbrineris latreilli were characterized as AMBI Fisheries Review Group III "species insensitive to fisheries in which the bottom is disturbed; their populations do not show a significant decline or increase" (Gittenberger & Van Loon, 2011). Mediomastus fragilis was characterized as Group IV, "second-order opportunistic species, which are sensitive to fisheries in which the bottom is disturbed; their populations recover relatively quickly however and benefit from the disturbance, causing their population sizes to increase significantly in areas with intense fisheries". Nucula nitidosa was characterized as Group II, "species sensitive to fisheries in which the bottom is disturbed, but their populations recover relatively quickly" (Gittenberger & Van Loon, 2011).  Bradshaw et al. (2000) noted that the abundance of Lumbrinereis gracilis and Cirratulidae indet. increased in experimental scallop dredged vs. undredged plots.  

Sensitivity assessment. Abrasion is likely to damage epifauna and may damage a proportion of the characterizing species. Therefore, resistance is assessed as ‘Medium’. Hence, resilience is assessed as ‘High’ as opportunistic species are likely to recruit rapidly and some damaged characterizing species may recover or recolonize 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). Further detail

Evidence

Comparative studies between disturbed and undisturbed areas indicate that abrasion and disturbance from bottom trawling on coarse gravels and sands, reduce the abundance of organisms, biomass and species diversity (Collie et al., 1997). Undisturbed sites contain more calcareous tube worms, bryozoans and hydroids and small fragile polychaetes and brittlestars.  

Experiments in shallow, wave-disturbed areas, using a toothed, clam dredge, found that deposit-feeding polychaetes were more impacted than carnivorous species. Dredging resulted in reductions of >90% of Spiophanes bombyx immediately post dredging compared with before impact samples and the population reduction persisting for 90 days (although results may be confounded by storm events within the monitoring period which caused sediment mobility). Some predatory polychaete taxa were enhanced by fishing. Protodorvillea kefersteini was one of these. Large increases in abundance in samples were detected post-dredging and persisting over 90 days. The passage of the dredge across the sediment floor will have killed or injured some organisms that will then be exposed to potential predators/scavengers (Frid et al., 2000; Veale et al., 2000) providing a food source to mobile scavengers including these species. Protodorvillia kefersteini also showed a rapid increase in abundance at 21 days after sediment disturbance (Thrush, 1986).

Bergman & Hup (1992) carried out a pre and post-experimental investigation using a 12 m beam trawl. The area was trawled three times over 2 days and samples were taken up to 2 weeks after trawling. Some benthic species showed a 10-65% reduction in density after trawling the area three times. There was a significant lowering of densities (40-60%) of echinoderms Asterias rubens and small Echinocardium cordatum, and of polychaete worms Lanice conchilega and Spiophanes bombyx. No change in the total density of Owenia fusiformis was observed (Bergman & Hup, 1992). Bradshaw et al. (2000) noted that the abundance of Lumbrinereis gracilis and Cirratulidae indet. increased in experimental scallop dredged vs. undredged plots.  

Aphelochaeta marioni, and Lumbrineris latreilli were characterized as AMBI Fisheries Review Group III "species insensitive to fisheries in which the bottom is disturbed; their populations do not show a significant decline or increase" (Gittenberger & Van Loon, 2011). Mediomastus fragilis was characterized as Group IV, "second-order opportunistic species, which are sensitive to fisheries in which the bottom is disturbed; their populations recover relatively quickly however and benefit from the disturbance, causing their population sizes to increase significantly in areas with intense fisheries". Nucula nitidosa was characterized as Group II, "species sensitive to fisheries in which the bottom is disturbed, but their populations recover relatively quickly" (Gittenberger & Van Loon, 2011). 

Sensitivity assessment. The trawling studies and Gittenberger & Van Loon (2011) suggest that the biological assemblage present in this biotope is characterized by species that are relatively resistant to penetration and disturbance of the sediments or recover quickly. Therefore, resistance is assessed as ‘Medium’ as some species will be displaced and may be predated or injured and killed. Hence, resilience is assessed as ‘High’ as most species will recover rapidly and sensitivity is assessed as ‘Low’. 

Changes in suspended solids (water clarity)

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

Evidence

No direct evidence was found to assess impacts on the characterizing species.  A change in turbidity at the pressure benchmark is assessed as an increase from intermediate 10-100 mg/l to medium (100-300 mg/l) and a change to clear (<10 mg/l). An increase or decrease in turbidity may affect primary production in the water column and indirectly alter the availability of phytoplankton food available to species in filter-feeding mode. However, phytoplankton will also be transported from distant areas and so the effect of increased turbidity may be mitigated to some extent. 

Changes in turbidity and seston are not predicted to directly affect burrowing polychaetes that live within sediments. The biotope is dominated by deposit-feeding species that are unlikely to be directly affected by increases or decreases in suspended sediments. Therefore, biotope resistance is assessed as ‘High’, resilience as ‘High’, and sensitivity is assessed as ‘Not sensitive' albeit with 'Low' confidence. 

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 addition of fine material will alter the character of this habitat by covering it with a layer of dissimilar sediment and will reduce its suitability for the species associated with this feature. Recovery will depend on the rate of sediment mixing or removal of the overburden, either naturally or through human activities. Recovery to a recognisable form of the original biotope will not take place until this has happened. In areas where the local hydrodynamic conditions are unaffected, fine particles will be removed by wave action moderating the impact of this pressure. The rate of habitat restoration would be site-specific and would be influenced by the type of siltation and rate. The long-term or permanent addition of fine particles would lead to the reclassification of this biotope type (see physical change pressures).

Little direct evidence of the effects of smothering on the characteristic species was found. Lumbrineris latreilli was characterized as AMBI sedimentation Group III: "species insensitive to higher amounts of sedimentation, but don’t easily recover from strong fluctuations in sedimentation" (Gittenberger & Van Loon, 2011). Mediomastus fragilis, Aphelochaeta marioni and Nucula nitidosa were characterized as Group IV, "second-order opportunistic species, insensitive to higher amounts of sedimentation; although they are sensitive to strong fluctuations in sedimentation, their populations recover relatively quickly and even benefit; this causes their population sizes to increase significantly in areas after a strong fluctuation in sedimentation" (Gittenberger & Van Loon, 2011). Ranchor (1976) suggested that Nucula nitidosa was tolerant of anaerobic conditions and their mobility, allowed them to survive when covered by sediments during stormy weather. Ranchor (1976) also reported that Nucula nitidosa was abundant in an area subject to sewage sludge dumping. 

Sensitivity assessment. This biotope (SS.SMx.IMx.MedCirr) is exposed to strong to weak tidal streams in moderately wave exposed to sheltered conditions, so that fine sediments may be removed quickly. The biotope is dominated by infaunal deposit-feeding polychaetes and bivalves that are likely to survive short periods under 5 cm of rapidly deposited sediment. Therefore, resistance is assessed as 'High', resilience as 'High' and sensitivity assessed as '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 addition of fine material will alter the character of this habitat by covering it with a layer of dissimilar sediment and will reduce its suitability for the species associated with this feature. Recovery will depend on the rate of sediment mixing or removal of the overburden, either naturally or through human activities. Recovery to a recognisable form of the original biotope will not take place until this has happened. In areas where the local hydrodynamic conditions are unaffected, fine particles will be removed by wave action moderating the impact of this pressure. The rate of habitat restoration would be site-specific and would be influenced by the type of siltation and rate. The long-term or permanent addition of fine particles would lead to the reclassification of this biotope type (see physical change pressures).

Little direct evidence of the effects of smothering on the characteristic species was found. Lumbrineris latreilli was characterized as AMBI sedimentation Group III: "species insensitive to higher amounts of sedimentation, but don’t easily recover from strong fluctuations in sedimentation" (Gittenberger & Van Loon, 2011). Mediomastus fragilis, Aphelochaeta marioni and Nucula nitidosa were characterized as Group IV, "second-order opportunistic species, insensitive to higher amounts of sedimentation; although they are sensitive to strong fluctuations in sedimentation, their populations recover relatively quickly and even benefit; this causes their population sizes to increase significantly in areas after a strong fluctuation in sedimentation" (Gittenberger & Van Loon, 2011). Ranchor (1976) suggested that Nucula nitidosa was tolerant of anaerobic conditions and their mobility, allowed them to survive when covered by sediments during stormy weather. Ranchor (1976) also reported that Nucula nitidosa was abundant in an area subject to sewage sludge dumping. 

Sensitivity assessment. This biotope (SS.SMx.IMx.MedCirr) is exposed to strong to weak tidal streams in moderately wave exposed to sheltered conditions, so that fine sediments may be removed quickly. The biotope is dominated by infaunal deposit-feeding polychaetes and bivalves that are likely to survive short periods of smothering by fine sediment. However, the deposition of 30 cm of fine sediment may take several tidal cycles to be removed and some of the more sensitive species may be reduced in abundance within the affected area. Therefore, resistance is assessed as 'Medium', resilience as 'High' and sensitivity assessed as 'Low' at the benchmark level.  

Litter

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

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No evidence.

Underwater noise changes

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

Evidence

'Not relevant'.

Introduction of light or shading

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

Evidence

Invertebrate species such as the bivalves and polychaetes may possess rudimentary eyes and be able to perceive light and dark. Changes in light levels are unlikely to affect adult stages, especially burrowing, infaunal species. This pressure is therefore assessed as ‘Not relevant’. 

Barrier to species movement

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

Evidence

Not relevant. This pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of larval stages or propagules. However, the dispersal of larval stages or propagules is not considered under the pressure definition and benchmark.

Death or injury by collision

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

Evidence

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

Visual disturbance

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

Evidence

'Not relevant'. Invertebrate species such as the bivalves and polychaetes may possess rudimentary eyes and be able to perceive light and dark but are unlikely to respond to visual disturbance as defined by this pressure. 

Genetic modification & translocation of indigenous species

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

Evidence

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

Introduction or spread of invasive non-indigenous species

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

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40) although it prefers ca 30 (Tillin et al., 2020). 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 in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020, Fitzgerald, 2007, Blanchard, 2009, Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015, Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets, that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy species between or within shells, resulting in a homogeneous Crepidula dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m². Where slipper limpet stacks are abundant, few other bivalves can live amongst them (Fretter & Graham, 1981; Blanchard, 1997). 

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330-1300 ind./m²) but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula with only moderate or low abundances (11 ind./m²) and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension–feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm) but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action which may be less suitable (Tillin et al., 2020). 

King scallop (Pecten maximus) and Queen scallop (Aequipecten opercularis) in the Bay of Brest, have been reported to decrease in the presence of Crepidula, largely due to silting and biodeposition that changes the habitat (Stiger Pouvreau & Thouzeau, 2015; Thouzeau et al., 2000). The scallop post larvae are unable to settle and survive on muddy Crepidula substrata. Crepidula could potentially be the main competitor for Pecten maximus, specially creating competition for space (Menesguen & Gregoris, 2018; Ragueneau et al., 2018). However, no direct competition for food was observed between Crepidula and the scallops (Thouzeau et al., 2000, Chauvaud et al. 2000) and scallop shell growth rates did not decrease with increasing Crepidula populations. Therefore, although Crepidula populations will likely impact scallop post larvae settlement, it does not affect shell growth rates or adult survivorship (Thouzeau et al., 2000). Models show that competition for space between the species does not impact the abundance of Crepidula, but does lower the abundance of Pecten sp. (Menesguen & Gregoris, 2018). 

The colonial ascidian Didemnum vexillum is present in the UK but appears to be restricted to artificial surfaces such as pontoons, this species may, however, have the potential to colonize and smother offshore gravel habitats. Valentine et al. (2007) describe how Didemnum sp. appears to have rapidly colonized gravel areas on the Georges Bank (US/Canada boundary). Colonies can coalesce to form large mats that may cover more than 50% of the seabed in parts.  

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile and unsuitable for most of the invasive non-indigenous species currently recorded in the UK. The above evidence suggests that Crepidula could colonize mixed sediment habitats in the subtidal, typical of this biotope, due to the presence of gravel, shells, cobbles, or any other hard substrata that can be used for larvae settlement (Tillin et al., 2020). Bohn et al. (2015) demonstrated that Crepidula had a preference for gravelly habitats, while De Montaudouin & Sauriau (1999) and Bohn et al. (2015) noted that Crepidula densities were low in intertidal coarse sediments. Therefore, Crepidula has the potential to colonize, and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020), as occurs in maerl gravels (Grall & Hall-Spencer, 2003) resulting in the loss of the biotope. This is a moderately exposed to very sheltered habitat, so storms may mobilise the sediment (JNCC, 2022), which may also mitigate or prevent colonization by Crepidula at high densities, although it has been recorded from areas of strong tidal streams (Hinz et al., 2011). Therefore, the habitat may be more suitable for Crepidula in wave sheltered areas of the biotope and where water movement is mediated by tidal flow rather than wave action, e.g., the deeper examples of the biotope. Didemnum sp. may also emerge as a threat to this biotope.

Therefore, resistance is assessed as 'Medium' in examples where wave action is high and subject to storms but 'Low' in wave sheltered areas dominated by tidal flow. Resilience is assessed as 'Very low' as it would require the removal of Crepidula, probably by artificial means. Hence, sensitivity is assessed as 'High' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope so the confidence in the assessment is 'Low' and 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

No evidence was found for the characterizing polychaete species.

Removal of target species

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

Evidence

The characteristic polychaete species are not directly targeted by fisheries. Hence, this pressure is assessed as '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

Species within the biotope are not functionally dependent on each other, although biological interactions will play a role in structuring the biological assemblage through predation and competition. Removal of adults may support the recruitment of juveniles by reducing competition for space and consumption of larvae. Removal of species would also reduce the ecological services provided by these species such as secondary production and nutrient cycling.

Sensitivity assessment. Species within the biotope are relatively sedentary or slow-moving, although the infaunal position may protect some burrowing species from removal. Therefore, resistance is assessed as ‘Low’ and resilience as ‘High’, as the habitat is likely to be directly affected by removal and some species will recolonize rapidly. Hence, sensitivity is assessed as 'Low'.