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

Amphipods were reported to have a low tolerance to temperature changes (Bousfield, 1973) although lethal limits were not given. However, the amphipods that occur within this habitat are mobile and can avoid unfavourable conditions to some extent. Ampelisca brevicornis is widespread in the North East Atlantic but is also recorded from the White Sea, south to the Iberian Peninsula and the Mediterranean, and as far as the coasts of South Africa, India, China and the Korean Peninsula (OBIS, 2020). It has been recorded in areas where sea surface temperatures (SST) range from 5 to 30°C, although most records occur in the 10-15°C range (OBIS, 2020).  Similarly, Ampelisca typica is also recorded from the White Sea, through the North East Atlantic to the Iberian Peninsula, and in areas where the SST ranged from 5-25°C although most records occurred in 10-15°C (OBIS, 2020). However, other species, e.g. Ampelisca sarsi have more limited distribution south of the British Isles (OBIS, 2020). Photis longicaudata is also widespread in the North East Atlantic but is also recorded from the Barents Sea, south to the Iberian Peninsula and the Mediterranean, and as far as the coasts of South Africa, India, China and the Korean Peninsula, and in the west Atlantic from Florida south to Uruguay and in areas where the SST ranged from minus 5-0°C and 5-30°C although most records occurred in 10-15°C (OBIS, 2020).

Murina (1997) categorized Polydora ciliata as a eurythermal species because of its ability to spawn in temperatures ranging from 10.6-19.9°C. This is consistent with a wide distribution in north-west Europe, which extends into the warmer waters of Portugal and Italy (Pardal et al., 1993; Sordino et al., 1989). In the western Baltic Sea, Gulliksen (1977) recorded high abundances of Polydora ciliata at temperatures of 7.5 to 11.5°C and in Whitstable, in Kent, where sea temperatures varied between 0.5 and 17°C (Dorsett, 1961). Growth rates may increase if the temperature rises. For example, at Whitstable in Kent, Dorsett (1961) found that a rapid increase in growth coincided with the rising temperature of the seawater during March. No information was found regarding the intolerance of Spiophanes bombyx to temperature. Spiophanes bombyx is found in the Mediterranean (Hayward & Ryland, 1995b), which is likely to be warmer than the waters around Britain and Ireland.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4-19°C (Huthnance, 2010). The characteristic tube mat forming species are distributed to the north and south of the British Isles and are likely to be able to resist a long-term increase in temperature of 2°C and may resist a short-term increase of 5°C in UK waters. Resistance and resilience are, therefore, assessed as 'High' and the biotope 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

Amphipods were reported to have a low tolerance to temperature changes (Bousfield, 1973) although lethal limits were not given. However, the amphipods that occur within this habitat are mobile and can avoid unfavourable conditions to some extent. Ampelisca brevicornis is widespread in the North East Atlantic but is also recorded from the White Sea, south to the Iberian Peninsula and the Mediterranean, and as far as the coasts of South Africa, India, China and the Korean Peninsula (OBIS, 2020). It has been recorded in areas where sea surface temperatures (SST) range from 5 to 30°C, although most records occur in the 10-15°C range (OBIS, 2020).  Similarly, Ampelisca typica is also recorded from the White Sea, through the North East Atlantic to the Iberian Peninsula, and in areas where the SST ranged from minus 5-0°C and 5-25°C although most records occurred in 10-15°C (OBIS, 2020). However, other species, e.g. Ampelisca sarsi have more limited distribution south of the British Isles (OBIS, 2020). Photis longicaudata is also widespread in the North East Atlantic but is also recorded from the Barents Sea, south to the Iberian Peninsula and the Mediterranean, and as far as the coasts of South Africa, India, China and the Korean Peninsula, and in the west Atlantic from Florida south to Uruguay and in areas where the SST ranged from minus 5-0°C and 5-30°C although most records occurred in 10-15°C (OBIS, 2020).

Murina (1997) categorized Polydora ciliata as a eurythermal species because of its ability to spawn in temperatures ranging from 10.6-19.9°C. This is consistent with a wide distribution in north-west Europe. In the western Baltic Sea, Gulliksen (1977) recorded high abundances of Polydora ciliata at temperatures of 7.5 to 11.5°C and in Whitstable, Kent, abundance was high when winter water temperatures dropped to 0.5°C (Dorsett, 1961). During the extremely cold winter of 1962/63, Polydora ciliata was unaffected (Crisp, 1964).  Spiophanes bombyx is found in waters off Denmark (Thorson, 1946), which are likely to be colder than British and Irish waters.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4-19°C (Huthnance, 2010). The characteristic tube mat forming species are distributed to the north and south of the British Isles and are likely to be able to resist a long-term decrease in temperature of 2°C and may resist a short-term decrease of 5°C in UK waters. Resistance and resilience are, therefore, assessed as 'High' and the biotope assessed as 'Not Sensitive' 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

Monitoring at a Spanish desalination facility where discharges close to the outfall reached a salinity of 53, found that amphipods, including Ampelisca spp. were sensitive to the increased salinity and that species free-living in the sediment were most sensitive (De-la-Ossa-Carretero et al., 2016).  Roberts et al. (2010b) reported that the effects of brine discharge were dependent on the receiving environment but that the effects were limited to with 10s of metres of the outfall. In their review, they reported that polychaete abundance and diversity decreased adjacent to a brine outfall in Alicante, Spain and that the family Ampharaetidae were the most sensitive while the family Paraonidae were the least sensitive (Ruso et al., 2008 cited in Roberts et al., 2010b).  Polydora ciliata is a euryhaline species inhabiting fully marine and estuarine habitats. It is unlikely that Spiophanes bombyx would experience hypersaline conditions, therefore unlikely to be adapted to such conditions.

Sensitivity assessment. The characterizing species of this biotope are euryhaline and likely to be resistant to increases in salinity. However, the biotope occurs full saline conditions (Connor et al., 2004) and is unlikely to experience hypersaline conditions. Therefore, exposure to hypersaline effluent (>40) might result in the death of a portion of the individuals in the population, especially Ampelisca and some polychaetes species. Therefore, resistance is assessed as 'Low' but with 'Low' confidence. Resilience is likely to be High, so the biotope is assessed as 'Low' sensitivity to an increase in salinity at the pressure benchmark.

Salinity decrease (local)

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

Evidence

Ampelisca brevicornis was recorded from areas where the sea surface salinity (SSS) ranged between 5 and 35 psu although most records were in the range 15-35 psu and Ampelisca typica was recorded from areas where the SSS ranged between 15 and 35 psu although most records were in the range 30-35 psu (OBIS, 2020). Photis longicaudata was recorded from areas where the sea surface salinity ranged between 10 and 35 psu although most records were in the range 30-35 (OBIS, 2020).

Polydora ciliata is a euryhaline species inhabiting fully marine and estuarine habitats. In an area of the western Baltic Sea, where bottom salinity was between 11.1 and 15.0 psu Polydora ciliata was the second most abundant species with over 1000 individuals per m² (Gulliksen, 1977). Spiophanes bombyx is a euryhaline species (Bailey-Brook, 1976; Maurer & Lethem, 1980), inhabiting fully saline and estuarine habitats.

Sensitivity assessment. SS.SMu.ISaMu.AmpPlon occurs in full salinity conditions (Connor et al., 2004). The evidence from distribution records (OBIS, 2020) suggests that the charactersitic amphipods vary in salinity tolerance but prefer full salinity conditions, except perhaps Ampelisca brevicornis, whereas the typical polychaetes are euryhaline. Therefore, a change in salinity regime from 'full' (30-35) to 'reduced' (18-30) might result in a reduction in the amphipod abundance and degradation of the tube mat. Hence, resitance is assessed as 'Medium' to represent the possible loss of a proportion of the resident community but with 'Low' confidence.  Resilience is probably 'High' so the biotope sensitivity is assessed as 'Low' to a decrease in salinity 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

Changes in water flow are likely to change the sediment characteristics in which the biotope occurs, primarily by resuspending and preventing deposition of finer particles (Hiscock, 1983). A decrease in water flow is unlikely to cause any impact on the biotope as species are adapted to incremental deposition, typical of low energy environments such as those where the sandy muds typical of this biotope occur. However, an increase would likely result in a decrease in tube-building material for the characterizing species, and the lack of deposition of particulate matter at the sediment surface would reduce food availability for the deposit feeders in the biotope. The resultant energetic cost over one year would be likely to result in some mortality of tube-builders and infauna. For example, Polydora ciliata was present and colonized test panels in Helgoland in three areas; two exposed to strong tidal currents and one site sheltered from currents (Harms & Anger, 1983). However, very strong water flows may sweep away Polydora colonies, often in a thick layer of mud on a hard substratum. 

The most damaging effect of increased flow rate would be the erosion of the medium to fine muddy sand substratum as this could eventually lead to loss of the habitat. Amphipods are thought to stabilize the intertidal sediments in which they reside (Mouritsen et al., 1998; MacKenzie et al., 2006). Mills (1967, 1969) noted that the tubes of Ampelisca increased the surface complexity of the sandflat in Barnstable Harbour, Massachusetts. However, their feeding decreased the median grain size of the sediment, towards fine particulates, and resulted in instability. Mills (1969) stated that minor breaks in the mat of Ampelisca tubes were sufficient to cause large areas to wash out, especially on windy days in a rising or falling tide, although no flow rates were given. Emergent species, such as the Polydora ciliata tubes that characterize this biotope, may create turbulent flow leading to particle resuspension. Additionally, where a change in water flow rate changes sediment characteristics, with increased deposits of coarser sediments, characterizing species may no longer be supported due to particular substratum preferences. 

Sensitivity assessment. SS.SMu.ISaMu.AmpPlon is recorded on sandy muds, which are typical of low energy environments i.e. wave sheltered and/or weak (<0.5 m/s) and very weak (negligible) tidal streams (Connor et al., 2004). Sand particles are likely to be eroded at about 0.20 m/s (based on Hjulström-Sundborg diagram, Sundborg, 1956), although the sandy muds are more cohesive and require higher flow rates to resuspend. Also, the tube mat may help to stabilize the sediment surface, initially, until itself destabilizes due to the build-up of finer particulates (Mills, 1967, 1969). Although a decrease in water flow rate is likely to be irrelevant, an increase in water flow at the pressure benchmark is may result in loss of parts of the characterizing mat of tubes formed by Ampelisca spp., and Polydora.  Furthermore, stronger currents (e.g. due to storms) are likely to wash away the community of semi-permanent tube-building amphipods and polychaetes that characterize the biotope. Therefore, resistance is assessed as 'Low' and resilience as 'High' so that sensitivity is assessed as 'Low' sensitivity to a change in water flow at the pressure 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

SS.SMu.ISaMU.AmpPlon is recorded from 0-30 m (Connor et al., 2004) and is predominately subtidal. Therefore, this pressure is probably 'Not relevant'.  

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

Where the biotope occurs in the shallow subtidal, it is likely to be affected by winter storms. Storms may cause dramatic changes in the distribution of macro-infauna by washing out dominant species, opening the sediment to recolonization by adults and/or available spat/larvae (Eagle, 1975; Rees et al., 1976; Hall, 1994) and by reducing the success of recruitment by newly settled spat or larvae (see Hall, 1994 for review). 

Feeding of the characterizing species may be impaired in strong wave action and changes in wave exposure may also influence the supply of particulate matter for tube-building polychaetes and amphipods. Mills (1967, 1969) noted that the tubes of Ampelisca increased the surface complexity of the sandflat in Barnstable Harbour, Massachusetts. Also, their feeding decreased the median grain size of the sediment, towards fine particulates, and resulted in instability. Mills (1969) stated that minor breaks in the mat of Ampelisca tubes were sufficient to cause large areas to wash out, especially on windy days in a rising or falling tide and reported that Ampelisca flats in Barnstable, Massachusetts, were damaged noticeably by winter storms (Mills, 1967, 1969). Mackenzie et al. (2006) reported that dense mats of Ampelisca persisted in Raritan Bay, New Jersey for at least five years because the mats occupied deeper waters (ca 7 m) and were sheltered from wave action. 

Sensitivity assessment. SS.SMu.ISaMu.AmpPlon is recorded on sandy muds, which are typical of low energy environments i.e. wave sheltered and/or weak (<0.5 m/s) and very weak (negligible) tidal streams (Connor et al., 2004). The tube mat that characterizes this biotope is probably very susceptible to damage or removal by during winter storms or on windy days, especially in the shallow examples of the habitat. Mills (1969) considered that the Ampelisca dominated tube mat demonstrated 'dynamic instability' due to its need to move from areas of physical disturbance and sediment modification.  Although the biotope occurs in low energy conditions (Connor et al., 2004) it is uncertain if a 3-5% change in significant wave height (the benchmark level) would be significant. Therefore, resistance is assessed as 'High', resilience as 'High' and sensitivity 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 any evidence is presented where available.

Experimental studies with various species suggest that polychaete worms are quite resistant to heavy metals (Bryan, 1984). Polydora ciliata occurred in an area of the southern North Sea polluted by heavy metals but was absent from sediments with very high heavy metal levels (Diaz-Castaneda et al., 1989). For most metals, toxicity to crustaceans increases with decreased salinity and elevated temperature, therefore marine species living within their normal salinity range may be less susceptible to heavy metal pollution than those living in salinities near the lower limit of their salinity tolerance (McLusky et al., 1986). In laboratory investigations, Hong & Reish (1987) observed 96-hour LC₅₀ water column concentrations of between 0.19 and 1.83 mg/l of cadmium (Cd) for several species of amphipod. Corophium volutator is highly intolerant of metal pollution at levels often found in estuaries from industrial outfalls and contaminated sewage. A concentration of 38 mg Cu/l was needed to kill 50% of Corophium volutator in 96-hour exposures (Bat et al., 1998). Other metals are far more toxic to Corophium volutator, e.g. zinc is toxic over 1 mg/l and toxicity to metals increases with increasing temperature and salinity (Bryant et al., 1985b). Mortality of 50% is caused by 14 mg/l (Bat et al., 1998). Although exposure to zinc may not be lethal, it may affect the perpetuation of a population by reducing growth and reproductive fitness. Mercury was found to be very toxic to Corophium volutator, e.g. concentrations as low as 0.1 mg/l caused 50% mortality in 12 days. Other metals are known to be toxic include cadmium, which causes 50% mortality at 12 mg/l (Bat et al., 1998); and arsenic, nickel and chromium which are all toxic over 2 mg/l (Bryant et al., 1984; Bryant et al., 1985; 1985b).

Hydrocarbon & PAH contamination

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

Evidence

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

In general, soft-sediment inhabitants, especially infaunal polychaetes, are particularly affected by oil pollution (Suchanek, 1993). For example, Jacobs (1980) investigated the effects of the Amoco Cadiz oil spill in 1978 and noted that the numbers of spionidae polychaetes decreased after the spill. In an analysis of kelp holdfast fauna following the Sea Empress oil spill in Milford Haven the fauna present, including Polydora ciliata, showed a strong negative correlation between numbers of species and distance from the spill (SEEEC, 1998). After the extensive oil spill in West Falmouth, Massachusetts, Grassle & Grassle (1974) followed the settlement of polychaetes in the disturbed area. Species with the most opportunistic life histories, including Polydora ligni, were able to settle in the area. This species has some brood protection which enables larvae to settle almost immediately in the nearby area (Reish, 1979). Furthermore, Gray et al. (1990) found that Scoloplos armiger was a dominant species in uncontaminated soft sediments at a case study site adjacent to the Ekofisk oil field but was not present at contaminated sites, suggesting Scoloplos armiger are also intolerant to hydrocarbon contaminates.

Amphipods in general and ampeliscid amphipods, in particular, seem particularly intolerant of contamination with oil. Dauvin (1998) reported reductions in abundance, biomass and production of Ampelisca sp. following the Amoco Cadiz oil spill. Furthermore, light fractions (C10 - C19) of oils are much more toxic to Corophium volutator than heavier fractions (C19 - C40). In exposures of up to 14 days, light fraction concentrations of 0.1 g/kg sediment caused high mortality. It took 9 g/kg sediment to achieve similar mortalities with the heavy fraction (Brils et al., 2002). In the Forth Estuary, Corophium volutator was excluded for several hundred metres around the outfalls from hydrocarbon processing plants. Roddie et al. (1994) found high levels of mortality of Corophium at sites contaminated with crude oil.

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 any evidence is presented where available.

In general, crustaceans are widely reported to be intolerant of synthetic chemicals (Cole et al., 1999) and intolerance to some specific chemicals has been observed in amphipods. Species of a different genus are likely to differ in their susceptibility to synthetic chemicals and that this may be related to differences in their physiology (Powell, 1979). Corophium volutator is paralysed by pyrethrum based insecticide sprayed onto the surface of the mud (Gerdol & Hughes, 1993) and pyrethrum would probably cause significant mortalities if it found its way into estuaries from agricultural runoff. Nonylphenol is an anthropogenic pollutant that regularly occurs in water bodies, it is an oestrogen mimic that is produced during the sewage treatment of non-ionic surfactants and can affect Corophium volutator (Brown et al., 1999). Nonylphenol is a hydrophobic molecule and often becomes attached to sediment in water bodies. This will make nonylphenol available for ingestion by Corophium volutator in estuaries where much of the riverine water-borne sediment flocculates and precipitates out of suspension to form mudflats. Nonylphenol is not lethal to Corophium volutator but does reduce growth and has the effect of causing the secondary antennae of males to become enlarged, which can make the amphipods more vulnerable to predators (Brown et al., 1999). Corophium volutator is killed by 1% ethanol if exposed for 24 hours or more but can withstand higher concentrations in short pulses. Such short pulses, however, have the effect of rephasing the diel rhythm and will delay the timing of swimming activity for the duration of the ethanol pulse (Harris & Morgan, 1984b).

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 Wilding & Hughes, 2010). OSPAR (2000) stated that, at that time, ivermectin was not licensed for use in mariculture but was incorporated into the feed as a treatment against sea lice at some farms. Ivermectin has the potential to persist in sediments, particularly fine-grained sediments at sheltered sites. 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. Furthermore, Polydora ciliata was abundant at polluted sites close to acidified, halogenated effluent discharge from a bromide-extraction plant in Amlwch, Anglesey (Hoare & Hiscock, 1974). Spionid polychaetes were found by McLusky (1982) to be relatively resistant of distillation and petrochemical industrial waste in Scotland.

Radionuclide contamination

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

Evidence

Corophium volutator readily absorbs radionuclides such as americium and plutonium from water and contaminated sediments (Miramand et al., 1982). However, the effect of contamination of the individuals was not known but accumulation through the food chain was assumed (Miramand et al., 1982). There was 'No evidence' on which to base an assessment.

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

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. Polychaetes appeared to be sensitive to hypoxia, as only 10% of polychaetes survived. 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 that may have reduced mortalities compared to other observations.

Amphipods appear not to be tolerant of reduced oxygenation. For example, Ampelisca agassizi was reported to be intolerant of hypoxia (Diaz & Rosenberg, 1995) and Jassa falcata, another tube building amphipod species, was absent from Californian harbours with low oxygen concentrations (0-2.5 mg/l).  Polydora ciliata is repeatedly found at localities with oxygen deficiency (Pearson & Rosenberg, 1978). For example, in polluted waters in Los Angeles and Long Beach harbours Polydora ciliata was present in the oxygen range 0.0-3.9 mg/l and the species was abundant in hypoxic fjord habitats (Rosenberg, 1977). Furthermore, in a study investigating a polychaete community in the north-west Black Sea, Polydora ciliata was observed in all four study sites, including those severely affected by eutrophication and hypoxia as a result of the discharge of wastewaters (Vorobyova et al., 2008). However, Polydora ciliata is unlikely to be able to resist anoxic conditions. Hansen et al. (2002) reported near total extinction of all metazoan in the Mariager Fjord (Denmark), including Polydora spp. after a severe hypoxia event that resulted in complete anoxia in the water column for two weeks. Additionally, Como & Magni (2009) investigated seasonal variations in benthic communities known to be affected by episodic events of hypoxia. The authors observed that the abundance of Polydora ciliata varied seasonally, decreasing during the summer months, and suggested it could be explained by the occurrence of hypoxic/anoxic conditions and sulphidic sediments during the summer. No details of the levels of dissolved oxygen leading to these community responses were provided. Other polychaetes in the biotope are also likely to be able to deal with hypoxia. For example, during low tide, the polychaete Scoloplos armiger survives deoxygenation by ascending into the oxidative layer where it can maintain aerobic metabolism. In laboratory conditions, Scoloplos armiger survived low oxygen conditions for 40 hours (Schöttler & Grieshaber, 1988).

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. 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.

Sensitivity assessment. Cole et al. (1999) suggested possible adverse effects on marine species exposed to dissolved oxygen concentrations below 4 mg/l and probable adverse effects below 2 mg/l. The characteristic polychaete Polydora ciliata is repeatedly found at localities with oxygen deficiency (Pearson & Rosenberg, 1978) and seems to only be affected by severe deoxygenation episodes. Other polychaetes in the biotope are likely to behave similarly. However, the mortality of tube building Ampelisca spp. species is likely. Therefore, resistance to deoxygenation at the pressure benchmark level is assessed as 'Low' but resilience is likely to be 'High so that biotope sensitivity is assessed as 'Low' sensitivity to exposure to a dissolved oxygen concentration of less than or equal to 2 mg/l for 1 week.

Nutrient enrichment

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

Evidence

Connor et al. (2004) suggested that this biotope was associated with moderate organic enrichment. Amphipods appear to be tolerant of and indeed prefer, high nutrient levels. Diaz et al. (2008) reported the change in the abundance of Ampelisca tube mats in Boston Harbour, USA when wastewater outfalls were moved from onshore to offshore. Boston Harbour became changed from anaerobic to more aerobic between 1992 and 2006.  Ampelisca mats became widespread in 1992 and then declined by 2000. Diaz et al. (2008) suggested that the optimal organic loading for the maintenance of large areas of amphipod tube mats around 500 gC per square meter per year; above or below which levels, the mats declined. 

Polydora ciliata is often found in environments subject to high levels of nutrients. For example, the species was abundant in areas of the Firth of Forth exposed to high levels of sewage pollution (Smyth, 1968), in nutrient-rich sediments in the Mondego Estuary, Portugal (Pardal et al., 1993), and the coastal lagoon Lago Fusaro, Naples (Sordino et al., 1989). The extensive growths of Polydora ciliata in mat formations were recorded at West Ganton, in the Firth of Forth, prior to the introduction of the Sewage Scheme (Read et al., 1983). The abundance of the species was probably associated with their ability to use the increased availability of nutrients as a food source and silt for tube building.

Sensitivity assessment. This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. The characterizing species of this biotope are likely to be able to resist and be favoured by nutrient enrichment where increased availability of nutrients may be used as a source of food (Hiscock et al., 2005a).  Nevertheless, the biotope is assessed as 'Not Sensitive' at the pressure benchmark level, which is set at compliance with Water Framework Directive (WFD) criteria for good status, based on nitrogen concentration (UKTAG, 2014).

Organic enrichment

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

Evidence

Connor et al. (2004) suggested that this biotope was associated with moderate organic enrichment. Amphipods appear to be tolerant of and indeed prefer, high nutrient levels. Diaz et al. (2008) reported the change in the abundance of Ampelisca tube mats in Boston Harbour, USA when wastewater outfalls were moved from onshore to offshore. Boston Harbour became changed from anaerobic to more aerobic between 1992 and 2006.  Ampelisca mats became widespread in 1992 and then declined by 2000. Diaz et al. (2008) suggested that the optimal organic loading for the maintenance of large areas of amphipod tube mats around 500 gC per square meter per year; above or below which levels the mats declined.  Gittenberger & Van Loon (2011) assigned Ampelisca brevicornis to AMBI Group II "Species indifferent to enrichment, always present in low densities with non-significant variations with time (from initial state, to slight unbalance). These include suspension feeders, less selective carnivores and scavengers".

Polydora ciliata is often found in environments subject to high levels of nutrients. For example, the species was abundant in areas of the Firth of Forth exposed to high levels of sewage pollution (Smyth, 1968), in nutrient-rich sediments in the Mondego Estuary, Portugal (Pardal et al., 1993), and the coastal lagoon Lago Fusaro in Naples (Sordino et al., 1989). The extensive growths of Polydora ciliata in mat formations were recorded at West Ganton, in the Firth of Forth, prior to the introduction of the Sewage Scheme (Read et al., 1983). The abundance of the species was probably associated with their ability to use the increased availability of nutrients as a food source and silt for tube building. In colonization experiments in an organically polluted fjord receiving effluent discharge from Oslo, Polydora ciliata had settled in large numbers within the first month (Green, 1983). However, Callier et al. (2007) investigated the spatial distribution of macrobenthos under a suspended mussel culture, in eastern Canada, where the sedimentation of organic matter to the bottom was approx. 1-3 gC/m²/day. Polydora ciliata was recorded as absent in the sites under the suspended mussel farm after one year and as dominant in reference areas of the study.  Como & Magni (2009) investigated seasonal variations in benthic communities known to be affected by episodic events of sediment over-enrichment. The authors observed that abundance of Polydora ciliata varied seasonally, and suggested this could be a result major accumulation of organic carbon-binding fine sediments in the study site.  Studies by Almeda et al. (2009) and Pedersen et al. (2010) investigated larval energetic requirements for Polydora ciliata and suggested maximum growth rates were reached at food concentrations ranging from 1.4 to 2.5 μg C/ml depending on larval size, and energetic carbon requirements of 0.09 to 3.15 μg C l/d, respectively. Borja et al. (2000) and Gittenberger & Van Loon (2011) both assigned Polydora ciliata to their AMBI Group IV ‘second-order opportunistic species (slight to pronounced unbalanced situations)’, However, Polydora ciliata can also occur in organically poor areas (Pearson & Rosenberg, 1978).

Sensitivity assessment. The evidence presented suggests that the characterizing species of this biotope are likely to be stimulated by enrichment and only affected by excessive organic enrichment (above the benchmark level). Therefore, resistance and resilience are assessed as 'High', and the biotope is assessed as 'Not Sensitive' to organic enrichment involving deposition of 100 gC/m²/yr.  It should be noted that a decrease in organic enrichment may be detrimental as the abundance and extent of Ampelisca tube mats require moderate organic enrichment (see Diaz et al., 2008) but a reduction in organic enrichment is not addressed by this pressure. 

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

SS.SMu.ISaMu.AmpPlon is characterized by sandy mud substratum.  A change to a rock or artificial substratum would result in the loss of suitable habitat and the characterizing species, significantly altering the character of the biotope. The biotope would be lost and/or reclassified.

Sensitivity assessment. Resistance to the pressure is considered 'None', and resilience 'Very Low' based on the permanent loss of suitable substratum to support the community of the characterizing tube-building polychaete and amphipod species. Sensitivity has been assessed as '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 sediment type)

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

Evidence

SS.SMu.ISaMu.AmpPlon is characterized by sandy muds only (Connor et al., 2004). A change in sediment type by one Folk Class (based on the Long, 2006 simplification) would result in an increase in the fraction of sand and gravel in the substratum.  Ampelisca tube mat communities are recorded from sandflats (e.g. Mills, 1967, 1969) and could potentially survive a change to a fine sand substratum. However, the biotope would probably transition into its sandy equivalent SS.SSa.IFiSa.TbAmPo and be reclassified. Mackenzie et al. (2006) reported that Ampelisca abdita did not occur on medium to coarse sand because they had difficulty using in to build their tubes.  A change to coarse sand or gravels would no longer support the characteristic species and the biotope would be lost and/or reclassified. 

Sensitivity assessment. Resistance to the pressure is considered 'None', and resilience 'Very Low' based on the permanent loss of suitable substratum to support the community of the characterizing species. Sensitivity has been assessed as 'High'.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Removal of the substratum to 30 cm would result in the loss of the tube mat created by Ampelisca spp., and its associated species e.g. Polydora ciliata tubes. 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 deposition of finer sediments than those of the surrounding substrate (Desprez, 2000).

Sensitivity assessment. Resistance is assessed as 'None' as the extraction of the sediment will remove the characterizing and associated species present. Resilience is assessed as 'Medium' (see resilience section) and sediments may need to recover (where exposed layers are different). Biotope sensitivity is, therefore, 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

The tubes of the polychaetes and amphipods are bound only with mucous and are, therefore, likely to be damaged or removed by abrasion. The Ampelisca dominated tube mat sits in the surface of the sediment. Mills (1967, 1969) noted that the tubes of Ampelisca increased the surface complexity of the sandflat in Barnstable Harbour, Massachusetts. However, their feeding decreased the median grain size of the sediment, towards fine particulates, and resulted in instability. Mills (1969) stated that minor breaks in the mat of Ampelisca tubes were sufficient to cause large areas to wash out, especially on windy days in a rising or falling tide.  The tube mat would probably be removed easily by the passage of bottom gears.  Therefore, resistance to abrasion is assessed as 'Low'  but resilience is probably  'High' so that sensitivity to surface abrasion 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

Activities that penetrate below the surface are likely to tear up and remove a significant proportion of the tube building community that characterize this biotope. Bergman & Van Santbrink (2000) found that direct mortality of gammarid amphipods, following a single passage of a beam trawl (in silty sediments where penetration is greater) was 28%. Furthermore, stomach analysis of fish caught scavenging in the tracks of beam trawls found parts of Ampelisca spp. indicating that these had been damaged and exposed by the trawl (Kaiser & Spencer, 1994). 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). 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. Bergman & Hup (1992) carried out a pre and post-experimental investigation using a 12 m beam trawl. The area was trawled three times over two days and samples taken up to two weeks after trawling. Some benthic species showed a 10-65% reduction in density after trawling the area three times. There was a significant lower of density (40-60%) of polychaete worms, including Spiophanes bombyx.

The Ampelisca dominated tube mat sits in the surface of the sediment. Mills (1967, 1969) noted that the tubes of Ampelisca increased the surface complexity of the sandflat in Barnstable Harbour, Massachusetts. However, their feeding decreased the median grain size of the sediment, towards fine particulates, and resulted in instability. Mills (1969) stated that minor breaks in the mat of Ampelisca tubes were sufficient to cause large areas to wash out, especially on windy days in a rising or falling tide.  The tube mat would probably be removed easily by the passage of bottom gears. 

Sensitivity assessment. The evidence presented suggests that the tube mat community and associated infauna may suffer significant mortality (>75%) as a result of penetrative activities of the seabed. Biotope resistance is, therefore, assessed as 'None' and recovery is assessed as 'Medium so that sensitivity is assessed as 'Medium'.

Changes in suspended solids (water clarity)

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

Evidence

The biotope is likely to occur in relatively turbid waters that allow sediment deposition to support the community of characterizing tube-building polychaetes and amphipods, and, therefore, the species in the biotope are likely to be adapted to turbid conditions. Amphipods are tolerant of high turbidity and gather suspended sediment for the construction of tubes. Mills (1967) reported that feeding by Ampelisca vadorum and Ampelisca abdita were initiated by the turbidity of the water surrounding the tubes. However, the feeding structures of suspension feeders such as Ampelisca spp. may become clogged by large increases in suspended sediment or feeding may be terminated, compromising growth. However, Mackenzie et al. (2006) noted that the Ampelisca mat modified the mud surface substantially. The tube mat stabilized the mud surface, minimised the resuspension of mud by strong currents and the surface of the sediment becomes covered by their faecal pellets that produce little turbidity when in the water column.  Mackenzie et al. (2006) also noted that suspension feeding by the dense population of Ampelisca spp. reduces turbidity locally by capturing silt in their faeces.

Tube-building polychaetes are likely to be tolerant of high turbidity as they normally inhabit waters with high levels of suspended sediment which they actively fix in the process of tube making. For example, in the Firth of Forth, Polydora ciliata formed extensive mats in areas that had an average of 68 mg/l suspended solids and a maximum of approximately 680 mg/l indicating the species can tolerate different levels of suspended solids (Read et al., 1982; Read et al., 1983). Daro & Polk (1973) reported that the success of Polydora is directly related to the quantities of muds of any origin carried along by rivers or coastal currents. Deposit feeders and tube builders rely on siltation of suspended sediment. A decrease in suspended sediment will reduce this supply and therefore may compromise growth and reproduction.  Spiophanes bombyx is found in estuarine regions which experience high levels of turbidity. Spiophanes bombyx is a surface deposit feeder and relies on a supply of nutrients at the sediment surface. An increase in turbidity, reducing light availability may reduce primary production by phytoplankton in the water column. Although productivity in the biotope is secondary, a reduction in primary production in the water column may result in reduced food supply to deposit and suspension feeders, which in turn may affect growth rates and fecundity.

Sensitivity assessment. An increase in suspended solids at the pressure benchmark level is unlikely to affect the characterizing species of this biotope. However, a decrease in the suspended matter in the biotope could result in limitation of material for tube building activities and the loss of suitable substratum for colonization by new recruits of Polydora ciliata, in particular. Therefore, resistance is assessed as 'Low' (loss of 25-75%) and resilience is 'High' so that sensitivity is assessed as 'Low' at the benchmark level. 

Smothering and siltation rate changes (light)

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

Evidence

The tubes of Ampelisca spp. are probably several centimetres in length, depending on species, and extend above the sediment surface. For example, the tubes of Ampelisca abdita in Barnstable Harbour were about 3.5 cm in length a protruded 1 cm above the sediment surface (Mills, 1967). The tube mats occur in sedimentary, muddy environments so that they are probably adapted to steady rates of sedimentation and require sediment to continue to feed and build their tubes. However, no information on their ability to burrow or 'grow up' through deposited sediment was found. Ampelisca brevicornis was characterized by Gittenberger & Van Loon (2011) in their index of sedimentation tolerance as Group II "species sensitive to high sedimentation. They prefer to live in areas with some sedimentation, but don’t easily recover from strong fluctuations in sedimentation". 

A Polydora mud can be up to 50 cm thick, but the animals themselves occupy only the first few centimetres. They either elongate their tubes or have left them to rebuild close to the surface. Munari & Mistri (2014) investigated the spatio-temporal variation pattern of a benthic community following deposition of dredged material, at a maximum thickness of 30–40 cm. Polydora ciliata was amongst the first colonizers of the newly deposited sediments. The authors suggested that individuals may have migrated vertically through the deep layer of dredged sand. This was based on the results of Roberts et al. (1998) who suggested 15 cm as the maximum depth of overburden through which benthic infauna can successfully migrate. After one year, no adverse impact of sand disposal on the benthic fauna was detected on the study site (Munari & Mistri, 2014).  Spiophanes bombyx was Group IV species: ‘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). 

Sensitivity assessment. The Ampelisca tube mat is probably of a similar height to a deposit of 5 cm of fine sediment.  Polydora ciliata and other characterizing polychaetes are likely to resist or relocate after smothering by 5 cm of sediment. However, this ‘light’ deposition of fine sediment is may cause some mortality of Ampelisca spp.. based on Gittenberger & Van Loon (2011).  Therefore, resistance is assessed as 'Medium' albeit with 'Low' confidence.  Resilience is probably 'High' and biotope sensitivity is assessed as 'Low' sensitivity to a ‘light’ deposit of up to 5 cm of fine material in a single discrete event.

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 tubes of Ampelisca spp. are probably several centimetres in length, depending on species, and extend above the sediment surface. For example, the tubes of Ampelisca abdita in Barnstable Harbour were about 3.5 cm in length a protruded 1 cm above the sediment surface (Mills, 1967). The tube mats occur in sedimentary, muddy environments so that they are probably adapted to steady rates of sedimentation and require sediment to continue to feed and build their tubes. However, no information on their ability to burrow or 'grow up' through deposited sediment was found. Ampelisca brevicornis was characterized by Gittenberger & Van Loon (2011) in their index of sedimentation tolerance as Group II "species sensitive to high sedimentation. They prefer to live in areas with some sedimentation, but don’t easily recover from strong fluctuations in sedimentation". 

A Polydora mud can be up to 50 cm thick, but the animals themselves occupy only the first few centimetres. They either elongate their tubes or have left them to rebuild close to the surface. Munari & Mistri (2014) investigated the spatio-temporal variation pattern of a benthic community following deposition of dredged material, at a maximum thickness of 30–40 cm. Polydora ciliata was amongst the first colonizers of the newly deposited sediments. The authors suggested that individuals may have migrated vertically through the deep layer of dredged sand. This was based on the results of Roberts et al. (1998) who suggested 15 cm as the maximum depth of overburden through which benthic infauna can successfully migrate. After one year, no adverse impact of sand disposal on the benthic fauna was detected on the study site (Munari & Mistri, 2014).  Spiophanes bombyx was Group IV species: ‘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). 

Sensitivity assessment. The Ampelisca tube mat is probably only a few centimetres in height and would be completely buried under a 30 cm deposit of sediment.  Polydora ciliata and other characterizing polychaetes are likely to resist or relocate after smothering by 30 cm of sediment. However, this ‘heavy’ deposition of fine sediment is may cause significant mortality of Ampelisca spp.. based on Gittenberger & Van Loon (2011).  Therefore, resistance is assessed as 'Low' albeit with 'Low' confidence.  Resilience is probably 'High' and biotope sensitivity is assessed as 'Low' sensitivity to a ‘heavy' deposit of up to 30 cm of fine material in a single discrete event.

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 was available on which to assess this pressure. However, Arendse & Barendregt (1981) manipulated magnetic fields to alter the geomagnetic orientation of the talitrid amphipod Orchestia cavimana.

Underwater noise changes

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

Evidence

There is no evidence to suggest that any of the species that characterize the biotope respond to noise or vibration at the level of the benchmark, so the biotope is assessed as Not relevant.

Introduction of light or shading

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

Evidence

SS.SMU.ISaMu.AmpPlon is a sublittoral biotope (Connor et al., 2004) and therefore not directly dependent on sunlight. Although, the characteristic species may respond to light orientation or shading they do not depend on light for feeding. Shading may decrease the abundance of benthic diatoms and euglenoids but the characteristic Ampelisca spp. are suspension-feeders capable of taking a range of organic particulates. The biotope is considered to have 'High' resistance and, by default, 'High' resilience and is, therefore, is assessed as 'Not Sensitive' to the introduction of light.

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 to biotopes restricted to open waters.

Death or injury by collision

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

Polydora ciliata exhibits shadow responses and withdraws its palps into its burrow, which is believed to be a defence against predation. The withdrawal of the palps interrupts feeding and possibly respiration, although the species also shows habituation of the response (Kinne, 1970).  Polydora is unlikely to be sensitive to visual disturbance caused by passing shipping but may respond to passing divers at close range.  Other characterizing polychaetes, such as Spiophanes bombyx, also inhabit a tube so its visual range is probably very limited.  No information on the visual responses of Ampelisca spp. was found. Nevertheless, visual disturbance (as defined by the benchmark) is probably '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

The important characterizing species in the biotope are not cultivated or likely to be translocated. This pressure is, therefore considered 'Not relevant'.

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. 

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). 

The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula function as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). However, Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas of homogenous fine sediment and areas dominated by boulders. Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. Blanchard (2009) noted that sandy areas in the Bay of Saint-Mont Michel were not colonized by Crepidula because of surface sand mobility. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at the Swansea east site (Powell-Jennings & Calloway, 2018). 

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 this biotope is unsuitable for the colonization of Crepidula fornicata due to a lack of gravel, shells, or any other hard substrata used for larvae settlement (Tillin et al., 2020). The aggregations of Ampelisca spp. characterizing this biotope might exclude the settlement of Crepidula rather than provide a suitable substratum as the amphipods may eat the Crepidula larvae and their tube mats may exclude limpet spat. Hence, resistance is assessed as 'High' and resilience as 'High' so that the biotope is assessed as 'Not sensitive', 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

Introduced organisms (especially parasites or pathogens) are a potential threat in all coastal ecosystems. However, no information was found on microbial pathogens affecting Polydora ciliata. Amphipods may be infected by a number of parasites or pathogens that alter population numbers through changes in host condition, growth, behaviour and reproduction (Green Extabe & Ford, 2014). For example, infection by acanthocephalan larvae may alter the behaviour and responses of gammarid amphipods (Bethel & Holmes, 1977). The amphipod Orchestia gammarellus is host to the parasitic protist Marteilia that has a feminizing effect on populations, with higher ratios of females and intersex males in infected, estuarine populations (Ginsburger-Vogel & Desportes, 1979). Studies conducted in the Baltic Sea suggested that increased parasitism by trematode species has a detrimental effect on local amphipods (Meissner & Bick, 1999; Mouritsen & Jensen, 1997; cited in Shim et al., 2013).  

Sensitivity assessment. There are no records of the biotope being affected by the introduction of microbial pathogens or parasites in the British Isles, and there is not enough evidence to assess sensitivity. 

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 is known to be targeted directly by commercial or recreational fisheries or harvesting. Therefore, 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

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. The characterizing species in this biotope are highly likely to be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures).

Sensitivity assessment. Loss of the characterizing species of this biotope is likely to occur as a result of unintentional removal. Removal of the characterizing species would result in the biotope being lost. Thus, the biotope is considered to have a resistance of 'Low' to this pressure.  However, resilience is probably  'High' so that sensitivity is assessed as 'Low'.