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

The species are widely distributed. Kurtiella bidentata is widespread around the British Isles and its distribution ranges from Norway to west Africa and the Mediterranean (Carter, 2008). Abra spp. are also widespread around the British Isles, with Abra alba occurring from the Norwegian Sea and the Baltic, south to the Iberian Peninsula, into the Mediterranean and Black seas, and south along the coast of Africa to Senegal (Budd, 2007), and Abra nitida from Norway south to Mediterranean and north-west Africa (Hayward & Ryland, 1995b). Melinna palmata spreads from northern Spain with scattered records in the Mediterranean, Morocco, Black Sea and Arabian Gulf (Hayward & Ryland, 1995b). Magelona filiformis and Magelona mirabilis are widespread on the coasts of Britain and Northern Ireland (Rayment, 2007b; Richards, 2007), with the latter also extending to north-west Europe to Mediterranean (Hayward & Ryland, 1995b). Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007).

No specific information concerning temperature tolerances of the biotopes and the characterizing species was found, but inferences may be made. For example, Kurtiella bidentata (studied as Mysella bidentata) was recorded in Kinsale Harbour at temperatures ranging from 7.7-18.8°C (O’Brien & Keegan, 2006), and Künitzer (1989) reported that the main factor affecting the growth rate of Kurtiella bidentata (studied as Mysella bidentata) was temperature.

Wilson (1981) investigated temperature tolerances of six bivalve species from Dublin Bay. The author concluded that species variations in tolerance to increased temperature varied seasonally and with distribution along tidal height. Lethal temperatures for all six bivalve species in the study varied greatly and were, in most cases, well above 20°C. The maximum sea surface temperatures around the British Isles rarely exceed 20°C (Hiscock, 1998).

Sensitivity assessment. The characterizing species of the biotopes are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4-19°C (Huthnance, 2010). No information was found on the maximum temperature tolerated by the characterizing species. Elevated temperatures may affect growth of some of the characterizing species, but no mortality is expected. It is therefore likely that Kurtiella bidentata, Abra spp., Melinna palmata, Magelona spp. and Thyasira spp. are able to resist a long-term increase in temperature of 2°C. However, Thyasira spp. may suffer some mortality as a result of an acute increase in temperature so resistance is therefore assessed as Medium (loss <25%). Resilience is likely to be High, so the biotopes are considered to have Low sensitivity to an increase in temperature at the pressure benchmark level. 

Temperature decrease (local)

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

Evidence

There is no information on the response of the biotopes to a decrease in temperature, but species are widely distributed. Kurtiella bidentata ranges from Norway to west Africa and the Mediterranean (Carter, 2008). Abra alba is distributed from the Norwegian Sea and the Baltic, south to the Iberian Peninsula, into the Mediterranean and Black seas, and south along the coast of Africa to Senegal (Budd, 2007), and Abra nitida is distributed from Norway south to Mediterranean and north-west Africa (Hayward & Ryland, 1995b). Melinna palmata spreads from northern Spain with scattered records in the Mediterranean, Morocco, Black Sea and Arabian Gulf (Hayward & Ryland, 1995b). Magelona filiformis and Magelona mirabilis are widespread on the coasts of Britain and Northern Ireland (Rayment, 2007b; Richards, 2007), with the latter also extending to north-west Europe to Mediterranean (Hayward & Ryland, 1995b). Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007).

Kurtiella bidentata (studied as Mysella bidentata) was among the species that suffered high losses that could be related to low temperatures in the Wadden Sea area in 1979, where the temperature was 3°C below average for 3 months (Beukema, 1979).

Short-term acute periods of extreme cold and icing conditions are likely to cause stress and some mortality in bivalve populations (Dame, 1996). For instance, during the 1978/79 winter which was very cold with severe ice conditions, water temperature in the outer Weser estuary, Germany, remained below 0°C on 45 successive days. Populations of the characteristic species of the benthos, including Abra alba were considerably damaged (Buhr, 1981). Equally, the abundance of Magelona mirabilis experienced a sharp decline following the severe winter of 1995/1996 in the Wadden Sea, the Netherlands (Armonies et al., 2001), which was 2.7°C and 3.7°C below the mean water temperatures for the area (Strasser & Günther, 2001).

No specific information on temperature tolerances of Melinna palmata and Thyasira spp. was found. However, Dauvin (2000) reported Abra-alba-Melinna communities in the Bay of Morlaix, France, where bottom temperatures range from 8°C in winter to 16°C in summer, but during severe winters the temperature is < 6°C like in 1986.

Coyle et al. (2007) analysed temporal differences in benthic infaunal samples from the south-eastern Bering Sea shelf. Significant differences were observed for specific functional groups, namely carnivores, omnivores and surface detritivores which suggested a mechanistic link between temperature changes and infaunal biomass, both of which were elevated relative to warm periods due to elevated carbon flux to the benthos and exclusion of benthic predators on infaunal invertebrates by the cold bottom water on the shelf.

Sensitivity assessment. The characterizing species of the biotope are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4-19°C (Huthnance, 2010). Although no information was found on the minimum temperature tolerated by the characterizing species, it is likely that Kurtiella bidentata, Abra spp., Melinna palmata, Magelona spp. and Thyasira spp. are able to resist a long-term decrease in temperature of 2°C. However, characterizing species may suffer some mortality as a result of an acute decrease in temperature, so resistance is therefore assessed as Low (25-75% loss) and resilience is likely to be High, so the biotopes are considered to have Low sensitivity to a decrease in temperature at the pressure 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

SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThys are found within fully marine subtidal locations. The former also occurs occasionally in variable salinity (Connor et al., 2004). However, it is highly unlikely that the biotopes would experience conditions of hypersalinity.

Sensitivity assessment. No direct evidence was found to assess the effects of changes in salinity and OBIS data (OBIS, 2014) was used as the basis of the assessment. The minimum and maximum range of salinities for the characterizing species are 18.6 - 38.6 pps for Kurtiella bidentata; 30.2-38.6 pps for Abra alba; 17.7 – 39.0 pps for Melinna palmata; 33.5 - 35.6 pps for Magelona filiformis and 31.8-39.1 pps for Thyasira flexuosa. This data suggests Kurtiella bidentata and Melinna palmata are able to tolerate wider salinity ranges, but no records of any of the species occurring in hypersaline conditions was found. It is therefore likely that key components of the biotopes communities would not be resistant of an increase in salinity to >40 psu, resulting in mortality of the characterizing species. Resistance is therefore assessed as Low (loss of 25-75%) but with low confidence. Once normal conditions are resumed, resilience is probably High so that sensitivity is therefore assessed as Low.

Salinity decrease (local)

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

Evidence

SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThys are found within fully marine subtidal locations. The former also occurs occasionally in variable salinity (Connor et al., 2004).

According to OBIS data (OBIS, 2014), the minimum and maximum range of salinities for the characterizing species are 18.6 - 38.6 pps for Kurtiella bidentata; 30.2-38.6 pps for Abra alba; 17.7 – 39.0 pps for Melinna palmata; 33.5 - 35.6 pps for Magelona filiformis and 31.8-39.1 pps for Thyasira flexuosa. This data suggests Kurtiella bidentata and Melinna palmata are able to tolerate wider salinity ranges, which confirm records of Kurtiella bidentata (studied as Mysella bidentata) in Kinsale Harbour at salinities ranging from 19.3-35 S (O’Brien & Keegan, 2006). However, Gogina et al. (2010a) reported that Kurtiella bidentata (studied as Mysella bidentata) showed a strong positive correlation with salinity varying at a factor of 8.30-27.10 psu. According to Budd (2007), the change would be likely to cause inhibition of growth and reproduction and exposure to low salinity may result in some mortality of Abra alba. Magelona mirabilis occurs in the Baltic Sea (Fiege et al., 2000) where salinity is typically lower than in the open ocean. It is likely that some populations of Magelona mirabilis are adapted to reduced salinity habitats, however, no information on the effects of an overall decrease in salinity were found. Thyasira spp. inhabit waters of reduced salinity with 25-30 psu being optimal. However, adults exposed to lower than optimal salinities produced non-viable or slow developing eggs (Jackson, 2007). There is insufficient information regarding the effects of salinity on adults.

Sensitivity assessment. SS.SMu.ISaMu.MelMagThys may occasionally occur in variable (18-35 psu) salinity probably due to the fact that the biotope occurs in sheltered marine inlets. The evidence presented suggests that not all characterizing species of both biotopes are likely to resist a decrease in salinity at the pressure benchmark level. Resistance is therefore assessed as Low (loss of 25-75%) but with low confidence. Once normal conditions are resumed, resilience is probably High so that sensitivity is therefore assessed as Low.

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

The hydrographic regime, including flow rates, is an important structuring factor in sedimentary habitats. The low energetic environments where the biotopes occur are therefore likely to be important in allowing for the development of the sandy mud substrata which characterize both biotopes. The most damaging effect of increased flow rate would be the erosion of the substratum as this could eventually lead to loss of the habitat. Increased water flow rates are likely to change the sediment characteristics in which the species live, primarily by resuspending and preventing deposition of finer particles (Hiscock, 1983). This may be particularly relevant for tube building species occurring in the biotopes such as characterizing species Melinna palmata. Furthermore, increased water flow rate may prevent settlement of larvae and therefore reduce recruitment. Mature adults buried at depth are likely to be unaffected as muddy substrata are particularly cohesive. Additionally, the consequent lack of deposition of particulate matter at the sediment surface would reduce food availability.

Decreased water movement would result in increased deposition of suspended sediment (Hiscock, 1983). An increased rate of siltation resulting from a decrease in water flow may result in an increase in food availability for the characterizing species and therefore growth and reproduction may be enhanced, but only if food was previously limiting.

Sensitivity assessment. Sand particles are most easily eroded and likely to be eroded at about 0.20 m/s (based on Hjulström-Sundborg diagram, Sundborg, 1956). Although having a smaller grain size than sand, clays and silts require greater critical erosion velocities because of their cohesiveness. SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThy occur in areas of very weak (negligible) and moderately strong (up to 1.5 m/s) tidal streams, respectively (Connor et al., 2004). The characterizing species in these biotopes occur in a range of tidal regimes, varying from moderately strong to very weak (negligible - 1.5 m/s) (Tillin & Hull, 2013c; Tillin & Tyler-Walters, 2014 references therein). Although changes in water flow would be likely to change the sedimentary regime in the biotope, the cohesive nature of the sandy muds that characterize the biotope is likely to provide some protection to changes in water flow at the pressure benchmark. Additionally, the range of flow rates the characterizing species tolerate is likely to fall within the benchmark. Resistance and resilience are therefore assessed as High and the biotopes considered Not Sensitive 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

Some records of SS.SMu.ISaMu.KurAbr occur in the shallow subtidal and so may be vulnerable to an increase in the emergence regime, which may suppress the upper distribution of the biotope. On the other hand, the biotope could benefit from a decrease in the emergence regime.

Sensitivity assessment. A change in emergence regime may affect the biological community on the upper range of SS.SMu.ISaMu.KurAbr. However, the characterizing species live infaunally and are likely to be protected and the sublittoral element of the biotope is not likely to be lost. Mid-range populations are unlikely to be affected by a change in emergence regime so resistance and resilience are considered to be High and SS.SMu.ISaMu.KurAbr is assessed as Not Sensitive at the pressure benchmark level. Changes in emergence regime are Not Relevant to SS.SMu.ISaMu.MelMagThy, which is protected by the depth at which the biotope occurs.

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

Potentially the most damaging effect of increased wave action would be the erosion of the fine sediment substratum as this could eventually lead to loss of the habitat that characterizes the biotopes. Decreased exposure will probably lead to increased siltation and reduced grain size (muddy sediment). Changes in wave exposure may therefore influence the supply of particulate matter for tube building and feeding activities of the characterizing species. Food supplies may also be reduced, affecting growth and fecundity of the species. Strong wave action may cause damage or withdrawal of the siphons and delicate feeding structures, resulting in loss of feeding opportunities and compromised growth for the characterizing species. Additionally, individuals may be dislodged by scouring from sand and gravel mobilized by increased wave action (Budd, 2007; Rayment, 2007b). During winter gales along the North Wales coast, large numbers of Abra alba were cast ashore and over winter survival rate was as low as 7% in the more exposed locations (Rees et al., 1977). Additionally, changes in wave exposure may also interfere with larval dispersal of the characterizing species. Olivier et al. (1996) reported that the post-larvae and juveniles of Abra alba were most abundant in the near-bottom water stratum at flood tides. Therefore, increased wave action could result in enhanced resuspension and dispersal of early life stages, whereas a reduction in wave exposure may lead to a decrease in dispersal.

Magelona mirabilis is adapted to life in areas with strong currents, high wave exposure and unstable sediments (Lackshewitz & Reise, 1998). On the other hand, Thyasira gouldii lives in rather wave sheltered areas at the heads of sea lochs (Jackson, 2007). Increases in wave exposure may disrupt the sediment in which they live, cause continual displacement and physical damage to the shells which are thin and fragile.

Sensitivity assessment. Records indicated that the characterizing species occur in a range of wave exposures. Kurtiella bidentata occurs in sheltered, very sheltered, and moderately exposed; Abra spp. in extremely sheltered, very sheltered, sheltered, moderately exposed, exposed, extremely exposed and very exposed; Melinna palmata in extremely sheltered, very sheltered, sheltered; and Thyasira flexuosa in extremely sheltered, very sheltered, sheltered, and moderately exposed (Connor et al., 2004). An increase in wave action may result in modification of the sediment and/or an increase in the probability that characterizing species would be removed by storms. However, as the biotope occurs in wave sheltered conditions, a further decrease in wave action is unlikely to be significant. However, an increase or decrease in wave height at the pressure benchmark (3-5% of significant wave height) is therefore considered to fall within the natural range of conditions experienced by mid-range populations of the biotopes, especially as the effect of wave action is reduced with depth. Resistance and resilience are therefore assessed as High, and the biotope is considered Not Sensitive at the benchmark level but with low confidence.

Transition elements & organo-metal contamination

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

Evidence

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

There is little or no information on the resistance of the key species in the biotopes. Experimental studies with various species suggests that polychaete worms are quite tolerant of heavy metals (Bryan, 1984).

Abra alba can live in polluted sediments (Dauvin, pers. comm.), for example, near Calais where high densities of Abra alba were found in sediment containing 8 mg/g iron and 4 mg/g titanium (Dewarumez et al., 1976, cited in Budd, 2007). The capacity of bivalves to accumulate heavy metals in their tissues, far in excess of environmental levels, is well known. Bryan (1984) states that Hg is the most toxic metal to bivalve molluscs while Cu, Cd and Zn seem to be most problematic in the field. In bivalve molluscs, Hg was reported to have the highest toxicity, mortalities occurring above 0.1-1 g/l after 4-14 days exposure (Crompton, 1997), toxicity decreasing from Hg > Cu and Cd > Zn > Pb and As > Cr ( in bivalve larvae, Hg and Cu > Zn > Cd, Pb, As, and Ni > to Cr).

Boilly & Richard (1978) stated that the presence of Magelona mirabilis is indicative of sediments which have been contaminated with iron. Studies on a dredge spoil disposal site in the harbours of Boulogne and Dunkerque in France (Bourgain et al., 1988) found higher densities of Magleona mirabilis three months after the dumping of dredge spoil than after five months, that is, when the metal contamination of the sediments was higher. No information regarding the effect of other metals on this species was found.

Hydrocarbon & PAH contamination

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

Evidence

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

Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected. Sub-lethal concentrations may produce substantially reduced feeding rates and/or food detection ability, probably due to ciliary inhibition. Respiration rates may increase at low concentrations and decrease at high concentrations. Generally, contact with oil causes an increase in energy expenditure and a decrease in feeding rate, resulting in less energy available for growth and reproduction. However, the Abra alba population affected by the 1978 Amoco Cadiz benefited from the nutrient enrichment caused by the oil pollution. The biomass of the fine-sand community remained low in 1979, a year after the spill, owing to the decimation of the Ampelisca amphipod population, but the biomass then doubled as a result of an increase in Abra alba abundance in 1980 and Abra alba remained a dominant species over the 20 year duration, over which recovery of the community was monitored (Dauvin, 1998).

Synthetic compound contamination

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

Evidence

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

Abra alba demonstrated alterations of its behaviour in response to exposure to marine sediments contaminated with pesticides (6000 ppm parathion, 200 ppm methyl parathion, 200 ppm malathion). No burrowing occurred in the most contaminated sediment, whilst burrowing was impaired in the moderately contaminated sediment with a median effective burrowing time (ET₍₅₀₎) of 9.0 (±3.0 - 28) minutes in comparison to a control time of 4.5 (±2.8 - 7.2) minutes (Møhlenberg & Kiørboe, 1983). There is no evidence relating directly to the effects of synthetic chemicals on the remaining characterizing species.

Radionuclide contamination

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

Evidence

No Evidence is available on which to assess this pressure.

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

A number of animals have behavioural strategies to survive periodic events of reduced dissolved oxygen. These include shell closure and reduced metabolic rate in bivalve molluscs and either decreased burrowing depth or emergence from burrows for sediment dwelling crustaceans, molluscs and annelids.

At oxygen concentrations below ca 0.4 mg O₂/l, Kurtiella bidentata eventually emerged from the substratum (Ockelmann & Muus, 1978). Nilsson & Rosenberg (1994) investigated hypoxic responses of benthic communities and reported Kurtiella bidentata (studied as Mysella bidentata) leaving the sediment at oxygen concentrations of 1.7 mg/l. According to the authors, this is a behaviour that occurs at hypoxic oxygen concentrations slightly higher than those causing mortality, suggesting high levels of stress caused to the organisms.

Abra alba is typically found in organically enriched sediments where it may be present in high densities (Dauvin & Gentil, 1989). Experimental examination of the interactions between eutrophication and oxygen deficiency (2.4-3.5 mg O₂/l over a 93 day experimental period) revealed that Abra alba became inefficient in its use of the available organic matter under prolonged conditions of hypoxia (Hylland et al., 1996). Abra alba was also reported to be sensitive to lowered oxygen concentrations off the Swedish west coast (Rosenberg & Loo, 1988; Weigelt & Rumohr, 1986, both cited in Rees & Dare, 1993).

Melinna palmata has been reported to survive temporary periods of hypoxia, withstand large variation in oxygen concentration, and tolerate temporary anaerobiosis and the presence of hydrogen sulphides (Grehan, 1991).

Niermann et al. (1990) reported that Magelona sp. in a fine sand community in the German Bight area, exposed to regular seasonal hypoxia, remained abundant during a period of hypoxia (1-3 mg/l), and decreased slightly in abundance on resumption of normoxia (Rayment, 2008).

López-Jamar et al. (1987) stated that Thyasira flexuosa is adapted to living in reduced sediments and also is found in organically enriched sediments. However, Dando & Spiro (1993) found that numbers of the congeners Thyasira equalis and Thyasira sarsi decreased rapidly following the deoxygenation of bottom water in the deep basin of the Gullmar fjord in 1979-80.

Rosenberg et al. (1991) exposed benthic species from the NE Atlantic to oxygen concentrations of around 1 mg/l for several weeks, including species of small bivalves. After 11 days in hypoxic conditions, bivalve individuals were still alive, although individuals showed increased stretching of syphon out of the sediment. In a meta-analysis study of hypoxia, median sub-lethal oxygen concentrations reported in experimental assessments, although no specific data was reported for all the characterizing species of these biotopes, the thresholds of hypoxia for different benthic groups was LC50 1.42 mg/l for bivalves, and sub-lethal (SLC50) of 1.20 mg/l for annelids (Vaquer-Sunyer & Duarte, 2008). For Kurtiella bidentata (studied as Mysella bidentata), the median sub-lethal oxygen concentrations reported in experimental assessments was 1 mg/l, and for Abra spp. was 0.57 mg/l (Vaquer-Sunyer & Duarte, 2008).

Sensitivity assessment. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Different species in the biotope will have varying responses to de-oxygenation. Based on the evidence presented, the characterizing species are likely to only be affected by severe deoxygenation episodes. However, some mortality of Thyasira spp. might occur in near anoxic (0% oxygen) conditions. Resistance to deoxygenation is therefore assessed as Medium. Resilience of the biotopes is likely to be High and the biotopes are therefore considered to have Low sensitivity to exposure to 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

Increased nutrients are most likely to affect abundance of phytoplankton which may include toxic algae (OSPAR, 2009). This primary effect resulting from elevated nutrients will impact upon other biological elements or features (e.g. toxins produced by phytoplankton blooms or deoxygenation of sediments) and may lead to ‘undesirable disturbance’ to the structure and functioning of the ecosystem. With enhanced primary productivity in the water column, organic detritus that falls to the seabed may also be enhanced, which may be utilized by the deposit feeders in the community.

In a sewage dumping region of the North Sea, a great increase in the abundance of Abra alba occurred in much of the dumping area because of the ecological adaptations of the species enabled it to exploit the greatly increased supply of nutrients (Caspers, 1981). For example, the Amoco Cadiz oil spill in March 1978 caused vast disturbance to the fine-sand communities of the Bay of Morlaix, France (Dauvin, 1982). Drastic qualitative and quantitative changes in species abundance, diversity and biomass were recorded after the spill. However, the Abra alba population persisted in the disturbed environment under eutrophic conditions and as an 'opportunistic species' (Hily & Le Bris, 1984), rapidly adapting its reproductive strategy to three spawnings per year. Increased growth and abundance was attributable to increased food availability and vacant ecological niches (Dauvin & Gentil, 1989).

As a surface deposit feeder, Magelona mirabilis relies on a supply of organic matter at the sediment surface. Increased nutrient levels in the water column would be expected to result in increased deposition of organic matter at the sediment surface, and therefore moderate nutrient enrichment may be beneficial to Magelona mirabilis. Indeed, Kröncke (1990) postulated that the increase in certain species, including Magelona sp., on the Dogger Bank between 1951 and 1987 may be due to eutrophication. However, Niermann (1996) noted that Magelona sp. decreased in abundance following a nutrient enrichment event in the North Sea, probably because the species were adapted to living in sediments with low or moderate amounts of organic carbon.

Enrichment from pulp mills is believed to have been the cause of the death of two populations of Thyasira spp. in west Scotland sea lochs. Thyasira flexuosa was recorded at densities of up to 4000 per square metre in enriched areas (Jackson, 2007).

In a report to identify seabed indicator species to support implementation of the EU habitats and water framework directives Kurtiella bidentata, Melinna palmata and Thyasira spp. were reported as likely to be favoured by nutrient enrichment, whereas Abra spp. and Magelona spp. were assessed as intolerant (Hiscock et al., 2005a). The community, and hence the biotopes, may change to one dominated by nutrient enrichment resistant species, in particular polychaete worms.

However, the biotopes are considered to be Not Sensitive at the pressure benchmark that assumes compliance with WFD criteria for good status.

Organic enrichment

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

Evidence

In a sewage dumping region of the North Sea, a great increase in the abundance of Abra alba occurred in much of the dumping area because of the ecological adaptations of the species enabled it to exploit the greatly increased supply of nutrients (Caspers, 1981). For example, the Amoco Cadiz oil spill in March 1978 caused vast disturbance to the fine-sand communities of the Bay of Morlaix, France (Dauvin, 1982). Abra alba persisted in the disturbed environment under eutrophic conditions and as an 'opportunistic species' (Hily & Le Bris, 1984), with increased growth and abundance attributed to increased food availability and vacant ecological niches (Dauvin & Gentil, 1989 cited in Budd, 2007).

The spread of Melinna palmata in the Romanian sublittoral described by Gomoiu (1982, cited from Grehan, 1991) was correlated with an increase in siltation following increased eutrophication linked phytoplankton blooms. Guillou & Hily (1983), in their study on the Bay of Brest, considered that the population benefited through enhanced growth from organic enrichment through sewage disposal.

As a surface deposit feeder, Magelona mirabilis relies on a supply of organic matter at the sediment surface. Increased nutrient levels in the water column would be expected to result in increased deposition of organic matter at the sediment surface. Kröncke (1990) postulated that the increase in certain species, including Magelona sp., on the Dogger Bank between 1951 and 1987 may be due to eutrophication. However, Niermann (1996) noted that Magelona sp. decreased in abundance following a nutrient enrichment event in the North Sea, probably because the species were adapted to living in sediments with low or moderate amounts of organic carbon.

Thyasira spp. are characteristic of organically enriched offshore sediments with Capitella capitata (Connor et al., 2004) and have been identified as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975, cited in Gray, 1979).

Borja et al. (2000) and Gittenberger & Van Loon (2011) assigned Abra alba, Melinna palmata and Thyasira flexuosa to their Ecological Group III ‘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)’. Kurtiella bidentata (as Mysella bidentata) and Magelona mirabilis were characterized as AMBI Group I – ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’.

Sensitivity assessment. No direct evidence of the characterizing species’ specific tolerances to organic enrichment was found. The evidence presented suggests that the majority of the characterizing species in the biotopes are likely to be able to utilize additional organic load as food. However, the evidence also suggests that Kurtiella bidentata and Magelona spp. May experience decreases in abundance as a result of organic enrichment, which can lead to shifts in community composition towards one dominated by tolerant species, such as polychaete worms (Pearson & Rosenberg, 1978). Additionally, Forrest et al. (2009) identified that the recovery of muddy sediments beneath fish farms from enrichment can be highly variable and may be many years at poorly flushed sites, suggesting that the low energy environments that characterize these biotopes may allow for prolonged periods of organic load sedimentations  and enhance the adverse effects to sensitive characterizing species. Resistance is therefore assessed as Low (loss 25-75%), but with low confidence. Resilience is likely to the High and the overall sensitivity of the biotopes judged 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

If the sediment that characterizes the biotopes were replaced with rock substrata, this would represent a fundamental change to the physical character of the biotopes. The characterizing species would no longer be supported and the biotopes would be lost and/or reclassified.

Sensitivity assessment. Resistance to the pressure is considered None, and resilience Very Low, given the permanent nature of this pressure. 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

Records indicate that SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThys occur in cohesive sandy muds (Connor et al., 2004). The characterizing species within these biotopes have wide ranges of sediment preferences. Kurtiella bidentata lives in muddy sand or fine gravel (Carter, 2008); Abra spp. prefer mud, muddy gravel, muddy sand, sandy mud (Budd, 2007); Magelona spp. occur in coarse clean sand and fine clean sand (Rayment, 2007b); and Thyasira spp. prefer mud, muddy sand, sandy mud (Jackson, 2007). Melinna palmata lives in a mucus lined tube covered with sediment particles in mud substrata (Fauchald & Jumars, 1979).

Sensitivity assessment. In SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThys, a change in Folk class from mud and sandy mud to sand or muddy sand would probably not affect the characterizing species which all have habitats preferences that would fall within this range. However, a change in one Folk class to gravelly mixed sediment is likely to result in some of the characterizing species not being supported anymore, and a reclassification of the biotopes. Resistance is therefore assessed as None and resilience is considered Very Low given the permanent nature of this pressure. Sensitivity is therefore assessed as High.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Sedimentary communities are likely to be highly intolerant of substratum removal, which will lead to partial or complete defaunation, expose underlying sediment, which may be anoxic and/or of a different character, and lead to changes in the topography of the area (Dernie et al., 2003). Any remaining species, given their new position at the sediment / water interface, may be exposed to unsuitable conditions. Newell et al. (1998) state that removal of 0.5 m depth of sediment is likely to eliminate benthos from the affected area. Some epifaunal and swimming species may be able to avoid this pressure. Removal of 30 cm of sediment will remove species that occur at the surface and within the upper layers of sediment, such as the characterizing species of these biotopes. For example, Abra alba is a shallow burrower and has a fragile shell (Tebble, 1976). Thyasira species are found 2-8 cm below the sediment surface (Dando & Southward, 1986)

Recovery of the sedimentary habitat would occur via infilling, although some recovery of the biological assemblage may take place before the original topography is restored, if the exposed, underlying sediments are similar to those that were removed. Newell et al. (1998) indicated that local hydrodynamics (currents and wave action) and sediment characteristics (mobility and supply) strongly influence the recovery of soft sediment habitats.

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotopes so resistance is assessed as None. SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThys occur in low energy environments, so resilience is therefore judged as Medium, based on the recruitment dispersal limitation of the characterizing species (see resilience section). Sensitivity has been 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 characterizing species of SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThys are infaunal and hence have some protection against surface disturbance. Bivalves and other species require contact with the surface for respiration and feeding. Siphons and delicate polychaete feeding structures may be damaged or withdrawn as a result of surface disturbance, resulting in loss of feeding opportunities and compromised growth.

Abra alba is a shallow burrower with a fragile shell (Tebble, 1976), and has been considered amongst the list of bivalve species most vulnerable to trawling (Bergmann & Van Santbrink, 2000) who reported between <0.5% and 18% mortality of Abra alba due to trawling in the southern North Sea. However, the small size of Abra alba relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993). This is likely to also be the case for small infaunal bivalve Kurtiella bidentata. Magelona mirabilis is a soft bodied organism which exposes its palps at the surface while feeding. The species usually lives within a few centimetres of the sediment surface. Physical disturbance, such as dredging or dragging an anchor, would likely cause physical damage to Magelona mirabilis. Thyasira spp. are small bivalves, the shells are thin and fragile and abrasion is likely to lead to damage and mortality within the population depending on the force (Jackson, 2007). Sparks-McConkey & Watling (2001) found that trawler disturbance resulted in a decline of Thyasira flexuosa in Penobscot Bay, Maine. However, the population recovered after 3.5 months. 

Furthermore, SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThys occur in cohesive sandy muds (Connor et al., 2004). Abrasion events caused by a passing fishing gear, or scour by objects on the seabed surface are likely to have marked impacts on the substratum and cause turbulent resuspension of surface sediments. When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see change in suspended solids and smothering pressures). The effects of trawling on infauna are greater in areas with low levels of natural disturbance compared to areas of high natural disturbance (e.g. Hiddink et al., 2006). In a meta-analysis of the impacts of different fishing activities on the benthic biota of different habitats, muddy sands were found to be vulnerable to the impacts of fishing activities, with recovery times predicted to take years (Kaiser et al., 2006). The long recovery time for muddy sands is due to the fact that these habitats are mediated by a combination of physical, chemical and biological processes (compared to sand habitats which are dominated by physical processes and recovery time takes days to months).

Sensitivity assessment. The characterizing species live infaunally and are considered to have some protection against surface disturbance. However, the evidence presented suggests that soft bodied organisms and fragile shells are likely to be damaged and removed by abrasion. Resistance to abrasion is therefore considered Low. Resilience of the biotopes is likely to be High. The biotopes are therefore considered to have Low sensitivity to abrasion or disturbance of the surface of the seabed.

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 disturb the surface and penetrate below the surface would remove /damage infaunal species such as the characterizing species within the direct area of impact. The footprint of the impact will depend on the type of gear used (Hall et al., 2008).

Bergman & Van Santbrink (2000) estimated the direct mortality of benthic macrofauna caused by the single pass of commercial beam and otter trawls. The results showed that a single pass of a 4 m or 12 m beam trawl or an otter trawl, in shallow sandy areas and deep silty sand areas (with 3-10% silt) in the North Sea caused a mortality of 20-65% of bivalves and 5-40% of gastropods, starfish, small-medium sized crustaceans and annelid worms. Of the characterizing species mentioned, mortality of Kurtiella bidentata (studied as Mysella bidentata) was reported as 4%, and Magelona sp. 30%. Some mortality was not caused directly by the passage of the trawl, but instead by disturbance, exposure and subsequent predation. Ball et al. (2000) reported on the short-term effects of fishing on benthos from a mud patch in the north-western part of the Irish Sea investigated in 1994–1996 by means of samples taken both before and shortly after (ca 24 hr) fishing activity. Kurtiella bidentata (studied as Mysella bidentata) was one of the species that was common at the inshore site and for which estimates of mortality were calculated and was uncommon or totally absent on the offshore fishing ground. Direct mortality from passage of an otter trawl was estimated as 70%. The delicate shells of Abra spp. are vulnerable to physical damage (e.g. by otter boards), but small size relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993). Equally, Thyasira spp. also have delicate shells and are likely to be vulnerable to physical disturbance. Following experimental beam trawl disturbance in an area that had previously been closed to fishing, populations of Melinna palmata increased by 41% (Tuck et al., 1998). The area was repeatedly disturbed over an 18 month period and recovery tracked for a further 18 months.

Furthermore, penetrative events caused by a passing fishing gear are also likely to have marked impacts on the substratum and cause turbulent resuspension of surface sediments (see abrasion pressure). When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). Trawling can create suspended sediment plumes up to 10 m above the bottom (Churchill, 1989 cited in Clarke & Wilber, 2000). Shrimp trawlers in Texas have increased suspended sediment concentrations to between 100 and 550 mg/l at 2 m above the bottom and 100 m astern of trawls (Schubel et al., 1978, cited in Clarke & Wilbur, 2000). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see change in suspended solids and smothering pressures). A meta-analysis of over 100 experimental fishing impact studies showed that beam trawling, scallop dredging and otter trawling all had significant short-term impacts in muddy sand habitats, with most severe effect on suspension feeders (Kaiser et al., 2006). Jennings et al. (2001) found that trawling in the muddy sand region led to significant decreases in infaunal biomass and production in the North Sea, with the abundance of larger individuals depleted more than smaller ones.

Sensitivity assessment. A large proportion of the characterizing species in these biotopes is likely to be lost or severely damaged, depending on the scale of the activity (see abrasion pressure). Therefore, a resistance of Low is suggested. Muddy sand habitats have been reported as having the longest recovery times, whilst mud habitats had an ‘intermediate’ recovery time (compared to clean sand communities which had the most rapid recovery rate) (Dernie et al., 2003). Resilience is probably High and, therefore, the biotopes’ sensitivity to this pressure if likely to be 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

For most benthic deposit feeders, food is suggested to be a limiting factor for populations (Levinton, 1979; Hargrave, 1980). Consequently, an increase in suspended particulates and subsequent increased deposition of organic matter in sheltered environments where sediments have high mud content will increase food resources to deposit feeders. Burrowing infauna are unlikely to be affected by an increase in suspended sediment. There may be possible clogging of feeding organs in suspension feeders and there may be some energetic cost to clear their feeding and respiration organs. However, characterizing bivalve species such as Kurtiella bidentata and Abra spp. are able to switch between feeding methods (Carter, 2008; Budd, 2007). If the suspended sediment has a high organic content, some suspension feeding organisms may benefit. Increased siltation may benefit deposit feeders, such as characterizing species Magelona spp.. An increase in turbidity resulting from an increase in suspended solids may reduce primary production in the water column and on sediment surfaces therefore reducing the availability of diatom food, both for suspension feeders and deposit feeders. Deposit feeders and tube builders rely on deposition of suspended sediment. A decrease in suspended sediment will reduce this supply and therefore may compromise growth and reproduction.

Lethal effects on Kurtiella bidentata are considered unlikely given the occurrence of this species in estuaries where turbidity is frequently high from suspended organic and inorganic matter. Melinna palmata has been reported to increase in response to decreased turbidity (Warwick et al., 1991, cited in Hiscock et al., 2004), suggesting that increased turbidity may be detrimental to this species. Thyasira is the only known obligate suspension feeder of the characterizing species, so additional cleaning costs may be incurred for feeding apparatus but this are not considered to be lethal (Jackson, 2007).

According to Widdows et al. (1979), growth of filter-feeding bivalves may be impaired at suspended particulate matter (SPM) concentrations >250 mg/l. For instance, the abundance of Abra alba declined over two years within 1 km of an outfall pipe discharging fine-grained mineral waste from the china-clay industry at a rate of 450,000 tons per year to Mevagissey Bay, Cornwall. However, it was argued that persistent sediment instability was the more significant source of stress to the predominantly deposit-feeding community than the suspended sediment concentration (Probert, 1981).

Sensitivity assessment. The main environmental effects of increased turbidity levels as a result of increase in suspended solids are a reduction in penetration of light into the water column, suspended-sediment impacts on filter-feeding organisms and increased deposition of particulates in low-energy environments. Buchanan & Moore (1986) found that a decline in quantities of organic matter changed the infauna of a deposit feeding community which is essentially food limited. This may lead to a shift in community structure with increased abundance of deposit feeders and a lower proportion of suspension feeders (as feeding is inhibited where suspended particulates are high and the sediment is destabilised by the activities of deposit feeders (Rhoads & Young, 1970). The evidence presented suggests that the characterizing species are likely to resist a change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate. But some species, such as Melinna palmata, Thyasira spp. and Abra spp., would be adversely impacted where a change from intermediate to medium (100-300 mg/l) turbidity occurred. Resistance is therefore assessed as Low. Resilience is likely to be High, so the biotopes are considered to have Low sensitivity to a change in suspended solids at the pressure 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

Bijkerk (1988 cited from Essink, 1999) indicated that the maximal overburden through which small bivalves could migrate was 20 cm in sand for Donax and approximately 40 cm in mud for Tellina sp. and approximately 50 cm in sand. No further information was available on the rates of survivorship or the time taken to reach the surface. Powilleit et al. (2009) studied responses to smothering for three bivalves; Arctica islandica, Macoma balthica and Mya arenaria. These successfully burrowed to the surface of a 32 – 41 cm deposited sediment layer of till or sand/till mixture and restored contact with the overlying water. These high escape potentials could partly be explained by the heterogeneous texture of the till and sand/till mixture with ‘voids’. In comparison to a thick coverage, thin covering layers (i.e. 15 - 16 cm and 20 cm) increased the chance of the organisms to reach the sediment surface after burial. This suggests that characterizing bivalve species such as Kurtiella bidentata, Abra spp. and Thyasira spp. are likely to be able to burrow through similar overburdens, although sudden smothering with 5 cm of sediment would temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure. Furthermore, Thyasira flexuosa have highly extensible feet (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Melinna palmata was reported to colonize the Bay of Seine, France, following increase of the fine sediment in the eastern part of the bay (Dauvin et al., 2007). The tubes of Melinna palmata project above the surface and this will provide some protection against siltation. The effects of siltation will depend on the amount and rate that particles are added. The species is sedentary and adults are judged unlikely to have any mechanism to escape from large inputs. The spread of Melinna palmata in the Romanian sublittoral described by Gomoiu (1982, cited from Grehan, 1991) was correlated with an increase in siltation following increased eutrophication linked phytoplankton blooms. As a surface deposit feeder, Melinna palmata lives in accreting environments and are likely to be able to tolerate low and constant levels of siltation.

Magelona mirabilis lives infaunally in fine sand and moves by burrowing. It deposit feeds at the surface by extending contractile palps from its burrow. An additional 5 cm layer of sediment would result in a temporary cessation of feeding activity, and therefore growth and reproduction are likely to be compromised. However, Magelona mirabilis would be expected to quickly relocate to its favoured depth (Rayment, 2007b).

The character of the overburden is an important factor determining the degree of vertical migration. Individuals are more likely to escape from a covering similar to the sediments in which the species is found than a different type. 

Sensitivity assessment. Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). However, the biotopes are likely to resist smothering at the benchmark level since the majority of associated fauna are burrowing infauna. Resistance is therefore assessed as High, and resilience is also High (by default) so that the biotopes are considered Not Sensitive to a ‘light’ deposition of up to 5 cm of fine material added to the seabed 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

Bijkerk (1988, cited from Essink, 1999) indicated that the maximal overburden through which small bivalves could migrate was 20 cm in sand for Donax and approximately 40 cm in mud for Tellina sp. and approximately 50 cm in sand. No further information was available on the rates of survivorship or the time taken to reach the surface. Powilleit et al. (2009) studied responses to smothering for three bivalves; Arctica islandica, Macoma balthica and Mya arenaria.  These successfully burrowed to the surface of a 32 – 41 cm deposited sediment layer of till or sand/till mixture and restored contact with the overlying water. These high escape potentials could partly be explained by the heterogeneous texture of the till and sand/till mixture with ‘voids’. In comparison to a thick coverage, thin covering layers (i.e. 15 - 16 cm and 20 cm) increased the chance of the organisms to reach the sediment surface after burial. This suggests that characterizing species Kurtiella bidentata, Abra spp. and Thyasira spp. are likely to be able to burrow through similar overburdens, although sudden smothering with 30 cm of sediment would temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure. Furthermore, Thyasira flexuosa have highly extensible feet (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Melinna palmata was reported to colonize the Bay of Seine, France, following increase of the fine sediment in the eastern part of the bay (Dauvin et al., 2007). The tubes of Melinna palmata project above the surface and this will provide some protection against siltation. The effects of siltation will depend on the amount and rate that particles are added. The species is sedentary and adults are judged unlikely to have any mechanism to escape from large inputs. Where inputs are at low rates and similar to background sediments then adults may be able to extend tubes to reach the surface to feed. The spread of Melinna palmata in the Romanian sublittoral described by Gomoiu (1982, cited from Grehan, 1991) was correlated with an increase in siltation following increased eutrophication linked phytoplankton blooms. As a surface deposit feeder, Melinna palmata lives in accreting environments and are likely to be able to tolerate low and constant levels of siltation.

Magelona mirabilis lives infaunally in fine sand and moves by burrowing. It deposit feeds at the surface by extending contractile palps from its burrow. An additional 30 cm layer of sediment would result in cessation of feeding activity, and therefore growth and reproduction are likely to be compromised (Rayment, 2007b).

The character of the overburden is an important factor determining the degree of vertical migration. Individuals are more likely to escape from a covering similar to the sediments in which the species is found than a different type. 

Sensitivity assessment. Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). Bivalve and polychaete species have been reported to migrate through depositions of sediment greater that the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Bijkerk, 1988; Powilleit et al., 2009; Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Resistance is therefore assessed as Low (25-75% loss) and resilience as High and the biotopes are considered to have Low sensitivity to a ‘heavy’ deposition 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 is available on which to assess this pressure.

Underwater noise changes

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

Evidence

Species in the biotopes may respond to vibrations from predators or excavation by retracting their palps into their tubes or by burrowing deeper into the sediment. However, the characterizing species are unlikely to be affected by noise pollution and so the biotopes are assessed as Not Sensitive.

Introduction of light or shading

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

Evidence

SS.SMu.ISaMu.KurAbr and SS.SMu.ISaMu.MelMagThy are sublittoral biotopes (Connor et al., 2004). Although eelgrass Zostera marina may occasionally occur in SS.SMu.ISaMu.KurAbr in low densities (Connor et al., 2004), the biotopes are not characterized by the presence of primary producers and therefore, not directly dependent on sunlight.

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

The characterizing species of the biotopes live infaunally, so are likely to have poor or no visual perception and unlikely to be affected by visual disturbance such as shading.

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 biotopes 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. The slipper limpet was first recorded in Northern Ireland at Belfast Lough in 2009. 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). Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and possibly burial due to deposition, which could mitigate Crepidula density.

Pacific oysters were first brought to Northern Ireland as part of aquaculture development. They have been grown in Northern Ireland since the early 1970s. Populations of Cassostrea gigas have formed solid reefs in soft sediment habitats such as the mudflats of the Wadden Sea (Ruesink et al., 2005; Kochmann et al., 2008, cited in OSPAR, 2009). 

The Manila clam (Tapes philippinarium), which was introduced to Poole Harbour for aquaculture in 1998, has become a naturalised population on the intertidal mudflats, occurring at densities of 60 clams/m² in some locations within the harbour (Jensen et al., 2007 cited in Caldow et al., 2007). Densities of clam species such as Cerastoderma edule increased since the introduction of the Manila clam, so Caldow et al. (2007) concluded that within Poole Harbour there was no evidence yet of species replacement by the Manila clam.

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. Subtidal muds are considered highly sensitive to non-native invasive species due to the potential for habitat and ecological change when these are established and the difficulty in removing them. The above evidence suggests that Crepidula fornicata could colonize sandy mud habitats in the subtidal, typical of this biotope, due to the presence of small shells and any other hard substrata that can be used for larvae settlement (Tillin et al., 2020). 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 very sheltered habitat which may be more suitable for Crepidula. The biotope is recorded in the Wash which already harbours Crepidula but is noted for its sediment mobility (Quinn, 2018). 

Therefore, resistance is assessed as 'Medium', as Crepidula may colonize in low densities as the availability of shell is limited, and sediment mobility in areas such as the Wash. Resilience is assessed as 'Very low', as it would require the removal of Crepidula, probably by artificial means. Hence, sensitivity is assessed as 'Medium' 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

More than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996).

A viral infection of the mutualist bacterium living on the gills of Thyasira gouldii was suggested as the reason for a major decline in the Loch Etive population (Jackson, 2007,) but no information specifically concerning the effects of microbial pathogens and parasites on the viability of the characterizing species was found.

Sensitivity assessment. No direct evidence of the biotopes being affected by the introduction of microbial pathogens was found as with which to assess this pressure.

Removal of target species

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

Evidence

It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction. This pressure is therefore considered Not Relevant.

Removal of non-target species

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

Evidence

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. Species in these biotopes, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Loss of these species would alter the character of the biotope resulting in re-classification, and would alter the physical structure of the habitat resulting in the loss of the ecosystem functions such as secondary production performed by these species.

Sensitivity assessment. Removal of the characterizing species would result in the biotopes being lost or re-classified. Therefore, the biotopes are considered to have a resistance of Low to this pressure and to have High resilience, resulting in the sensitivity being judged as Low.