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

Intertidal species are exposed to extremes of high and low air temperatures during periods of emersion. They must also be able to cope with sharp temperature fluctuations over a short period during the tidal cycle. In winter air temperatures are colder than the sea, conversely in summer air temperatures are much warmer than the sea. Species that occur in the intertidal are therefore generally adapted to tolerate a range of temperatures, with the width of the thermal niche positively correlated with the height of the shore that the animal usually occurs at (Davenport & Davenport, 2005). The geographic distribution of species characteristic of this biotope extends south of the British Isles, further suggesting these species are likely to be resistant to an increase in temperature. Infaunal species are likely to be protected to some extent from direct effects of acute increases in temperature by sediment buffering, although increased temperatures may affect infauna indirectly by stimulating increased bacterial activity and increased oxygen consumption.

Emery & Stevensen (1957) reported that Nephtys spp. could withstand summer temperatures of 30-35°C so is likely to withstand the benchmark acute temperature increase. An acute increase in temperature at the benchmark level may result in physiological stress endured by the infaunal species but is unlikely to lead to mortality. Nephtys cirrosa is an active worm that can swim short distances and, therefore, it could avoid short-term changes in temperature by migrating away from localised warmer spots.

No direct evidence was found to assess the sensitivity of Paraonis fulgens, however, this species is recorded in warmer waters than the UK in the Gulf of Mexico. Paraonis fulgens was one of the most abundant macrobenthic organisms collected in the shallow waters off Perdido Key, Florida, where winter water temperatures average 22 °C (Gaston et al. 1992). Angulus tenuis is found off the Norwegian coasts to the Mediterranean and north-west coast of Africa and is likely to be resistant to temperature changes at the pressure benchmark.

Spiophanes bombyx is found in the Mediterranean (Hayward & Ryland, 1995), which is likely to be warmer than the waters around Britain and Ireland.

Capitella capitata is a cosmopolitan species in coastal marine and estuarine soft sediment systems. The global population is made up of several genetically distinct (and genetically isolated) sibling species whose distributions overlap such that local Capitella capitata populations consist of several co-occurring sibling species (Grassle & Grassle, 1976). Within the complex tolerances may vary and local acclimation is possible. Capitella capitata has also been recorded in extreme environments around hydrothermal vents (Gamenick & Giere, 1997), which suggests that the species complex would be relatively tolerant to an increase in temperature. Experimental evaluation of the effects of combinations of varying salinities and temperature on Capitella capitata were carried out by Redman (1985) and Warren (1977). Redman (1985) found that length of life decreased as follows: 59 weeks at mid-temperature and salinity (15°C, 25ppt); 43 weeks at high temperature & high salinity (18°C, 30 ppt); 33 weeks at lower temperature & high salinity (12°C, 30 ppt); 17 weeks at high temperature and low salinity (18°C, 20ppt). Redman (1985) also found that net reproduction (Ro: the mean number of offspring produced per female at the end of the cohort) decreased as follows: 41.75 control; 36.69 under high salinity, high temperature; 2.19 high temperature, low salinity; 2.16 low temperature, high salinity. Therefore, a combination of changes in temperature and salinity may decrease the viability of the population. Warren (1977) used individual worms collected from Warren Point (south-west England) to test response to high and low temperatures. Worms were acclimated to 10°C for 10 days and subsequently heated in a water bath at 1°C per 5 min. When the temperature had reached 28°C worms were removed at 0.5°C intervals and returned to a constant temperature of 10°C. The percentage mortality after 24 h was calculated. Larvae were removed from the maternal tube and tested using the same method. The experiments indicated that temperatures above 30°C were most critical; the upper lethal temperature was 31.5°C for adult worms and a little higher for the larvae.

The amphipods that occur within this habitat are mobile and can avoid unfavourable conditions to some extent. Bathyporeia life cycles vary between locations and this is related to temperature (Mettam, 1989). Preece (1971) tested the temperature tolerances of Bathyporeia pilosa in the laboratory. Individuals acclimated to 15°C for 24 hours were exposed to temperature increases (water temperature raised by 0.2°C/minute). As the test temperatures were reached individuals were removed, placed in seawater at 4°C and allowed to recover for 24 hours at which point mortalities were tested. Amphipods were also allowed to bury into sediments and held at test temperatures for 24 hours of 32.5°C, 31.8°C and 29.5°C before being allowed to recover in fresh seawater at 15°C for a further 24 hours, before mortalities were assessed. Upper lethal temperatures (the temperature at which 50% of individuals died for adult males and gravid females of Bathyporeia pilosa were 39.4°C. These tests measured short-term exposure only and species had a lower tolerance for longer-term (24-hour exposure). No mortality occurred for Bathyporeia pilosa individuals held at 29.5°C and 30.8°C; however, 15% of individuals exposed to water temperatures of 31.8°C and 96% at 32.5°C died.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010). A chronic increase in temperature throughout the year of 2°C may fall within the normal temperature variation and an acute increase in water temperatures from 19 to 24°C for a month may be tolerated by the characterizing species supported by deeper burrowing and/or migration. It is likely that the characterizing species are able to resist a long-term increase in temperature of 2°C and may resist a short-term increase of 5°C. Resistance and resilience are, therefore assessed, as ‘High’ and the biotope is assessed as ‘Not Sensitive’ at the benchmark level.

Temperature decrease (local)

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

Evidence

Intertidal species are exposed to extremes of high and low air temperatures during periods of emersion. They must also be able to cope with sharp temperature fluctuations over a short period during the tidal cycle. In winter air temperatures are colder than the sea, conversely in summer air temperatures are much warmer than the sea. Species that occur in the intertidal are therefore generally adapted to tolerate a range of temperatures, with the width of the thermal niche positively correlated with the height of the shore that the animal usually occurs at (Davenport & Davenport, 2005). Some of the characterizing species are found in colder waters that the UK suggesting these can tolerate colder waters than typically encountered. Paraonis fulgens occurs in colder waters than Irish and UK seas, such as the Bay of Fundy, Canada where winter temperatures are between 0 and 4 °C (Risk & Tunnicliffe 2006). Spiophanes bombyx is found in the waters off Denmark (Thorson, 1946) which are likely to be colder than British and Irish waters. Macomangulus (syn. Angulus) tenuis is found off the Norwegian coasts to the Mediterranean and north-west coast of Africa and is likely to be resistant to temperature changes at the pressure benchmark. However, Nephtys cirrosa reaches its northern limit in Scotland and German Bight of the North Sea. A decrease in temperature may result in loss of the species from the biotope in these areas.

Wu et al. (1988) collected Capitella capitata individuals at seawater temperatures of -2° that harboured mature oocytes indicating reproductive activity even under low temperatures. Warren (1977) used Capitella captitata adults collected from Warren Point (south-west England) to test response to high and low temperatures. Worms were acclimated to 10°C for 10 days before testing. The worms were cooled in a water bath to experience a decrease in temperature of 1°C per 5 min. When the final temperature was reached worms were removed at 0.5 °C intervals and returned to a constant temperature of 10°C. The percentage mortality after 24 hr was calculated. Each experiment was repeated once. Larval Capitella capitata were removed from the maternal tube and tested using the same method. Both adults and larvae of Capitella capitata were tolerant of low temperatures, 50% of the adults and 65% of the larvae surviving at -1°C.

Crisp (1964) reported that species of amphipod seemed to be unharmed by the severe winter of 1962-1963. This may be due to burial in sediments buffering temperature or seasonal migration to deeper waters to avoid freezing. In the winter migrations have also been observed for Bathyporeia spp. (Fish & Fish, 1978; Fish & Preece, 1970). Preece (1971) tested the temperature tolerances of Bathyporeia pilosa in the laboratory. Individuals acclimated to 15°C for 24 hours were placed in a freezer in wet sediment. As test temperatures were reached individuals were removed and allowed to recover for 24 hours at which point mortalities were tested. Amphipods were also allowed to bury into sediments and held at test temperatures of -1°C, -3°C and -5°C for 24 hours before being allowed to recover in fresh seawater at 15°C for a further 24 hours before mortalities were assessed. The lower lethal short-term tolerances of Bathyporeia pilosa were -13.6°C. Bathyporeia pilosa individuals could withstand temperatures as low as -1°C for 24 hours, at -3°C, 5% of Bathyporeia pilosa died but this rose to 82% at -5°C.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4-19°C (Huthnance, 2010). A chronic decrease in temperature throughout the year of 2°C may fall within the normal temperature variation but an acute decrease in water temperatures from 4°C to -1°C at the coldest part of the year may lead to freezing and lethal effects but may be tolerated by the characterizing species through deeper burrowing and/or migration. However, the abundance of Nephtys cirrosa may be reduced in northern examples of the biotope or severe winters. Therefore, biotope resistance is assessed as Medium. However, resilience is probably ‘High’ and sensitivity is assessed as Low.

Salinity increase (local)

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

Evidence

This biotope is found in full salinity (30-35 ppt) habitats (18-35 ppt) (JNCC, 2015).  A change at the pressure benchmark is therefore assessed as a change to hypersaline conditions (>40 ppt) from full salinity. Little evidence was found to assess responses to hypersalinity. However, monitoring at a Spanish desalination facility where discharges close to the outfall reached a salinity of 53, found that amphipods 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. (2010) concluded that the reported effects of brine discharges were limited and difficult to compare but identified some trends. Hypersaline effluents tend to disperse quickly in well flushed environments like the habitat this biotope occurs in. However, sediment communities were affected in the immediate vicinity of brine discharges. For example, one of the studies reviewed found that the sediment became dominated by nematodes, with polychaetes, crustaceans and molluscs only fond at a distance from the outfall. Another study noted that the diversity of polychaete communities decreased adjacent to the outfall and that the Ampharetidae were the most sensitive while the Paranoidae were the least sensitive.  

Sensitivity assessment. No direct evidence was found to assess biotope sensitivity. However, if the biotope was exposed to hypersaline effluents then a proportion of the community may be lost and species diversity and abundances are likely to decrease. Therefore, a biotope resistance of 'Low' is suggested. Resilience is probably 'High' (following the restoration of the usual salinity regime) so that sensitivity is 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

Nephtys cirrosa were most abundant in salinities >30 psu in the German Bight (south-east North Sea) (Meißner et al., 2008), while Spiophanes bombyx is a euryhaline species (Bailey-Brook, 1976; Maurer & Lethem, 1980), inhabiting fully saline and estuarine habitats. Spio martinensis was a characterizing species in estuarine channels and inlets in Germany, suggesting resistance to lower salinities (Nehmer et al., 2003)

Sensitivity assessment Nephtys cirrosa is possibly the most sensitive characteristic species to the lower range of the ‘variable’ or ‘reduced’ salinity category, although as a mobile species it will be resistant through being able to move lower down the shore or away from freshwater run-off. Nephtys cirrosa displays resistance to the pressure as the species occur at the mouths of estuaries and estuarine lagoons where salinity may fall below 20 psu (Barnes, 1994), so are unlikely to be significantly impacted by a reduction in salinity. In addition, as this biotope LS.LSa.FiSa.Po.Pful is recorded from both full and variable (18-30) salinity regimes it would probably tolerate 'reduced' salinity conditions for a year. Hence, resistance and resilience are both ‘High’ and sensitivity is, therefore, ‘Not sensitive’.

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 biotope and sub-biotopes occur on moderately exposed or sheltered beaches. Tidal flow velocities from very weak to moderately strong occur in the biotope LS.LSa.FiSa.Po suggesting changes in flow velocity at the benchmark level are unlikely to impact the biotope as characterizing species are likely to be resistant to a very weak to moderately strong flow velocities. Changes in flow velocity are more likely to lead to changes between sub-biotopes. For instance, 21% of records of LS.LSa.FiSa.Po.Ncir occur in moderately strong flow velocities compared to 8% of records of LS.LSa.FiSa.Po.Pful (Paraonis fulgens, Capitella capitata, Pygospio elegans) suggesting a change to the LS.LSa.FiSa.Po.Ncir sub-biotope is more likely under an increase in flow velocity.  

Sensitivity assessment. The LS.LSa.FiSa.Po biotope (and sub-biotopes) occur in water flow velocities from very weak to moderately strong and in a moderately strong to sheltered wave climate The fine sand in low in mud and organic matter, which suggests it is well sorted and oxygenated by water movement. Species characterizing the biotope are likely to be resistant to changes at the pressure benchmark level (a 0.1-0.2 m/s change in flow) although changes between sub-biotopes may occur if silt or mud content of the substratum changes. Resistance and resilience are assessed as ‘High’ and sensitivity is assessed as ‘Not Sensitive’.

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

Spiophanes bombyx is found in the intertidal so may be tolerant to some emersion of the substratum. Nephtys cirrosa dominated the low intertidal species community sampled at DePanne, Belgium. There were good affinities with the subtidal Nephtys cirrosa species assemblage further offshore, suggesting a decrease in emergence will have a limited impact on the species and the associated biotopes (Degraer et al., 1999).  A decrease in emergence may allow the biotope to extend up the shore is suitable habitat exists. However, a decrease in emergence may result in drying of sediment between tides at the upper limit of the biotope and result in an extension of the BarSa biotope (Connor et al., 2004). 

Sensitivity assessment.  A decrease in emergence, and hence, drying of the sediment at the upper limit of the biotope may cause the upper limit of this biotope to move down the shore, reducing its extent. Therefore, resistance is assessed as 'Medium'. Hence, resilience is High' and sensitivity is assessed as 'Low'.

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

The biotope and its sub-biotopes occur on moderately exposed or sheltered beaches. Increases and decreases in wave exposure may lead to increased erosion or deposition. Species in moderately exposed examples of the biotope are likely to be resistant to the dynamic nature of substratum.

Increased wave exposure is likely to resuspend finer material and may lead to reduced abundance of species, such as Capitella capitata that are absent when there is no mud content in the substratum. The circulatory motion of wave action may also wash infauna such as Nephthys cirrosa and Capitella capitata from the sediment in most exposed locations. Although increased wave action is likely to wash some individuals from the sediment, recovery would be rapid.

Sensitivity assessment. An increase in significant wave height at the benchmark level is unlikely to create a noticeable impact, where initial conditions are sheltered. Where conditions are moderately exposed, infauna, such as Nephthys cirrosa, are likely to be washed from the sediment by the largest waves. However, the biotope was reported to be naturally disturbed by winter storms (Connor et al., 2004) and a 3-5% change in significant wave height (the benchmark) is unlikely to affect the biotope adversely, Therefore, resistance and resilience are assessed as ‘High’, and the biotope 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.

Levels of contaminants that exceed the pressure benchmark may cause impacts. Bryan & Gibbs (1983) reported lower sediment-metal concentrations in sandy areas than mud near the mouth of Restronguet Creek, a branch of the Fal Estuary system which is heavily contaminated with metals. Although heavy metals may not accumulate in the substratum to the extent that they would in muddy substrata, characterizing infauna are likely to be susceptible. Bryan & Gibbs (1983) suggested that in populations of polychaetes exposed to heavy metal contamination for a long period, metal resistance could be acquired. For example Nephtys hombergii from Restronguet Creek seemed able to regulate copper. The head end of the worm became blackened and x-ray microanalysis by Bryan & Gibbs (1983) indicated that this was caused by the deposition of copper sulphide in the body wall. In the same study, Bryan & Gibbs (1983) presented evidence that Nephtys hombergii from Restronguet Creek possessed increased tolerance to copper contamination. Specimens from the Tamar Estuary had a 96 h LC50 of 250 µg/l, whilst those from Restronguet Creek had a 96 h LC50 of 700 µg/l (35 psu; 13°C).

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.

Contamination at levels greater than the pressure benchmark may adversely influence the biotope. 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. Oil spills resulting from tanker accidents have caused deterioration of sandy communities in the intertidal and shallow sublittoral. Subtidal sediments, however, may be at less risk from oil spills unless oil dispersants are used, or if wave action causes dispersion of oil into the water column and sediment mobility drives oil into the sediment (Elliott et al., 1998). Microbial degradation of the oil within the sediment would increase the biological oxygen demand and oxygen within the sediment may become significantly reduced. Species within the biotope have been reported to be intolerant of oil pollution, e.g. amphipods (Suchanek, 1993). After the Amoco Cadiz oil spill, there was a reduction in both the number of amphipod species and the number of individuals (Cabioch et al., 1978). Initially, significant mortality would be expected, attributable to toxicity. Amphipod populations have been reported not return to pre-spill abundances for five or more years, which is most likely related to the persistence of oil within sediments (Southward, 1982). Nephtys species were amongst the fauna that was eradicated from sediments following the 1969 West Falmouth spill of Grade 2 diesel fuel documented by Sanders (1978). Multivariate analysis showed that the Prestige oil spill scarcely affected the macroinfaunal community structure during the study period (2003-2009) and its effect was limited just to the first campaign (2003), six months after the Prestige accident (Junoy et al., 2013). Opportunistic species such as Capitella capitata have been shown to increase in abundance close to sources of contamination. High numbers of Capitella capitata have been recorded in hydrocarbon contaminated sediments (Ward & Young, 1982; Olsgard, 1999; Petrich & Reish, 1979) and colonization of areas defaunated by high hydrocarbon levels may be rapid (Le Moal, 1980). After a major spill of fuel oil in West Virginia, Capitella capitata increased dramatically alongside large increases in Polydora ligni and Prionospio sp. (Sanders et al. 1972, cited in Gray 1979).

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.

Boon et al. (1985) reported that Nephtys species in the North Sea accumulated organochlorines but, based on total sediment analyses, organochlorine concentrations in Nephtys species were not correlated with the concentrations in the (type of) sediment which they inhabited.

Radionuclide contamination

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

Evidence

No evidence was found 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

No information concerning the reduced oxygen tolerance of Nephtys cirrosa was found but evidence (Alheit, 1978; Arndt & Schiedek, 1997; Fallesen & Jørgensen, 1991) indicated a similar species, Nephtys hombergii, to be very tolerant of episodic oxygen deficiency and at the benchmark duration of one week. Nephtys cirrosa and Spio spp. were classified by Borja et al. (2000) as being indifferent to enrichment, suggesting some resilience to de-oxygenation. Dense Capitella capitata populations are frequently located in areas with greatly elevated organic content, even though eutrophic sediments are often anoxic and highly sulfidic (Tenore 1977; Warren 1977; Tenore & Chesney 1985; Bridges et al. 1994). The polychaetes Capitella capitata, Pygospio elegans and Scoloplos armiger have all been reported to recolonize habitats following periods of anoxia and hypoxia. 

Scoloplos armiger has been described as being present in low oxygen areas and as a dominant species in the recolonization of previously anoxic areas (Pearson & Rosenberg, 1978). Intertidal Scoloplos armiger is, in contrast to subtidal specimens, subject to hypoxia when tidal flats are without oxygenated seawater during low tide (Kruse et al., 2004). Tolerance against hypoxia and sulfide is low (Kruse et al., 2004), and worms may ascend into the oxic layer during low tide (Schoettler & Grieshaber, 1988).  Capitella capitata exhibits a relatively high tolerance for sediment hypoxia, hydrogen sulphide concentration, and other sediment conditions avoided by many infauna (Henriksson, 1969). Forbes & Lopez (1990) experimentally demonstrated that reduced oxygen concentrations (pO₂ = 20 mm Hg or less) led to decreased Capitella capitata growth rates and cessation of burrowing and feeding activity even when an abundance of food was provided. The authors hypothesize that animals rely solely on anaerobic metabolism once this threshold is crossed. Magnum & Van Winkle (1973) similarly observed that Capitella capitata oxygen uptake ceased when pO₂ fell to between 0-34 mm Hg. The fact that experimental worms lost body mass under these conditions supports the contention that full aerobic metabolism cannot be sustained at very low ambient oxygen conditions despite a very high affinity of Capitella capitata haemoglobin for oxygen. Diaz & Rosenberg (1995) listed Capitella capitata as resistant of moderate hypoxia.

Arenicola marina is subject to reduced oxygen concentrations regularly at low tide and is capable of anaerobic respiration. The transition from aerobic to anaerobic metabolism takes several hours and is complete within 6-8 hrs, although this is likely to be the longest period of exposure at low tide. Fully aerobic metabolism is restored within 60 min once oxygen returns (Zeber & Schiedek, 1996). This species was able to survive anoxia for 90 hrs in the presence of 10 mmol/l sulphide in laboratory tests (Zeber & Schiedek, 1996). Hydrogen sulphide (H₂S) produced by chemoautotrophs within the surrounding anoxic sediment and may, therefore, be present in Arenicola marina burrows. Although the population density of Arenicola marina decreases with increasing H₂S, Arenicola marina can detoxify H₂S in the presence of oxygen and maintain a low internal concentration of H₂S. At high concentrations of H₂S in the lab (0.5, 0.76 and 1.26 mmol/l) the lugworm resorts to anaerobic metabolism (Zeber & Schiedek, 1996). At 16°C Arenicola marina survived 72 hrs of anoxia but only 36 hrs at 20°C. Tolerance of anoxia was also seasonal, and in winter anoxia tolerance was reduced at temperatures above 7°C. Juveniles have a lower tolerance of anoxia but are capable of anaerobic metabolism (Zebe & Schiedek, 1996). However, Arenicola marina was reported to be unaffected by short periods of anoxia and to survive for 9 days without oxygen (Borden, 1931 and Hecht, 1932 cited in Dales, 1958; Hayward, 1994). Diaz & Rosenberg (1995) listed Arenicola marina as a species resistant of severe hypoxia.

Sensitivity assessment.   The species characterizing the biotope are mobile and able to migrate vertically to escape unsuitable conditions.  The biotope is characterized by well-sorted and oxygenated sands, where the anoxic layer occurs below 10 cm and is patchy where it occurs (Connor et al., 2004). This suggests that the resident species may not be adapted to low oxygen levels but also that deoxygenation of the water column may be short-lived, especially as the biotope is exposed at low tide.  Therefore, while some members of the community are known to be tolerant, other species may be lost or reduced in abundance and resistance is assessed as of 'Medium'. Resilience is probably ‘High’ and sensitivity is assessed as Low. However, hypoxia or anoxia caused by the bacterial decomposition of organic matter may be detrimental.

Nutrient enrichment

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

Evidence

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations.  The benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014).  In-situ primary production is limited to microphytobenthos within and on sediments and the high levels of sediment mobility may limit the level of primary production as abrasion would be likely to damage diatoms (Delgado et al., 1991).

Sensitivity assessment.  The nutrient level is not a key factor structuring the biotope at the pressure benchmark.   In general, however, primary production is low, this biotope is species-poor, and characterizing species may be present at low abundances (depending on wave exposure). ​

Organic enrichment

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

Evidence

Benthic responses to organic enrichment have been described by Pearson & Rosenberg (1978) and Gray (1981).  In general, moderate enrichment increases food supply and increases productivity and abundance. Nephtys cirrosa and Spio spp. were classified by Borja et al. (2000) as being indifferent to enrichment. Dense Capitella capitata populations are frequently located in areas with greatly elevated organic content such as areas of sewage disposal and below fish farms and mussel long lines, even though eutrophic sediments are often anoxic and highly sulfidic (Gray, 1979; Tenore, 1977;  Warren, 1977; Tenore & Chesney, 1985; Bridges et al., 1994; Haskoning, 2006; Callier et al., 2007).

Sensitivity assessment. At the benchmark levels, resistance was assessed as ‘High’ as the main characterizing species are tolerant of organic enrichment and an input at the pressure benchmark is unlikely to lead to gross pollution effects. A resilience of ‘High’ is assigned (by default) and the biotope is assessed as ‘Not sensitive’.

Physical loss (to land or freshwater habitat)

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

Evidence

All marine and estuarine habitats and benthic species within them 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

A change to natural or artificial hard substratum would remove this sedimentary biotope and the species. If pockets of fine sediment accumulate in pockets within the substrata then these areas may be re-colonised by species associated with this biotope but these pockets of sediment would not be equivalent to the biotope. Recovery will depend on the reinstatement of suitable habitat.

Sensitivity assessment. Based on the loss of suitable habitat, biotope resistance to this pressure is assessed as ‘None’. Resilience is assessed as ‘Very low’ as the pressure benchmark refers to a permanent change. Biotope sensitivity is, therefore ‘High’.

Physical change (to another sediment type)

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

Evidence

The benchmark for this pressure refers to a change in one Folk class.  The pressure benchmark originally developed by Tillin et al. (2010) used the modified Folk triangle developed by Long (2006) which simplified sediment types into four categories: mud and sandy mud, sand and muddy sand, mixed sediments and coarse sediments.  The change referred to is, therefore, a change in sediment classification rather than a change in the finer-scale original Folk categories (Folk, 1954).  The change in one Folk class is considered to relate to a change in classification to adjacent categories in the modified Folk triangle (Long, 2006). As this biotope occurs within fine sands and muddy sands (JNCC, 2015), the change at the pressure benchmark refers to a potential change to coarse sediments, mixed sediments, sand and muddy sands or mud.

The particle size of sediments and correlated physical and chemical factors (such as drainage, organic matter content and hydrodynamic regime), is a key determinant of the structure of benthic invertebrate assemblages (Van Hoey et al., 2004; Yates et al., 1993).  Infauna can be affected by changes in sediment as many are adapted to burrow through certain grades of sediment (Trueman & Ansell, 1969), decreased fine fractions will reduce habitat suitability for species that maintain permanent burrows. Changes in sedimentary features may also influence the proportions of suspension and deposit-feeding animals (Sanders, 1968), with deposit feeders favoured by increases in the proportion of silts and clays. In North America, cultivation of clam species including the Manila clam, Tapes philippinarum usually involves some form of habitat modification in the form of adding gravel or gravel and crushed shell over mud and sand beaches, to create a more productive clam habitat (referred to as ‘gravelled clam plots’). Such habitat modifications lead to alterations in the local environment and consequently faunal composition. Simenstad and Fresh (1995, cited in Kaiser & Beadman, 2002) reported that the application of gravel to intertidal sediments resulted in a shift from a polychaete to a bivalve and nemertean dominated community, but emphasised that changes are likely to be site-specific.

Responses are also likely to be species-specific and depend on habitat preferences. Pygospio elegans prefers fine sediments such as sand and mud; increased sediment coarseness is likely to render sediments unsuitable for this species. Empirical evidence supporting this view is provided by Bolam (1999) where experimental manipulation of sediments by implanting macroalgae mats led to increased fine sediment fractions (with associated increased organic and water content) which led to the establishment of Pygospio elegans. Capitella capitata was found in fine and medium grain size sediments and was almost completely absent in sediments without mud in the Belgium part of the North Sea (Degraer et al., 2006). This suggests that a change to muddy sand is likely to result in increased abundance but a change to coarser or gravelly sand is likely to lead to reduced abundance.

Nepthys cirrosa occurs in fine to coarser sands, with the greatest abundance in the Belgium part of the North Sea recorded in medium grain sizes (Degraer et al., 2006). A change to gravelly sand is unlikely to impact the species, however, a change to muddy sand may limit the species abundance as the species displays a slight preference for low mud content levels < 10% (Degraer et al., 2006). 

Changes to finer sand are likely to result in increased abundance of Angulus tenuis and changes to the sub-biotope LS.LSa.FiSa.Po.Aten, particularly in the low intertidal where the substratum remains damp at low tide.

Sensitivity assessment. Individual members of the community are found in a range of different sediment types, at different abundances. The character of the habitat is largely determined by the sediment type, changes to this would lead to habitat re-classification. The addition of coarse sand particles or fine particles in sufficient quantities would lead to the development of a different habitat type.   Changes in sediment characteristics can lead to changes in community structure. An increase in coarse sediments would lead to the development of a community typical of mixed sediments, clean sands and/or gravels depending on the degree of change.  In general, an increase to very coarse sediments may favour some amphipod species rather than burrowing polychaetes and sessile tube-dwelling polychaetes. This change would alter the character of the biotope present leading to re-classification, biotope resistance is assessed as 'None' and, as the change is permanent, resilience is assessed as 'Very Low'. Biotope sensitivity is, therefore '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

The process of extraction is considered to remove all biological components of the biotope group.  If extraction occurred across the entire biotope, loss of the biotope would occur. Recovery would require substratum to return to sand and with a finer silt fraction.

Sensitivity assessment. The resistance of the biotope to extraction is probably ‘None’. Resilience differs between species with slower recovery likely to be displayed by Nephtys cirrosa. Resilience is assessed as ‘High’ (although if the substratum changed recovery could be prolonged) and biotope sensitivity is assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

This biotope is present in disturbed and well sorted sands, the associated species are generally present in low abundances and adapted to frequent disturbance. Therefore, resistance to surface abrasion is probably ‘High’. The polychaete Nephtys cirrosa is adapted to life in unstable sediments and survives through rapid burrowing (McDermott, 1983, cited from Elliott et al., 1998). This characteristic is likely to protect this species from surface abrasion.

Paraonis fulgens were found to reduce in abundance in experimental areas exposed to trampling (Reyes-Martínez et al., 2015), suggesting a lower resistance of this species to abrasion or surface disturbance. Chandrasekara and Frid (1996) found that some species including Capitella capitata and Scoloplos armiger reduced in abundance in intertidal muds, along a pathway heavily trampled for five summer months (ca 50 individuals a day Bonsdorff & Pearson (1997) found that sediment disturbance forced Capitella capitata deeper into the sediment, although the species was able to burrow back through the sediment to the surface again. Juveniles and adults of Scoloplos armiger stay permanently below the sediment surface and freely move without establishing burrows. While juveniles are only found a few millimetres below the sediment surface, adults may retreat to 10 cm depth or more (Reise, 1979; Kruse et al., 2004) and are likely to be more protected. The egg cocoons are laid on the surface and hatching time is 2-3 weeks during which these are vulnerable to surface abrasion.

Several studies have assessed the effects of trampling on other intertidal amphipods and these assessments are used as a proxy. Comparisons between shores with low and high levels of trampling found that the amphipod Bathyporeia pelagica is sensitive to human trampling, other species including Pontocrates arenarius and the isopod Eurydice affinis also decreased in response to trampling but Bathyporeia pelagica appeared to be the most sensitive  (Reyes-Martínez et al., 2015). Changes in abundance of talitrid amphipods on urban beaches subject to high levels of recreational use was also observed by Bessa et al. (2014), this study compared abundances between samples taken ten years apart and thus the trends observed were not directly attributable to trampling vs beach cleaning or other pressures although they illustrate a general trend in density patterns as recreational use increases. Ugolini et al. (2008) carried out a controlled trampling experiment on Talitrus saltator. Plastic cylinders of 110 cm diameter (area 0.95 m²) were placed in the sand and all individuals trapped and counted, and 400 steps were made in a cylinder in 15 minutes after the amphipods had reburied. The trampling rate was based on the observed number of beach users and therefore represents a realistic level of exposure. Live individuals were counted at the end of the experiment and 24 hours after. Trampling significantly reduced abundance of the amphipods and after 24 hours the percentage of surviving amphipods dropped to almost zero, while survival rates of control (untrampled) amphipods were unaffected. Abrasion and compaction can, therefore, kill buried amphipods within sediments.

Sensitivity assessment. The characterizing species Paraonis fulgens Capitella capitata and Scoloplos armiger are reduced following abrasion impacts (trampling). However, species in the biotope are adapted to disturbance. Hence, resistance is assessed as ‘Medium’. The resilience of Capitella capitata and other opportunistic species is very high but Nephtys cirrosa is likely to show longer recovery times but overall resilience is assessed as ‘High’, although the potential for longer recovery of Nephtys cirrosa should be accounted for. Sensitivity is, therefore, 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

Nephtys cirrosa and Spiophanes bombyx were characterizing species of infauna assemblages in both control and impact sample sites on the Thornton Bank Belgium (North Sea), before and after dredging occurred as part of the construction process for an offshore wind farm (Coates et al. 2015). Recovery of assemblages occurred within one to two years at individual dredged sites. The species potentially display resilience to dredging activities as past aggregate dredging had also occurred before wind farm construction.

Nephtys cirrosa was found to be sensitive to experimental trawling disturbance over 18 months (Tuck et al., 1998). Nephtys cirrosa is also likely to be vulnerable to dredging but can probably accommodate limited sediment deposition from the dredging process (MES, 2010). Collie et al. (2000) found that the abundance of Nephtys hombergii was negatively affected by fishing activities. Mean response of infauna and epifauna communities to fishing activities was also much more negative in mud and sand communities (such as this biotope) than other habitats. Nephtys hombergii abundance also significantly decreased in areas of the Solent, UK, where bait digging had occurred (Watson et al. 2007). Similarly, Nephtys hombergii abundance was reduced by 50% in areas where tractor towed cockle harvesting was undertaken on experimental plots in Burry inlet, south Wales, and had not recovered after 86 days (Ferns et al., 2000).

Capitella capitata, are soft-bodied, relatively fragile species inhabiting mucus tubes close to the sediment surface. Abrasion and compaction of the surficial layer may damage individuals. Capitella capitata and Pygospio elegans were categorised as AMBI fisheries Group IV- as ‘second-order opportunistic species, which are sensitive to fisheries in which the bottom is disturbed. Their populations recover relatively quickly however and benefit from the disturbance, causing their population sizes to increase significantly in areas with intense fisheries’ (Gittenberger & Van Loon 2011). 

Spio filicornis is a soft-bodied organism that exposes its palps at the surface while feeding. It lives infaunally in sandy sediment and any physical disturbance that penetrates the sediment, for example, dredging or dragging an anchor, would lead to physical damage of Spio filicornis. However, adult worms can burrow up to 10 cm and may escape the disturbance. Juveniles can only burrow up to 2 cm into the sediment and are likely to be affected. However, individuals are likely to pass through a passing scallop dredge due to their small size. Bergman & Hup (1992) reported that the total density of spionids actually increased with increased fishing disturbance presumably due to their ability to colonize newly exposed substratum. Hall et al. (1990) investigated the impact of hydraulic dredging for razor clams. They reported that any effects only persisted for a short time, with the community restored after approximately 40 days in stormy conditions. The population density of Spio filicornis was slightly reduced in the dredged site relative to the control site but its abundance had increased over that of the control site after 40 days. However, the control site showed a similar level of variation in abundance.

Bergman & 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%. Similar results were reported from experiments s in shallow, wave disturbed areas, using a toothed, clam dredge. Bathyporeia spp. experienced a reduction of 25% abundance in samples immediately after intense clam dredging, abundance recovered after one day (Constantino et al., 2009). Experimental hydraulic dredging for razor clams resulted in no statistically significant differences in Bathyporeia elegans abundances between treatments after one or 40 days (Hall et al., 1990), suggesting that recovery from effects was very rapid. Ferns et al. (2000) examined the effects of a tractor-towed cockle harvester on benthic invertebrates and predators in intertidal plots of muddy and clean sand. Harvesting resulted in the loss of a significant proportion of the most common invertebrates from both areas. In the muddy sand, the population of Bathyporeia pilosa remained significantly depleted for more than 50 days, whilst the population in clean sand recovered more quickly. These results agree with other experimental studies that clean sands tend to recover more quickly than other habitat types with higher proportions of fine sediment (Dernie et al., 2003).

Bergman & Hup (1992) found that worm species (including Scoloplos armiger) showed no change in total density after trawling a subtidal habitat. Conversely, a later study by Bergman & Santbrink (2000) found that the direct mortality of Scoloplos armiger from a single passage of a beam trawl in subtidal silty grounds was 18% of the population. Rostron (1995) undertook experimental dredging of sandflats with a mechanical cockle dredger, including a site comprised of stable, poorly sorted fine sands with small pools and Arenicola marina casts with some algal growths. At this site, post-dredging Scoloplos armiger had disappeared from some dredged plots. Ferns et al. (2000) used a tractor-towed cockle harvester, to extract cockles from intertidal plots of muddy sand and clean sand, to investigate the effects on non-target organisms; 31% of the population of Scoloplos armiger (initial density of 120 per m2) were removed. Populations of Scoloplos armiger remained significantly depleted in the area of muddy sand for more than 50 days after harvesting. Ball et al. (2000) found that species including Scoloplos armiger showed a significant decrease in abundance of between 56-27% after 16 months of otter trawling at a previously unfished Scottish sea loch. Chandrasekara and Frid (1996, cited in Tyler-Walters & Arnold, 2008) found that along a pathway heavily used for five summer months (ca. 50 individuals/day), Scoloplos armiger reduced in abundance. Recovery took place within 5-6 months. These studies suggest that Scoloplos armiger is likely to be impacted by sediment disturbance and that recovery to previous densities may require more than two years.

A number of studies have found that the abundance of the polychaete Pygospio elegans is reduced by simulated cockle dredging (Hall & Harding, 1998; Moore, 1990; Ferns et al., 2000; Rostron, 1995).  Ferns et al. (2000) found that tractor towed cockle harvesting removed 83% of Pygospio elegans (initial density 1850/ m2).  In muddy sand habitats, Pygospio elegans had not recovered to the original abundance after 174 days (Ferns et al.,2000). Rostron (1995) also found that Pygospio elegans had not recovered to pre-dredging numbers after six months. Conversely, Hall & Harding, (1998) found that the abundance of Pygospio elegans increased significantly over 56 days following suction dredging.  Pygospio elegans inhabits a fragile tube that projects above the sediment surface and is probably more vulnerable to physical disturbance and abrasion than other, more deeply buried, infaunal species.

Sensitivity assessment.  The evidence suggests that many of the characteristic species could suffer a significant loss in abundance due to penetrative gear. Therefore, the resistance of the biotope is assessed as ‘Low’, as a proportion of the population of characterizing species may be removed. However, species in the biotope are adapted to disturbance and recover quickly so that resilience is assessed as ‘High ’ and sensitivity as ‘Low’.

Changes in suspended solids (water clarity)

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

Evidence

The characterizing species live within the sand and are unlikely to be directly affected by an increased concentration of suspended matter in the water column. Within the mobile sands habitat storm events or spring tides may re-suspend or transport large amounts of material and therefore species are considered to be adapted to varying levels of suspended solids. Bathyporeia spp. feed on diatoms within the sand grains (Nicolaisen & Kanneworff, 1969), an increase in suspended solids that reduced light penetration could alter food supply. However, diatoms are able to photosynthesize while the tide is out and therefore a reduction in light during tidal inundation may not affect this food source, depending on the timing of the tidal cycle. Bathyporeia spp. may be regular swimmers within the surf plankton, where the concentration of suspended particles would be expected to be higher (Fincham, 1970a).

However, the biotope is characterized by a low amount of organic matter and an increase in suspended solids may cause a change in this factor if this is coupled with changes in hydrodynamics that reduce particle re-suspension. Increased suspended solids are unlikely to have a direct impact on infauna but increased organic matter may result in an increase in the abundance of opportunistic species such as Capitella capitella. Biotope resistance is assessed as ‘High’ and resilience as ‘High’ (by default) so that the biotope is assessed to be ‘Not sensitive’.

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 characterizing species Pygospio elegans is limited by high sedimentation rates (Nugues et al., 1996) and the species does not appear to be well adapted to oyster culture areas where there are high rates of accumulation of faeces and pseudo faeces (Sornin et al., 1983; Deslous-Paoli et al., 1992; Mitchell, 2006 and Bouchet & Sauriau, 2008).  Pygospio elegans is known to decline in areas following the re-deposition of very fine particulate matter (Rhoads & Young, 1971; Brenchley, 1981). Experimental relaying of mussels on intertidal fine sands led to the absence of Pygospio elegans compared to adjacent control plots. The increase in fine sediment fraction from increased sediment deposition and biodeposition alongside possible organic enrichment and decline in sediment oxygen levels was thought to account for this (Ragnarsson & Rafaelli, 1999).

Mobile and/or burrowing species (including molluscs and polychaetes such as Nephtys spp., and Scoloplos armiger) are generally considered to be able to reposition following periodic siltation events or low levels of chronic siltation. Nephthys cirrosa occurs in fine to coarser sands, with the greatest abundance in the Belgium part of the North Sea recorded in medium grain sizes (Degraer et al., 2006). A light deposition of fine sediment may lead to small but insignificant changes in abundance as it will reduce the available preferred habitat with medium grain size. As the tidal flow is strong in this biotope, a light deposition of finer sediment is likely to be resuspended. Resistance is likely to be high for Nephthys cirrosa at the benchmark level as this species is likely to be able to reposition within sediments.

Capitella capitata was categorised as AMBI sedimentation Group IV as a ‘second-order opportunistic species, insensitive to higher amounts of sedimentation. Although they are sensitive to strong fluctuations in sedimentation, their populations recover relatively quickly and even benefit. This causes their population sizes to increase significantly in areas after a strong fluctuation in sedimentation’ (Gittenberger & Van Loon 2011).

Sensitivity assessment. None of the characterizing species is considered likely to be significantly impacted by deposition of up to 5 cm of fine material. Resistance is assessed as ‘High’, resilience as ‘High’ and sensitivity is assessed as ‘Not sensitive’.

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

Studies have found that beach ‘replenishment’ or ‘nourishment’ that involves the addition of sediments on beaches can have a number of impacts on the infauna (Peterson et al., 2000; Peterson et al., 2006). Impacts are more severe when the sediment added differs significantly in grain size or organic content (Nelson et al., 1989; Peterson et al., 2000). For example, Maurer et al. (1981) found that the amphipod Parahaustorius longimerus, which occurs intertidally in clean, well-sorted sands and is an active, effective burrower, was able to regain the surface after being buried by sand far more easily than when buried under silt/clay mixtures. 

Nephtys cirrosa is a large infaunal species, with an adult size between 6 cm and 10 cm and capable of moving through the sediment, suggesting some resilience to smothering. Nephtys cirrosa is an active worm which demonstrates the characteristic swimming motion (a rapid lateral wriggling, starting from the rear and increasing in amplitude towards the head) of the Nephtyidae. Deposition of up to 30 cm of fine material is likely to bury some individuals beyond the typical 5 to 15 cm depth of tunnels. It is likely Nephtys cirrosa close to the surface may be capable of relocating in the sediment although feeding and reproduction activities are likely to be interrupted.

Nephthys cirrosa occurs in fine to coarser sands, with the greatest abundance in the Belgium part of the North Sea recorded in medium grain sizes (Degraer et al., 2006). Presence of fine material may lead to small but insignificant changes in abundance as it will reduce the available preferred habitat with medium grain size. As the tidal flow is strong in this biotope, a light deposition of finer sediment is likely to be resuspended. Resistance is likely to be high to the presence of finer material for Nephthys cirrosa but initial smothering is likely to cause some mortality and interrupt feeding and reproduction activity at the benchmark level.

Capitella capitata has been categorised through expert and literature review, as AMBI sedimentation Group IV – a second-order opportunistic species, insensitive to higher amounts of sedimentation. Although they are sensitive to strong fluctuations in sedimentation, their populations recover relatively quickly and even benefit. This causes their population sizes to increase significantly in areas after a strong fluctuation in sedimentation (Gittenberger & Van Loon 2011).

 Bijkerk (1988, results cited from Essink, 1999) found that the maximal overburden through which Bathyporeia could migrate was approximately 20 cm in mud and 40 cm in sand. No further information was available on the rates of survivorship or the time taken to reach the surface and no information was available for other characterizing species. 

Sensitivity assessment. Overall smothering by 30 cm of fine sediments may result in mortality of characterizing species. Although some polychaetes may be able to reposition after sedimentation at the pressure benchmark this will depend on the characteristics of the overburden and sedentary species such as Pygospio elegans are likely to suffer high levels of mortality. The introduction of fine sediment may also alter the sediment typical of the biotope causing a temporary shift in the abundance of species. However, the opportunistic species occurring in the biotope are likely to recover rapidly following sediment recovery. Biotope resistance is, therefore, assessed as ‘Low’, resilience is assessed as  ‘High’, following habitat recovery to fine sands and biotope sensitivity is assessed as ‘Low’.

Litter

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

Evidence

Plastic debris breaks up to form microplastics. Microplastics have been shown to occur in marine sediments and to be ingested by detritivores such as the amphipod Orchestia gammarellus, deposit feeders such as Arenicola marina and holothurians, as well as by suspension feeders, e.g. Mytilus edulis (Wright et al., 2013b; Browne et al., 2015).

Wright et al. (2013) showed that the presence of microplastics (5% UPVC) in a lab study significantly reduced feeding activity when compared to concentrations of 1% UPVC and controls. As a result, Arenicola marina showed significantly decreased energy reserves (by 50%), took longer to digest food, and as a result decreased bioturbation levels, which would be likely to impact the colonization of sediment by other species, reducing diversity in the biotopes the species occurs within. Wright et al. (2013) suggested that in the intertidal regions of the Wadden Sea, where Arenicola marina is an important ecosystem engineer, Arenicola marina could ingest 33 m³ of microplastics a year.

In a similar experiment, Browne et al. (2013) exposed Arenicola marina to sediments with 5% PVC particles or sand presorbed with pollutants nonylphenol and phenanthrene for 10 days. PVC is dense and sinks to the sediment. The experiment used Both microplastics and sand transferred the pollutants into the tissues of the lugworm by absorption through the gut. The worms accumulated over 250% more of these pollutants from sand than from the PVC particulates. The lugworms were also exposed to PVC particulates presorbed with plastic additive, the flame retardant PBDE-47 and antimicrobial Triclosan. The worms accumulated up to 3,500% of the concentration of theses contaminants when compared when to the experimental sediment. Clean sand and PVC with contaminants reduced feeding but PVC with Triclosan reduced feeding by over 65%. In the PVC with Triclosan treatments, 55% of the lugworms died.  Browne et al. (2013) concluded that the contaminants tested reduced feeding, immunity, response to oxidative stress, and survival (in the case of Triclosan).

Sensitivity assessment. Impacts from the pressure ‘litter’ would depend on upon the exact form of litter or man-made object introduced.  Browne et al. (2015) suggested that if effects in the laboratory occurred in nature, they could lead to significant changes in sedimentary communities as Arenicola marina is an important bioturbator and ecosystem engineer in sedimentary habitats. Arenicola marina does not reach high abundances in this biotope but other deposit-feeding polychaetes could potentially ingest microplastics, although no evidence is available at present. This pressure is 'Not assessed' as no benchmark has been defined for this pressure.

Electromagnetic changes

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

Evidence

Electric and magnetic fields generated by sources such as marine renewable energy device/array cables may alter behaviour of predators and affect infauna populations. Evidence is limited and occurs for electric and magnetic fields below the benchmark levels, confidence in evidence of these effects is very low.

Field measurements of electric fields at North Hoyle wind farm, North Wales recorded 110µ V/m (Gill et al. 2009). Modelled results of magnetic fields from typical subsea electrical cables, such as those used in the renewable energy industry produced magnetic fields of between 7.85 and 20 µT (Gill et al. 2009; Normandeau et al. 2012). Electric and magnetic fields smaller than those recorded by in-field measurements or modelled results were shown to create increased movement in thornback ray Raja clavata and attraction to the source in catshark Scyliorhinus canicular (Gill et al. 2009).

Flatfish including dab Limanda limanda and sole Solea solea are predators of many polychaete species.  They have been shown to decrease in abundance in a wind farm array or remain at distance from wind farm towers (Vandendriessche et al., 2015; Winter et al. 2010). However, larger plaice increased in abundance (Vandendriessche et al., 2015). There have been no direct causal links identified to explain these results.

Sensitivity assessment. No evidence was found on the effects of electric and magnetic fields on the characterizing species. However, responses by flatfish and elasmobranchs suggest changes in predator behaviour are possible. There is no evidence currently but if electromagnetic fields affect predator-prey dynamics as further marine renewable energy devices are deployed, these are likely to be over small spatial scales and unlikely to significantly impact the biotope.

Underwater noise changes

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

Evidence

Species within the biotope can probably detect vibrations caused by noise. However, at the benchmark level the community is unlikely to be sensitive to noise and this pressure is therefore ‘Not relevant’.

Introduction of light or shading

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

Evidence

As this feature is not characterized by the presence of primary producers shading is not considered likely to alter the character of the habitat. As the characterizing biological assemblage occurs within the sediment, an increase in light or shading is considered ‘Not relevant’. However, shading may reduce the microphytobenthos component of this infralittoral biotope. Mucilaginous secretions produced by these algae may stabilize fine substrata (Tait & Dipper, 1998). Shading will prevent photosynthesis leading to death or migration of sediment microalgae, which may alter sediment cohesion and food supply to higher trophic levels.

Sensitivity assessment. Changes in light are not considered to directly affect the biotope.  However, some changes in behaviour or food supply for characterizing species could result. Overall, resistance is assessed as High. Therefore, resilience is High and the biotope is assessed as Not sensitive. 

Barrier to species movement

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

Evidence

Nephtys cirrosa produce pelagic larvae. Barriers that limit tidal excursion and flushing may reduce connectivity but help to retain larvae. Capitella capitata and the associated species Pygospio elegans are capable of both benthic and pelagic dispersal. In the sheltered waters where this biotope occurs, with reduced water exchange, in-situ reproduction may maintain populations rather than long-range pelagic dispersal. As the tubificid oligochaetes that occur in this biotope have benthic dispersal strategies via egg cocoons laid on the surface (Giere & Pfannkuche, 1982), water transport is not a key method of dispersal over wide distances. The biotope is considered to have ‘High’ resistance to the presence of barriers that lead to a reduction in tidal excursion, resilience is assessed as ‘High’ (by default) and the biotope is considered to be ‘Not sensitive’

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

Characterizing species may have some, limited, visual perception. As they live in the sediment the species will most probably not be impacted at the pressure benchmark and this pressure is considered '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

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

Introduction or spread of invasive non-indigenous species

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

Evidence

Coastal and estuarine areas are among the most biologically invaded systems in the world, especially by molluscs such as the slipper limpet Crepidula fornicata and the Pacific oyster Magallana gigas (OSPAR, 2009b). The two species have not only attained considerable biomasses from Scandinavian to Mediterranean countries but have also generated ecological consequences such as alterations of benthic habitats and communities, or food chain changes. 

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Baron, 2010; GBNNSS, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Baron, 2010; Padilla, 2010). Magallana gigas is recognised as a beneficial and important species in aquaculture worldwide (Padilla, 2010). It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSS, 2011, 2012; Humphreys et al., 2014 cited in Alves et al., 2021; Hansen et al., 2023).

Since introduction, the species has invaded and established self-sustaining natural populations throughout Europe from the North Sea, Wadden Sea and Scandinavian coastlines to the Atlantic coastlines of Spain and Portugal, as well as the Mediterranean and Adriatic Sea (Wrange et al., 2010; GBNNSS, 2011, 2012; Ezgeta-Balic et al., 2019; Spagnolo et al., 2019; Bergstrom et al., 2021; Hansen et al., 2023). In the UK, the species predominantly occurs around the southern and western coastlines (OBIS, 2024; NBN, 2024). Shipping activity has also been associated with the introduction of Magallana gigas in the northeastern Adriatic Sea, where it was not introduced for aquaculture (Ezgeta-Balic et al., 2019). It was also suggested that some Magallana gigas populations were established in southwest England from France possibly via fouling on ships (GBNNSS, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).

Magallana gigas has a high fecundity, a long-lived pelagic larval phase (2 to 4 weeks) and can produce up to 200 million eggs during spawning (Herbert et al., 2012, 2016; Alves et al., 2021; Wood et al., 2021; Hansen et al., 2023). Hence, as a broadcast spawner, it has a high dispersal potential of more than 1000 km (Padilla, 2010; Wood et al., 2021). Larval mortality can be as large as 99%, as larvae are sensitive to environmental conditions (Alves et al., 2021). However, adults are long-lived so populations can survive with infrequent recruitment (Padilla, 2010). Larval dispersal and mass spawning events have facilitated the settlement and establishment of Pacific oysters, as seen in the Oosterschelde estuary, Netherlands (Hansen et al., 2023). It has been suggested that the spread of the Pacific oyster in Scandinavia is due to northward larval drift on tidal and wind-driven currents (Hansen et al., 2023). Wood et al. (2021) suggested that larval dispersal of the Pacific oyster from populations within and outside the UK was possible via unaided (passive) transport by currents, but that aquaculture and offshore structures (e.g. windfarms) increased the risk of the invasive species spreading and the geographical extent of spread.

Magallana gigas is an ecosystem engineer and can dramatically change habitat structure when it invades. Once successfully settled, groups of Pacific oysters may form dense aggregations, potentially forming a reef, which in some regions can reach densities of 700 individuals m² (Herbert et al., 2012, 2016). Once, the density of live or dead Pacific oysters reaches or exceeds 200 ind./m² little of the underlying substratum remains visible (Herbert et al., 2016). These reefs can stabilize the sediment surface locally (Troost, 2010). When such reefs are formed or, particularly when the species colonizes soft sediments such as mud or sand, it can change and affect local communities, by creating hard substrata for mobile species, which might not otherwise be present before the invasion (Padilla, 2010). However, Hansen et al. (2023) suggested that no immediate ecosystem risk is observed where the Pacific oyster occurs sporadically.

Magallana gigas also colonizes littoral intertidal biogenic reefs formed by the blue mussel Mytilus edulis or honeycomb worm Sabellaria alveolata (GBNNSS, 2011, 2012; Kochmann, 2012; Kochmann et al., 2013; Herbert et al., 2016; Tillin et al., 2020). Evidence suggests the Pacific oyster can out-compete Mytilus edulis, particularly for food and space, as the faster growth rates of the oyster make it more competitive when food or space is limiting (Nehls et al., 2006; Padilla, 2010; Tillin et al., 2020; Joyce et al., 2021). The invasion of Magallana gigas may alter the structure and function of these intertidal reefs but can create a multi-layered structure of a mixture of oysters and blue mussels that is more resilient and accumulates a higher biodiversity of flora and fauna and supports the densities of other native species such as Littorina littorea (Reise et al., 2017; Andriana et al., 2020; Cornelius & Buschbaum, 2020; Hansen et al., 2023).

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann, 2012; Kochmann et al., 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). It also prefers mudflats with mixed sediment composed of shingle and sand, attaching to whatever hard substrata are available within otherwise unsuitable fine muddy sediment (Spencer et al., 1994; Mckinstry & Jensen, 2013; Tillin et al., 2020).

Magallana gigas has been reported from estuaries growing on intertidal mudflats, sandflats, and other soft sediments (Padilla, 2010; Herbert et al., 2016; Cabral et al., 2020). The settlement of spat on hard substrata within sediments has been observed in the estuaries of the River Dart, Exe, Fal, Fowey, Tamar, Teign, and Yealm in Devon and Cornwall, the Menai Straits, Wales and large estuaries of Lough Swilly, Lough Foyle and the Shannon in Ireland, and the Tagus Estuary in Portugal (Spencer et al., 1994; Kochmann, 2012; Kochmann et al., 2013; Cabral et al., 2020). In Lough Swilly, Lough Foyle and the Shannon, the Pacific oyster was often associated with intertidal mud or sandflats (Kochmann et al., 2013). In contrast, the Pacific oysters were absent from sandflat areas in Poole Harbour (Mckinstry & Jensen, 2013).

Although shorelines comprised of mainly mud were suggested to be unsuitable for spat settlement (Spencer et al., 1994), the presence of smaller hard substrata, such as shells or pebbles, can enable larvae to settle (Tillin et al., 2020). For example, in the River Teign estuary, Pacific oyster settlement was observed on shell-covered ground mainly attached to mussel shells, and occasionally attached to cockles, stones and common periwinkle (Littorina littorea) shells on a mud flat in the estuarine intertidal zone otherwise mainly comprised of sand and mud (Spencer et al., 1994). In addition, the Blue Lagoon on the north shore of Poole Harbour had the highest abundance of oysters on mud mixed with shingle and shell (Mckinstry & Jensen, 2013). Outside of the Blue Lagoon, oysters were also recorded on mixed substrata composed of mud, gravel, and shell (McKinstry & Jensen, 2013). Tillin et al. (2020) concluded that while successful invasions occurred on mudflats, Magallana gigas prefers mixed substrata. Fine mud sediments without hard substrata (such as small stones, gravel, and shell) are unlikely to be suitable (Tillin et al., 2020). The speed of Magallana gigas reef formation on soft substrata seems to be dependent on the amount of hard substrata present, developing quicker once there is a sufficient amount (Troost, 2010). Bergstrom et al. (2021) reported that the presence of Magallana gigas was partially dependent on increasing gravel content up to 15% but remained stable with increasing percentages (measured up to 80%).

The oyster reefs, in the Wadden Sea and Brittany, on littoral muddy and sandy habitats formed predominantly at lower tidal levels from Mean Low Water levels to the shallow subtidal (Herbert et al., 2012, 2016). Pacific oyster spatfall was recorded in the estuarine intertidal zone on areas with hard substrata of stone and shell, particularly between the low water of spring tides and high water of neap tides, such as in the Menai Strait (Spencer et al., 1994).

At high densities the Pacific oyster reef smothers sediment, provides hard substrata in an otherwise sedimentary environment with additional niches for colonization by other species that require hard substratum (e.g. barnacles), and changes surface roughness and local hydrography (Troost, 2010; Herbert et al., 2012, 2016; Tillin et al., 2020). Lejart & Hily (2011) found the surface available for epibenthic species in the Bay of Brest, increased 4-fold when oysters were present on mud, for every 1 m² of colonized substrata the oyster reef added 3.87 m² of surface area on mud sediment. An increase in available settlement substrata, free of epibiota, could be the reason oyster reefs see an increase in the macrofaunal abundance. This can change the community composition and habitat structure in reefs on soft mud sediments, creating new habitats for an increasing abundance of infaunal and epibenthic mobile species (Kochmann et al., 2008; Lejart & Hily, 2011; Zwerschke et al., 2018). Results have shown 38% of species present in the oyster reefs on mud were characteristic of rocky substratum habitats (Lejart & Hily, 2011).

In the Bay of Brest, Pacific oyster reefs had a higher diversity and species richness than surrounding mud habitats, including the mud underneath the reefs, where the population was dominated by carnivores rather than suspension the feeders found on the mudflats (Lejart & Hily, 2011; Herbert et al., 2012). In addition, in muddy habitats around the UK, Ireland and Northern France, macrofaunal diversity increased as Pacific oyster density increased but epifaunal diversity decreased as oyster densities increased (Zwerschke et al., 2018). It was suggested that the reason for the decrease in epifaunal diversity was the decrease in settlement space and the increase in habitat fragmentation because of dense oyster assemblages (Zwerschke et al., 2018).

Green & Crowe (2014) examined the effects of Magallana gigas density in experimental plots (0.25 m²) in Lough Swilly and Lough Foyle, Ireland. The number of species and species diversity increased with oyster cover on mudflats, depending on site and duration. The assemblage also changed due to the increased abundance of barnacles and bryozoans on the oyster shells and polychaetes within the sediment (Green & Crowe, 2014). Zwerschke et al. (2020) suggested that Pacific oyster beds could replace the ecosystem services provided by native oysters, in areas where native oysters had been lost. Morgan et al. (2021) suggested that the smothering of sediment habitats could prevent fish and bird species from feeding on infauna like worms, molluscs, and crustaceans. Also, the development of tidepools within mixed Pacific oyster and blue mussel reefs in soft sediment intertidal sites has been observed in the Wadden Sea, which can create new microhabitats within the reefs (Weniger et al., 2022).

Pacific oysters have been found to reduce the proportion of fine particles and increase the proportion of large particles in the mud under the reef (Lejart & Hily, 2011). The evidence suggests that Pacific oyster reefs change sediment characteristics, by affecting nutrient cycling and increasing the organic content of sediment, sand-to-silt ratio and levels of porewater ammonium (Kochmann et al., 2008; Padilla, 2010; Wagner et al., 2012 cited in Tillin et al., 2020; Green & Crowe, 2014; Herbert et al., 2012, 2016; Zwerschke et al., 2020; Hansen et al., 2023).  Zwerschke et al. (2020) found no significant differences in nutrient cycling rates of native oyster beds or Magallana gigas beds or their associated benthic communities, in experimental plots in Ireland. Persistent changes in the rates of nutrient cycling were driven by the density and presence of oysters (Zwerschke et al., 2020).

The deposition of faeces and pseudo-faeces by Magallana gigas can increase the toxic levels of sulphide in sediments and associated hypoxic sediment conditions, which can reduce photosynthesis and growth in eelgrass (Kelly & Volpe, 2007). Faecal deposition and hypoxia have also been suggested to explain a reduction in species diversity in the sediment underlying high-density oyster reefs (Green & Crowe, 2013, 2014; Herbert et al., 2016). However, Lejart & Hily (2011) observed no organic or silt enrichment by Pacific oysters in mud beneath oyster reefs in the Bay of Brest, and no significant difference in the amount of organic matter found in the mud underneath oyster reefs and on bare mud not colonized by the oyster. The biodeposits excreted by the oyster may be washed away by powerful tides and currents seen in the Bay of Brest and the effects of organic enrichment at oyster reefs might be minimal due to wave action (Lejart & Hily, 2011).

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). Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and possibly burial due to deposition, which could mitigate Crepidula density. In addition, in the 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 but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). However, gravel formed the base of most stacks of Crepidula in the intertidal, which suggested that initial colonization occurred on available hard substrata (i.e., gravel) in the absence of adult shells of Crepidula (Bohn et al., 2015).

Crepidula is recorded from the lower intertidal to ca 160 m in depth but it is most common in the shallow subtidal and low water springs (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020). Bohn et al. (2012, 2013a, 2013b, 2015) suggested that extreme conditions in intertidal limited its upward distribution due to early post-settlement mortality. It reached its highest densities on the lower shore (below ca 0.7 m) and was absent from the high tidal level (ca 1.8 m) in the MHW (Bohn et al., 2015). Bohn et al. (2013b) noted that Crepidula spat in their experimental intertidal panels suffered high mortality of 78-100% during emersion by low water spring tides. Thieltges et al. (2003) noted that Crepidula abundance at the intertidal to the subtidal transition zone (ca 21/ m²) was significantly higher than in the upper, mid, and lower intertidal ca <3/ m²). Similarly, Diederich & Pechenik (2013) noted that Crepidula densities were not significantly different in the low intertidal (+0.2 m) and shallow subtidal (-1 m) but became lower at +0.4 and were absent above +0.6 m in Bissel Cove, Rhode Island where the mean high water was +1.38 m. They reported that intertidal adults experienced temperatures of ca 42°C, which were 15°C higher than subtidal adults. However, there was no significant difference in the tolerance of subtidal and intertidal adults with a lethal range of 33-37°C after three hours in the laboratory. Diederich & Pechenik (2013) suggested that adult Crepidula were living close to their upper thermal limit in Rhode Island and would be driven into the subtidal due to climate change. Diederich et al. (2015) reported that most juvenile Crepidula died after aerial exposure under laboratory conditions (20°C, 75% relative humidity), while adults from the intertidal and subtidal survived (26°C, 75% relative humidity). Franklin et al. (2023) noted that the body mass index of adult Crepidula did not decrease significantly in winter months in New Hampshire, USA, but did decrease in spring and summer, probably due to its investment in reproduction. 

The density of Crepidula populations in northern Europe (Germany, Denmark, and Norway) was significantly lower (ca <100/ m²) than in southern waters. Thieltges et al. (2004) reported that the population of Crepidula was affected strongly by cold winters in the Wadden Sea. The winters of 2001 and 2003 resulted in ca 56-64% mortality of intertidal Crepidula and up to 97% on one mussel bed, compared to only 11-14% in southern areas without frost. Crepidula almost vanished from the Wadden Sea after the 1978/79 winter and took ten years to recover due to moderate winters which regularly affected the population. Similarly, 25% mortality was observed in Crepidula populations on the south coast of the UK after the extreme 1962/63 winter (Crisp, 1964, Bohn et al., 2012). Thieltges et al. (2003) suggested that global warming may allow Crepidula populations to become more abundant in northern Europe.

Ensis siliqua occurs on some North Wales shores  (Connor et al., 2004). This species could co-occur with or be replaced by a similar, but non-native species Ensis directus. However, such a change is unlikely to alter the character of the biotope. 

Sensitivity assessment. The above evidence suggests that this biotope is unsuitable for colonization by Crepidula fornicata due to a lack of gravel, shells, or any other hard substrata used for larvae settlement (Tillin et al., 2020). This biotope is dominated by fine mud and a shell fraction is not recorded in the description (Connor et al., 2004) but if artificial hard debris (e.g. litter) was introduced to the habitat then it may provide an initial point for the colonization of Crepidula in particular. Although it would probably take many years, colonization by Crepidula would result in the complete modification of the habitat, reclassification and loss of the biotope, although polychaete populations may survive in the sediment itself. Sediment mobility due to winter storms may also prevent colonization. Therefore, colonization by Crepidula fornicata is unlikely due to the lack of hard substrata and resistance is assessed as 'High'. Hence, resilience is 'High' and the biotope is assessed as 'Not sensitive'. 

The above evidence also suggests that this biotope is unsuitable for the colonization of Magallana gigas due to a lack of gravel, shell, or any other hard substrata required for successful settlement and establishment (Kochmann, 2012; Kochmann et al., 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). Although Magallana gigas has been found in intertidal mudflats and sandflats, the Pacific oysters were absent from sandflat areas in Poole Harbour (Mckinstry & Jensen, 2013). Sediment mobility due to winter storms may also restrict colonization or its density. Therefore, resistance to colonization by Magallana gigas is assessed as 'High' due to the lack of hard substrata in this biotope, resilience is assessed as 'High' and the biotope is assessed as 'Not sensitive'. The confidence in the assessment is 'Low' because the sensitivity of this biotope to Crepidula and Magallana is potentially site-specific, there is a risk of its introduction by artificial means, and there is a lack of direct evidence of Crepidula being reported to occur in the biotope. 

Introduction of microbial pathogens

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

Evidence

No evidence was found.

Removal of target species

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

Evidence

Nephtys cirrosa is targeted by bait diggers, there is limited information on the effect of targeted removal on Nephtys cirrosa populations, however, there is evidence on effects on Nephtys hombergii. Nephtys hombergii is directly removed through commercial bait digging and by recreational anglers and abundance significantly decreased in areas of the Solent, UK, where bait digging (primarily for Nereis virens) had occurred (Watson et al. 2007). Recovery of Nephtys hombergii has been assessed to be high as re-population would occur initially relatively rapidly via adult migration and later by larval recruitment. Dittman et al. (1999) observed that Nephtys hombergii was amongst the macrofauna that colonized experimentally disturbed tidal flats within two weeks of the disturbance that caused defaunation of the sediment. However, if sediment is damaged recovery is likely to be slower, for instance, Nephtys hombergii abundance was reduced by 50% in areas where tractor towed cockle harvesting was undertaken on experimental plots in Burry inlet, south Wales, and had not recovered after 86 days (Ferns et al., 2000).

Removal of Nephtys cirrosa by bait digging may cause short-term loss of food resources for predators such as fish species including Limanda limanda and Pleuronectes platessa. As recovery is medium to high, the long-term impacts on populations are likely to be small but will be dependent upon the scale and frequency of bait digging activities.

Sensitivity assessment. Confidence in this assessment about the removal of Nephtys cirrosa is 'Low' as it is based on evidence of removal of Nephtys hombergii. However, biotope resistance is assessed as ‘Low’ based on direct removal of a characterizing species, Resilience is assessed as ‘High’ as habitats that are not regularly harvested may recover rapidly, although it should be noted that continued harvesting will inhibit recovery. Biotope sensitivity to a single harvesting event is assessed as ‘Low’. It is important to consider that the spatial extent and duration of harvesting is important to consider when assessing this pressure as smaller scale extraction may not impact the entire extent of the biotope but greater scale extraction over a long period would cause longer-term impacts.

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 this biotope, 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).

Collie et al. (2000) identified that intertidal communities (such as this biotope) suffered impacts from impact from fishing activities. The review concluded that there were ecologically important impacts from removal of >50% of fauna from bottom towed fishing activity (dredge and trawls) (Collie et al., 2000). Kaiser et al. (2001) carried out experimental hand raking, similar to that used in intertidal cockle fisheries. Both small and large raked plots showed changed communities in comparison to control plots, smaller plots recovered in 56 days, whilst larger plots remained in an altered state.

Collie et al. (2000) found that the abundance of a Nephtys hombergii was negatively affected by fishing activities. Mean response of infauna and epifauna communities to fishing activities was also much more negative in mud and sand communities (such as this biotope) than other habitats. Nephtys hombergii abundance also significantly decreased in areas of the Solent, UK, where bait digging had occurred (Watson et al. 2007). Similarly, Nephtys hombergii abundance was reduced by 50% in areas where tractor towed cockle harvesting was undertaken on experimental plots in Burry inlet, south Wales, and had not recovered after 86 days (Ferns et al., 2000).

Sensitivity assessment. The incidental damage or removal of a proportion of the population (e.g. by commercial bait digging) may change the character of the community temporarily.  The biotope is disturbed seasonally by storms, (Connor et al., 2004) and may recover quickly.  However, long-term disturbance from repeated events e.g. by periodic bait digging (see above) may prolong recovery. Biotope resistance is assessed as ‘Low’ based on removal or damage of characterizing species, that on commercial scales can remove a large proportion of the population and lead to an impacted community. Resilience is assessed as ‘High’ but it should be noted that continued harvesting will impact the population and Nephtys cirrosa will take longer to recover if harvesting is overextended spatial scales. Biotope sensitivity is assessed as ‘Low’. It is important to consider that the spatial extent and duration of areas impacted by removal is important to consider when assessing this pressure, as smaller scale extraction may not impact the entire extent of the biotope but greater scale extraction over a long period would cause longer-term impacts. The type of fishing activity is also important to consider in relation to the type and severity of the impact.