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

Arenicola marina is recorded from shores of western Europe, Norway, Spitzbergen, north Siberia, and Iceland. In the western Atlantic, it has been recorded from Greenland, along the northern coast from the Bay of Fundy to Long Island. Its southern limit is about 40°N (see Arenicola marina review), although OBIS (2016) includes a few records from the Atlantic coast of Africa and the Mediterranean.

Sommer et al. (1997) examined sub-lethal effects of temperature in Arenicola marina and suggested a critical upper and lower temperature of 20°C and 5°C respectively in North Sea specimens. Above or below these critical temperatures, specimens resort to anaerobic respiration. Sommer et al. (1997) noted that specimens could not acclimate to a 4°C increase above the critical temperature. De Wilde & Berghuis (1979) reported 20% mortality of juveniles reared at 5°C, negligible mortality at 10 and 15°C but 50% mortality at 20°C and 90% at 25°C.

Schroeer et al. (2009) identified a shift in the thermal tolerance of Arenicola marina, with an optimum moving towards higher temperatures with decreasing latitudes, suggesting the species may adapt to long-term shifts such as 2°C but over time. Therefore, Arenicola marina in UK and Irish populations will occupy an optimum temperature range in relation to UK and Irish latitudes. An upper limit above 20°C may occur in more southerly populations. In studies in Whitley Bay, Tyne and Wear, UK, Arenicola marina was most active in spring and summer months, with a mean rate of cast production fastest in spring and particularly slow in autumn and winter, suggesting feeding rate is greatest in higher temperatures (Retraubun et al., 1996). Retraubun et al. (1996) also showed that cast production by specimens in lab experiments increased with temperature, peaking at 20°C before declining. Rates of cast production at 30°C were still higher than at 10°C, suggesting UK populations may have greater tolerance to higher temperatures than populations studied in more northerly latitudes.

Temperature change may affect maturation, spawning time and synchronisation of spawning and reproduction in the long-term (Bentley & Pacey, 1992; Watson et al., 2000). Spawning can be inhibited in gravid adults maintained above 15°C (Watson et al., 2000). However, spawning success would remain dependent upon spring and autumn temperatures remaining below 15°C. Additionally, an impact from temperature change at the substratum surface may be mitigated as Arenicola marina is protected from direct effects by their position in the sediment.

Little information was found concerning temperature tolerance in synaptid holothurians. Both Labidoplax media and Leptosynapta bergensis are essentially boreal species, being restricted to the northern shores of the British Isles. Therefore, these species may be intolerant of increase of temperature, especially short-term acute change.

Sensitivity assessment. Arenicola marina is probably not resistant of a short-term acute change in temperature of 5°C, although it is unlikely to be directly affected due to its infaunal habit and can migrate down the shore to deeper waters to avoid the changes in temperature (Reise et al., 2001).  Hence, a resistance of Medium is suggested to represent a loss of some of the Arenicola population and especially juveniles, and a proportion of the synaptid holothurians.  Resilience is probably High and sensitivity is assessed as Low.

Temperature decrease (local)

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

Evidence

Arenicola marina is recorded from shores of western Europe, Norway, Spitzbergen, north Siberia, and Iceland. In the western Atlantic, it has been recorded from Greenland, along the northern coast from the Bay of Fundy to Long Island. Its southern limit is about 40°N (see Arenicola marina review), although OBIS (2016) includes a few records from the Atlantic coast of Africa and the Mediterranean.

Arenicola marina displays a greater tolerance to decreases in temperature than to increases, although optimum temperatures are reported to be between 5°C and 20°C. Reise et al. (2001) stated that Arenicola marina was known to be a winter hardy species and that its abundance and biomass were stable even after severe winters.  Sommer et al. (1997) report populations in the White Sea (sub-polar) acclimatised to -2°C in winter. Populations in the North Sea (boreal) were less tolerant of temperatures below 5°C, although in laboratory experiments on individual lugworms from North Sea populations worms survived a temperature drop from 6 or 12°C to -1.7°C for more than a week (Sommer & Portner, 1999).

Temperature change may affect maturation, spawning time and synchronisation of spawning and reproduction in the long-term (Bentley & Pacey, 1992; Watson et al., 2000). Spawning success is dependent upon spring and autumn temperatures, the seasons when spawning occurs in relation to spring and neap tides, remaining below 13-15°C. De Wilde & Berghuis (1979) reported 20% mortality of juveniles reared at 5 °C, negligible mortality at 10 °C and 15 °C but 50% at 20°C and 90% mortality at 25°C.

Evidence from the Sylt in the North Sea suggests that the effects of severe winters on Arenicola marina populations are small and brief (Reise, et al., 2001) The severe winter of 1995/1996 disrupted the usual juvenile settlement cycle in the sand flats of the Sylt, North Sea (Reise et al., 2001).  In the severe winter, the adult population of Arenicola marina migrated down the shore, to deeper, waters to avoid low temperatures and 66 days of ice on the intertidal sand flats.  Although, the adult population was halved, and no dead lugworms were observed on the surface or in the sediment. The post-larvae hibernate in the deep water channels (subtidal) in shell gravel and mussel beds. In summer the juveniles were not restricted to the upper shore but settled over a wider area of the flats, in the space left by the adult population. Reise et al. (2001) concluded that the enhanced recruitment demonstrated that the post-larvae did not suffer increased mortality during the winter, probably as their subtidal hibernation sites did not experience ice cover.  Similarly, Arenicola marina was listed as ‘apparently unaffected’ by the severe 1962/63 winter in the UK (Crisp, 1964).

Little information was found regarding temperature tolerance in synaptid holothurians. Both Labidoplax media and Leptosynapta bergensis are essentially boreal in distribution being essentially limited in distribution to northern habitats in the British Isles and may not be affected by a long-term decrease in temperature. The shallow, enclosed situations they are found in are subject to significant decreases in temperature driven by air temperatures because of their enclosed site. Therefore, they may be resistant of sharp decreases in temperature.

Sensitivity assessment. Arenicola marina populations are distributed to the north of the British Isles, exhibit regional acclimation to temperature, are known to be winter hardy, and can migrate to deeper water to avoid change in temperature and even ice.  The synaptid holothurians are boreal species.  Therefore, the biotope is probably resistant of a short to long-term decrease in temperature at the benchmark level and a resistance of High is suggested. Hence, resilience is High and the biotope is assessed as Not sensitive at the benchmark level.

Salinity increase (local)

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

Evidence

The biotope occurs in ‘full’ (35 ppt) salinity so that in an increase in salinity would result in hypersaline conditions (>40 ppt). Hypersaline conditions are only likely because of hypersaline effluents (brines). Arenicola marina loses weight when exposed to hyperosmotic shock (47 psu for 24 hrs) but are able to regulate and gain weight within 7-10 days (Zebe & Schiedek, 1996).

Echinoderms are generally regarded as stenohaline and most species are exclusively marine (Binyon, 1966; Pawson, 1966; Stickle & Diehl, 1987; Lawrence, 1996; Russell, 2013). Leptosynapta inhaerens was reported to be euryhaline but only known in full salinity waters in European waters (Binyon, 1966) while Leptosynapta chela was reported from hypersaline waters (55 pps) in the Black Sea (Russell, 2013).

Sensitivity assessment. Arenicola marina was able to survive and adapt to short-term exposure to 47 psu (Zebe & Schiedek, 1996) but no evidence of the effect of long-term increases in salinity of hypersaline effluents was found. The synaptid holothurians may not as species vary in their resistance to changes in salinity but no direct evidence was found. Therefore, no assessment was made.   

Salinity decrease (local)

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

Evidence

This biotope is recorded from full salinity (Connor et al., 2004).  However, this biotope was reported to occur at 24-35 psu and 18-35 psu in Scottish lagoons (Thorpe et al. 1998; Thorpe, 1998) Arenicola marina was recorded in biotope from ‘full’ to reduced salinity (Connor et al., 2004).

Once the salinity of the overlying water drops below about 55% seawater (about 18psu) Arenicola marina stops irrigation and compresses itself at the bottom of its burrow. It raises its tails to the head of the burrow to 'test' the water at intervals, about once an hour. Once normal salinities return they resume usual activity (Shumway & Davenport, 1977; Rankin & Davenport,1981; Zebe & Schiedek, 1996). This behaviour, together with their burrow habitat, enabled the lugworm to maintain its coelomic fluid and tissue constituents at a constant level, whereas individuals exposed to fluctuating salinities outside their burrow did not (Shumway & Davenport, 1977). Environmental fluctuations in salinity are only likely to affect the surface of the sediment, and not deeper organisms since the interstitial or burrow water is little affected. However, lugworms may be affected by low salinities at low tide after heavy rains. Arenicola marina was able to osmoregulate intracellular and extracellular volume within 72 - 114 hrs by increased urine production and increased amino acid concentration in response to hypo-osmotic shock (low salinity) (see Zebe & Schiedek, 1996). Hayward (1994) suggested that Arenicola marina is unable to tolerate salinities below 24 psu and is excluded from areas influenced by freshwater runoff or input (e.g. the head end of estuaries) where it is replaced by Hediste diversicolor .  However, Barnes (1994) reported that Arenicola marina occurred at salinities down to 18 psu in Britain, but survived as low as 8 psu in the Baltic, whereas Shumway & Davenport (1977) reported that this species cannot survive less than 10 psu in the Baltic. However, Arenicola marina was also found in the western Baltic where salinities were as low as 10 ppt, and Baltic specimens survived at 6 ppt (Zebe & Schiedek, 1996).  Therefore, regional populations can adapt to brackish conditions.

Echinoderms are generally regarded as stenohaline and most species are exclusively marine (Binyon, 1966; Pawson, 1966; Stickle & Diehl, 1987; Lawrence, 1996; Russell, 2013). Leptosynapta inhaerens occurred at 18.46 psu in the Black Sea (Russell, 2013). Leptosynapta sp. may derive some protection from a short-term reduction in salinity due to their burrowing habitat, however, Labidoplax media may be more vulnerable.

Sensitivity assessment.  The evidence suggests that a reduction in salinity from ‘full’ to ‘reduced’ for a year is unlikely to adversely affect the resident Arenicola population. Mobile members of the community typically occur in the intertidal and are also probably unaffected, although the synaptid holothurians may migrate out of the biotope.  Therefore, a resistance of High is suggested. Hence, resilience is High and the biotope is assessed as Not sensitive at the benchmark level.

Water flow (tidal current) changes (local)

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

Evidence

The biotope is found in weak (<0.5 m/s) to very weak (negligible) flow in very sheltered, shallow, soft muds in sheltered basins of sea lochs and lagoons, and is typified by very conspicuous mounds and casts (Connor et al., 2004).  

In 36-65 day mesocosm studies of the effects of Arenicola marina bioturbation, Wendelboe et al. (2013) found that the surface of the sediment was dominated faecal mounds and feeding pits at a flow rate of 0.11 m/s but was more eroded and the surface was more even at 0.25 m/s. At the low flow (0.11 m/s) there was no change in the sediment. But at 0.25 m/s, there was a substantial reduction in the silt and clay fractions of the sediment (a 36% reduction) and in the organic content of the sediment (a 42% reduction). At 0.25 m/s the material ejected into faecal casts was eroded (once the mucilaginous coating had eroded) and the water surface became turbid, resulting in loss of both silt/clay fractions and organic matter. Wendelboe et al. (2013) concluded that at ‘high’ flow (0.25 m/s) bioturbation by Arenicola (or other fauna) could lead to a gradual change in the sediment in the bioturbated sediment layer (i.e. the upper few centimetres).  However, their experiment was a closed system, whereas the biotope is likely to receive regular input of organic matter.

Arenicola marina is generally absent from sediments with a mean particle size of <80µm and abundance declines in sediments >200µm (fine sand) because they cannot ingest large particles. Its absence from more fluid muddy sediments is probably because they do not produce large amounts of mucus with which to stabilise their burrows. Populations are greatest in sands of mean particle size of 100 µm. Between 100 and 200 µm the biomass of Arenicola marina increases with increasing organic content (Longbottom, 1970; Hayward, 1994). However, it is recorded from a variety of sediments from fine muds to muddy sands and sandy muds, clean sand and mixed sediments (Connor et al., 1997b).

Sensitivity assessment.  The biotope occurs in weak to very weak flow so that any further reduction is not relevant. An increase in water flow could modify the sediment. A significant increase may result in a change in the sediment from fine muds to sand muds and the fine particulates are removed.  The experimental evidence suggests that a change in the flow of 0.11 m/s to 0.25 m/s was enough to alter the sediment and the appearance of the biotope within 65 days. Therefore, a change in the flow of 0.1-0.2 m/s may alter the sediment and the appearance of the biotope. The Arenicola population would persist although the biotope may come to resemble a muddy sand biotope (e.g. IMuSa.AreISa) and be reclassified.  Therefore, a resistance of None is suggested. Resilience is probably High as, once the low energy conditions return, it may only take a couple of years for the mud to deposit in these otherwise sheltered and isolated habitats. Hence, sensitivity is assessed as Medium.

Emergence regime changes

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

Evidence

Changes in emergence is only relevant to intertidal and sublittoral fringe biotopes.

Wave exposure changes (local)

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

Evidence

The biotope is recorded in extremely or very wave sheltered conditions and weak or very weak flow. The low energy habitat is probably crucial for the accumulation of the fine muds that typify the biotope. A further decrease in wave action is not relevant. However, an increase in wave action (e.g. due to an increase in average storminess or a loss of the barrier that creates the isolated situations in which theses habitats form) would probably result in modification of the sediment and a change to more sandy conditions, depending on the magnitude of the increase. If muddy or sandy sediments remain, the Arenicola population would probably persist, but the biotope would be reclassified and lost. Nevertheless, a 3-5% increase in significant wave height (the benchmark) is unlikely to be significant and the biotope is assessed as Not sensitive (resistance and resilience are High) 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

Sediment may act as a sink for heavy metals contamination so that deposit feeding species may be particularly vulnerable to heavy metal contamination through ingestion of particulates. At high concentrations of Cu, Cd or Zn the blow lug left the sediment (Bat & Raffaelli, 1998). The following toxicities have been reported in Arenicola marina:

• no mortality after 10 days at 7 µg Cu/g sediment, 23 µg Zn/g and 9 µg Cd/g;
• median lethal concentrations (LC₅₀) of 20 µg Cu/g, 50 µg Zn/g, and 25 µg Cd/g (Bat & Raffaelli, 1998).

However, Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. Short-term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se the least toxic. Little is known about the effects of heavy metals on echinoderms. Bryan (1984) reported that early work had shown that echinoderm larvae were intolerant of heavy metals, e.g. the intolerance of larvae of Paracentrotus lividus to copper (Cu) had been used to develop a water quality assessment. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu). Sea urchins, especially the eggs and larvae, are used for toxicity testing and environmental monitoring (reviewed by Dinnel et al. 1988). Crompton (1997) reported that mortalities occurred in echinoderms after 4-14day exposure to above 10-100 µg/l Cu, 1-10 mg/l Zn and 10-100 mg/l Cr but that mortality occurred in echinoderm larvae above10-100 µg/ l Ni.
Therefore, although the polychaete members of the biotope may be relatively resistant to heavy metal contamination it appears that echinoderms and their larvae may be more intolerant.

Nevertheless, this pressure is Not assessed but evidence is presented where available.

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

Sheltered embayments and lagoons, where this biotope is found, are particularly vulnerable to oil pollution, which may settle onto the sediment and persist for years (Cole et al., 1999). Subsequent digestion or degradation of the oil by microbes may result in nutrient enrichment and eutrophication (see nutrients below). Although protected from direct smothering by oil by its depth, the biotope is relatively shallow and would be exposed to the water soluble fraction of oil, water soluble PAHs, and oil adsorbed onto particulates.

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. Crude oil and oil: dispersant mixtures were shown to cause mortalities in Arenicola marina (see review). Prouse & Gordon (1976) found that blow lug was driven out of the sediment by waterborne fuel oil concentrations of >1 mg/l or sediment concentration of >100 µg/g. Crude oil and refined oils were shown to have little effect on fertilization in sea urchin eggs but in the presence of dispersants, fertilization was poor and embryonic development was impaired (Johnston, 1984). Sea urchin eggs showed developmental abnormalities when exposed to 10-30mg/l of hydrocarbons and crude oil : Corexit dispersant mixtures have been shown to cause functional loss of tube feet and spines in sea urchins (Suchanek, 1993). Although no direct information on synaptid holothurians was found, it seems likely that adults and especially larvae are intolerant of hydrocarbon contamination.

Nevertheless, this pressure is Not assessed.

Synthetic compound contamination

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

Evidence

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

The xenobiotic ivermectin was used to control parasitic infestations in livestock including sea lice in fish farms, degrades slowly in marine sediments (half-life >100 days). Ivermectin was found to produce a 10 day LC50 of 18µg ivermectin /kg of wet sediment in Arenicola marina. Sub-lethal effects were apparent between 5 - 105 µg/kg. Cole et al. (1999) suggested that this indicated a high intolerance. Arenicola marina has shown negative responses to chemical contaminants, including damaged gills following exposure to detergents (Conti, 1987), and inhibited the action of esterases following suspected exposure to point source pesticide pollution in sediments from the Ribble estuary, UK (Hannam et al., 2008).

Little information on the toxicity of synthetic chemicals to synaptid holothurians was found. Newton & McKenzie (1995) suggested that echinoderms tend to be very intolerant of various types of marine pollution but gave no detailed information. Cole et al. (1999) reported that echinoderm larvae displayed adverse effects when exposed to 0.15mg/l of the pesticide Dichlorobenzene (DCB). Smith (1968) demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus. Therefore, the larvae of synaptid holothurians may also be intolerant of synthetic chemicals.
Overall, therefore, members of this biotope may be sensitive synthetic chemicals to varying degrees and adverse effects on larvae may reduce recruitment in the log term resulting in the loss of a proportion of the population.

Radionuclide contamination

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

Evidence

Reports on littoral sediment benthic communities at Sandside Bay, adjacent to Dounray nuclear facility, Scotland, (where radioactive particles have been detected and removed) reported Arenicola marina were abundant (SEPA, 2008). Kennedy et al. (1988) reported levels of ¹³⁷Cs in Arenicola spp. of 220-440 Bq/kg from the Solway Firth.  However, no information on the effects of radionuclide contamination was found.

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

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 is able to 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 has been found 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.

Nilsson & Rosenberg (1994) exposed sediment cores, containing Labidoplax buskii and other benthic species to moderate (1mg O₂/l) and severe (0.5mg O₂/l) hypoxia for 20 days. Labidoplax buskii was seen to leave the sediment when the oxygen concentration was reduced to 1.6 mg O₂/l (16% saturation) and were dead by the end of the experiment in both moderate and severe hypoxia experiments. However, little other information on tolerance of hypoxia in synaptid holothurians was found.

Sensitivity assessment.  The fine muddy sediments found in this biotope are organic rich and the benthic macrofauna is probably adapted to a degree of hypoxia. Burrowing species such as Arenicola marina and Leptosynapta sp. burrow into anoxic sediment and may be tolerant of hypoxia. Using Labidoplax buskii and Arenicola marina as examples, the characterizing species would probably survive exposure to 2mg O₂/l for one week (the benchmark) although they may incur a metabolic cost or reduced feeding during exposure. Therefore, resistance is assessed as High, resilience as High (by default) and the biotope is probably Not sensitive at the benchmark level.

Nutrient enrichment

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

Evidence

The abundance and biomass of Arenicola marina increase with increased organic content in their favoured sediment (Longbottom, 1970; Hayward, 1994). Therefore, moderate nutrient enrichment may be beneficial.

Indirect effects may include algal blooms and the growth of algal mats (e.g. of Ulva sp.) on the surface of the intertidal flats. Algal mats smother the sediment, and create anoxic conditions in the sediment underneath, changes in the microphytobenthos, and with increasing enrichment, a reduction in species richness, the sediment becoming dominated by pollution tolerant polychaetes, e.g. Manayunkia aestuarina. In extreme cases, the sediment may become anoxic and defaunated (Elliot et al., 1998). Algal blooms have been implicated in mass mortalities of lugworms, e.g. in South Wales where up to 99% mortality was reported (Boalch, 1979; Olive & Cadman, 1990; Holt et al. 1995). Feeding lugworms were present and exploitable by bait diggers within 1 month, suggesting rapid recovery, probably by migration from surrounding areas or juvenile nurseries.

Nevertheless, this biotope is considered to be Not sensitive at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Organic enrichment

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

Evidence

The abundance and biomass of Arenicola marina increase with increased organic content in their favoured sediment (Longbottom, 1970; Hayward, 1994). Moderate enrichment increases food supplies, enhancing productivity and abundance. Gray et al. (2002) concluded that organic deposits between 50 to 300 gC m^–2 yr^–1, are efficiently processed by benthic species. Substantial increases > 500 g C m^-2 yr^-1 would likely to have negative effects, limiting the distribution of organisms and degrade the habitat, leading to eutrophication, algal blooms, and changes in community structure to a community dominated by opportunist species (e.g. capitellids) with increased abundance but reduced species richness,  and eventually to abiotic anoxic sediments (Pearson & Rosenberg, 1978; Gray, 1981; Snelgrove et al.,1995; Cromey et al., 1998).

Borja et al. (2000) and Gittenberger & loon (2011) placed Arenicola marina into the AMBI pollution group III, defined as ‘Species tolerant to excess organic matter enrichment. These species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)’.

Sensitivity assessment.  The biotope is probably rich in organic matter as it occurs in sheltered, isolated areas. Therefore, a resistance of High is suggested at the benchmark level. Hence, resilience is High and the biotope is assessed as Not sensitive at the benchmark level.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

This biotope is only found in sediment and the burrowing organisms, Arenicola marina and synaptid holothurians would not be able to survive if the substratum type was changed to either a soft rock or hard artificial type, and the biotope would be lost. 

Sensitivity assessment.  The resistance to this change is ‘None’, and the resilience is assessed as ‘Very low’ as the change at the pressure benchmark is permanent. The biotope is assessed to have a ‘High’ sensitivity to this pressure at the benchmark.

Physical change (to another sediment type)

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

Evidence

This biotope (IFiMu.Are) is only found in shallow, sheltered conditions and is defined by the presence of fine mud.  A change in sediment type by one Folk class (using the Long 2006 simplification) would change the sediment to muddy sand or sand. Although the Arenicola population would persist, the biotope would be lost and reclassified, probably as IMuSa.ArelSa.  Therefore, a resistance of None is recorded, resilience is Very low (the pressure is a permanent change) and sensitivity is assessed as High.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Extraction of sediment to a depth of 30 cm would remove the community within the affected area. Therefore, a resistance of None is suggested. Resilience is probably Medium, due to the isolated nature of the sea lochs and lagoons in which this biotope is found, and sensitivity is assessed as Medium.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Arenicola marina lives in sediment to a depth of 20-40 cm and is, therefore, protected from most sources of abrasion and physical disturbance caused by surface action. However, it is likely to be damaged by any activity (e.g. anchors, dredging) that penetrates the sediment (see below). Leptosynapta sp. also lives in burrows in the sediment. Labidoplax media lives in the top few cm of the sediment are, therefore, more vulnerable to abrasion but also likely to burrow to safety.

There are few studies on the effects of trampling on sedimentary habitats. Most studies suggest that the effects of trampling across sedimentary habitats depend on the relative proportion of mud to sand (sediment porosity), the dominant infauna (nematodes and polychaetes vs. bivalves) and the presence of burrows (Tyler-Walters & Arnold, 2008). Recovery from impact is relatively fast as shown by Chandrasekara & Frid (1996), where no difference was reported between samples in winter following summer trampling. Wynberg & Branch (1997) suggest that trampling effects are most severe in sediments dominated by animals with stable burrows, as these collapse and the sediment becomes compacted. Rossi et al. (2007) examined trampling across intertidal mudflats but were not able to show a significant difference in Arenicola abundance between trampled and control sites due to the natural variation in abundance between study sites.

Rees (1978 cited in Hiscock et al., 2002) assessed pipe laying activity in Lafan Sands, Conwy Bay, Wales. The pipe was laid in a trench dug by excavators. The spoil from the trenching was then used to bury the pipe. The trenching severely disturbed a narrow zone, but a zone some 50 m wide on each side of the pipeline was also disturbed by the passage of vehicles. The tracked vehicles damaged and exposed shallow-burrowing species such as the bivalves Cerastoderma edule and Macoma balthica, which were then preyed upon by birds. Deeper-dwelling species were apparently less affected; casts of the lugworm Arenicola marina and feeding marks made by the bivalve Scrobicularia plana were both observed in the vehicle tracks. During the construction period, the disturbed zone was continually re-populated by mobile organisms, such as the gastropod Hydrobia ulvae. Post-disturbance recolonization was rapid. Several species, including the polychaetes Arenicola marina, Eteone longa and Scoloplos armiger recruited preferentially to the disturbed area. However, the numbers of the relatively long-lived Scrobicularia plana were markedly depressed, without signs of obvious recruitment several years after the pipeline operations had been completed.

Sensitivity assessment.  Although this biotope is found in the subtidal it is shallow (0-10 m) so that it is theoretically possible for vehicles or pedestrians to traverse the biotope. Nevertheless, the evidence suggests that Arenicola is little affected by abrasion in the form of trampling or vehicle compaction.  Therefore, a resistance of High is suggested so that resilience is also High (by default) and the biotope is probably Not sensitive to abrasion due to trampling or vehicular access.

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

Mendonça et al. (2008) studied populations of the polychaete Arenicola marina at Culbin Sands lagoon, Moray Firth, in NE Scotland. An unprecedented and unexpected cockle harvesting event took place,1.5 years after the start of the sampling programme, which dramatically disturbed the sediment as it was conducted using tractors with mechanical rakes in some areas, and by boats using a suction dredge in other areas. Therefore, there was an opportunity to compare annual biomass fluctuations “before” and “after” the disturbance. Arenicola marina was observed to return to normal activities just a few hours after the disturbance of the sediment during the harvesting event.

Rees (1978 cited in Hiscock et al., 2002) assessed pipe laying activity in Lafan Sands, Conwy Bay, Wales. The pipe was laid in a trench dug by excavators. The spoil from the trenching was then used to bury the pipe. The trenching severely disturbed a narrow zone, but a zone some 50 m wide on each side of the pipeline was also disturbed by the passage of vehicles. The tracked vehicles damaged and exposed shallow-burrowing species such as the bivalves Cerastoderma edule and Macoma balthica, which were then preyed upon by birds. Deeper-dwelling species were apparently less affected; casts of the lugworm Arenicola marina and feeding-marks made by the bivalve Scrobicularia plana were both observed in the vehicle tracks. During the construction period, the disturbed zone was continually re-populated by mobile organisms, such as the gastropod Hydrobia ulvae. Post-disturbance recolonization was rapid. Several species, including the polychaetes Arenicola marina, Eteone longa and Scoloplos armiger recruited preferentially to the disturbed area. However, the numbers of the relatively long-lived Scrobicularia plana were markedly depressed, without signs of obvious recruitment several years after the pipeline operations had been completed.

McLusky et al. (1983) examined the effects of bait digging on blow lug populations in the Forth estuary. Dug and infilled areas and unfilled basins left after digging re-populated within 1 month, whereas mounds of dug sediment took showed a reduced population. The basins accumulated fine sediment and organic matter and showed increased population levels for about 2-3 months after digging.

Fowler (1999) reviewed the effects of bait digging on intertidal fauna, including Arenicola marina. Diggers were reported to remove 50 or 70% of the blow lug population. Heavy commercial exploitation in Budle Bay in winter 1984 removed 4 million worms in 6 weeks, reducing the population from 40 to <1 per m². Recovery occurred within a few months by recolonization from surrounding sediment (Fowler, 1999). However, Cryer et al. (1987) reported no recovery for 6 months over summer after mortalities due to bait digging. Mechanical lugworm dredgers were used in the Dutch Wadden Sea where they removed 17-20 million lugworms/year. However, when combined with hand digging the harvest represented only 0.75% of the estimated population in the area. A near doubling of the lugworm mortality in dredged areas was reported, resulting in a gradual substantial decline in the local population over a 4 year period. The effects of mechanical lugworm dredging are more severe and can result in the complete removal of Arenicola marina (Beukema, 1995; Fowler, 1999). Beukema (1995) noted that the lugworm stock recovered slowly reached its original level in at least three years.

Sensitivity assessment.  Penetrative gear would probably damage or remove a proportion of the population of Arenicola but given its potential density, the effects may be minor. Similarly, recreational bait digging may have a limited effect. However, if commercial bait digging occurred in the shallow sublittoral, then a significant proportion of the population may be removed. Hence, a resistance of Low is suggested. Resilience is probably Medium, due to the isolated nature of the sea lochs and lagoons in which this biotope if found, and sensitivity is assessed as Medium.

Changes in suspended solids (water clarity)

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

Evidence

This biotope occurs in fine silts and muds that accumulate in low energy environments (wave sheltered and weak flow) in shallow isolated water bodies.  Deposit feeders are unlikely to be perturbed by increased concentrations of suspended sediment since they live in sediment and are probably adapted to re-suspension of sediment by wave action, during storms or runoff.

In 36-65 day mesocosm studies of the effects of Arenicola marina bioturbation, Wendelboe et al. (2013) found that the surface of the sediment was dominated by faecal mounds and feeding pits at a flow rate of 0.11 m/s, but was more eroded and the surface was more even at 0.25 m/s. At the low flow (0.11 m/s) there was no change in the sediment. However, at 0.25 m/s, there was a substantial reduction in the silt and clay fractions of the sediment (a 36% reduction) and in the organic content of the sediment (a 42% reduction). At 0.25 m/s the material ejected into faecal casts was eroded (once the mucilaginous coating had eroded) and the water surface became turbid, resulting in loss of both silt/clay fractions and organic matter.  

Sensitivity assessment.  The evidence from Wendelboe et al. (2013) suggests that an increase in water movement due to storms, or runoff is likely to disturb the sediment surface regularly, especially in winter months, so that the biotope is probably not affected by changes in suspended sediment.  In addition, Arenicola marina occurs at high abundances in mudflats and sandflats in estuaries where suspended sediment levels may reach grammes per litre. Therefore, a resistance of High is suggested so that resilience is High (by default) and the biotope is assessed as Not sensitive at the benchmark level.

Smothering and siltation rate changes (light)

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

Evidence

Arenicola marina is a sub-surface deposit feeder that derives the sediment it ingests from the surface. It rapidly reworks and mixes sediment. It grows to 12-20 cm in length and lives in burrows to a depth of 20-40 cm. It is unlikely to be perturbed by smothering by 5 cm of sediment. Juveniles may be more susceptible but both adults and juveniles are capable of leaving the sediment and swimming (on the tide) up or down the shore (see Reise et al., 2001). If present, Leptosynapta bergensis has a similar lifestyle and is also unlikely to be intolerant of smothering at the level of the benchmark. Labidoplax media lives in the top few cm of sediment and will incur an energetic cost, due to its relatively small size.  In addition, Gittenberger & Loon (2011) placed Arenicola marina into their AMBI Sedimentation Group III, defined as ‘species insensitive to higher amounts of sedimentation, but don’t easily recover from strong fluctuations in sedimentation’.

Sensitivity assessment. This biotope occurs in a depositional environment, where sedimentation is likely, to be high due to the low energy of the habitat. Therefore, resistance to a deposit of 5 cm of fine sediment is assessed as High. Hence, resilience is High (by default) and the biotope is probably Not sensitive at the benchmark level.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Arenicola marina is a sub-surface deposit feeder that derives the sediment it ingests from the surface. It rapidly reworks and mixes sediment. It grows to 12-20 cm in length and lives in burrows to a depth of 20-40 cm. Adults may be able to resist smothering by 30 cm of sediment but juveniles may be more susceptible.  Both adults and juveniles are capable of leaving the sediment and swimming (on the tide) up or down the shore (see Reise et al., 2001). If present, Leptosynapta bergensis has a similar lifestyle and is also unlikely to be intolerant of smothering at the level of the benchmark. Labidoplax media lives in the top few cm of sediment and will incur an energetic cost, due to its relatively small size.  In addition, Gittenberger & Loon (2011) placed Arenicola marina into their AMBI sedimentation Group III, defined as ‘species insensitive to higher amounts of sedimentation, but don’t easily recover from strong fluctuations in sedimentation’.

Sensitivity assessment. This biotope occurs in a depositional environment, where sedimentation is likely, to be high due to the low energy of the habitat. However, the deposit of 30 cm in a single event is probably greater than the normal range of sedimentation and, in theses sheltered habitats, likely to remain.  Therefore, a proportion of the adults and a greater proportion of the juveniles may not be able to realign themselves with the surface of the sediment and resistance is assessed as Medium but at ‘Low’ confidence due to the lack of direct evidence. Hence, resilience is probably High and sensitivity is assessed as Low at the benchmark level.

Litter

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

Evidence

Plastic debris breaks up to form microplastics. Microplastics have been shown to occur in marine sediments and to be ingested by 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 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 33m³ 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 nonylophenol 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 upon the exact form of litter or man-made object being 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 bioturbators and ecosystem engineer in sedimentary habitats. Nevetheless, while significant impacts have been shown in laboratory studies, impacts at biotope scales are still unknown and this pressure is  ​Not assessed.

Electromagnetic changes

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

Evidence

No evidence was found

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 and in response may retreat into the sediment for protection. However, at the benchmark level, the community is unlikely to be respond to noise and therefore is Not sensitive.

Introduction of light or shading

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

Evidence

All characterizing species live in the sediment and do not rely on light levels directly to feed or so limited direct impact is expected. As this biotope is not characterized by the presence of primary producers it is not considered that shading would alter the character of the habitat directly.

Beneath shading structures, there may be changes in microphytobenthos abundance. This biotope may support microphytobenthos on the sediment surface and within the sediment. Mucilaginous secretions produced by these algae may stabilise fine substrata (Tait & Dipper, 1998). Shading will prevent photosynthesis leading to death or migration of sediment microalgae altering sediment cohesion and food supply to deposit feeders like Arenicola and synaptid holothurians, although they fed on a range of organic matter within the sediment.

Sensitivity assessment. Therefore, biotope resistance is assessed as ‘High’ and resilience is assessed as ‘High’ (by default) and the biotope is considered to be ‘Not sensitive’.

Barrier to species movement

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

Evidence

Not relevant - this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of seed.  But seed dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

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

Evidence

Not relevant to seabed habitats. 

Visual disturbance

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

Evidence

Not relevant.

Genetic modification & translocation of indigenous species

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

Evidence

Important characterizing species within this biotope are not genetically modified or translocated. Therefore, This pressure is considered ‘Not relevant’ to this biotope group.

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). Padilla (2010) predicted that Magallana gigas could either displace or overgrow mussels on rocky and sedimentary habitats of low or high energy.  However, Padilla (2010) also noted that there were no examples of Magallana gigas invading sedimentary habitats where there are no native ecosystem engineers (bivalves or Sabellaria). In the Wadden Sea and the North Sea, Magallana gigas overgrows mussel beds in the intertidal zone (Diederich 2005, 2006; Kochmann et al. 2008), although they did show a preference for settling on conspecifics before the mussels and struggled to settle on mussels with a fucoid covering. However, recruitment of Magallana gigas was significantly higher in the intertidal than the shallow subtidal, although the survival of adult oysters or mussels in the subtidal is limited by predation. Crepidula fornicata larvae may out-compete oyster (Magallana gigas) larvae during summer months where the two species co-occur. Trophic competition between adult Crepidula fornicata and Magallana gigas was reported in France during winter and spring. 

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.

Sensitivity assessment. The sediments characterizing this biotope are likely to be unsuitable for most of the invasive non-indigenous species currently recorded in the UK. However, the above evidence also suggests that mud and fine sediments are unsuitable for the colonization of Crepidula fornicata due to the lack of gravel, shells, or any other hard substrata used for larvae settlement (Bohn et al., 2015; Tillin et al., 2020). This is a sheltered to extremely sheltered habitat which would be otherwise suitable for Crepidula but the absence of suitable hard substrata and the presumed high deposition rate of fine particulates is likely to prevent colonization by Crepidula. 

Magallana gigas was predicted to invade sedimentary habitats, although no direct examples exist to date and Magallana gigas recruitment is lower in the subtidal (Diederich 2005, 2006; Padilla, 2010). Magallana prefers to settle on conspecifics over mussels but probably requires some form of hard substrata to gain a foothold in the habitats. As with Crepidula, this biotope is dominated by soft muds that are probably not suitable for settlement by Pacific oyster larvae.  Hence, the resistance to colonization by Crepidula and Magallana is assessed as 'High' and resilience as 'High' so the biotope is assessed as 'Not sensitive'. 

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

Ashworth (1904) recorded the presence of distomid cercariae and Coccidia in Arenicola marina from the Lancashire coast. However, no information concerning infestation or disease related mortalities 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

McLusky et al. (1983) examined the effects of bait digging on blow lug populations in the Forth estuary. Dug and infilled areas and unfilled basins left after digging re-populated within 1 month, whereas mounds of dug sediment took showed a reduced population. The basins accumulated fine sediment and organic matter and showed increased population levels for about 2-3 months after digging.

Fowler (1999) reviewed the effects of bait digging on intertidal fauna, including Arenicola marina. Diggers were reported to remove 50 or 70% of the blow lug population. Heavy commercial exploitation in Budle Bay in winter 1984 removed 4 million worms in 6 weeks, reducing the population from 40 to <1 per m². Recovery occurred within a few months by recolonization from surrounding sediment (Fowler, 1999). However, Cryer et al. (1987) reported no recovery for 6 months over summer after mortalities due to bait digging. Mechanical lugworm dredgers were used in the Dutch Wadden Sea where they removed 17-20 million lugworms/year. However, when combined with hand digging the harvest represented only 0.75% of the estimated population in the area. A near doubling of the lugworm mortality in dredged areas was reported, resulting in a gradual substantial decline in the local population over a 4 year period. The effects of mechanical lugworm dredging are more severe and can result in the complete removal of Arenicola marina (Beukema, 1995; Fowler, 1999). Beukema (1995) noted that the lugworm stock recovered slowly reached its original level in at least three years.

Sensitivity assessment.  Recreational bait digging may remove a proportion of the population of Arenicola but given its potential density, the effects may be minor.  However, if commercial bait digging occurred in the shallow sublittoral, then a significant proportion of the population may be removed.  The physical effects of removal are addressed under penetration above. However, Arenicola marina is a bioturbator and ecosystem engineer and its removal would probably have a significant effect on the nature of the sediment and the other species that could inhabit the sediment. Hence, a resistance of Low is suggested. Resilience is probably Medium, due to the isolated nature of the sea lochs and lagoons in which this biotope if found, and sensitivity is assessed as Medium.

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

Arenicola marina is a bioturbator and ecosystem engineer and its incidental removal would probably have a significant effect on the nature of the sediment and the other species that could inhabit the sediment.  Hence, a resistance of Low is suggested. Resilience is probably Medium, due to the isolated nature of the sea lochs and lagoons in which this biotope if found, and sensitivity is assessed as Medium.