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

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the Mediterranean Sea and Indian Ocean (Farke, 1979). Therefore, the species must be capable of tolerating higher temperatures than it experiences in Northern Europe. For example, Covazzi Harriague et al. (2007) reported Aphelochate marioni occurring in the Rapallo Harbour (Ligurian Sea, NW Mediterranean) at 24°C. Furthermore, Aphelochaeta marioni lives infaunally and so is likely to be insulated from rapid temperature change. An increase in temperature would be expected to cause some physiological stress but no mortality.

Deeper burrowing oligochaetes are protected from fluctuations in temperature by the overlaying sediments which dampen changes if poorly drained (Giere & Pfannkuche, 1982).  Bamber & Spencer (1984) observed that Tubificoides were dominant species in an area affected by thermal discharge in the River Medway estuary. Sediments were exposed to the passage of a temperature front of approximately 10^oC between heated effluent and estuarine waters during the tidal cycles. Tubificoides benedii has increased in abundance in mudflat habitats in Jade Bay, North Sea between 1930 and 2009 (Schueckel & Kroencke, 2013). Climate warming as well as decreasing nutrient loads and species introductions have occurred in the region since the 1970s, suggesting the species may adapt to temperature increases at the benchmark level. Tubificoides benedii is considered an opportunist that is adapted to rapid environmental fluctuations and harsh conditions in estuaries (Gogina et al., 2010), suggesting resistance would be high to this pressure at the benchmark level.

Found from the northern Atlantic, from such areas as the Barents Sea, the Baltic and the North Sea, to the Mediterranean, Nephtys hombergii has been reported from as far south as South Africa, suggesting the species can tolerate a 5°C increase in temperature in the UK and Irish coasts. Records are limited but Emery et al. (1957) found that Nephtys hombergii could withstand summer temperatures of 30-35°C.

Environmental factors, such as temperature, day length, and tidal or lunar cycles, have been implicated in the timing of spawning of Nephtyidae, in particular the spring tide phase of the lunar cycle (Bentley et al., 1984). In the Tyne Estuary, spawning of Nephtys hombergii occurred in May and September, whilst in Southampton Water the species spawned throughout the year with peaks in July and November (Wilson, 1936; Oyenekan, 1986). In Århus Bay, Denmark, Nephtys hombergii spawned in August and September, but a decrease in the number of individuals bearing gametes in May and June suggested that at least part of the population spawned in early summer (Fallesen & Jørgensen, 1991). A 5°C increase in temperature for one month period, or 2°C for one year is likely to impact the timing of reproduction in these areas. A combination of environmental factors appears to influence timing (in particular spring tides), therefore, temperature may not be the most significant cue but evidence is limited to identify the significance of temperature in relation to other factors (or cues).

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4-19°C (Huthnance, 2010). Most organisms in the biotopes are distributed to the north and south of Britain and Ireland and unlikely to be affected adversely by long-term temperature change. In addition, subtidal and especially infaunal species are likely to be protected from acute temperature change. Furthermore, the dominance of Tubificoides spp. when exposed to a heated effluent suggests that this genus would be highly resistant to an increase in temperature at the pressure benchmark. Resistance and resilience of the biotopes are therefore assessed as High and the biotopes are judged as Not Sensitive.

Temperature decrease (local)

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

Evidence

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the western Baltic Sea, south Atlantic Ocean and North Sea (Farke, 1979). Therefore the species must be capable of tolerating low temperatures. Aphelochaeta marioni lives buried in sediment and is therefore well insulated from decreases in temperature. In the Wadden Sea, the population was apparently unaffected by a short period of severe frost in I973 (Farke, 1979). Kędra et al. (2010) reported Aphelochaete marioni occurring in the Svalbard Archipelago where temperatures below zero may be experienced in the winter. A decrease in temperature would be likely to cause some physiological stress but no mortality.

Most littoral oligochaetes, including tubificids, can survive freezing temperatures and can survive in frozen sediments (Giere & Pfannkuche, 1982). Tubificoides benedii (studied as Peloscolex benedeni) recovered after being frozen for several tides in a mudflat (Linke, 1939).

Nephtys hombergii are found as far north as the Barents Sea, and would be expected to be resistant to a 5°C decrease in temperature for one month period, or 2°C for one year. Environmental factors, such as temperature, day-length, and tidal or lunar cycles, have been implicated in the timing of spawning of the Nephtyidae, in particular the spring tide phase of the lunar cycle (Bentley et al., 1984). Olive et al. (1997) found that relative spawning success in a North Sea (Newcastle on Tyne) population of Nephtys hombergii was positively correlated with winter sea and air temperatures. This suggests a 5°C decrease in temperature for a one month period, occurring in winter, or 2°C for one year are likely to impact spawning success.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4-19°C (Huthnance, 2010). Based on the evidence presented, Aphelochaeta marioni, Tubificoides benedii and Nephtys hombergii are likely to be able to resist a long-term decrease in temperature of 2°C and may resist a short-term decrease of 5°C. Temperature may act as a spawning cue and an acute or chronic decrease may result in some delay in spawning, however, this is not considered to impact the adult populations and may be compensated by later spawning events. Resistance and resilience are therefore assessed as High and the biotopes judged as Not Sensitive.

Salinity increase (local)

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

Evidence

Environmental fluctuations in salinity are only likely to affect the surface of the sediment, and not deeper buried organisms, since the interstitial or burrow water is less affected. However, under longer term or permanent increase in salinity, sediment waters would also adjust.

Populations of Aphelochaeta marioni inhabit the open coast where seawater is at full salinity. Covazzi Harriague et al. (2007) reported Aphelochaeta marioni in the Rapallo Harbour (Ligurian Sea, NW Mediterranean) at salinities above 38 psu. Farke (1979) studied the effects of changing salinity on Aphelochaeta marioni (studied as Tharyx marioni) in a microsystem in the laboratory. Over several weeks, the salinity in the microsystem was increased from 25-40 psu and no adverse reaction was noted. However, when individuals were removed from the sediment and displaced to a new habitat, they only dug into their new substratum if the salinities in the two habitats were similar. If the salinities differed by 3-5 psu, the worms carried out random digging movements, failed to penetrate the sediment and died at the substratum surface after a few hours. This would suggest that Aphelochaeta marioni can tolerate salinity changes when living infaunally but may not resist an increase in salinity when removed from its habitat.

Oligochaete dominated biotopes are recorded from a range of salinity regimes from full (LS.LSa.MoSa.Ol; LS.LSa.MoSa.Ol.FS), variable (SS.SMu.SMuVS.CapTubi) to low (SS.SMu.SMuVS.LhofTtub) habitats (Connor et al., 2004). The species characterizing these biotopes are likely to vary. Giere & Pfannkuche (1982) identified how species change over a hypothetical salinity gradient with marine stenohaline species present at full salinities replaced by more euryhaline oligochaete species including Tubificoides benedii and Tubificoides pseudogaster. Studies in the Rhine delta have found that Tubificoides benedii is tolerant of a range of salinities (Verdonschot et al., 1982).

Nephtys hombergii is considered to be a brackish water species (Barnes, 1994), but where the species occurs in open coastal locations the species would have to tolerate salinities of 25 psu and above. Within a few months of the closure of a dam across the Krammer-Volkerak estuary in the Netherlands, Wolff (1971b) observed that species with pelagic larvae or a free-swimming phase, including Nephtys hombergii expanded rapidly with a concomitant increase of salinity to 9-15 psu everywhere. Prior to the closure of the dam, the estuary demonstrated characteristics of a typical 'salt-wedge' estuary with a salinity gradient from 0.3 to 15 psu. Hence, Nephtys hombergii is likely to survive increases in salinity within estuarine environments. Nephtys hombergii may still be found in fully marine locations but, may be competitively inferior to other species of Nephtyidae (e.g. Nephtys ciliata and Nephtys hystricis) and occur in lower densities. An increase to fully marine (30-40 ‰) would therefore be likely to lead to a reduction in density of Nephtys hombergii.

Sensitivity assessment. Based on that all species occur in greater salinities than those that occur in these biotopes, the biological assemblages associated with the biotopes are considered to have High resistance and High resilience and is therefore considered to be Not Sensitive to an increase in salinity at the pressure benchmark level.

Salinity decrease (local)

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

Evidence

Environmental fluctuations in salinity are only likely to affect the surface of the sediment, and not deeper buried organisms, since the interstitial or burrow water is less affected. However, under longer term or permanent increase in salinity, sediment waters would be expected to also adjust.

Aphelochaeta marioni thrives in estuaries and is, therefore, likely to be tolerant of decreases in salinity. It has been recorded from brackish inland waters in the southern Netherlands with a salinity of 16 psu, but not in areas permanently exposed to lower salinities (Wolff, 1973). However, it also penetrates into areas exposed to salinities as low as 4 psu for short periods at low tide when freshwater discharge from rivers is high (Farke, 1979). In the Severn Estuary, Aphelochaeta marioni (studied as Tharyx marioni) characterized the faunal assemblage of very poorly oxygenated, poorly sorted mud with relatively high interstitial salinity (Broom et al., 1991).

Nephtys hombergii is considered to be a brackish water species, and has been reported to extend in to estuarine locations where salinity is less than 18 psu (Barnes, 1994). Clark & Haderlie (1960) found Nephtys hombergii in the Bristol Channel at salinities between 15.9 psu and 25.1 psu. If the salinity were to become intolerable to the polychaete, it is likely that as a mobile species, able to both swim and burrow, Nephtys hombergii would avoid the change in salinity by moving away and localized densities would decline.

In Ria de Averio, western Portugal, Tubificoides benedii characterized communities in estuarine sample sites further upstream with lower salinity, suggesting a high resistance to a decrease in salinity (Rodrigues et al., 2011).

Overall, the important characterizing species are likely to tolerate a short-term change in salinity from e.g. variable to low salinity and a long-term change from variable to reduced salinity. The species richness of the biotopes may decline but the biotopes will probably not be adversely affected.

Sensitivity assessment. Records indicate SS.SMu.SMuVS.AphTubi occurs in areas of full (30-35 ppt) and variable (18-35 ppt) salinities, whereas SS.SMu.SMuVS.NhomTubi occurs in areas of variable (18-35 ppt) salinity (Connor et al., 2004). Nephtys hombergii is possibly the most sensitive to the lowest salinity levels within the ‘low’ 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. The remaining characterizing species Aphelochaeta marioni and Tubificoides spp. occur in estuarine areas experiencing low salinities, so are therefore likely to resist a decrease in salinity at the pressure benchmark level. Resistance of the biotopes is therefore assessed as High and resilience as High (by default), and the biotopes are considered Not Sensitive to a decrease in salinity at the pressure 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 hydrographic regime is an important structuring factor in sedimentary habitats. An increase in water flow rate is not likely to affect Nephtys hombergii, Aphelochaeta marioni and Tubificoides spp. and other characterizing species as they live infaunally. The most damaging effect of increased flow rate could be the erosion of the substratum as this could eventually lead to loss of the habitat. Orvain et al. (2007) investigated the spatio-temporal variations in intertidal mudflat erodibility in western France and suggested a potential link between Polychaeta and bed erodibility given the high polychaete abundances observed in the study.

Increased water flow rates is likely to change the sediment characteristics in which the species live, primarily by resuspending and preventing deposition of finer particles (Hiscock, 1983). The characterizing species prefer habitats with silty/muddy substrata which would not occur in very strong tidal streams. Coarser sediments are likely to remain in areas of strongest flow velocity (where finer particles have been resuspended) (Coates et al., 2014). Species such as Tubificoides benedii and other opportunist polychaetes that tolerate coarser particle size will possibly increase in abundance. Aphelochaeta marioni has been recorded in the Wadden Sea with flow rates up to 0.45 m/s (Compton et al., 2013), and in the Westerscheld estuary (SW Netherlands) where spring current velocities vary between 0.1-0.58 m/s (Van Colen et al., 2010a).

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

Sensitivity assessment. Sand particles are most easily eroded and likely to be eroded at about 0.20 m/s (based on Hjulström-Sundborg diagram, Sundborg, 1956). Although having a smaller grain size than sand, clays and silts require greater critical erosion velocities because of their cohesiveness. The biotopes occur most often in weak, moderately strong and strong tidal streams (Connor et al., 2004), suggesting the species characterizing the biotopes can adapt to a range of tidal currents, aided by each species burrowing traits. A change in water flow rate at the pressure benchmark level of 0.1-0.2 m/s is considered to fall within the range of flow speeds experienced by populations in the middle of their range. Resistance and resilience are, therefore, assessed as High and the biotopes considered Not Sensitive to a change in water flow at the pressure benchmark level.

Emergence regime changes

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

Evidence

The biotopes occur in the infralittoral so that only the upper extent of shallow examples are likely to be emersed on extreme low tides. All characterizing species would probably survive an increase in emergence. However, the species can only feed when immersed and therefore likely to experience reduced feeding opportunities. Over the course of a year, the resultant energetic cost is likely to cause some mortality. In addition, increased emergence is likely to increase the vulnerability to predation from shore birds. A decrease in emergence is likely to allow the biotopes to extend their upper limits, where suitable substrata exist.

Opportunistic, deposit feeding polychaetes and oligochaetes, such as Aphelochaeta marioni and Tubificoides benedii are likely to tolerate stressful conditions, and often out-compete more sensitive species in intertidal environments due to greater tolerances (Gogina et al., 2010). Nephtys hombergii is sufficiently mobile to rapidly burrow and seek damper substrates during periods when emergence increases. For instance, Vader (1964) observed that the worm relocates throughout the tidal cycle.

For instance, Tubificoides benedii is capable of penetrating the substrate to depths of 10 cm, shows resistance to hypoxia and is often typified as an ‘opportunist’ that is adapted to the rapid environmental fluctuations and harsh conditions in estuaries (Gogina et al., 2010). Highest abundances were predicted by Gogina et al. (2010) to be related to depth with an optimum of 10 m to 20 m. Further case studies were not returned by literature searches on Tubificoides benedii. The studies returned by searches suggest abundance may be limited by a decrease in high water level or a change in time (increase) where substrate is not covered by the sea. An increase in the time the biotopes are covered by the sea is likely to result in increased abundance of Tubificoides benedii.

Sensitivity assessment. Some mortality of the characterizing species is likely to occur because of emergence regime changes. Resistance is therefore assessed as Medium and resilience is likely to be High, so the biotopes are considered to have Low sensitivity to emergence regime changes at the pressure benchmark level.

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

Strong wave action is likely to cause damage or withdrawal of delicate feeding and respiration structures resulting in loss of feeding opportunities and compromised growth for the characterizing species. Decreases in wave exposure may influence the supply of particulate matter because wave action may have an important role in resuspending the sediment. Furthermore, Aphelochaeta marioni characteristically inhabits soft sediments in sheltered areas (Broom et al., 1991), so individuals may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action.

Disturbance of sediment by waves may reduce oligochaete abundance (Giere, 1977) and oligochaetes may be absent from very wave exposed shores (Giere & Pfannkuche, 1982). 

Nephtys hombergii lives infaunally but may sometimes partially emerge to seek and capture food but does not present a significant surface area to wave action to sustain physical damage. Clark & Haderlie (1960) and Clark et al. (1962) suggested that strong wave action limited the distribution of Nephtys hombergii. Increased wave action for long durations (e.g. 1 year) may ultimately change the nature of the substratum that the polychaete inhabits and its distribution may consequently alter.

Sensitivity assessment. Hydrographic regimes are an important structuring factor in sedimentary habitats, and an increase in wave exposure could result in fine sediments being eroded (Hiscock, 1983), resulting in the likely reduction of the habitat and a decrease in food availability. Some erosion will occur naturally and storm events may be more significant in loss and damage of the substratum than changes in wave height at the pressure benchmark. Limited zoobenthic biomass has been recorded in areas exposed to strong currents and wave action (Beukema, 2002), limiting food availability to species such as Nephtys hombergii, however, impacts from this pressure at the benchmark level may be low for the biotopes, as they are limited to sheltered locations. Increases in wave action may therefore remain within the limits of the species tolerances but factors such as sediment redistribution may alter the physical biotopes. The biotopes are found in moderately exposed, sheltered, very sheltered and extremely sheltered sites (Connor et al., 2004) and a change at the benchmark level is likely to fall within the range experienced by the mid-range examples. The biotopes are therefore considered to have High resistance to changes at the pressure benchmark, where these do not lead to increased erosion of the substratum. Resilience is therefore assessed as High and the biotopes are considered to be Not Sensitive at the pressure benchmark.

Transition elements & organo-metal contamination

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

Evidence

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

Evidence suggests that polychaetes are fairly resistant to the effects of heavy metals (Bryan, 1984). Aphelochaeta marioni is tolerant of heavy metal contamination occurring in the heavily polluted Restronguet Creek (Bryan & Gibbs, 1983) and it is also an accumulator of arsenic (Gibbs et al., 1983). Nephtys hombergii is also recorded in Restronguet Creek. Rygg (1985) classified Tubificoides spp. as highly tolerant species, common at the most copper polluted stations (>200 mg/kg) in Norwegian fjords.

Hydrocarbon & PAH contamination

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

Evidence

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

Cirratulids seem to be mostly immune to oil spills, probably because their feeding tentacles are protected by a heavy secretion of mucus (Suchanek, 1993). This is supported by observations of Aphelochaeta marioni following the Amoco Cadiz oil spill in March, 1978 (Dauvin, 1982, 2000). Prior to the spill, Aphelochaeta marioni (studied as Tharyx marioni) was present in very low numbers in the Bay of Morlaix, western English Channel. Following the spill, the level of hydrocarbons in the sediment increased from 10 mg/kg dry sediment to 1443 mg/kg dry sediment 6 months afterwards. In the same period, Aphelochaeta marioni increased in abundance to a mean of 76 individuals/m², which placed it among the top five dominant species in the faunal assemblage. It was suggested that the population explosion occurred due to the increased food availability because of accumulation of organic matter resulting from high mortality of browsers. Six years later, abundance of Aphelochaeta marioni began to fall away again, accompanied by gradual decontamination of the sediments.

Tubificoides benedii was found in UK waters near oil refineries as the sole surviving member of the macrofauna. Populations were, however, apparently reduced and the worms were absent from areas of oil discharge and other studies indicate sensitivity to oiling (Giere & Pfannkuche, 1982).

McLusky (1982) found that petrochemical effluents, including organic solvents and ammonium salts, released from a point source to an estuarine intertidal mudflat of the Forth Estuary, Scotland, caused severe pollution in the immediate vicinity. Beyond 500 m distance, the effluent contributed to an enrichment of the fauna in terms of abundance and biomass similar to that reported by Pearson & Rosenberg (1978) for organic pollution; Nephtys hombergii was found in the area with maximum abundance of species and highest total biomass at 500 m from the discharge. It seems likely that significant hydrocarbon contamination would kill affected populations of the species. On return to prior conditions, recolonization is likely via adult migration and larval settlement.

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.

There is little evidence directly relating to the effects of synthetic chemicals on Aphelochaeta marioni. Waldock et al. (1999) reported that the species diversity of polychaete infauna, including Aphelochaeta marioni, in the Crouch Estuary increased in the three years after the use of TBT was banned within the estuary, suggesting that TBT had suppressed their abundance previously. Furthermore, Beaumont et al. (1989) concluded that TBT had a detrimental effect on the larval and/or juvenile stages of infaunal polychaetes.

No evidence concerning the specific effects of chemical contaminants on Nephtys hombergii was found. 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 is available on which to assess this pressure.

Introduction of other substances

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

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Connor et al. (1997a) described sediments in which Aphelochaeta marioni is commonly found as usually with a "black anoxic layer close to the sediment surface." Broom et al. (1991) recorded that Aphelochaeta marioni (studied as Tharyx marioni) characterized the faunal assemblage of very poorly oxygenated mud in the Severn Estuary. They found Aphelochaeta marioni to be dominant where the redox potential at 4 cm sediment depth was 56 mV and, therefore, concluded that the species was tolerant of very low oxygen tensions. It is likely that feeding, growth and reproduction would be impaired under sustained low oxygen conditions.

Oligochaetes, such as Tubificoides spp. vary in their tolerance of hypoxia and associated high sulphide levels. Most enchytraaids and naidids are sensitive to hydrogen sulphide and hypoxia while tubificids are often more resistant (Giere, 2006). Tubificoides benedii has a high capacity to tolerate anoxic conditions, its extreme oxygen tolerance is based on an unusually low respiration rate (Giere et al., 1999). Respiration rates of Tubificoides benedii measured at various oxygen concentrations showed that aerobic respiration is maintained even at very low oxygen concentrations (Giere et al., 1999). Birtwell & Arthur (1980) showed that Tubificoides benedii could tolerate anoxia in the Thames Estuary (LT₅₀ = 58.8 hours at 20^oC, 26.6 hours at 25^oC  and 17.8 hours at 30^oC in experiments with worms acclimated to 20^oC.)

Nephtys hombergii inhabits intertidal areas where resistance to low oxygen is needed and sulphide levels can reach up to 1 mM (Giere, 1992; Thierman et al., 1996). As with other characterizing polychaete species, Nephtys hombergii exhibits the ability to switch from aerobic to anerobic respiration, which provides some protection from the toxic effects of sulphide. Nephtys hombergii has adapted to such conditions by utilizing several strategies. Arndt & Schiedek (1997) found Nephtys hombergii to have a remarkably high content of phosphagen (phosphoglycocyamine), which is the primary energy source during periods of environmental stress. With increasing hypoxia, energy is also provided via anaerobic glycolysis, with strombine as the main end-product. Energy production via the succinate pathway becomes important only under severe hypoxia, suggesting a biphasic response to low oxygen conditions which probably is related to the polychaete's mode of life. The presence of sulphide resulted in a higher anaerobic energy flux and a more pronounced energy production via glycolysis than in anoxia alone. Nevertheless, after sulphide exposure under anaerobic conditions of <24 h, Arndt & Schiedek (1997) observed Nephtys hombergii to recover completely. Although Nephtys hombergii appears to be well adapted to a habitat with short-term fluctuations in oxygen and appearance of hydrogen sulphide, its high energy demand as a predator renders it likely to limit its survival in an environment with longer lasting anoxia and concomitant sulphide exposure. For instance, Fallesen & Jørgensen (1991) recorded Nephtys hombergii in localities in Århus Bay, Denmark, where oxygen concentrations were permanently or regularly low, but in the late summer of 1982 a severe oxygen deficiency killed populations of Nephtys species (Nephtys hombergii and Nephtys ciliata) in the lower part of the bay. However, Nephtys hombergii recolonized the affected area by the end of autumn the same year. Alheit (1978) reported a LC50 at 8°C of 23 days for Nephtys hombergii maintained under anaerobic conditions.

Nephtys hombergii have tolerated extreme hypoxia, leaving the sediment only after 11 days (Nilsson & Rosenberg, 1994). Nephtys hombergii in artificially created anoxic conditions were shown to survive for at least 5 days (Schöttler, 1982) and do not switch from aerobic to anerobic metabolic pathways until oxygen saturation decreases <12% (Schöttler, 1982).

Sensitivity assessment. The characterizing species are likely to only be affected by severe deoxygenation episodes. Resistance to deoxygenation at the pressure benchmark level is likely to be High. Resilience of the biotopes is likely to also be High and the biotopes are therefore considered Not Sensitive to exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for 1 week.

Nutrient enrichment

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

Evidence

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). The biotopes are, therefore, considered 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

Kędra et al. (2010) reported Aphelochaeta mariori that occurred in the Hornsundfjord, Svalbard, where primary production has been recorded as 120 gC/m²/yr. Covazzi Harriague et al. (2007) reported Aphelochaeta mariori at sites with organic matter sedimentation as high as 359 mg/m²/hr in the Ligurian Sea, Italy. Furthermore, Markert et al. (2010) compared macrofaunal communities in the Wadden Sea in reefs dominated by Mytilus edulis and Cassostrea gigas and found Aphelochaeta mariori as a dominate species throughout the study site, which suggested the species was unlikely to be affected by organic enrichment.

Tubificoides benedii is very tolerant of high levels of organic enrichment and often dominate sediments where sewage has been discharged or other forms of organic enrichment have occurred (Pearson & Rosenberg, 1978; Gray, 1971; McLusky et al., 1980). Their tolerance for organic enrichment is attributed to their adaptation to live in and feed on enriched organic deposits (Pearson & Rosenberg, 1978), and their high population densities in such areas is enhanced by the lack of predation and competition. Tubificoides benedii are abundant in mussel beds (mussel relaying may be the source of smothering) which has been attributed to their tolerance of organically rich deoxygenated sediment (Commito & Boncavage, 1989). Tubificoides benedii has also been found in elevated abundances in areas of organic enrichment around fish farms (Haskoning, 2006).

The infaunal habit of Nephtys hombergii and its ability to burrow relatively rapidly through, and into the substratum are likely to aid the species in its avoidance of unsuitable conditions. Nephtys spp. were present in organically enriched sediments  along the Catalan Spanish coast (Cardell et al., 1998), suggesting the species is likely to be resistant to some organic enrichment.

Sensitivity assessment. The evidence presented suggest the characterizing species may not be affected by organic enrichment at the benchmark level. Resistance and resilience are therefore assessed as High and the biotopes are considered Not Sensitive to organic enrichment (deposit of 100 gC/m²/yr).

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

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

Sensitivity assessment. Resistance to the pressure is considered None and resilience Very Low. Sensitivity has been assessed as High. Although no specific evidence is described, confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another sediment type)

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

Evidence

Aphelochaeta marioni has been recorded from a variety of different sediment types. In the intertidal area of the Wadden Sea, it achieved highest abundance where the sediment fraction smaller than 0.04 mm diameter was greater than 10% of the total sediment (Farke, 1979). Tubificoides benedii (studied as Peloscolex benedeni) is found in a range of substratum types from sandy mixed habitats, fine sands and coarse sands (Giere & Pfannkuche, 1982). High densities of Nephtys hombergii were found in substrata of 0.3% particles >0.25 mm and 5.8% <0.125 mm in diameter, but the worm tolerated up to 3.8% 0.25 mm and 2.2-15.9% <0.125 mm (Clark et al., 1962). Nephtys hombergii may be found in higher densities in muddy environments and this tends to isolate it from Nephtys cirrosa, which is characteristic of cleaner, fairly coarse sand. Degraer et al. (2006) summarized that the higher the medium grain size of the sediment, the lower the relative occurrence of Nephtys hombergii and in grain sizes over 0.5 mm the species was absent in the Belgium part of the North Sea.

Sensitivity assessment. The characterizing species of these biotopes occur in a range of substrata and are likely to resist a change in sediment type by 1 Folk class from, for example, sandy mud to muddy sand (based on the Long, 2006 simplification). However, this would probably represent a fundamental change in the character of the biotopes, and a change in the abundance of the characteristic species, resulting in the loss and/or reclassification of the biotopes. Resistance is therefore assessed as None and resilience as Very Low given the permanent nature of the pressure and the biotopes are considered to have High sensitivity to a change in seabed type by one Folk class.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Sedimentary communities are likely to be highly intolerant of substratum removal, which will lead to partial or complete defaunation, and will expose underlying sediment which may be anoxic and/or of a different character or bedrock and lead to changes in the topography of the area (Dernie et al., 2003). Any remaining species, given their new position at the sediment/water interface, may be exposed to conditions to which they are not suited. Removal of 30 cm of surface sediment will remove the infaunal community and other species present in the biotopes. For instance, Aphelochaeta marioni lives buried in soft sediments with the majority of individuals found in the upper 4 cm of the sediment (Rayment, 2007a). Removal of the substratum to 30 cm would result in the loss of the characterizing species. Recovery of the biological assemblage may take place before the original topography is restored, if the exposed, underlying sediments are similar to those that were removed. Hydrodynamics and sedimentology (mobility and supply) influence the recovery of soft sediment habitats (Van Hoey et al., 2008).

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotopes. Resistance of the biotopes is assessed as None. Resilience of SS.SMu.SMuVS.AphTubi is likely to be Medium given the low energy environment where the biotope occurs, whereas resilience of SS.SMu.SMuVS.NhomTubi is likely to be High. Both biotopes sensitivity are therefore assessed as Medium.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Aphelochaeta marioni is a soft bodied organism which exposes its palps and cirri at the surface while feeding (Rayment, 2007a). The species lives infaunally in soft sediment, usually within a few centimetres of the sediment surface. Physical disturbance, such as dredging or dragging an anchor, would be likely to penetrate the upper few centimetres of the sediment and cause physical damage to Aphelochaeta marioni. De Biasi & Pacciardi (2008) compared macrobenthic communities in a commercial fishing ground exploited by otter trawling with an area closed to fishing for over 10 years in the Adriatic Sea. The authors found that polychaetes, including Aphelochaeta spp. were among the species dominating the disturbed areas, which is likely to result from the ability of the species to recolonize disturbed areas rapidly, rather than indicate that the polychaetes are resistant to disturbance of the seabed surface.

Tubificoides benedii can be relatively deeply buried and could avoid direct exposure to abrasion although sediment disturbance and compaction could damage these soft-bodied species, and oligochaetes in general are not found in high abundances in sediments with high levels of disturbance from wave action. Experimental studies on crab-tiling impacts have found that densities of Tubificoides benedii and Tubificoides pseudogaster were higher in non-trampled plots (Sheehan et al., 2010), indicating that these oligochaetes have some sensitivity to trampling. Whomersley et al. (2010) conducted experimental raking on intertidal mudflats at two sites (Creeksea Crouch Estuary England, and Blackness lower Forth Estuary, Scotland), where Tubificoides benedii were dominant species. For each treatment, 1 m² plots were raked twice to a depth of 4 cm (using a garden rake). Plots were subject to either low intensity treatments (raking every four weeks) or high (raking every two weeks). The experiment was carried out for 10 months at Creeksea and a year at Blackness. The high and low raking treatments appeared to have little effect on Tubificoides benedii (Whomersley et al., 2010).

Collie et al. (2000) found that Nephtys hombergii displayed a negative effect on abundance as a result of fishing activities and mean response of infauna and epifauna communities to fishing activities was much more negative in mud and sand communities than other habitats. Nephtys hombergii abundance also significantly decreased in areas of the Solent, UK, where bait digging (primarily for Nereis virens) 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 biotopes will be impacted if damage to seabed surface features is widespread, however, motile and opportunistic fauna, such as Tubificoides benedii may recover quickly. Nephtys hombergii shows a greater negative impact (Collie et al., 2000; Ferns et al., 2000). Resistance is assessed as Low and resilience is assessed as High, providing a sensitivity assessment of Low.

Penetration or disturbance of the substratum subsurface

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

Evidence

Activities that penetrate below the surface would remove a significant proportion of the characterizing species within the direct area of impact (see evidence under ‘abrasion’ above). Resistance of the biotopes is therefore assessed as Low, although the significance of the impact for the bed will depend on the spatial scale of the pressure footprint. Resilience is assessed as High, and sensitivity is assessed as Low.

Changes in suspended solids (water clarity)

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

Evidence

The biotopes are probably exposed to the high levels of suspended sediment characteristic of estuarine conditions. Therefore, the resident species are probably adapted to high suspended sediment levels. Aphelochaeta marioni lives infaunally and is a surface deposit feeder (Rayment, 2007a), therefore relying on a supply of nutrients at the sediment surface. An increased rate of siltation may result in an increase in food availability and therefore growth and reproduction of Aphelochaeta marioni. However, food availability would only increase if the additional suspended sediment contained a significant proportion of organic matter and the population would only be enhanced if food was previously limiting. A decrease in the suspended sediment would result in a decreased rate of deposition on the substratum surface and therefore a reduction in food availability for Aphelochaeta marioni. This would be likely to impair growth and reproduction.

Tubificoides benedii displays preference for fine organic enriched sediments and may, therefore, increase in abundance if suspended solids settle and lead to increased organic enrichment (Gogina et al., 2010).

Sensitivity assessment. Changes in light penetration or attenuation associated with this pressure are not relevant to Nephtys hombergii, Aphelochaeta marioni and Tubificoides benedii. As the species live in the sediment they are also likely to be adapted to increased suspended sediment (and turbidity). However, alterations in the availability of food or the energetic costs in obtaining food or changes in scour could either increase or decrease habitat suitability for these characterizing species. The following sensitivity assessment relies on expert judgement, utilizing evidence of species traits and distribution and therefore confidence has been assessed as low. Resistance is High as no significant negative effects are identified and potential benefits from increased food resources may occur. Resilience is also High as no recovery is required under the likely impacts. Sensitivity of the biotopes is therefore assessed as 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

Aphelochaeta marioni lives infaunally in soft sediments and moves by burrowing. It deposit feeds at the surface by extending contractile palps from its burrow. An additional 5 cm layer of sediment would result in a temporary cessation of feeding activity, and therefore growth and reproduction are likely to be compromised. However, Aphelochaeta marioni would be expected to quickly relocate to its favoured depth, with no mortality. Kędra et al. (2010) reported Aphelochaeta mariori to occur in the Hornsundfjord, Svalbard, where sedimentation rates can vary between 0.1-35 cm/yr.

Tubificoides live relatively deeply buried and can tolerate periods of low oxygen that may occur following the deposition of a fine layer of sediment. In addition, the presence of this species in areas experiencing deposition, such as estuaries, indicate that this species is likely to have a high tolerance to siltation events. Tubificoides spp. showed some recovery through vertical migration following the placement of a sediment overburden 6 cm thick on top of sediments (Bolam, 2011). Whomersley et al. (2010) experimentally buried plots on intertidal mudflats at two sites (Creeksea Crouch Estuary, England, and Blackness lower Forth Estuary, Scotland), where Tubificoides benedii were dominant species. For each treatment, anoxic mud was spread evenly to a depth of 4 cm on top of each treatment plot. The mud was taken from areas adjacent to the plots, and was obtained by scraping off the surface oxic layer and digging up the underlying mud from approximately 20 cm depth. Plots were subject to either low intensity treatments (burial every four weeks) or high (burial every two weeks). The experiment was carried out for 10 months at Creeksea and a year at Blackness. At Creeksea numbers of Tubificoides benedii increased in both burial treatments until the third month (high burial) and sixth month (low burial). At Blackness, increased numbers of Tubificoides benedii were found in both burial treatments after one month (Whomersley et al., 2010).

In the low energy environment where the biotopes occur, a ‘light’ deposition of sediments is likely to be cleared in a few tidal cycles.

Sensitivity assessment. The characterizing species Nephtys hombergii, Aphelochaeta marioni and Tubificoides benedii live in the sediment, to depths to 15 cm and are considered likely to resist smothering by 5 cm of sediment. Resistance and resilience are, therefore, assessed as High and the biotopes are considered Not Sensitive to a ‘light’ deposition of up to 5 cm of fine material in a single discrete event.

Smothering and siltation rate changes (heavy)

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

Evidence

Aphelochaeta marioni lives infaunally in soft sediments and moves by burrowing. It deposit feeds at the surface by extending contractile palps from its burrow. An additional layer of sediment would result in a temporary cessation of feeding activity, and therefore growth and reproduction are likely to be compromised. However, Aphelochaeta marioni would be expected to quickly relocate to its favoured depth, with no mortality. Kędra et al. (2010) reported Aphelochaeta mariori to occur in the Hornsundfjord, Svalbard, where sedimentation rates can vary between 0.1-35 cm/yr. Furthermore, Do et al. (2012) studied the macrobenthos recovery in the Arcachon Bay (France) following a deposition of sediment up to 10 cm thick that resulted from dredging activities. The authors reported Aphelochaeta marioni as considerably reduced or absent from impacted areas characterized mainly by mud substrata.

The pressure benchmark (30 cm deposit) represents a significant burial event and the deposit may remain for some time in a sheltered mudflat. Some impacts on some characterizing species may occur and it is considered unlikely that significant numbers of the population could reposition, based on Bolam (2011). Polychaete species have been reported to migrate through depositions of sediment greater that the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Placement of the deposit will, therefore, result in a defaunated habitat until the deposit is recolonized.

Sensitivity assessment. Resistance is assessed as Low and resilience as High and the biotopes are considered to have Low sensitivity to a ‘heavy’ deposition of up to 30 cm of fine material in a single discrete event.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

No Evidence is available on which to assess this pressure.

Underwater noise changes

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

Evidence

None of the species in the biotopes are likely to be sensitive to noise or vibration at the benchmark level and no information was found concerning the intolerance to noise. Species within the biotopes can probably detect vibrations caused by noise and in response may retreat in to the sediment for protection. However, the characterizing species are unlikely to be affected by noise pollution and so the biotopes are assessed as Not Sensitive.

Introduction of light or shading

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

Evidence

Changes in light availability impact primary production by phytoplankton in the water column and by the microphytobenthos on the sediment surface, which in turn may affect food availability for the characterizing species. Farke (1979) noted that Aphelochaeta marioni is intolerant to visual disturbance in a microsystem in the laboratory possibly due to its nocturnal life habits (Farke, 1979). In order to observe feeding and breeding in the microsystem, the animals had to be gradually acclimated to lamp light. Even then, additional disturbance, such as an electronic flash, caused the retraction of palps and cirri and cessation of all activity for some minutes. Visual disturbance, in the form of direct illumination during the species' active period at night, may therefore result in loss of feeding opportunities, which may compromise growth and reproduction.

Sensitivity assessment. However, the characterizing community of the biotopes live infaunally and are unlikely to be directly dependent on sunlight so the biotopes are considered to have High resistance and, by default, High resilience and therefore is Not Sensitive to the introduction of light or shading.

Barrier to species movement

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

Evidence

Not Relevant to biotopes restricted to open waters.

Death or injury by collision

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

Evidence

Not Relevant to seabed habitats. NB. Collision by grounding vessels is addressed under surface abrasion.

Visual disturbance

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

Evidence

Most species will respond to the shading caused by the approach of a predator, however, their visual acuity is probably very low. Farke (1979) noted that Aphelochaeta marioni was intolerant to visual disturbance in a microsystem in the laboratory possibly due to its nocturnal life habits (Farke, 1979). In order to observe feeding and breeding in the microsystem, the animals had to be gradually acclimated to lamp light. Even then, additional disturbance, such as an electronic flash, caused the retraction of palps and cirri and cessation of all activity for some minutes. Most aquatic oligochaetes, such as Tubificoides spp. have no eyes, although a few have simple ocelli (eyspots) which are light receptors. However, as the characterizing species live in the sediment, the species will most probably not be impacted at the pressure benchmark.

Sensitivity assessment.  At the pressure benchmark, resistance is assessed as High and resilience as High and the biotopes are assessed as 'Not sensitive'.

Genetic modification & translocation of indigenous species

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

Evidence

The important characterizing species in the biotopes are not cultivated or likely to be translocated. This pressure is therefore considered Not Relevant.

Introduction or spread of invasive non-indigenous species

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

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. 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.

Commito (1987) found that the population density of Tubificoides benedii was the same or higher in mussel beds than in open areas, suggesting that colonization of sediments by the Pacific oyster Magallana gigas would not necessarily impact the population (although it would alter the character of the biotopes). Tang & Kristensen (2010) found that the abundance of macrofauna, including Tubificoides, was lower in marsh invaded by the hybrid cordgrass Spartina anglica than in mudflats. Colonization of upper mudflats by this species would alter the character of the biotopes resulting in loss and reclassification.

Sensitivity assessment. The above evidence suggests that Crepidula could colonize cohesive mud habitats in the subtidal, typical of this biotope, where shells, stones, or any other hard substrata can be used for larvae settlement (Tillin et al., 2020). Crepidula has the potential to colonize, and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020), as occurs in maerl gravels (Grall & Hall-Spencer, 2003) resulting in the loss of the biotope. This is a sheltered to extremely sheltered habitat, which is suitable for Crepidula. There is evidence of Crepidua occurring within this biotope, but it is unclear how much Crepidula is recorded and if the slipper limpet will come to dominate. The relative abundance of hard substrata might limit the spread of Crepidula and the possibility of burial in a high deposition habitat might limit its abundance. The biotopes may be sensitive to invasion by Spartina anglica which would alter the character of the mudflat and the biological assemblage. 

Therefore, resistance is assessed as 'Medium' on the basis that Crepidula shells will provide a hard substratum for the expansion of Crepidula populations. Resilience is assessed as 'Very low', as it would require the removal of Crepidula, probably by artificial means. Hence, the biotope is assessed as 'Medium'. Further evidence is required on the presence of Crepidula within this biotope, so the confidence in the assessment is 'Low'.

Introduction of microbial pathogens

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

Evidence

Introduced organisms (especially parasites or pathogens) are a potential threat in all coastal ecosystems. Little information was found regarding microbial infection of polychaetes, although Gibbs (1971) recorded that nearly all of the population of Aphelochaeta marioni in Stonehouse Pool, Plymouth Sound, was infected with a sporozoan parasite belonging to the acephaline gregarine genus Gonospora, which inhabits the coelom of the host. No evidence was found to suggest that gametogenesis was affected by Gonospora infection and there was no apparent reduction in fecundity.

Marine oligochaetes host numerous protozoan parasites without apparent pathogenic effects even at high infestation levels (Giere & Pfannkuche, 1982).

Sensitivity assessment. The biotopes are judged to have High resistance to this pressure. By default, resilience is assessed as High and the biotopes are classed as Not Sensitive.

Removal of target species

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

Evidence

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 very high as repopulation would initially occur 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).

Sensitivity assessment. Resistance is Low for SS.SMu.SMuVS.NhomTubi due to direct removal of a characterizing species Nephtys hombergii, that on commercial scales can remove a large proportion of the population. For instance, Neves de Carvalho et al. (2013) calculated that bait digging activities in the Douro estuary, Portugal may remove up to 9.9 tonnes of Hediste diversicolor. Resilience is assessed as Medium, as regions that are not regularly harvested may recover rapidly but continued harvesting will impact the population. Sensitivity of SS.SMu.SMuVS.NhomTubi is assessed as Medium. 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.

On the other hand, it is extremely unlikely that species indicative of sensitivity of SS.SMu.SMuVS.AphTubi would be targeted for extraction, so this pressure is therefore considered Not Relevant for SS.SMu.SMuVS.AphTubi.

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 the biotopes, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Hall & Harding (1997) demonstrated that commercial cockle harvesting by suction dredging had significant effects on soft-sediment infaunal communities. Following dredging, species numbers were reduced by up to 30% and abundances by up to 50%.

Collie et al. (2000) identified that well established intertidal communities suffered the greatest impact from bottom towed 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). However, the burrowing and tunnelling traits of the characterizing species may provide some resistance to this pressure. 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 Nephtys hombergii abundance 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 than other habitats. Nephtys hombergii abundance also significantly decreased in areas of the Solent, UK, where bait digging (primarily for Nereis virens) 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).

Aphelochaeta marioni is a soft bodied organism which exposes its palps and cirri at the surface while feeding (Rayment, 2007a). The species lives infaunally in soft sediment, usually within a few centimetres of the sediment surface. Physical disturbance, such as dredging or dragging an anchor, would be likely to penetrate the upper few centimetres of the sediment and cause physical damage to Aphelochaeta marioni.

Sensitivity assessment. Resistance is Low due to direct removal or damage of characterizing species that on commercial scales can remove a large proportion of the population. Resilience is assessed as Medium as regions that are not regularly harvested may recover rapidly but continued harvesting will impact the population. Sensitivity is assessed as Medium. It is important to consider that the spatial extent and duration of areas impacted by fishing gear effort is important to consider when assessing this pressure as smaller scale extraction may not impact the entire extent of the biotopes 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.