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

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 temperatures above, even a 5°C increase in 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).

Streblospio shrubsolii have been shown to reproduce in a temperature range of 7.5°C – 30°C with highest reproduction levels occurring between 16°C – 21°C (Levin & Creed, 1986, Da Fonseca - Genevois & Cazaux, 1987, Chu & Levin, 1989, Lardicci et al., 1997). The evidence  was based on Mediterranean sites, limiting confidence for UK and Irish seas. The timing of reproduction and growth, although occurring throughout the year, increased in late spring and early summer but were strongly reduced during periods of higher temperatures in summer and disappeared or were strongly reduced at lower temperatures in winter (Lardicci et al., 1997). The timing of growth and reproduction in Streblospio shrubsolii depended on the synergistic effects of temperature and photoperiod, so that cues may differ at locations at different latitudes (Chu & Levin, 1989). Both a 5°C increase in temperature for one month period, or 2°C for one year are within the temperature range reproduction occurs within (7.5°C – 30°C) and within the temperature range where highest reproduction levels occur (16°C – 21°C), suggesting limited impact from the pressure at benchmark pressures is likely.  

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the Mediterranean Sea and Indian Ocean (Hartmann-Schröder, 1974; Rogall, 1977; both cited in Farke, 1979) and 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 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.

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.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010). It is likely that the important characteristic species are able to resist a long-term increase in temperature of 2°C and may resist a short-term increase of 5°C. Resistance and Resilience are therefore assessed as ‘High’ and the biotope is 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

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.

Streblospio shrubsolii has been shown to reproduce in a temperature range of 7.5°C – 30°C with highest reproduction levels occurring between 16°C – 21°C (Levin & Creed, 1986; Da Fonseca - Genevois & Cazaux, 1987; Chu & Levin, 1989; Lardicci et al., 1997). Reproductive activity disappeared or strongly reduced at lower temperatures in winter in a Mediterranean case study (Lardicci et al., 1997). This case study suggests reproduction would be delayed in UK and Irish populations that experienced both a  5°C decrease in temp for one month period, or 2°C for one year.

Aphelochaeta marioni is distributed over a wide temperature range. It has been recorded from the Western Baltic Sea, South Atlantic Ocean and North Sea (Hartmann-Schröder, 1974; Rogall, 1977: both cited in Farke, 1979) and 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, metabolic activity should quickly return to normal when temperatures increased.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010). Aphelochaeta marioni, Streblospio shrubsolii 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 population and may be compensated by later spawning events. 'Resistance' and 'Resilience' are therefore assessed as ‘High’ and the biotope assessed 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

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 (1971) observed that species with pelagic larvae or a free-swimming phase, 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. In fully marine locations Nephtys hombergii may still be found 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.

Streblospio shrubsolii occurred in subtidal areas of the Thames estuary as well as intertidal flats, suggesting the species is resistant to higher salinities than the ‘variable’ levels occurring higher in estuaries (Attrill, 1998). Likewise Tubificoides benedii has been recorded  in high abundance in offshore areas of the North Sea (Gray et al., 1990). Although evidence was limited on response of these species to rapid increases in salinity it is likely they would be resistant to an increase to the fully marine category (30-40 ‰).

Conde et al. (2013) found that Streblospio shrubsolii were a dominant species in low salinity, estuarine conditions (5-9‰) in the Tagus estuary, Portugal. In Ria de Averio, western Portugal Streblospio shrubsolii and Tubificoides benedii were characterizing species of communities in estuarine sample sites further upstream with lower salinity, suggesting a high resistance to a decrease in salinity (Rodrigues et al., 2011).

Streblospio shrubsolii occurred in subtidal areas of the Thames estuary as well as intertidal flats, suggesting the species is resistant to higher salinities than the ‘variable’ levels occurring higher in estuaries (Attrill, 1998). Likewise Tubificoides benedii has been recorded  in high abundance in offshore areas of the North Sea (Gray et al., 1990). Although evidence was limited on response of these species to rapid increases in salinity it is likely they would be resistant to an increase to the fully marine category (30-40 ‰).

Sensitivity assessment. Nephtys hombergii are likely to decrease in abundance. Resistance is assessed as 'Low', Resilience is assessed as  'High' (following restoration of salinity regime) and biotope sensitivity is assessed as  'Low'. Increase in salinity is likely to lead to changes to the biotope LS.LMu.MEst.HedLim As the associated HedLim communities occur further down estuaries towards the open coast, in more saline conditions. The infauna in LS.LMu.MEst.HedLim is similar, though the ragworm Hediste diversicolor is always abundant, and both Nephtys hombergii and Streblospio shrubsolii are often absent. The bivalve assemblage tends to be more diverse LS.LMu.MEst.HedLim (Conner et al., 2004).

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 occurs on the mid and lower shore of sheltered estuaries, with salinities within the MNCR categories of reduced (18-30ppt) to variable (18-35 ppt) (Connor et al., 2004). Maximum salinity would be expected to be approximately 18-35ᵒ⁄ₒₒ.  A decrease of one MNCR salinity category would be to ‘Low’ category <18‰.

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.

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.

Conde et al. (2013) found that Streblospio shrubsolii were a dominant species in low salinity, estuarine conditions (5-9‰) in the Tagus estuary, Portugal. In Ria de Averio, western Portugal Streblospio shrubsolii and Tubificoides benedii were characterizing species of communities in estuarine sample sites further upstream with lower salinity, suggesting a high resistance to a decrease in salinity (Rodrigues et al., 2011).

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).

Sensitivity assessment 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. Aphelochaeta marioni, Streblospio shrubsolii and Tubificoides benedii survive in much lower salinities suggesting resistance and resilience of these species are higher. As decreasing salinity is likely to lead to a change in biotope to one dominated by Hediste diversicolor Resistance is assessed as ‘Low’ , Resilience is ‘High’ given return to reduced or variable salinities and sensitivity is therefore, ‘Low'. 

Water flow (tidal current) changes (local)

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

Evidence

The biotope occurs most often in weak or moderately strong tidal streams (Conor et al., 2004), suggesting the species characterizing the biotope can adapt to a range of tidal currents, aided by each species burrowing traits.

The hydrographic regime is an important structuring factor in sedimentary habitats. An increase in water flow rate is not likely to affect Nephtys hombergii, Streblospio shrubsolii, Aphelochaeta marioni and other characterizing species as they live infaunally The most damaging effect of increased flow rate (above the pressure benchmark) 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 re-suspending 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 re-suspended) (Coates et al., 2014). Species such as Tubificoides benedii and other opportunist polychaetes that tolerate coarser particle size will possibly increase in abundance.

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.  As 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 the characterizing species, Resistance and Resilience are assessed as ‘High’ and the biotope 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 biotope and characterizing species occurs in the mid to low intertidal. 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 biotope to extend its upper limit, where suitable substrata exist.

Opportunistic, deposit feeding polychaetes, such as, Streblospio shrubsolii, 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, show resistance to hypoxia and are 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 biotope is 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 as a result of emergence regime changes. Resistance is therefore assessed as ‘Medium’ and resilience is likely to be ‘High’, so the biotope is 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

As this biotope occurs in sheltered estuaries a change in nearshore significant wave height >3% but <5% is unlikely to lead to wave heights that will cause greater erosion. The following impacts are only likely to occur in the most exposed examples of the biotope, greatest impacts would occur within very exposed area, where increased wave exposure is likely to cause erosion of the substrata and consequently, loss of habitat.

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, Alder & McIntyre (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.

Aphelochaeta marioni characteristically inhabits soft sediments in sheltered areas (Broom et al., 1991). 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. Furthermore, individuals may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action.

Recovery would be influenced by the length of time it would take for the habitat to return to a suitable state for recolonization by adult and juvenile specimens from adjacent habitats, and the establishment of a breeding population. This may take between one and three years, as populations differ in reaching maturity (Dales, 1950; Mettam et al.,1982; Olive & Garwood, 1981), from the time that the habitat again becomes suited to the species.

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 this biotope, as the biotope is limited to sheltered locations. Increases in wave action may therefore remain within the limits of the species tolerance but factors such as sediment redistribution may alter the physical biotope.

Sensitivity assessment. Resistance to a change in nearshore significant wave height >3% but <5% of the two main characterizing species Nephtys hombergii, Aphelochaeta marioni, Streblospio shrubsolii and Tubificoides benedii  is ‘High’, given that the biotope occurs in very sheltered locations and an increase in nearshore significant wave height of >3% but <5% would continue to result in sheltered conditions which are within the species tolerance limits. At the highest benchmark pressure (5% increase) the species exhibit ‘High’ resistance through their traits to live in the sediment. Resilience (recoverability) is also ‘High’ giving a Sensitivity of ‘Not Sensitive’. Due to limited evidence, confidence in this assessment is Low and the potential for long-term changes to the substratum following continued increase in wave action should be considered.

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. The following review discusses impacts at higher concentrations than the pressure benchmark.

Characterizing species display some resistance to heavy metal concentration. Aphelochaeta marioni is tolerant of heavy metal contamination occurring in the heavily polluted Restronguet Creek, Falmouth, UK (Bryan & Gibbs, 1983) and it is also an accumulator of arsenic (Gibbs et al., 1983). Nephtys hombergii is also recorded in Restronguet Creek.

The levels of Cu, Zn, As and Sn in the sediments of Restronguet Creek are highly contaminated, in the order of 1500-3500 µg/g (Bryan & Gibbs, 1983). Concentrations of dissolved Zn typically range from 100-2000 µ g/l, Cu from 10-100 µg/l and Cd from 0.25-5.0 µg/l (Bryan & Gibbs, 1983). Analyses of organisms from Restronguet Creek revealed that some species contained abnormally high concentrations of heavy metals. Nephtys hombergii from the middle and lower reaches of the creek contained appreciably higher concentrations of Cu (2227 µg/g dry wt), Fe and Zn than comparable specimens of Hediste diversicolor (as Nereis diversicolor). However, amongst polychaetes within the creek, there was evidence that some metals were regulated. In Nephtys hombergii the head end of the worm became blackened and x-ray microanalysis by Bryan & Gibbs (1983) indicated that this was caused by the deposition of copper sulphide in the body wall. In the same study, Bryan & Gibbs (1983) presented evidence that Nephtys hombergii from Restronguet Creek possessed increased tolerance to copper contamination. Specimens from the Tamar Estuary had a 96 h LC50 of 250 µg/l, whilst those from Restronguet Creek had a 96 h LC50 of 700 µg/l (35 psu; 13°C). Bryan & Gibbs (1983) suggested that since the area had been heavily contaminated with metals for > 200 years, there had been adequate time for metal-resistant populations to develop especially for relatively mobile species.

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 tributyltin (TBT) was banned within the estuary, suggesting that TBT had suppressed their abundance previously.

At concentrations of 1-3 µg/l of TBT there was no significant effects on the abundance of Hediste diversicolor or Cirratulus cirratus after 9 weeks in a microcosm. However, no juvenile polychaetes were retrieved from the substratum suggesting that TBT had an effect on the larval and/or juvenile stages of these polychaetes, effects may therefore also be present on juveniles of characterizing species of this biotope.

In general, for estuarine animals, heavy metal toxicity increases as salinity decreases and temperature increases (McLusky et al., 1986). As this biotope experiences variable salinity conditions, it is likely that some polychaete species in the biotope might be adversely affected by high contamination by heavy metals.

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. The following review discusses impacts at higher concentrations than the pressure benchmark.

Contamination at levels greater than the pressure benchmark may adversely influence the biotope. Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected.

The 1969 West Falmouth Spill of Grade 2 diesel fuel, documented by Sanders (1978), illustrates the effects of hydrocarbons in a sheltered habitat with a soft mud/sand substrata (Suchanek, 1993). The entire benthic fauna was eradicated immediately following the spill and remobilization of oil that continued for a period >1 year after the spill contributed to much greater impact upon the habitat than that caused by the initial spill. Effects are likely to be prolonged as hydrocarbons incorporated within the sediment by bioturbation will remain for a long time owing to slow degradation under anoxic conditions. Oil covering the surface and within the sediment would prevent oxygen transport to the infauna and promote anoxia as the infauna utilise oxygen during respiration. Although Nephtys hombergii is relatively tolerant of hypoxia and periods of anoxia, a prolonged absence of oxygen would probably result in the death of it and other infauna.

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.

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 are often abundant in polluted coastal muds (Giere & Rhode, 1987) and increase in abundance in highly polluted sites compared to sites at a distance from contamination (oil drilling waste) (Gray et al., 1990), suggesting this species would actually increase in abundance under this pressure.

Overall, hydrocarbon contamination is likely to adversely affect some members of the community, and more resistant (or opportunistic) species to increase in abundance, resulting in a reduction in species richness.

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 following review discusses impacts at higher concentrations than the pressure benchmark.

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.

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. Specific effects of synthetic chemicals have been reported for other species of polychaete. Exposure of Hediste diversicolor and Arenicola marina to Ivermecten resulted in significant mortality (Collier & Pinn, 1998).

At concentrations of 1-3 µg/l of TBT there was no significant effects on the abundance of Hediste diversicolor or Cirratulus cirratus after 9 weeks in a microcosm. However, no juvenile polychaetes were retrieved from the substratum suggesting that TBT had an effect on the larval and/or juvenile stages of these polychaetes, effects may therefore also be present on juveniles of characterizing species of this biotope.

Radionuclide contamination

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

Evidence

Insufficient information was available in relation to characterizing species to assess this pressure. Limited evidence is available on species with similar traits. Beasley & Fowler (1976) and Germain et al., (1984) examined the accumulation and transfers of radionuclides in Hediste diversicolor from sediments contaminated with americium and plutonium derived from nuclear weapons testing and the release of liquid effluent from a nuclear processing plant. Both concluded that the uptake of radionuclides by Hediste diversicolor was small. Beasley & Fowler (1976) found that Hediste diversicolor accumulated only 0.05% of the concentration of radionuclides found in the sediment. Both also considered that the predominant contamination pathway for Hediste diversicolor was from the interstitial water.

Sensitivity assessment. There is insufficient information available on the biological effects of radionuclides to comment further upon the intolerance of characterizing species to radionuclide contamination. Assessment is given as ‘No Evidence’.

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.

Some, all be it, limited evidence was retuned by searches on activated carbon (AC). AC is utilised in some instances to effectively remove organic substances from aquatic and sediment matrices. Lillicrap et al. (2015) demonstrate that AC may have physical effects on benthic dwelling organisms at environmentally relevant concentrations at remediated sites.

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. (1997) 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. Therefore, it was 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.

Nephtys hombergii inhabits intertidal areas where resistance to low oxygen is needed and sulphide levels can reach up to 1mM (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 utilising 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).

Streblospio shrubsolii is identified as a characteristic species communities in polluted environments (Cooksey & Hyland, 2007) and in Ria de Averio, western Portugal Streblospio shrubsolii and Tubificoides benedii were characterizing species of communities in estuarine sample sites further upstream where exposure to dissolved oxygen concentration was likely to be lowest (Rodrigues et al., 2011).

Sensitivity assessment. The characterizing species are adapted to intertidal areas where resistance to low dissolved oxygen concentration is required and therefore resistance is assessed as ‘High’ and resilience as ‘High’ and the biotope is assessed as ‘Not sensitive’ at the pressure benchmark level.

Nutrient enrichment

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

Evidence

The biotope is likely to be not sensitive to the pressure at the benchmark levels, however evidence was available on responses to further reductions in water quality.

Nephtys hombergii showed resistance to increased pollution in studies along the Spanish Catalan coast. Changes in species composition parallel to the shoreline showed greatest abundance of Nephtys hombergii occurred at sample sites closer to water sewage discharges and river outflow (in comparison to non-polluted sites) (Cardell et al., 1998). For the entire species communities, these sites contained greater biomass but reduced species diversity suggesting Nephtys hombergii was amongst a small number of species that could exploit these conditions. Nephtys hombergii lives infaunally between a depth of 5 and 15 cm where light is not transmitted. An increase in turbidity, is unlikely to have a detectable effect on the viability of the species.

Streblospio shrubsolii occurred amongst other pollution tolerant species including the polychaetes Capitella capitata, Polydora ciliata, and Manayunkia aestuarina and the oligochaetes Peloscolex benendeni and Tubifex pseudogaste in the Tees estuary, UK during periods of gross pollution in 1971-1973 (Gray, 1976).

Raman & Ganapati (1983) studied the distribution of Aphelochaeta marioni (studied as Tharyx marioni) in relation to a sewage outfall in Visakhaptnam Harbour, Bay of Bengal. Increased nutrients often derive from sewage inputs and presence of species such as Aphelochaeta marioni in such situations (for instance Broom et al., 1991) may reflect tolerance to high nutrients or to deoxygenated conditions or both. Aphelochaeta marioni was found to be dominant in the 'semi-healthy zone' characterized by low nutrients (nitrate 0.02 mg/l, phosphate 0.88 mg/l). Aphelochaeta marioni was not found in high numbers in the polluted zone close to the sewage outfall, characterized by high nutrients (nitrate 0.042-0.105 mg/l, phosphate 2.35-3.76 mg/l) (Rayment, 2007). This would suggest that Aphelochaeta marioni is intolerant of eutrophication. However, it would be expected that an increase in organic nutrients would lead to increased food availability for the deposit feeding Aphelochaeta marioni. Furthermore, Dauvin (1982, 2000) recorded an increase in abundance of Aphelochaeta marioni following an oil spill which resulted in an explosion of plant growth due to high mortality of grazers. Therefore, the available evidence on the resistance of Aphelochaeta marioni to nutrient changes does not allow consistent conclusions to be drawn.

Sensitivity assessment. The characterizing species show 'High' Resistance to increased pollution and turbidity, Resilience is therefore also 'High', and the biotope is characterized as ‘Not sensitive’.

Organic enrichment

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

Evidence

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., 1999) suggesting the species is likely to be resistant to some organic enrichment.

Kędra et al. (2010) reported Aphelochaeta mariori to occur 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/h 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, suggesting the species is unlikely to be affected by enhanced organic enrichment.

In Ria de Averio, western Portugal Streblospio shrubsolii and Tubificoides benedii were characterizing species of communities further upstream in estuarine sample sites, at sites with increased organic matter (Rodrigues et al., 2011). Streblospio shrubsolii are also considered characteristic species communities in polluted environments, suggesting the species is likely to be resistant to increased organic enrichment (Cooksey & Hyland, 2007).

Sensitivity assessment. The characterizing species show High resistance to increased organic enrichment, resilience is, therefore, also High, and the biotope is assessed as ‘Not sensitive’.

Physical loss (to land or freshwater habitat)

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

Evidence

All marine 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 assessment. Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’.  Although no evidence from direct studies was available, 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, in particular sandy mud or mud and the burrowing organisms, Nephtys hombergii, Aphelochaeta marioni, Streblospio shrubsolii and Tubificoides benedii would not be able to survive if the substratum type was changed to either a soft rock or hard artificial type. Consequently the biotope would be lost altogether if such a change occurred. 

Sensitivity assessment. 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 sediment type)

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

Evidence

The biotope occurs in littoral mud (JNCC, 2015). A change in 1 Folk class (based on UK SeaMap simplified classification) would result in a change to mixed, sandy or coarse sediments. The characterizing species may colonize a variety of substrata, providing some resistance to an increase in mud or gravel content. However, changes in gravel content may lead to decreased abundance of characterizing species and increased abundance of competing species.

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).

High densities of Nephtys hombergii were found in substrata of 0.3% particles >0.25mm and 5.8% <0.125mm in diameter, but the worm tolerated up to 3.8% 0.25mm and 2.2-15.9% <0.125mm (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. An increase in gravel content, although tolerated by Nephtys hombergii may lead to increased abundance of Nephtys cirrosa and decreased abundance of Nephtys hombergii. 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 gain sizes over 0.5mm the species was absent in the Belgium part of the North Sea.

Silva et al. (2006) found Streblospio shrubsolii in an estuarine site in western Portugal, were more closely associated with increasing mud content and decreasing gravel content.

Sensitivity assessment. Although the characterizing species are mobile and likely to move to occupy more suitable sediment, the biotope will be affected by a change from soft wet mud with a fine sand fraction if an increased in gravel content occurs, as characterizing species will reduce in abundance. The loss of the littoral mud that characterizes this habitat would change the character of the biotope and is likely to lead to reclassification. For example, a change to sands would likely result in the biotope reverting to a littoral sand biotope.  Based on a change in character, the biotope is considered to have ‘No’ resistance to this pressure, resilience is assessed as Very ’Low’ as a change at the pressure benchmark is permanent and biotope 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

The substratum of this biotope consists of soft wet mud with a fine sand fraction (Conner et al., 2004). The characterizing species burrow into the sediment, or tunnel to depths not exceeding 30 cm. The process of extraction is considered to remove all biological components of the biotope group and the sedimentary habitat although similar sediments may remain. For instance, Aphelochaeta marioni lives buried in soft sediments with the majority of individuals found in the upper 4 cm of the sediment (Rayment, 2007). Removal of the substratum to 30 cm would result in the loss of the characterizing species.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘High’. Sensitivity has been assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Damage to seabed surface features may occur due to human activities such as bottom towed fishing gear (trawling and dredging) and natural disturbance from storms.  Abrasion from boat moorings  were  demonstrated to also impact species communities close to the mooring buoy in a case study in the Fal and Helford estuaries (south-west UK). Coarser sediment was exposed close to mooring buoys, caused by  suspension  of fine sediments by movement of the chain (Latham et al., 2012). However, fine sand and muddy sediments displayed the least influence from disturbance from moorings, suggesting a smaller impact to this biotope than other intertidal biotopes.

Sensitivity assessment. The characterizing species are burrowing infauna and likely to be relatively protected from a single event of abrasion at the surface, biotope resistance is, therefore, assessed as 'Medium' and resilience as 'High' so that biotope sensitivity is assessed as 'Low'.

Penetration or disturbance of the substratum subsurface

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

Evidence

As the characterizing species are burrowing species the impact from damage to the sub-surface sea bed would be greater than damage to the sea bed surface.

Collie et al. (2000) found that abundance of Nephtys hombergii  declined 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.  Resistance of the biotope is assessed as ‘Low’, although the significance of the impact for the biotope 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

This biotope is 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, 2007), 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, Streblospio shrubsolii 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, utilising 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 biotope 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

The biotope is located mainly in soft wet mud with a fine sand fraction, on the mid and lower shore of sheltered estuaries (Connor et al., 2004). These locations would be likely to experience some redistribution of fine material during tidal cycles. Although the biotope occurs in sheltered locations some mixing from wave action may also be expected. The characterizing species Nephtys hombergii , Aphelochaeta marioni , Streblospio shrubsolii and Tubificoides benedii  live in the sediment, to depths to 15cm and would be expected to be well adapted to these conditions.

Longer term deposition of fine material (e.g. continuous deposition) would be expected to lead to higher densities of macrobenthic organisms. For example, in the North Sea (Belgium) deposition of fine particle sediment, disturbed by scour around the base of a wind farm tower led to higher macrobenthic densities and created a shift in macrobenthic communities around the wind farm tower (influenced by the  direction fine material had settled) (Coates et al., 2014).

Within a Marine Biotic Index compiled by Borja et al. (2000) the characterizing species Nephtys hombergii  is classified within‘Group II’ which includes species that are indifferent to enrichment, Streblospio shrubsolii is ‘Group III’ which includes species that tolerate disturbance and excess organic content  and Tubificoides benedii  ‘Group V’ which tolerate high enrichment and polluted conditions. (Aphelochaeta marioni was not assessed). Powilleit et al., (2009) studied the response of the polychaete Nephtys hombergii to smothering. This species successfully migrated to the surface of 32-41 cm deposited sediment layer of till or sand/till mixture and restored contact with the overlying water.  The high escape potential could partly be explained by the heterogeneous texture of the till and sand/till mixture with ‘voids’.  While crawling upward to the new sediment surfaces burrowing velocities of up to 20 cm/day were recorded for Nephtys hombergii. Similarly, Bijkerk (1988, results cited from Essink 1999) indicated that the maximal overburden through which species could migrate was 60 cm through mud for Nephtys and 90 cm through sand. No further information was available on the rates of survivorship or the time taken to reach the surface.

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.

Contamination, for example from hydrocarbons may be an added impact if deposited sediment has been carried from a source of pollution such as oil drilling sites. These impacts are considered in the ‘pollution and other chemical changes’ section.

Sensitivity assessment. As the exposure to the pressure is for a single discrete event resistance is assessed as ‘High’, resilience is also ‘High’ and sensitivity is assessed as ‘Not Sensitive’. Confidence in this assessment is lower as the assessment is based on traits of the species characterizing the biotope and the relevant direct case studies present examples where impacts are not from single discrete events.

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

Limited evidence was found on responses of characterizing species to a deposition of up to 30 cm of fine material. Limited evidence was found on responses of characterizing species to a deposition of up to 30cm of fine material. A thick layer of sediment has a smothering effect and in most instances buried species will die although some polychaetes can escape up to 90cm of burial In response to nourishment (Speybroek et al., 2007, references therein). Peterson  et al. (2000) found that the dominant macrofauna  were  reduced by 86-99% 5-10 weeks after the addition of sediment that was finer than the original sediments but with a high shell content. The pressure benchmark (30 cm deposit) represents a significant burial event and the deposit may remain for some time in sheltered habitats. 

Bolam (2011) showed that Streblospio shrubsolii vertical migration capability was reduced by deposition of just 6 cm simulated dredged material. Tubificoides benedii showed good recovery following deposition of material.

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.

Rosenberg (1977) found recruitment of benthic species was heavily reduced in the vicinity of a dredged area, suggesting the increased turbidity was likely to be responsible. Contamination, for example from hydrocarbons may be an added impact if deposited sediment has been carried from a source of pollution such as oil drilling sites (Gray et al., 1990). These impacts are considered in the ‘pollution and other chemical changes’ section.

Powilleit et al., (2009) studied the response of the polychaete Nephtys hombergii to smothering. This species successfully migrated to the surface of 32-41 cm deposited sediment layer of till or sand/till mixture and restored contact with the overlying water.  The high escape potential could partly be explained by the heterogeneous texture of the till and sand/till mixture with ‘voids’.  While crawling upward to the new sediment surfaces burrowing velocities of up to 20 cm/day were recorded for Nephtys hombergii. Similarly, Bijkerk (1988, results cited from Essink 1999) indicated that the maximal overburden through which species could migrate was 60 cm through mud for Nephtys and 90 cm through sand. No further information was available on the rates of survivorship or the time taken to reach the surface.

Sensitivity assessment. Deposition of up to 30 cm of fine material is likely to provide different impacts for the different species characterizing the biotope. Behavioural traits suggest the characterizing species are resistant to this pressure as they are mobile within the sediment, although recruitment and survival of juveniles is likely to be impacted. Case studies show Aphelochaeta marioni are likely to be considerably reduced and Tubificoides benedii are likely to be able to exploit the increased nutrient input and rapidly colonise the deposited sediment. Opportunistic species are likely to colonise the biotope if heavy deposition of fine material occurs. The deposited sediment is likely to release large quantities of organic materials enhancing population density but with the risk that pre-impacted communities will shift to a different state (Coates et al., 2014, Coates et al., 2015).  Resistance is assessed as ‘Medium’ as recruitment and survivability of juveniles of characterizing species may be impacted. Resilience is assessed as ‘High’ and biotope sensitivity is assessed as 'Low'.

Litter

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

Evidence

No evidence was returned on the impact of litter on characterizing species for this biotope, although studies show impacts from ingestion of microplastics by subsurface deposit feeding worms (Arenicola marina) and toxicants present in cigarette butts have been shown to impact the burrowing times and cause DNA damage in ragworms Hediste diversicolor.

Litter, in the form of cigarette butts has been shown to have an impact on ragworms. Hediste diversicolor showed increased burrowing times, 30% weight loss and a  >2 fold increase in DNA damage when exposed to water with toxicants (present in cigarette butts) in quantities 60 fold lower than reported from urban run-off (Wright et al., 2015). Studies are limited on impacts of litter on infauna and this UK study suggests health of infauna populations are negatively impacted by this pressure.

Studies of sediment dwelling, sub surface deposit feeding worms, a trait shared by species abundant in this biotope, showed negative impacts from ingestion of microplastics. For instance, Arenicola marina ingests microplastics that are present within the sediment it feeds within. Wright et al. (2013) carried out a lab study that displayed presence of microplastics (5% UPVC) 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 colonisation of sediment by other species, reducing diversity in the biotopes the species occurs within. Wright et al. (2013) also present a case study based on their results, that in the intertidal regions of the Wadden Sea, where Arenicola marina is an important ecosystem engineer, Arenicola marina could ingest 33 m² of microplastics a year.

This pressure is 'Not assessed' as no pressure benchmark has been defined.

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 on effects of electric and magnetic fields on the characterizing species.

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

Field measurements of electric fields at North Hoyle wind farm, North Wales recorded 110µ V/m (Gill et al., 2009). Modelled results of magnetic fields from typical subsea electrical cables, such as those used in the renewable energy industry produced magnetic fields of between 7.85 and 20 µT (Gill et al., 2009; Normandeau et al., 2011). Electric and magnetic fields smaller than those recorded by in field measurements or modelled results were shown to create increased movement in thornback ray Raja clavata and attraction to the source in catshark Scyliorhinus canicular (Gill et al., 2009).

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

Sensitivity assessment. ‘No evidence’ was available to complete a sensitivity assessment, however, responses by flatfish and elasmobranchs suggest changes in predator behaviour are possible. There is currently no evidence but effects may occur on predator prey dynamics as further marine renewable energy devices are deployed, these are likely to be over small spatial scales and not impact the biotope. 

Underwater noise changes

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

Evidence

Species within the biotope can probably detect vibrations caused by noise and in response may retreat in to the sediment for protection. However, at the benchmark level the community is unlikely to be sensitive to noise and this pressure is considered 'Not relevant'.

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 find prey so limited direct impact is expected.  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 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.

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. More general changes to the productivity of the biotope may, however, occur. Beneath shading structures there may be changes in microphytobenthos abundance. Littoral mud and sand support microphytobenthos on the sediment surface and within the sediment. The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow within the upper several millimetres of illuminated sediments, typically appearing only as a subtle brownish or greenish shading. 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 higher trophic levels. The impact of these indirect effects is difficult to quantify.

Sensitivity assessment. Based on the direct impact, 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

Barriers that reduce the degree of tidal excursion may alter larval supply to suitable habitats from source populations. Barriers may also act as stepping stones for larval supply over greater distances (Adams et al., 2014). Conversely, the presence of barriers at brackish waters may enhance local population supply by preventing the loss of larvae from enclosed habitats to environments, which are unfavourable, reducing settlement outside of the population. If a barrier (such as a tidal barrier) incorporated renewable energy devices such as tidal energy turbines, these devices may affect hydrodynamics and so migration pathways for larvae into and out of the biotope (Adams et al., 2014). Evidence on this pressure is limited.

Sensitivity assessment. Resistance to this pressure is assessed as 'High' and resilience as 'High' by default. This biotope is therefore considered to be 'Not sensitive'.

Death or injury by collision

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

Evidence

‘Not relevant’ to seabed habitats.  NB. Collision by interaction with bottom towed fishing gears and moorings are addressed under ‘surface abrasion’.

Visual disturbance

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

Evidence

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

Genetic modification & translocation of indigenous species

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

Evidence

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

Introduction or spread of invasive non-indigenous species

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

Evidence

Intertidal sediments may be colonized by a number of invasive non-indigenous species. Invasive species that alter the character of the biotope or that predate on characterizing species are most likely to result in significant impacts. Intertidal flats may be colonized by the invasive non-indigenous species Crepidula fornicata and the pacific oyster Crassostrea gigas. The two species have not only attained considerable biomasses from Scandinavian to Mediterranean countries but have also generated ecological consequences such as alterations of benthic habitats and communities and food chain changes (OSPAR, 2009b).

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

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

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

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

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

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

Although shorelines comprised of mainly mud were suggested to be unsuitable for spat settlement (Spencer et al., 1994), the presence of smaller hard substrata, such as shells or pebbles, can enable larvae to settle (Tillin et al., 2020). For example, in the River Teign estuary, Pacific oyster settlement was observed on shell-covered ground mainly attached to mussel shells, and occasionally attached to cockles, stones and common periwinkle (Littorina littorea) shells on a mud flat in the estuarine intertidal zone otherwise mainly comprised of sand and mud (Spencer et al., 1994). In addition, the Blue Lagoon on the north shore of Poole Harbour had the highest abundance of oysters on mud mixed with shingle and shell (Mckinstry & Jensen, 2013). Outside of the Blue Lagoon, oysters were also recorded on mixed substrata composed of mud, gravel, and shell (McKinstry & Jensen, 2013). In the Wadden Sea, the distribution of Magallana gigas on soft sediment shores can overlap with native bivalve species such as Cerastoderma edule, Macoma balthica and Scrobicularia plana (Troost, 2010; Herbert et al., 2012, 2016). However, these native species are likely to occur at higher shore elevations compared to the lower shore habitats preferred by the Pacific oyster (Troost, 2010; Herbert et al., 2012, 2016). For example, in the Wadden Sea greater densities of Cerastoderma edule and Macoma balthica were found above the level of Magallana gigas reef development (Herbert et al., 2012). Tillin et al. (2020) concluded that while successful invasions occurred on mudflats, Magallana gigas prefers mixed substrata. Fine mud sediments without hard substrata (such as small stones, gravel, and shell) are unlikely to be suitable (Tillin et al., 2020). The speed of Magallana gigas reef formation on soft substrata seems to be dependent on the amount of hard substrata present, developing quicker once there is a sufficient amount (Troost, 2010). Bergstrom et al. (2021) reported that the presence of Magallana gigas was partially dependent on increasing gravel content up to 15% but remained stable with increasing percentages (measured up to 80%).

The oyster reefs, in the Wadden Sea and Brittany, on littoral muddy and sandy habitats formed predominantly at lower tidal levels from Mean Low Water levels to the shallow subtidal (Troost, 2010; Herbert et al., 2012, 2016). Pacific oyster spatfall was recorded in the estuarine intertidal zone on areas with hard substrata of stone and shell, particularly between the low water of spring tides and high water of neap tides, such as in the Menai Strait (Spencer et al., 1994).

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

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

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

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

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

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40) although it prefers ca 30 (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020).

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020, Fitzgerald, 2007, Blanchard, 2009, Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015, Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets, that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy species between or within shells, resulting in a homogeneous Crepidula dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m².

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330-1300 ind./m²) but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula with only moderate or low abundances (11 ind./m²) and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension–feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm) but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action which may be less suitable (Tillin et al., 2020).

The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula function as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). However, Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas of homogenous fine sediment and areas dominated by boulders. Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. Blanchard (2009) noted that sandy areas in the Bay of Saint-Mont Michel were not colonized by Crepidula because of surface sand mobility. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at the Swansea east site (Powell-Jennings & Calloway, 2018). Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and possibly burial due to deposition, which could mitigate Crepidula density. In addition, in the MHW, the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell, or gravel or both but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). However, gravel formed the base of most stacks of Crepidula in the intertidal, which suggested that initial colonization occurred on available hard substrata (i.e., gravel) in the absence of adult shells of Crepidula (Bohn et al., 2015).

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

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

The Manila clam (Tapes philippinarium), which was introduced to Poole Harbour for aquaculture in 1998, has become a naturalised population on the intertidal mudflats (occurring at densities of 60 clams/m2 in some locations within the harbour (Jensen et al. 2004, cited in Caldow et al. 2007). Densities of Cerastoderma edule and Abra tenuis increased following the introduction of the Manila clam but the abundance of Macoma balthica declined (Caldow et al., 2005), although the decline of these species may have been caused by tri-butyl tin pollution (Langston et al., 2003) and may have facilitated the naturalization of the Manila clam. The predatory veined whelk (Rapana venosa) and Hemigrapsus takinei are not established in the UK (although Hemigrapsus takinei has been recorded at two locations) could become significant predators of Cerastoderma edule and other species associated with the biotope in the future.

Sensitivity assessment. Intertidal muddy sands may be exposed to invasive species which can alter the character of the habitat (primarily Crepidula fornicata at the sublittoral fringe and Magallana gigas), leading to re-classification of this biotope. The above evidence suggests that this biotope is unsuitable for the colonization of Crepidula fornicata due to a lack of gravel, shells, or any other hard substrata used for larvae settlement (Tillin et al., 2020). Despite the very sheltered to extremely sheltered conditions of the habitat that would otherwise be suitable for Crepidula, the mobility of the sediment is unsuitable and makes it unlikely for Crepidula to become established. There may be higher densities of Crepidula in the lower shore examples of the biotope, but the densities may be lower in the mid-shore, and it may be absent from the upper shore due to the unsuitable extreme conditions in the intertidal zone preventing Crepidula post-settlement recruitment and mitigating colonization (Bohn et al. 2015). In addition, the reduced salinity levels in this biotope might prevent colonization at high densities, because the majority of evidence records of Crepidula occur in salinities from 30-35 psu (OBIS, 2023). In addition, Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and possibly burial due to deposition, which could mitigate Crepidula density. Therefore, resistance to colonization by Crepidula fornicata is assessed as 'High' and resilience as 'High' so the biotope is assessed as 'Not sensitive'. The confidence in the assessment is 'Low' because the sensitivity of this biotope to Crepidula is potentially site-specific, there is a risk of its introduction by artificial means, and the is a lack of direct evidence of Crepidula being reported to occur in the biotope.

The above evidence suggests that this biotope is unsuitable for the colonization of Magallana gigas due to a lack of gravel, shells, or any other hard substrata required for successful settlement and establishment (Kochmann, 2012; Kochmann et al., 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). However, some examples of the biotope have low percentages of hard substrata (such as boulders, cobbles and shell), which could allow the settlement and establishment of Magallana gigas.  The evidence suggests the distribution of Magallana gigas can overlap with Macoma balthica and other native bivalve species (Troost, 2010; Herbert et al., 2012, 2016). The distribution of Magallana gigas can overlap with Macoma balthica and other native bivalve species (Troost, 2010; Herbert et al., 2012, 2016). However, the mid-shore and upper-shore extent of this biotope is probably not suitable for colonization of the Pacific oyster, which is found predominantly at the Mean Low Water levels to shallow subtidal (Troost, 2010; Herbert et al., 2012, 2016). But, the oyster could colonize the lower shore extent or examples of the biotope. Therefore, resistance to colonization by Magallana gigas is assessed as 'Medium' as a 'worst-case' scenario. Resilience is assessed as 'Very low' as the Magallana gigas population would need to be removed for recovery to occur. Hence, sensitivity is assessed as 'Medium'. The confidence in the assessment is 'Low' because the sensitivity of this biotope to Magallana gigas is potentially site-specific and there is a risk of its introduction by artificial means. 

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.

Sensitivity assessment. Insufficient information was available on introduction of microbial pathogens or metazoan disease vectors to assess this pressure. Although introduced microbial pathogens or metazoan disease vectors needs to be considered an conclusion of  ‘No Evidence’ is given in the assessment.

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 re-population would occur initially relatively rapidly via adult migration and later by larval recruitment. Dittman et al. (1999) observed that Nephtys hombergii was amongst the macrofauna that colonized experimentally disturbed tidal flats within two weeks of the disturbance that caused defaunation of the sediment. However, if sediment is damaged recovery is likely to be slower, for instance Nephtys hombergii abundance was reduced by 50% in areas where tractor towed cockle harvesting was undertaken on experimental plots in Burry inlet, south Wales, and had not recovered after 86 days (Ferns et al., 2000).

Sensitivity assessment. Resistance is ‘Low’ due to direct removal of a characterizing species, that on commercial scales can remove a large proportion of the population. Resilience is assessed as ‘High’ as regions that are not regularly harvested may recover rapidly but it should be noted that continued harvesting will impact the population. Sensitivity is assessed as 'Low'. It is important to consider that the spatial extent and duration of harvesting is important to consider when assessing this pressure as smaller scale extraction may not impact the entire extent of the biotope but greater scale extraction over a long period could cause more severe impacts.

Removal of non-target species

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

Evidence

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

Collie et al. (2000) 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 (such as this biotope) 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, 2007). 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 biotope but greater scale extraction over a long period would cause longer term impacts. The type of fishing activity is also important to consider in relation to the type and severity of the impact.