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 temp for one month period, or 2°C for one year is likely to impact the timing of reproduction in these areas, although the combination of environmental factors appearing to influence timing (in particular spring tides) may limit the impact of changes in temperature on timing of spawning events.

In Europe, Macoma balthica occurs as far south as the Iberian Peninsula and, hence, would be expected to tolerate higher temperatures than experienced in Britain and Ireland. Oertzen (1969) recorded that Macoma balthica could tolerate temperatures up to 49°C before thermal numbing of gill cilia occurred presumably resulting in death. (Ratcliffe et al., 1981) reported that Macoma balthica from the Humber Estuary, UK, tolerated six hours of exposure to temperatures up to 37.5°C with no mortality. However, Barda et al. (2014) found that increased temperature reduced growth rates in populations in the Baltic Sea. Beukema et al. (2014) also warn that increasing water temperatures as a result of global warming are likely to shorten the growing season (typically late winter to early spring) if warmer spring and summer water temperatures are experienced.  Jansen et al. (2007) suggest that temperature increases on the Spanish coast along the Bay of Biscay over the past 40 years have caused loss of Macoma balthica populations, due to short-term but frequent exposure to >30° C in the Spanish estuaries, which induced elevated maintenance rates in Macoma balthica, and ultimately starvation. Jansen et al., (2007) predict the southern limit of the species will progressively shift north if temperatures continue to rise.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010). It is likely that the 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 temp 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 temp for a one month period, occurring in winter, or 2°C for one year are likely to impact spawning success.

The geographical distribution of Macoma balthica suggests that it is very tolerant of low temperature. The species occurs in the Gulfs of Finland and Bothnia where the sea freezes for several months of the year (Green, 1968). It must therefore tolerate much lower temperatures than it experiences in Britain and Ireland. Furthermore, Macoma balthica was apparently unaffected by the severe winter of 1962/3 which decimated populations of many other bivalve species (Crisp, 1964), and De Wilde (1975) noted that Macoma balthica (as Macoma balthica) kept at 0°C maintained a high level of feeding activity. It is unlikely, therefore, that in seas around the UK and Ireland Macoma balthica would be intolerant of decreases in temperature at the benchmark level.

Sensitivity assessment Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010). 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 judged as ‘Not Sensitive’.

Salinity increase (local)

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

Evidence

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; Hiscock ed 1996). Maximum salinity would be expected to be approximately 18-35?⁄??.  An increase of one MNCR salinity category would be to fully marine 30-40 ‰. 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 (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 the 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 ‰).

Macoma balthica is found in brackish and fully saline waters but is more common in brackish waters (Clay, 1967b) so may tolerate a state of flux. Seitz (2011) found Macoma balthica distribution across a salinity gradient between a minimum and maximum of 8.8psu to 19 psu in Chesapeake Bay was not influenced by salinity. Instead, resource availability was the principal influence on Macoma balthica at a broad scale, suggesting changes in one MNCR salinity category would have limited impact. McLusky & Allan (1976) reported that Macoma balthica failed to grow at 41 psu. It is likely that Macoma balthica would be tolerant of an increase in salinity category to fully marine but further increases to >40‰ are likely to affect growth and condition.

Sensitivity assessment. Nephtys hombergii is likely to decrease in abundance and Macoma balthica growth rates are likely to reduce at the highest salinities. Resistance is assessed as 'Low', resilience 'Medium' and sensitivity as 'Medium'. An 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; Hiscock ed 1996). Maximum salinity would be expected to be approximately 18-35?⁄??.  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).

McLusky & Allan (1976) conducted salinity survival experiments with Macoma balthica (as Macoma balthica) over a period of 150 days. Survival times declined with decreased salinity. At 12 psu specimens survived 78 days, whilst specimens at 8.5 psu survived 40 days. Some specimens of Macoma balthica survived 2.5 days at 0.8 psu, which was apparently due to the animals ability to clamp its valves shut in adverse conditions. McLusky & Allan (1976) also reported that Macoma balthica failed to grow (increase shell length) at 15 psu. Macoma balthica is found in brackish and fully saline waters (Clay, 1967b) so may tolerate a state of flux. Its distribution in combination with the experimental evidence of McLusky & Allan (1976) suggests that Macoma balthica is likely to be very tolerant to a decreased salinity over a short period. A decline in salinity in the long-term may have implications for the species viability in terms of growth, and the distribution of the species may alter as specimens at the extremes retreat to more favourable conditions. Metabolic function should, however, return quickly to normal when salinity returns to original levels. Decreased salinity may also affect the ability of Macoma balthica to tolerate contaminants such as heavy metals (see Bryant et al., 1985 & 1985a). Usually, contaminants become more toxic at low salinity (Langston, W.J. pers comm.).

Sensitivity assessment. The characterizing species within the biotope occupy between ‘variable’ and ‘fully marine’ category salinities and can tolerate greater osmotic stress for short periods, caused by decreases in salinity below 18?⁄?? (the upper limit for ‘reduced’ category salinity). Resistance to this decrease in salinity from variable to low is probably ‘Low’, so that resilience is also ‘High’ (following habitat restoration) and the biotope is assessed as having 'Low' sensitivity at the benchmark level.

Water flow (tidal current) changes (local)

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

Evidence

The biotope occurs in weak tidal streams (JNCC comparative tables) 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, 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.

Macoma balthica is likely to experience greater impact from increased water flow as the species thrives in low energy environments, such as estuaries that characterize the biotope (Tebble, 1976). Macoma balthica also shows preference for substratum that has a high proportion of fine sediment (Budd & Rayment, 2001). Increased water flow rate is likely to change the sediment characteristics in this biotope, primarily by re-suspending and preventing deposition of finer particles (Hiscock, 1983). This would result in erosion of the preferred habitat, which may cause mortality of some portion of the population of Macoma balthica. Higher current velocity (0.18 m/s ) recorded in flume experiments conducted in the Isle of Sylt (North Sea) led to juvenile Macoma balthica being washed out of the sediment (Zuhlke & Reise, 1994). Green (1968) recorded that towards the mouth of an estuary where sediments became coarser and cleaner, Macoma balthica was replaced by another tellin species, Tellina tenuis.

Sensitivity assessment. A decrease in water flow may result in accretion of fine sand, and, thereby, a change to muddy sand and mud. As the biotope occurs in association with muddy sand or fine sand a decrease in water flow is unlikely to impact the biotope ( although under an excessive deposition of mud or silt, mud communities, e.g. HedLim, or UEst.Tben may replace the biotope, but this is unlikely at the benchmark levels).

Finer sediment has a predicted threshold velocity (flow velocity at which fine grain size sediment would be picked up from the sea bed) of ~0.05 m/s (Gray and Elliott, 2009), therefore an increase of 0.1-0.2 m/s may cause a significant change in grain size of sediments. Although Resistance is ‘None’ if cases occurred where Macoma balthica were replaced by another tellin species and the biotope would be altered, Resistance is assessed as 'Low', Resilience is ‘High’. The resulting sensitivity score is ‘Low’ given the potential scenario that an increase in peak mean spring bed flow velocity of between 0.1m/s to 0.2m/s for more than 1 year may result in a characterizing species, Macoma balthica being replaced by another species.

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

Not relevant to sublittoral biotopes.

Wave exposure changes (local)

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

Evidence

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.

Macoma balthica occurs in the upper regions of the intertidal (Tebble, 1976) and is therefore likely to be tolerant of prolonged emergence. It is a bivalve and can close tightly by contraction of the adductor muscle, storing moisture inside the shell. The silty sediments in which the species lives have a high water content and are therefore resistant to desiccation. Furthermore, Macoma balthica is mobile and able to relocate in the intertidal by burrowing (Bonsdorff, 1984) or floating (Sörlin, 1988). It would be expected to react to an increase in emergence by migrating down the shore to its preferred position. There may be an energetic cost to this migration but it is not expected that mortality would result. Macoma balthica should quickly recover from the energetic cost of relocation.

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 levels 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 main characterizing species Nephtys hombergii, and Macoma balthica 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

The biotope is likely to be 'not sensitive' at the pressure benchmark that assumes compliance with all relevant environmental protection standards. Although contamination at levels greater than the pressure benchmark may adversely affect the biotope.

The 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.

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.

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.

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 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.

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

Nephtys hombergii inhabits intertidal areas where resistance to low oxygen is needed and sulphide levels can reach up to 1mM (Giere, 1992; Thierman, 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 5d (Schöttler, 1982) and do not switch from aerobic to anerobic metabolic pathways until oxygen saturation decreases <12% (Schöttler, 1982).

Streblospio shrubsolii 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., 1999). 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, associated with 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 during periods of gross pollution in 1971-1973 (Gray, 1976).

Sensitivity assessment. The characterizing species show High resistance to increased pollution and turbidity, resilience is therefore also High, and the biotope is assessed 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.

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 specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

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

Evidence

This biotope is only found in sediment, in particular sandy mud and the burrowing organisms, Nephtys hombergii and  Macoma balthica, 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

At the pressure benchmark a change in one Folk class refers to a change in classification to adjacent categories in the modified Folk triangle (Long, 2006). For this biotope three adjacent categories are relevant, these include a change from sandy mud to i) muddy sand, mud or ii) gravelly mud or a change from sand to iii) gravelly sand (Folk 1954; cited in Long 2006).

High densities of Nephtys hombergii were found in substrata of 0.3% particles >0.25mm and 5.8% <0.125 mm in diameter but the worm tolerated up to 3.8% 0.25 mm and 2.2-15.9% <0.125 mm (Clark et al.,1962). Nephtys hombergii may be found in higher densities in muddy environments and this tends to isolate it from Nephtys cirrosa, which is characteristic of cleaner, fairly coarse sand. 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) summarise 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.

Sensitivity assessment. Characterizing species are resistant to increases in mud content although changes in lower salinity regions may lead to changes to the biotope . Increase in gravel content may have a greater impact on the biotope and lead to replacement of Nephtys hombergii by Nephtys cirrosa, which is characteristic of cleaner, fairly coarse sand. Resistance is assessed as 'Low', resilience as Very low (the pressure is a permanent change) and sensitivity is assessed as High.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

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 30cm. The process of extraction is considered to remove all biological components of the biotope group.  If extraction occurred across the entire biotope, loss of the biotope would occur. Recovery would require substratum to soft wet mud with a fine sand fraction.

The characterizing species Nephtys hombergii and Macoma balthica burrow into the sediment, to depths not exceeding 30cm. The process of extraction is considered to remove all biological components of the biotope group.  If extraction occurred across the entire biotope, loss of the biotope would occur. Recovery would require substratum to return to fine sand and muddy sand sediments with scattered pebbles, boulders and cobbles. Recovery of benthic infauna communities from an impact such as extraction of substratum (from activities such as use of bottom towed fishing gears, aggregate dredging or storm impacts) is predicted to follow succession from initial colonization community of opportunistic species that reproduce rapidly, have small body sizes, short lifespans and early reproductive ages, through to a transitional community and finally an equilibrium community of slower growing, longer lived, larger species (Newell et al., 1998). 

Arenicola marina and Macoma balthica are more likely to occur in the late transitional and the equilibrium communities that rely on more stable sediments that have recovered from disturbance (Newell et al., 1998). Therefore, even if hydrological conditions allow for re-establishment of fine sand and muddy sand, recovery times to an equilibrium community, from an impact such as dredging are predicted to be between 2-3 years minimum and often 5-10 years (Newell et al., 1998).

Hiddink (2003) showed that the density of Macoma balthica was reduced in areas in the Wadden sea (Netherlands) that had experienced suction dredging for cockles, which removes the surface sediment, The disturbance to the sediment also appeared to leave the habitat less suitable for settlement of young Macoma balthica (Hiddink, 2003).

Smaller scale extraction of patches of substratum through activities such as bait digging may have impacts over finer spatial scales within the biotope. If the impact is not spread over a larger area the effects are likely to occur within the dug area. McLusky et al. (1983) found that Arenicola marina rapidly recolonize basins created by bait digging but populations were reduced in the dug mounds. Macoma balthica populations were unaffected suggesting the biotope would recover from this impact if it occurred over a limited spatial scale.  

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very Low’ based on the loss of suitable substratum to support the community of the characterizing species. Sensitivity has been assessed as ‘High’. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure. 

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.

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

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

Aphelochaeta marioni, Streblospio shrubsolii and Tubificoides benedii rapidly colonise disturbed sediments, which will reduce recovery times. This was displayed by Biasi & Pacciardi (2008) in the Adriatic Sea, where polychaetes, including Aphelochaeta spp. were among the species dominating the areas disturbed by fishing activity (otter trawling).

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.

Scouring around wind farm and other renewable energy device bases has been shown to reveal coarser sediment close to bases and deposition of fine material in the direction of the dominant current.

Species poor communities were revealed in coarser material and  higher densities of macrobenthic organisms occurred where finer sediment was deposited (further from the base)  (Coates et al., 2014). This process created a shift in macrobenthic communities around the wind farm tower (influenced by the  direction fine material had settled) (Coates et al., 2014).

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 biotope will be impacted if damage to seabed surface features is widespread, as, although, motile and opportunistic fauna, such as Streblospio shrubsolii and Tubificoides benedii may recover quickly Nephtys hombergii  shows a greater negative impact (Collie et al., 2000, Ferns et al., 2000, Kaiser et al., 2001). Resistance is assessed as ‘Low’, Resilience is assessed as ‘Medium’, providing a sensitivity assessment of ‘Medium’.

It is important to consider the extent and duration of the cause of abrasion. For instance, long-term change in distribution of sediment grain sizes may occur following continual trawling effort or around the base of a renewable energy device, this would alter the biotope within the footprint of the activity and sensitivity would therefore be greater.

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

Penetration and or disturbance of the substratum would result in similar, if not identical results as ‘abrasion’ or ‘removal’ of this biotope.  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. Consequently the pressure has been given the same sensitivity assessment as removal.

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 ‘Low’, and sensitivity is assessed as ‘High’.

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.

Changes in light penetration or attenuation associated with this pressure are not relevant to Nephtys hombergii and Macoma balthica biotopes. 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 characterizing species.

Increases in turbidity may reduce benthic diatom productivity and productivity of phytoplankton in the water column. Increased clarity, however, may increase primary production. In cases of increased turbidity impacts may be small for Nephtys hombergii as the species feeds on a range of prey in the sediment and reductions in phytoplankton may be mitigated but may liit prey resources.    

An increase in suspended solids (inorganic or organic) may also increase food availability of deposit feeders if sediment containing meiofauna, bacteria or organic particles is transported in the water column. However, higher energetic expenditure to unclog the feeding apparatus may occur, which may alter habitat suitability.

An increase in food availability through either increased phytoplankton abundance (under increased water clarity) or increased food resources suspended in the water column (under increased turbidity) may enhance growth and reproduction of both suspension and deposit feeding species.

Sensitivity assessmentResistance is ‘High’ as no significant negative effects are identified and potential benefits from increased food resources may occur, based on expert judgement, utilising evidence of species traits and distribution.  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 15 cm 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).

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. Evidence is therefore assessed for evidence of deposits of fine material from sources such as dredge waste spoil.

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.

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.

The characterizing species Nephtys hombergii  is classified within AMBI ‘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  is ‘Group V’ which tolerate high enrichment and polluted conditions (Aphelochaeta marioni was not assessed) (Borja et al., 2000).

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 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). Recovery to pre-impact communities, given the pressure occurs as a single discrete event is likely to require succession through transitional communities before an equilibrium community is reached, taking up to 10 years (Newell et al., 1998).

Resistance is assessed as ‘Medium’ as recruitment and survivability of juveniles of characterizing species may be impacted. Resilience is assessed as ‘Medium’. The final sensitivity is therefore, ‘Medium’.

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 micro plastics by sub surface 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 micro plastics. For instance, Arenicola marina ingests micro-plastics that are present within the sediment it feeds within. Wright et al. (2013) carried out a lab study that displayed presence of micro-plastics (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 33m² of micro-plastics a year.

Sensitivtiy assessment. ‘No evidence’ was returned to complete a sensitivity assessment, however, both microplastics and the toxicants present in cigarette butts are likely to have negative impacts on the characterizing species.

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. 2012). Electric and magnetic fields smaller than those recorded by in field measurements or modelled results were shown to create increased movement in thornback ray Raja clavata and attraction to the source in catshark Scyliorhinus canicular (Gill et al. 2009).

Flatfish 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 into 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

There is no direct evidence of effects of changes in incident light on the characterizing species of this biotope. 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.  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 muddy sands 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.

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 trans-located. 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 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, or food chain changes (OSPAR, 2009). Colonization of the upper shore by cord grass, Spartina anglica, may result in loss of characterizing biotopes. This species is not strictly an invasive species as it is a fertile hybrid of native and non-native species.

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 the Wadden Sea, the Pacific oyster Magallana gigas has colonized intertidal flats (Smaal et al., 2005). This species may consume larvae including the pelagic larvae of Cerastoderma edule and Macoma balthica and other species reducing recruitment (Smaal et al., 2005). Troost et al. (2009) investigated whether Magallana gigas had a competitive feeding advantage over Cerastoderma edule and concluded that differences in feeding current characteristics were small and not considered to lead to significant differences in feeding efficiency.  The most severe effects are likely to occur from impacts on sediment, where Magallana gigas create reefs on sedimentary flats that will prevent recruitment of juveniles and will restrict access of infauna to the sediment-water interface impacting respiration and feeding.

The Manila clam (Tapes philippinarium), which was introduced to Poole Harbour for aquaculture in 1998, has become a naturalised population on the intertidal mudflats (occurring at densities of 60 clams/m² in some locations within the harbour (Jensen et al. 2007, cited in Caldow et al. 2007).  Densities of Cerastoderma edule and Abra tenuis increased since the introduction of the Manila clam although the abundance of Scrobicularia plana and 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 burrowing lifestyle of Eteone longa and other infaunal polychaetes may confer some protection from changes to the sediment surface and may provide some new habitat (as this species has been found among oyster banks).

The predatory veined whelk (Rapana venosa) and Hemigrapsus takinei are not established in the UK (although Hemigrapsus takinei has been recorded at two locations) and these species could become significant predators of bivalves and other invertebrates associated with the biotope in the future.

Sensitivity assessment. Intertidal mixed sediments 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 could be 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). However, in areas of the biotope that contain mixed sediment, there may be suitable hard substrata for the colonization of Crepidula. This is a sheltered to extremely sheltered habitat which may be suitable for Crepidula, especially where water movement is mediated by tidal flow rather than wave action. 

Therefore, resistance is assessed as 'Medium' in examples of the biotope that include mixed sediment. Resilience is assessed as 'Very low', as it would require the removal of Crepidula, probably by artificial means. Hence, sensitivity is assessed as 'Medium' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope so the confidence in the assessment is 'Low' and further evidence is required. 

Introduction of microbial pathogens

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

Evidence

No evidence for microbial pathogens on the characterizing polychaetes was found. Macoma balthica in Delaware Bay, north-east USA, was found to host Perkinsus genus pathogens Perkinsus andrewsi and Perkinsus marinus (Lindsay et al., 2007). Cerastoderma edule has been reported to host approximately 50 viruses, bacteria and fungi, and including turbellaria, digeneans and cestodes (Longshaw & Malham, 2012).

Sensitivity assessment. Based on the evidence for the Macoma balthica and Cerastoderma edule, it is likely that parasitic infection may indirectly alter the species composition of the biotope. Although less evidence was retuned for the characterizing mollusc species Macoma balthica, similar impacts from introduction of pathogens are likely to be experienced for this species. Resistance is assessed as 'Medium', Resilience as 'High' and Sensitivity as 'Low'.

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).

Hiddink (2003) found that the density of Macoma balthica was reduced in areas in the Wadden sea (Netherlands) that had experienced suction dredging for cockles, which removes the surface sediment, The disturbance to the sediment also appeared to leave the habitat less suitable for settlement of young Macoma balthica (Hiddink, 2003). This study provides evidence of loss of a characterizing species from the biotope and that recovery is unlikely to occur until the sediment characteristics have returned to pre-impact conditions. Removal of target species such as cockles Cerastoderma edule or bait digging for Arenicola marina is likely to impact the biotope. The extent of the impact will depend on the fishing / removal method and spatial extent.

Sensitivity assessment. Resistance is ‘Low’ due to direct removal of a characterising species, that on commercial scales can remove a large proportion of the population (for instance, Neves de Carvalho et al., 2013) calculated that bait digging activities in the Douro estuary, Portugal may remove up to 9.9 tonnes of Hediste diversicolor). Resilience is assessed as ‘Medium’ as regions that are not regularly harvested may recover rapidly but continued harvesting will impact the population. Sensitivity is assessed as ‘Medium’. The spatial extent and duration of harvesting is important to consider when assessing this pressure as smaller scale extraction may not impact the entire extent of the biotope but greater scale extraction over a long period would cause longer term imapcts.

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

Incidental removal of the characterizing species would alter the character of the biotope and the delivery of ecosystem services such as secondary production and bioturbation. Populations of characterizing species, such as Nephtys hombergii and Macoma balthica provide food for macroinvertebrates fish and birds so their removal could lead to wider ecological effects.

Sensitivity assessment. Removal of the characterizing species would alter the character of the biotope. Resistance is therefore assessed as ‘Low’ and resilience as ‘High’, so sensitivity is categorized as ‘Low’.