The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

Nephtys hombergii is recorded in the northern Atlantic, from the Barents Sea, the Baltic and the North Sea, to the Mediterranean. It has been reported as far south as South Africa, suggesting the species can tolerate temperatures above, even a 5°C increase, in the UK and Irish coasts. Records are limited, but Emery et al. (1957) found that Nephtys hombergii could withstand summer temperatures of 30 to 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, 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 the timing of spawning events.

Streblospio shrubsolii have been shown to reproduce in a temperature range of 7.5°C to 30°C, with highest reproduction levels occurring between 16°C and 21°C (Levin & Creed, 1986; Dafonsecagenevois & Cazaux, 1987; Chu & Levin, 1989; Lardicci et al., 1997). The evidence was based on Mediterranean sites, limiting confidence for the 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 temp for one month, or 2°C for one year, are within the temperature range reproduction occurs within (7.5°C to 30°C) and within the temperature range where the highest reproduction levels occur (16°C to 21°C), suggesting limited impact from the pressure at the benchmark level.  

In Europe, Macoma balthica occurs as far south as the Iberian Peninsula. Hence, it could 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. Wilson (1981) showed that the lethal temperatures for Macoma balthica change between seasons. Critical temperatures were studied for a Macoma balthica population in Dublin Bay, and a summer maximum of 37.5°C and a winter maximum of 27.5°C were reported (Wilson, 1981).  Temperature tolerances were also reported to change with height up the shore, which suggested that the species adapted to variable conditions.

Field observations and laboratory experiments showed that Macoma balthica spawns (criterion: 50% spent) in spring when the gradual increase of the mean (monthly averaged) water temperature surpasses 8.3°C. The success of spawning and recruitment is affected by the timing of the spring phytoplankton bloom and avoidance of the main settlement of the predator Crangon crangon on intertidal shores (Philippart et al., 2003). A mismatch in spawning cues due to an acute increase in temperature could result in low recruitment or recruitment failures.

On the other hand, some evidence suggested that Macoma balthica might be sensitive and have a low tolerance to temperature increase (Beukema et al., 2009; Garcia et al., 2016). Despite apparent adaptation to regional temperature ranges, increased temperature reduced growth rates and weight gain in populations of Macoma balthica in the Baltic Sea (Beukema et al., 2009; Barda et al., 2014; Beukema et al., 2017a; Beukema & Dekker, 2020). 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.

In the Baltic Sea, Macoma balthica annual mortality rates were significantly higher at high temperatures in summer and winter (Beukema et al., 2009; Beukema et al., 2017b&c; Beukema & Dekker, 2020). Jansen et al. (2007) suggest that temperature increases in the Spanish coast along the Bay of Biscay over the past 40 years caused loss of Macoma balthica populations, due to short-term but frequent exposure to over 30°C in the Spanish estuaries, which induced elevated maintenance rates in Macoma balthica, and ultimately starvation.

Repeated recruitment failure also occurred after mild winters in a comparable North Sea location, probably due to enhanced survival of predators, particularly shrimp and shore crabs feeding on post-larvae Macoma balthica (Beukema, 1992; Beukema et al., 2001; Schueckel & Kroencke, 2013; Beukema et al., 2017b). In the Baltic Sea, recruitment was more successful after cold winters than warm (Beukema et al., 2009; Beukema et al., 2017c; Beukema & Dekker, 2020). As a result, Jansen et al. (2007) predicted that the southern limit of the species would progressively shift north if temperatures continued to rise. In long-term monitoring (44 years) of Macoma balthica populations in the Wadden Sea, Beukema et al. (2017b;c) showed that warming alone could not explain population decline because, although recruitment and adult survival declined in warm winters, a sharp population decline in the mid-1990s and rapid spread of mortality between sites were consistent with a disease-like outbreak.

Ehrnsten et al. (2019a) found that increasing bottom-water temperatures reduced Macoma balthica (reported as Limecola balthica) biomass by increasing metabolic costs and increasing degradation of the food available, based on two models. Therefore, warming weakened the otherwise positive biomass response to increased organic matter sedimentation.

Staniek et al. (2025) found that 15-day exposure to short-term heatwave conditions, +2.8° C above ambient temperatures (22.2° C) in a mild heatwave treatment and +4.4° C above ambient temperatures in a strong heatwave treatment, did not affect the survival (97 to 99%) or size of Macoma balthica, and an increase in total biomass was observed. However, the strong heatwave treatment did cause a significant decrease in the chlorophyll and respiration rates in Macoma balthica, suggesting a potential long-term negative effect of heatwaves.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4 to 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’. However,  Macoma balthica may retreat north as a result of long-term warming due to climate change. 

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year (Temperature change pressure definition).

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, 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 one month, occurring in winter, or 2°C for one year are likely to impact spawning success.

Streblospio shrubsolii have been shown to reproduce in a temperature range of 7.5°C to 30°C, with highest reproduction levels occurring between 16°C and 21°C (Levin & Creed, 1986; Dafonsecagenevois & Cazaux, 1987; Chu & Levin, 1989; Lardicci et al., 1997). The evidence was based on Mediterranean sites, limiting confidence for the 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 temp for one month, or 2°C for one year, are within the temperature range reproduction occurs within (7.5°C to 30°C) and within the temperature range where the highest reproduction levels occur (16°C to 21°C), suggesting limited impact from the pressure at the benchmark level.  

Colder winter temperatures have been shown to benefit Macoma balthica population dynamics. Recruitment success increased following colder winters, and repeated recruitment failure occurred after mild winters in a comparable North Sea location (Beukema, 1992; Schueckel & Kroencke, 2013; Beukema et al., 2001). In Friedrichskoog, Germany, König (1943) found a high accumulation of dead Cerastoderma edule biomass after a severe winter 1936/1937, but high numbers of Macoma balthica (80,000 individuals/m²) spat in the following year (winter 1939). Winter water surface temperatures in the Wadden Sea (Netherlands) have increased 1.5°C since the 1980s (Oost et al., 2009). During milder winters, Macoma balthica showed increased loss of body weight and produced fewer and smaller eggs (van der Meer et al., 2003). However, reduced recruitment success during milder winters may also be due to increased predation as juvenile Crangon crangon have shown increased abundance in relation to milder winters (Beukema, 1992; Beukema et al., 2001; Beukema & Dekker, 2005; Schueckel & Kroencke, 2013; Beukema et al., 2017b). In the Baltic Sea, recruitment was more successful after cold winters than warm (Beukema et al., 2009; Beukema et al., 2017c; Beukema & Dekker, 2020).

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

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4 to 19°C (Huthnance, 2010). The important characteristic species show limited impacts from decreases in temperature. 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 for by later spawning events. Resistance and resilience are therefore assessed as ‘High’ and the biotope judged as ‘Not Sensitive’ at the benchmark level.

Salinity increase (local)

Benchmark. An increase in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition).

Evidence

This biotope occurs on the mid and lower shore of sheltered estuaries exposed to variable (18 to 35 ppt) salinity (JNCC, 2015). An increase of one MNCR salinity category, at the pressure benchmark, would be to fully marine 30 to 40 ppt. Short-term 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 a longer-term or permanent increase in salinity, sediment waters would also be expected to adjust.

Ruso et al. (2007) examined the effects of brine discharge on soft-bottom communities and found that at sites close to the discharge where salinity exceeded 39 psu, assemblages dominated by Polychaeta, Crustacea and Mollusca were replaced by communities characterized by nematodes. Evidence suggests that increased salinity can alter assemblage structure and reduce polychaete abundance, richness and diversity up to 400 m from the discharge site, although sensitivity to brine discharge differs among polychaete families (Ruso et al., 2007; 2008; Roberts et al., 2010). Little evidence was found for tolerances of elevated salinity, although populations are likely to be acclimated to short-term increased salinities in surface sediment layers, as evaporation on hot days or wind-driven desiccation leads to increased salinities.

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 to 15 psu everywhere. Before the closure of the dam, the estuary demonstrated characteristics of a typical 'salt-wedge' estuary with a salinity gradient from 0.3 to 15 psu. Hence, Nephtys hombergii is likely to survive increases in salinity within estuarine environments.

Nephtys hombergii may still be found in fully marine locations, but may be competitively inferior to other species of Nephtyidae (e.g. Nephtys ciliata and Nephtys hystricis) and occur in lower densities. An increase to fully marine (30 to 40 ppt) would likely lead to a reduction in the density of Nephtys hombergii. In the estuarine Bay of Seine, Nephtys hombergii positively correlated with euhaline environmental conditions (more than 30) (Dauvin et al., 2017). Nepthy cirrosa and Nephtys assimilis were also abundant in the Bay of Seine, occurring in medium to fine sand and muddy assemblages, respectively. 

Nephtys fluviatilis is an estuarine oligohaline and has been recorded to prefer low salinity conditions between 4 and 15 psu (Castellano et al., 2020; Mucciolo et al., 2021). Laboratory studies found that Nephtys fluviatilis tolerated salinities up to 25 for 24 hours, and maintained its body weight from salinity 3 to 15, but despite this, mortalities were observed at salinities 0 to 3 (freshwater) and 35 (full salinity) after 24 hours of exposure (Mucciolo et al., 2021).

Macoma balthica is found in brackish and fully saline waters but is more common in brackish waters (Clay, 1967b), so it may tolerate variation in salinity. A muddy assemblage dominated by Macoma balthica occurs in the polyhaline Lateral Bank and the polyhaline Great Mudflat of the Seine estuary (Lécuyer et al., 2024). Seitz (2011) found that the distribution of Macoma balthica across a salinity gradient between a minimum and maximum of 8.8 psu to 19 psu in Chesapeake Bay was not influenced by salinity. Instead, resource availability was the principal influence on Macoma balthica. McLusky & Allan (1976) reported that Macoma balthica failed to grow at 41 psu. Macoma balthica would probably be tolerant of an increase in salinity category to fully marine, but further increases to >40 ppt are likely to affect growth and condition. 

Gebruk et al. (2023) identified polychaete Macoma balthica and Nephtys longoestosa as dominant species at the marine end of a salinity gradient in Pechora Bay, where salinity increased from estuarine to near euhaline conditions (from around 2.6 to 26.3 psu surface salinity and around 10.7 to 29.4 psu near bottom salinity). In tidal channels in Jade Bay, Wadden Sea Macoma balthica and Nephtys hombergii were recorded as characteristic species in subtidal communities exposed to a constant salinity of 29 to 30, but Macoma balthica was also found in communities near the tidal gates, which could experience a low salinity of 18 (Schückel et al., 2015).

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, they would likely be resistant to an increase to the fully marine category (30 to 40 ppt).

Conde et al. (2013) reported that Streblospio shrubsolii was a dominant species in low salinity, estuarine conditions (5 to 9‰) in the Tagus estuary, Portugal. In Ria de Averio, western Portugal, Streblospio shrubsolii and Tubificoides benedii were characteristic 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). Avramidi et al. (2022) reported high abundances of Streblospio shrubsolii at both poorly flushed, fully marine stations with little to no current and well flushed stations, in variable salinities and strong tidal currents in the Port of Rotterdam, which were exposed to industrial discharge and/or brine outfalls.

Sensitivity assessment. Macoma balthica and Streblospio shrubsolii may be able to tolerate increased salinity levels due to their distribution in a range of salinities. However, Nephtys hombergii is likely to decrease in abundance following an increase in salinity. Resistance is assessed as 'Low', Resilience is assessed as  'High' (following restoration of salinity regime), and biotope sensitivity is assessed as  'Low'. An increase in salinity may lead to the replacement of this biotope by LS.LMu.MEst.HedMac, as the associated HedMac communities occur further down estuaries towards the open coast, in more saline conditions. The infauna in LS.LMu.MEst.HedMac 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 in LS.LMu.MEst.HedMac (JNCC, 2015, 2022).

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition detail).

Evidence

This biotope occurs on the mid and lower shore of sheltered estuaries exposed to variable (18 to 35 ppt) salinity (JNCC, 2015, 2022). Maximum salinity would be expected to be approximately 18 to 35‰.  A decrease of one MNCR salinity category at the pressure benchmark would be to the ‘Low’ salinity 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 a longer-term or permanent increase in salinity, sediment waters would also be expected to adjust.

Nephtys hombergii is considered to be a brackish water species, and has been reported to occur in 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, so that the population in the biotope would decline. 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. Before 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. 

In the estuarine Bay of Seine, Nephtys hombergii positively correlated with euhaline environmental conditions (more than 30) (Dauvin et al., 2017). 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. Nepthy cirrosa and Nephtys assimilis were also abundant in the Bay of Seine, occurring in medium to fine sand and muddy assemblages, respectively. 

Nephtys fluviatilis is an estuarine oligohaline and has been recorded preferring low salinity conditions between 4 and 15 psu (Castellano et al., 2020; Mucciolo et al., 2021). Laboratory studies found that Nephtys fluviatilis tolerated salinities up to 25 for 24 hours, and maintained its body weight from salinity 3 to 15, but despite this, mortalities were observed at salinities 0 to 3 (freshwater) and 35 (full salinity) after 24 hours of exposure (Mucciolo et al., 2021).

Macoma balthica is found in brackish and fully saline waters (Clay, 1967b), so it may tolerate salinity fluctuations, including decreases in salinity. For example, it can acclimatise in extremely brackish conditions in the Baltic Sea and is able to grow in oligohaline water between 0.5 and 3.0 ppt (Jansen et al., 2009 summary only; Pourmozaffar et al., 2020). Gebruk et al. (2023) identified polychaete Macoma balthica and Nephtys longoestosa as dominant species at the marine end of a salinity gradient in Pechora Bay, where salinity increased from estuarine to near euhaline conditions (from around 2.6 to 26.3 psu surface salinity and around 10.7 to 29.4 psu near bottom salinity). In tidal channels in Jade Bay, Wadden Sea Macoma balthica and Nephtys hombergii were recorded as characteristic species in subtidal communities exposed to a constant salinity of 29 to 30, but Macoma balthica was also found in communities near the tidal gates, which can experience a low salinity of 18 (Schückel et al., 2015).

McLusky & Allan (1976) conducted salinity survival experiments with 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 animal’s 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’s distribution in combination with the experimental evidence of McLusky & Allan (1976) suggests that Macoma balthica is likely to be resistant to 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, quickly return 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.).

Conde et al. (2013) found that Streblospio shrubsolii was a dominant species in low salinity, estuarine conditions (5 to 9‰) in the Tagus estuary, Portugal. In Ria de Averio, western Portugal, Streblospio shrubsolii and Tubificoides benedii were characteristic 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). Avramidi et al. (2022) reported high abundances of Streblospio shrubsolii at both poorly flushed, fully marine stations with little to no current and well flushed stations, in variable salinities and strong tidal currents in the Port of Rotterdam, which were exposed to industrial discharge and/or brine outfalls.

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‰. Resistance to this decrease in salinity from variable (18 to 35‰) to low (<18 ppt) is likely to lead to some species replacement by polychaetes or oligochaetes more tolerant of low salinity. Nephtys hombergii and oligochaetes are likely to remain, but Macoma balthica is likely to decrease in low salinity conditions. A similar biotope could remain where salinities were close to 18 ppt, but a severe reduction in salinity would probably lead to loss of the biotope. Resistance is therefore assessed as ‘Low’. Resilience (following restoration of typical conditions)  is ‘High’, and sensitivity is assessed as ‘Low’. It should be noted that resistance would be lower, and sensitivity greater, where salinity was reduced to a level close to freshwater. A reduction in salinity could lead to a change in biotope to LS.LMu.UEst.Hed.Ol, which tends to occur in more reduced salinities, further towards the head of estuaries. The polychaete assemblage is poorer, and molluscs are virtually absent. It is the presence of Macoma balthica and Hydrobia ulvae that primarily distinguishes NhomLimStr from Hed.Ol (Connor et al. 2014).

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s and 0.2 m/s for more than one year (Water flow pressure definition). 

Evidence

The hydrographic regime is an important structuring factor in sedimentary habitats. An increase in water flow rate is not likely to affect Nephtys hombergii, Streblospio shrubsolii 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.

Foulquier et al. (2020) reported that Nephtys cirrosa remained a characteristic species despite disturbance in the high-energy and naturally stressed Adour estuary coastal zone, off the French Basque coast. The site is exposed to strong hydrodynamic conditions, with water flow from estuarine discharge ranging from 120 m³/s to 400 m³/s, exceeding 1,000 m³/s during floods, and significant wave heights ranging from 0.2 m to 5.2 m. In Jade Bay, German Wadden Sea Nephtys homergii is associated with high tidal current velocity (average current velocity 0.27 to 0.39 m/s and maximum current velocity 1.34 m/s) (Schückel et al., 2015).

In the Port of Rotterdam, high abundances of Streblospio shrubsolii were recorded at stations exposed to little to no tidal current and at stations exposed to strong tidal currents (Avramidi et al., 2022).

Macoma balthica is likely to experience greater impact from increased water flow as the species thrives in low-energy environments, such as the extremely sheltered areas that characterize the biotope (Tebble, 1976). Increased water flow rate is likely to result in erosion of the preferred habitat, which may cause mortality of some portion of the population of Macoma balthica. Higher current velocity (18 cm/s or 0.18 m/s) recorded in flume experiments conducted in the Isle of Sylt (North Sea) caused juvenile Macoma balthica to be washed out of the sediment (Zuhlke & Reise, 1994). Green (1968) reported that, towards the mouth of an estuary where sediments became coarser and cleaner, Macoma balthica was replaced by another tellin species, Tellina tenuis. However, field studies have shown that Macoma balthica can be more abundant in macrofauna communities adapted to unstable sediments (Schückel et al., 2015). In Jade Bay, Wadden Sea, Macoma balthica had a higher mean abundance in a community associated with higher current velocity (mean 0.39 m/s and max 1.34 m/s) than in a community associated with lower tidal current velocity (mean 0.09 m/s and max 0.96 m/s) (Schückel et al., 2015). It is important to note that the mean abundances per site were low in both communities (4 and 2 individuals per site, respectively) (Schückel et al., 2015).

Increased water flow rates are likely to change the sediment characteristics in which the species live, primarily by resuspending and preventing deposition of finer particles (Hiscock, 1983). The characteristic species prefer habitats with silty/muddy substrata, which would not occur in very strong tidal streams. Coarser sediments are likely to remain in areas of strongest flow velocity (where finer particles have been resuspended) (Coates et al., 2014). Species such as Tubificoides benedii and other opportunist polychaetes that tolerate coarser particle size will possibly increase in abundance. In addition, 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 characteristic species, and therefore growth and reproduction may be enhanced, but only if food was previously limiting.

Sensitivity assessment. A decrease in water flow may reduce the sand component of the sediment but otherwise not affect this muddy biotope. Finer sediment has a predicted threshold velocity (flow velocity at which fine-grained sediment would be picked up from the seabed) of around 0.05 m/s (Gray & Elliott, 2009). Therefore, an increase of 0.1 to 0.2 m/s may cause a significant change in the grain size of sediments, resulting in a change from sand mud to muddy sand or fine sand-dominated biotopes. Resistance is assessed as 'Low', and resilience is assessed as ‘High’. The resulting sensitivity score is ‘Low’, given the potential scenario that an increase in peak mean spring bed flow velocity of between 0.1 m/s and 0.2 m/s for more than one year may result in a change in sediment composition and resultant reclassifcation of the biotope. 

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. (Emergence regime change pressure definition).

Evidence

The biotope and characterizing species occur 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, are 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.     

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. Philippe et al. (2016) reported that Nephtys hombergii was most abundant at an emersion time of around four hours.     

Opportunistic, deposit-feeding polychaetes, such as Streblospio shrubsolii 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). For instance, Tubificoides benedii is capable of penetrating the substratum to depths of 10 cm, shows resistance to hypoxia and is often typified as an ‘opportunist’ that is adapted to the rapid environmental fluctuations and harsh conditions in estuaries (Gogina et al., 2010). Highest abundances were predicted by Gogina et al. (2010) to be related to depth, with an optimum of 10 m  to 20 m. An increase in the time the biotope is covered by the sea is likely to result in increased abundance of Tubificoides benedii.

Macoma balthica occurs in the upper regions of the intertidal (Tebble, 1976) and is, therefore, likely to be tolerant of prolonged emergence. Philippe et al. (2016) found that Macoma balthica biomass and abundance increased with increasing emersion time, with the highest biomass and abundance found in sites experiencing the longest emersion time. 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. Macoma balthica also occurs in the intertidal and sublittoral zones down to depths of 190 m (Olafsson, 1986), although it is more abundant intertidally.  Hence, it would be expected to resist a decrease in the emergence regime. However, a case study, predicting changes in biomass of Macoma balthica in the Humber estuary, UK (western North Sea) under expected sea level rise conditions, displayed negative impacts. Coastal squeeze from sea level rise would produce steeper and more homogenous beach face profiles. The abundance of Macoma balthica was predicted to be lower on steeper beach faces, and the biomass of Macoma balthica was predicted to decrease (Fujii & Raffaelli, 2008).  The sensitivity assessment given in relation to the benchmark pressure should, therefore, be interpreted in relation to intertidal habitat availability following the relative sea level changes.  

Sensitivity Assessment. Some changes in biotope extent may occur as a result of emergence regime changes. Resistance is therefore assessed as ‘Medium’, and resilience (following restoration of the tidal regime) is likely to be ‘High’, so the biotope is considered to have ‘Low’ sensitivity to 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 (Wave action pressure definition). 

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 et al. (1962) suggested that strong wave action limited the distribution of Nephtys hombergii. Increased wave action for long durations (e.g. one year) may ultimately change the nature of the substratum that the polychaete inhabits, and its distribution may consequently alter. 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. 

Sensitivity assessment. This biotope is recorded in sandy muds in wave sheltered to extremely wave sheltered habitats, in estuaries. An increase in nearshore significant wave height of >3% but <5% may alter the wave climate of the extremely or very sheltered examples of the biotope to wave sheltered conditions, which are typical for the biotope. Hence, a change in significant wave height at the benchmark level is unlikely to significantly affect this biotope and its community. Hence, resistance and resilience are assessed as 'High' and sensitvity as 'Not sensitive' at the benchmark level, albeit with Low confidence due to the lack of direct evidence. 

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant Transitional metal or organometal (e.g. TBT) contaminants via uncontrolled releases or incidental spills (Transitional metals and organometals pressure definition). 

Evidence

The results of the Rapid Evidence Assessment on the effects of 'Transitional metal or organometal' contaminants on selected polychaete and mollusc species are summarized below. The full 'evidence reviews' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'Transitional metal or organometal' contaminant examined, together with an overall pressure assessment. 

Transitional metals. Bryan (1984) reported that short-term toxicity in polychaetes was highest to Hg, Cu, and Ag, declined with Al, Cr, Zn, and Pb, with Cd, Ni, Co, and Se being the least toxic. It was reported that polychaetes have a range of tolerances to heavy metal levels of Cu, Zn, As, and Sn, being in the order of 1500-3500 µg/g. An analysis of organisms from Restronguet Creek revealed that 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. Nephtys cirrosa is also recorded in Restronguet Creek (Bryan & Gibbs, 1993). 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-hour LC50 of 250 µg/l, whilst those from Restronguet Creek had a 96-hour LC50 of 700 µg/l (35 psu; 13°C). Bryan & Gibbs (1983) suggested that since the area had been heavily contaminated with metals for over 200 years, there had been adequate time for metal-resistant populations to develop, especially for relatively mobile species.

In addition, there is evidence that some polychaete species can adapt to metal contamination in the long term (Bryan & Hummerstone, 1971, 1973; Grant et al., 1989; Hateley et al., 1989; Mouneyrac et al., 2003; Alla et al., 2006b; Burlinson & Lawrence, 2007; McQuillan et al., 2014). McQuillan et al. (2014) suggested that some species (Nephtys hombergii) had developed metal-resistant populations as a functional genetic trait to copper homeostasis. Therefore, the resistance of Nephtys spp. to ‘transitional metals and organometals’ is assessed as ‘Low’, resilience as ‘High’ and sensitivity as ‘Low’ but with ‘Low’ confidence as it is based on a single study.  However, confidence in the assessment is 'Low' due to the limited evidence and its ability to adapt to transitional metal contamination in the long term. 

The evidence on the remaining polychaete species is limited to single observations from four studies. In all cases, only sublethal results were reported. The sensitivity of Eteone spp. and Tubificoides spp. to ‘Transitional metals and organometals’ is assessed as ‘Not sensitive’ but with ‘Low’ confidence due to the lack of evidence.

‘Severe’ or ‘significant’ mortality was reported in 52% of the results from studies in the evidence review of the effects of ‘Transitional metals and organometal’ exposure on Macoma spp. depending on the exposure concentration or duration. Copper and cadmium were reported to result in ‘Severe’ mortality, while arsenic, chromium, mercury, silver, zinc, and nickel were reported to result in ‘significant’ mortality, depending on concentration, duration, and environmental conditions. The remaining metals were reported to result in no mortality or sublethal effects. Barite (in the form of drilling mud barite) was shown to cause 100% mortality of M. balthica within 12 days at a depth of 2- and 3-mm dosage, but the cause may have been due to physical damage to their gill filaments rather than chemical toxicity (Barlow & Kingston, 2001). Overall, the evidence suggests that the worst-case resistance of Macoma spp. to ‘transitional metals’ exposure is ‘None’. Therefore, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’.

Organometals. The evidence on the effects of tributyltin was limited to two papers, one of which was not accessible (Walsh et al., 1984). Beaumont et al. (1989) introduced TBT into three microcosms at high (1-3 µg/l) concentrations and three microcosms at low (0.06-0.17 µg/l) concentrations. Non-introduced Nereis diversicolor, Arenicola marina, and Eteone sp. occurred in the low-level TBT and the control treatments, but not in the high-level TBT treatments. High mortalities of Nereis diversicolor were recorded in all microcosms, including the control, so the results were inconclusive. Overall, only sublethal effects were reported in the evidence reviewed. Therefore, the sensitivity of Nereis spp. to TBT is assessed as ‘Not sensitive’ but with ‘Low’ confidence due to the lack of evidence. No evidence of the effects of TBT on Nephtys spp. or Steblospio spp. was found. 

Beaumont et al. (1989) also examined the effects of tributyltin (TBT) exposure in Macoma balthica. However, no mortality was reported, and some juvenile Macoma recruited to the low (0.06-0.17 µg/l) TBT treatment mesocosm. Therefore, the worst-case sensitivity of Macoma spp. to TBT exposure is assessed as ‘Not sensitive’, but confidence in the assessment is ‘Low’ due to the limited number of studies reviewed.

Nanoparticulate metals. Sublethal effects and ‘no’ mortality were reported in all but one of the studies reviewed that examined the effects of nanoparticulate metals on polychaetes. The nanoparticulates did result in reduced burrowing speed or activity, where an effect was reported. While a reduction in burrowing ability may reduce feeding or increase the susceptibility to predation, no direct mortality was reported. However, Cozzari et al. (2015) reported that silver nanoparticles caused 8% and 16% mortalities in the 2.5 and 10 µg/g treatments after 4 days, respectively, and the 5 µg/g treatments caused 12% mortality by day seven. Therefore, the worst-case resistance of Nereis spp. to nanoparticulate metals is assessed as ‘Medium’, resilience as ‘High’ and sensitivity as ‘Low’ but with ‘Low’ confidence due to the lack of evidence.

Dai et al. (2013) investigated the effects of silver, silver oxide nanoparticulates, copper, and copper oxide nanoparticulates on Macoma balthica. Clams were exposed to sediment spiked with 200 µg/g of silver or copper for 35 days. No significant effects on mortality, condition index, or burrowing behaviour were observed for any of the metal forms. Therefore, the sensitivity of Macoma spp. to nanoparticulate copper or silver is assessed as ‘Not sensitive’, but confidence in the assessment is ‘Low’ due to the limited number of studies reviewed. 

Overall sensitivity assessment of 'Transitional metals and organometals'. Nephtys hombergii, Macoma balthica and Streblospio shrubsolii are the important characteristic species that define this biotope. The evidence collated suggests that both Nephtys spp. and Macoma spp. have been reported to experience 'severe' or 'significant' mortality due to exposure to a range of transitional metals, depending on the concentration of the metals, duration of exposure, and local conditions such as temperature and salinity. Nephtys hombergii and molluscs are reported to be able to detoxify metal contamination within their tissue, and Nephtys hombergii may also genetically adapt to local metal contamination. Therefore, the evidence suggests that the worst-case resistance of this biotope to ‘transitional metals’ exposure is ‘None’, based on the response of Macoma balthica to transitional metals, so resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’. However, confidence in the assessment is 'Low' due to the variation in response between the metals studied, changes in toxicity due to local conditions, and the potential for localised adaptation.  The effects of nanoparticulate metals on polychaetes and bivalve populations suggest that exposure could result in population decline due to a reduction in feeding and burrowing activity and the potential increase in predation. However, the evidence is limited and 'no' mortality was reported. 

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant hydrocarbon or polyaromatic hydrocarbon (PAH) contaminants via uncontrolled releases or incidental spills (Hydrocarbon & PAH pressure definition).

Evidence

The results of the Rapid Evidence Assessment on the effects of 'Hydrocarbon and PAH' contaminants on selected polychaete and mollusc species are summarized below. The full 'evidence reviews' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'Hydrocarbon and PAH' contaminant examined, together with an overall pressure assessment. 

Oil spills. The 1969 West Falmouth (America) spill of Grade 2 diesel fuel documented the effects of hydrocarbons in a sheltered habitat (Suchanek, 1993). The entire benthic fauna was eradicated immediately following the spill. The remobilization of oil continued for over one year after the spill and contributed to a much greater impact on the habitat than that caused by the initial spill. The 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 will prevent oxygen transport to the infauna and promote anoxia as the infauna utilize oxygen during respiration. McLusky (1982) found that petrochemical effluents released from a point source to an estuarine intertidal mudflat 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, and Hediste diversicolor was found amongst an impoverished fauna at 250 m from the discharge.

The Amoco Cadiz oil spill resulted in reductions in abundance, biomass, and production of the affected invertebrate communities. However, Nephtys hombergii and other polychaetes (cirratulids and capitellids) were largely unaffected by the Amoco Cadiz oil spill (Conan, 1982). The sediment rapidly recovered, and in 1981, benthic recruitment occurred under normal conditions (Dauvin, 1998).

Hydrocarbons and PAHs. No direct evidence of the effects of hydrocarbons or PAHs on Streblospio spp. was found. However, the effects on similar species might provide suitable proxies.  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 low numbers in the area with a maximum abundance of species and the highest total biomass at 500 m from the discharge. Its abundance was greatest at 1.5-2 km from the discharge, while Eteone spp. and spionids were most abundant at 1-1.5 km (McLusky, 1982). However, the petrochemical discharge polluted the sediment within 500 m of the discharge but beyond that the effects were due to organic enrichment rather than the toxicity of petrochemicals alone (McLusky, 1982).

Lewis et al. (2008) reported that both the WAF of crude oil and the PAH fluoranthene adversely affected fertilization success in Neries virens, while the WAF caused 'significant' larval mortality. No mortality due to PAH exposure was reported in the evidence reviewed. McLusky & Martins (1998) investigated the long-term effects of petrochemical discharge on the faunal composition of an estuarine mudflat over a 20-year period. The abundance of Neries diversicolor varied over the 20 years and but no significant trends were recorded.  Therefore, the resistance of Nereis spp. (and Hediste spp.) to petrochemical hydrocarbons and PAHs is assessed as ‘Low’ based on the worst-case results reviewed. Hence, resilience is ‘High’ and sensitivity is assessed as ‘Low’ on the assumption that fertilization failure could result in long-term population decline but with ‘Low’ confidence due to the limited evidence reviewed. 

Stekoll et al. (1980) exposed the Macoma balthica, to Prudhoe Bay crude oil in flowing seawater for six months at three concentrations: low 0.03 mg/l, medium 0.3 mg/l and high 3.0 mg/l and concluded that chronic exposure of Macoma balthica to oil-in-seawater concentrations even as low as 0.03 mg/l would lead to population decreases. The individuals in this study were not subjected to any of the stresses that normally occur in their natural environment on mudflats, such as changes in salinity, temperature, oxygen availability and wave action, therefore, it is possible that exposure of Macoma balthica to oil under field conditions results in higher mortality. Shaw et al. (1976) also reported mortality of Macoma balthica caused by exposure to crude oil following an experimental application of oil at a concentration of 1.2 µl oil/cm² and 5.0 µl oil/cm² to sediments, which equated to oil spills of one ton /20 km² and one ton/100 km². 'Significant' mortalities were observed after only two days following the application of the oil at a concentration of 5.0 µl oil/cm². Some specimens of Macoma balthica survived the application of oil in these experiments but were weakened (Shaw et al., 1976). Therefore, the worst-case resistance of Macoma spp. to petroleum hydrocarbons is assessed as ‘Low’, resilience as ‘High’ and sensitivity as ‘Low’ but with ‘Low’ confidence due to the limited number of studies reviewed.

Only one article (Farke & Gunther, 1984) examined the effects of dispersants on Macoma balthica. No mortality was reported, and it was unclear what effects the dispersant had on the population based on the data alone. Therefore, the evidence was not adequate to support an assessment of its sensitivity to dispersants.  No results on the impacts of ‘Hydrocarbons and PAHs’ on Scrobicularia plana were found in the evidence reviewed. 

Overall sensitivity assessment of this pressure. The evidence reviewed suggests that the important characteristic species Macoma balthica could experience 'significant' mortality due to exposure to petroleum hydrocarbons. Oil spills have the potential to significantly affect benthic fauna in sheltered soft sediments, but the effects vary depending on location and the extent, duration and type of oil spilt. Nephtys hombergii and other polychaetes were reported to be unaffected after the Amoco Cadiz spill, while the West Falmouth spill eradicated the benthos and petrochemical effluent excluded Nephtys from closer than 500 m to the discharge point.  Therefore, the resistance of this biotope to petrochemical hydrocarbons and PAHs is assessed as ‘Low’ based on the worst-case results reviewed in Nephtys spp. and Macoma spp. Hence, resilience is ‘High’ and sensitivity is assessed as ‘Low’ but with ‘Low’ confidence due to the limited evidence available. 

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more synthetic compound contaminants via uncontrolled releases or incidental spills (Synthetic compound contamination pressure definition).

Evidence

The results of the Rapid Evidence Assessment on the effects of 'Synthetic compounds' contaminants on selected polychaete and mollusc species are summarized below. The full 'evidence reviews' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'Synthetic compounds' contaminant examined, together with an overall pressure assessment. 

Pesticides/biocides. Nephtys spp., Eteone longa, and Streblospio benedicti were examined in only two of the articles reviewed. In all cases, only sublethal effects were reported. For example, Dumbauld et al. (2001) observed the effects of the application of the pesticide Carbaryl on the estuarine benthic community in oyster culture sites but did not detect trends or significant differences in the abundance of the polychaetes Streblospio benedicti and Eteone longa. Similarly, Scanes et al. (1993) reported no significant changes in polychaete abundance (inc. Nephtys spp.) after an accidental spill of the pesticide Aldrin on an estuarine beach in New South Wales, Australia. Therefore, the sensitivity of Nephtys spp., Eteone longa, and Streblospio benedicti to ‘pesticides/biocides’ is assessed as ‘Not sensitive based on this evidence alone. The confidence is ‘Low’ due to the lack of evidence.

However, the toxicity of different pesticides varies depending on the chemical used and the species affected. For example, seven articles examined the effects of ‘Pesticides/biocides’ on Hediste (Nereis) diversicolor. Collier & Pinn (1998) reported that Nereis diversicolor was the most sensitive of the species in their experiment, as 8.0 mg/m² Ivermectin caused 100% mortality within 14 days. Mayor et al. (2008) reported that the sea-lice insecticide Emamectin benzoate had significant effects on the survival of Hediste diversicolor with an LC50 value of 1,367.71 µg/kg (wet sediment). Scaps et al. (1997) reported that by day 21, 20% of Nereis diversicolor exposed to 10^-6 and 10^-8 M Malathion died, and 33.5 and 62.5% mortality occurred in the 10^-8 and 10^-6 M parathion-ethyl treatments. Only sublethal effects were reported in the other pesticides reviewed for effects on Hedsite spp. or Nereis spp. Underwood & Paterson (1993) reported that Nereis diversicolor was absent from areas of sediment treated with formaldehyde. Although not studied directly, the observation suggests that Nereis diversicolor was excluded from the treatment site due to the toxicity of the formaldehyde or the lack of microalgal food.  Therefore, the worst-case resistance of Nereis spp. and Hediste spp. to ‘Pesticides/biocides’ is assessed as ‘None’, resilience as ‘Medium’ and sensitivity assessed as ‘Medium’. The confidence is assessed as ‘low’ due to the limited evidence and the variation in toxicity between species and chemicals tested.

The effects of pesticides/biocides on Arenicola spp. were studied in six of the articles reviewed. Garnas et al. (1977) reported 100% mortality in Arenicola cristata exposed to 1000 µg/l Kepone for 5 days. Similarly, Rubenstein (1979) reported 100% mortality in A. cristata exposed to 29.5 µg/l of Kepone, for 144 hours. Exposure to 2.8, 4.5, 6.6, 7.4, and 29.5 µg/l Kepone for 144 hours also reduced feeding behaviour significantly. However, Mirex and sodium pentachlorophenate reduced feeding and burrowing behaviour significantly (Schoor & Newman, 1976; Rubinstein, 1978) in Arenicola cristata. Carbaryl, Ivermectin and Parathion-ethyl were reported to cause significant mortality in A. marina (Conti, 1987; Allen et al., 2007). Ivermectin had a significant effect on the survival of the lugworms; the 10-day LC50 was 17.9 µg IVM/kg wet sediment in Test 1 and 14.8 µg IVM/kg wet sediment in Test 2 and Ivermectin reduced the mean casting rate (Allen et al., 2007). Conti (1987) reported 48-hour LC50s of 2,700 µg/l for Parathion-ethyl and 7,200 µg/l for Carbaryl. Therefore, the worst-case resistance of Arenicola spp. to ‘Pesticides/biocides’ is assessed as ‘None’, resilience as ‘Medium’ and sensitivity assessed as ‘Medium’. The confidence is assessed as ‘low’ due to the limited evidence and the variation in toxicity between species and the chemicals tested.

Three articles examined the effects of ‘Pesticides/biocides’ on Macoma spp. Armstrong & Millemann (1974) exposed Macoma nasuta to Sevin at 15, 20, 25, and 30 mg/l for 96 hours. After 96 hours of exposure, around 50% of the clams had withdrawn one or both siphons, with estimated 48-hour and 96-hour EC50s of 27.5 and 17 mg/l Sevin, respectively. But no mortality was reported. Boese et al. (1990) reported sublethal effects on the physiology of Macoma spp. exposed to 5.2 – 7.8 µg/l hexachlorobenzene for 3-7 days. Dumbauld et al. (2001) examined the effects of pesticide treatment of oyster sites on benthic infauna. No significant effects on Macoma spp. abundance were observed 24 hours, 2 weeks, 1 month and 1 year after the application of Carbaryl at 5.6 kg/ha. The average density of Macoma spp. was significantly different from the control plots at 51 days post-exposure to 8.4 kg/ha Carbaryl but there were no significant differences after 2 days or 1 year (Dumbauld et al., 2001). Therefore, the worst-case sensitivity of Macoma spp. to the ‘Pesticides/biocides’ tested is assessed as ‘Not sensitive’ since only sublethal or transient effects were reported. However, confidence in the assessment is ‘Low’ due to the limited number of studies and pesticides tested.

Pharmaceuticals. No direct evidence of the effects of Pharmaceuticals on Nephtys spp., Eteone longa, Streblospio spp. or Macoma spp. was found.  However, eight articles examined the effects of several ‘Pharmaceuticals’ on Hediste (Nereis) diversicolor. For example, Maranho et al. (2014) reported that the survival of the Hediste diversicolor was negatively correlated with concentrations of Carbamazepine and 17α-ethynylestradiol. ‘Significant’ mortality (>25%) was observed in the Carbamazepine (0.05 and 0.5 µg/l), Fluoxetine (0.001 µg/l) and 17α-ethynylestradiol (0.01 and 0.1 µg/l) treatment groups. Pires et al. (2016) reported that Hediste diversicolor experienced 8.3% mortality at concentrations 0.3 and 3.0 µg/l after 28 days of exposure to carbamazepine, 25% mortality at 6.0 µg/l and 16.7% mortality at 18.0 µg/l after 28 days of exposure carbamazepine. ‘Some’ mortality occurred after exposure to the beta-blocker Propranolol (Maranho et al., 2014). However, the other studies reported only sublethal effects, including a reduction in burrowing activity. Therefore, the worst-case resistance of Nereis spp. and Hediste spp. to ‘Pharmaceuticals is assessed as ‘Low’, resilience as ‘High’, and sensitivity as ‘Low’. The confidence is assessed as ‘low’ due to the limited evidence and the variation in toxicity between the chemicals studied. 

Only two papers examined the effects of ‘Pharmaceuticals’ on Arenicola marina. Zanuri et al. (2017) investigated the impacts of Diclofenac, Ibuprofen, and Sildenafil citrate (Viagra®) on the fertilization biology of spawning marine invertebrates, including Arenicola marina. Sperm motility and swimming speed were reduced when exposed to >1 µg/l Diclofenac for over 90 minutes. Ibuprofen exposure significantly increased the swimming speed of the sperm when exposed to >10 µg/l for 30 minutes or longer. Diclofenac negatively affected the fertilization success of the polychaetes but neither Ibuprofen nor Sildenafil citrate exposure affected fertilization success significantly. Pre-incubation of both sperm and oocytes caused significant decreases in fertilization success of the polychaetes at Diclofenac concentrations of 1 µg/l and above. Ibuprofen caused significant reductions in fertilization success when gametes were exposed to Ibuprofen at 1000 µg/l. Pre-incubation with Sildenafil citrate had no effects on fertilization success.  Lewis & Galloway (2009) reported that methyl methanesulfonate exposure did not affect fertilization success at all tested concentrations but abnormal development occurred at all tested concentrations (18, 32, and 52 mg/l for 24 hours and 72 hours prior to the induction of spawning) in polychaete Arenicola marina. Therefore, the sensitivity of Arenicola marina to ‘Pharmaceuticals’ is assessed as ‘Not sensitive’, but with ‘Low’ confidence due to the limited evidence. However, reported effects on larval development may have long-term effects on recruitment and population dynamics. No direct evidence of the effects of other synthetic compounds on Nephtys spp., Eteone longa, Streblospio spp. or Macoma spp. was found. 

Overall sensitivity assessment for this pressure. The effects of pesticide/biocide exposure on the characteristic polychaetes and bivalve varies between studies and the chemical tested. No evidence of mortality due to pesticide/biocide exposure was reported in Nephtys spp., Eteone longa, Streblospio spp. but other polychaetes (Hediste spp. or Nereis spp. and Arencola spp. were reported to experience 'severe' or 'significant' mortality. The pharmaceuticals studied were reported to cause mortality in Nereis spp. but also a reduction in burrowing activity and a reduction in fertilization success in Arenicola spp. Therefore, the worst-case resistance of this biotope to 'Synthetic compound' contamination is assessed as 'Low' based on the assumption that polychaetes share similar biochemistry and physiology.  However, the confidence in the assessment is 'Low' due to the variation in response between species and the chemicals tested. Hence, resilience is assessed as 'High' and sensitivity as 'Low'. 

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels (Radionuclides contamination pressure definition).

Evidence

Insufficient information was available about 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 "other" substances (solid, liquid or gas) contaminants via uncontrolled releases or incidental spills (Introduction of other substances pressure definition). 

Evidence

The results of the Rapid Evidence Assessment on the effects of 'Other substances' contaminants on selected polychaete and mollusc species are summarized below. The full 'evidence reviews' should be consulted for details of the studies examined and their results. A sensitivity assessment is provided for each type or source of 'Other substances' contaminant examined, together with an overall pressure assessment. 

No direct evidence of the effects of 'Other substances' on Nephtys spp., Eteone longa, or Streblospio spp. was found.  Galloway et al. (2010) investigated the sublethal toxicity of nano-titanium dioxide and carbon nanotubes on the marine polychaete Arenicola marina. The lugworms were exposed through natural sediments to a 10-day OECD/ASTM 1990 acute toxicity test. Sediment was prepared with either single-walled carbon nanotubes (0.003–0.03 g/kg), nano-titanium dioxide (1–3 g/kg), or seawater alone. The lugworms were fed every other day, and feeding behaviour was monitored every 48 hours. Casts were collected, dried overnight and weighed, with seawater renewals every 48 hours following cast collection. After 10 days of exposure, the lugworms were removed from the exposure sediment, and the lugworm's ability to re-bury into clean sediment was assessed following the OECD/ICES A. marine burrowing bioassay. Sediment exposure to single-walled carbon nanotubes or nano-titanium dioxide had no effects on the burrowing behaviour of the lugworm. During the exposure period, single-walled carbon nanotubes did not affect the feeding behaviour of the lugworms. However, nano-titanium dioxide exposure caused a significant impact on feeding behaviour, with reductions in casting rate at 2 g/kg nano-titanium dioxide. Therefore, Arenicola marina is probably ‘Not sensitive’ to single-walled carbon nanotubes (at 0.003–0.03 g/kg). However, confidence in the assessment is ‘low’ due to the lack of evidence.

De Marchi et al. (2017) investigated the effects of different multi-walled carbon nanotubes (MWCNTs) (at 0.01, 0.10, and 1.00 mg/l) on Diopatra neapolitana and Hediste diversicolor. The effects on physiological and biochemical performance were assessed after 28 days of exposure. The respiration rate of Hediste diversicolor was measured after 28 days of exposure. Exposure to 0.01 mg/l MWCNTs increased the respiration rate compared to the controls. However, respiration rates significantly decreased at 0.1 mg/L MWCNTs compared to the control, but respiration rate increased at the highest tested concentration (1.00 mg/L) of MWCNTs. Mortality of H. diversicolor individuals exposed to 0.01, 0.10 and 1.00 mg/l was 11% at each of the tested concentrations. In the control treatment, 100% survival was recorded after the bioassay. Pires et al. (2022) investigated the effects of graphene oxide (GO) nanosheets on the behavioural, physiological, and biochemical responses of Hediste diversicolor. Polychaetes were exposed to a range of concentrations of graphene oxide nanosheets (10, 100, 1000, and 10,000 µg/l) for 28 days. The study assessed the effects on the behaviour, feeding activity, mucus production, regenerative capacity, antioxidant status, biochemical damage, and metabolism. Body regeneration was significantly influenced by GO exposure, with all individuals exposed to GO exhibiting reductions in the number of regenerated segments when compared to the controls. Feeding activity was influenced by GO exposure, with increased feeding times for exposed individuals. The segregation of mucus was significantly higher in individuals exposed to GO when compared to the controls. Burrowing rates of polychaetes exposed to GO were significantly slower than those in the control, with 20-35% of individuals in the highest tested concentrations unable to burrow by the end of the 30-minute assay. Mortality was around 40% in the GO-exposed treatments, with 30% mortality in the 10 and 100 µg/l treatments, but only 5% in the highest tested concentration of 10,000 µg/l. No mortality occurred in the control. Therefore, Hediste diversicolor probably has a resistance of ‘Low’ to graphene oxide nanosheets and ‘Medium’ resistance to multi-walled carbon nanotubes. Hence, resilience is probably ‘High’ and sensitivity is assessed as ‘Low’ but with ‘Low’ confidence.

Pires et al. (2016) investigated the biochemical effects of single and combined exposure to Carbamazepine and caffeine on Hediste diversicolor. Polychaetes were exposed to a range of concentrations of carbamazepine (0.3, 3.0, 6.0 and 9.0 µg/l) and caffeine (0.5, 3.0, and 18.0 µg/l) for 28 days. 8.3% of mortality at concentrations 0.3 and 3.0 µg/l after 28 days of exposure to carbamazepine, 25% at 6.0 µg/l and 16.7% at 18.0 µg/l. Specimens experienced 8.3% of mortality after 28 days of exposure to caffeine at 0.5 and 18.0 µg/l, and polychaetes exposed to 0.3 µg/l carbamazepine + 0.5 µg/l caffeine had 8.3% mortality. No mortality was recorded in the 3 µg/l caffeine or 3 µg/l caffeine + 6 µg/l carbamazepine treatment. The evidence suggests that Hediste diversicolor has a ‘Low’ sensitivity to caffeine exposure, but confidence in the assessment is ‘Low’ due to the lack of evidence.

Muller-Karanassos et al. (2021) investigated the effects of environmental concentrations of antifouling paint particles on sediment-dwelling invertebrates. Adult ragworms and cockles were exposed to three types of antifouling paint particles (APP), two biocidal (‘historic’ and ‘modern’) and one biocide-free (‘silicone’). Two laboratory-based 18-day and 5-day exposure experiments were carried out. The APPs ranged in particle size and included varying concentrations of Cu, Sn, Pb, Hg, and Zn. Trial experiments carried out using the maximum environmental APP concentration (18.8 g/l) caused 100% mortality of all ragworms and cockles in the modern treatment within 6 days. In the 18-day exposure, antifouling paint particle concentrations were 4.2 g/l for the historic biocidal treatment, 3.0 g/l for the modern biocidal treatment, and 2.1 g/l for the non-biocidal silicone treatment. The burrowing rate of the ragworms was reduced by 29% in the modern biocidal treatment. However, there were no significant differences between treatments. Ragworms decreased in weight and feeding rates significantly, but significant differences were only seen between the modern biocidal treatment and the control. Modern biocidal antifouling paint particles were used at concentrations ranging from 0 to 30 g/l (ragworms) and 0 to 6 g/l (cockles) to estimate the 5-day LC50 exposure. The 5-day LC50 values were 19.9 g/l for the ragworms and 2.3 g/l for cockles. The 5-day EC50 values were 14.6 g/l for the ragworms and 1.4 g/l for cockles. The evidence from Muller-Karanassos et al. (2021) suggests that antifouling paint particles remain toxic in the environment. Therefore, the resistance of Hediste diversicolor to APPs is assessed as ‘None’. Hence, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’, but confidence in the assessment is ‘Low’ due to the lack of evidence.

Caldwell et al. (1975) exposed Macoma balthica to 100, 330, 1000, 3,300, and 10,000 µg/l hydrogen sulphide for 96 hours. The longer the clams were exposed to hydrogen sulphide, the lower the concentration was required to cause 50% mortality. The LC50 at 24, 48, and 96 hours were 10,000, 8,000 and 6,000 µg/l, respectively. Crecelius (1979) examined the effect of bromate on Macoma inquinata and reported 100% mortality after 72 hours at 880 mg/l bromate. Therefore, the worst-case resistance of Macoma spp. to the inorganic chemicals tested is assessed as ‘None’, resilience as ‘Medium’ and sensitivity as ‘Medium’ but with ‘Low confidence due to the limited evidence. Wastewater discharge was shown to reduce the abundance of Macoma cumana in the affected area (Bergayou et al., 2019). Exposure to pulverised fuel ash was reported to result in 20% mortality of Macoma balthica in the 100% PFA treatment (Jenner & Bowmer, 1990). Therefore, the worst-case resistance of Macoma balthica to pulverised fuel ash exposure is assessed as ‘Medium’, resilience as ‘High’, and sensitivity as ‘Low’, but with ‘Low’ confidence. However, its resistance to wastewater discharge may be ‘Medium’ and its sensitivity ‘Low’, depending on the nature of the contaminants involved, but confidence is ‘Low’ due to the lack of evidence.

Overall sensitivity assessment for this pressure. No direct evidence of the effects of 'Other substances' on Nephtys spp., Eteone longa, or Streblospio spp. was found. The above evidence suggests that Hediste spp. may be sensitive to caffeine, graphene oxide nanosheets, and anti-fouling paint particles, while single-walled carbon nanotubes and nano-titanium dioxide had limited effects on Arenicola marina. Macoma balthica was reported to experience mortality due to exposure to pulverised fuel ash (PFA) and wastewater discharge. Therefore, the worst-case resistance of this biotope to 'Other substances' contamination is assessed as 'None' based on the effects of APP and the assumption that polychaetes share similar biochemistry and physiology.  However, the confidence in the assessment is 'Low' due to the variation in response between species and the chemicals tested. Hence, resilience is assessed as 'Medium' and sensitivity as 'Medium'. Note that the evidence on the effects of each chemical tested should be treated separately, and the above species-specific assessments used where appropriate.

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) (deoxygenation pressure definition).

Evidence

Nephtys hombergii inhabits intertidal areas where resistance to low oxygen is needed, and sulphide levels can reach up to 1 mM (Giere, 1992; Thierman, 1996). As with other characteristic polychaete species, Nephtys hombergii exhibits the ability to switch from aerobic to anaerobic 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, Nephtys hombergii recovered completely after sulphide exposure under anaerobic conditions for <24 hours (Arndt & Schiedek, 1997). Although Nephtys hombergii appears to be well adapted to a habitat with short-term fluctuations in oxygen and the 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 tolerated extreme hypoxia, leaving the sediment only after 11 days (Nilsson & Rosenberg, 1994). Nephtys hombergii in artificially created anoxic conditions were reported to survive for at least 5 days (Schöttler, 1982) and do not switch from aerobic to anaerobic metabolic pathways until oxygen saturation decreased below 12% (Schöttler, 1982).

Macoma balthica appears to be relatively tolerant of deoxygenation. Brafield & Newell (1961) frequently observed that Macoma balthica survived low oxygen concentrations (e.g. less than 1 mg O₂/l) and shell growth continued. Similarly, Jansson et al. (2015) found juvenile Macoma balthica exhibited the highest growth rate and significantly higher survival in exposure to low oxygen concentrations (3.0 mg/l) for 29 days in experimental conditions. An LC50 of dissolved oxygen of Macoma balthica was reported as 1.7 mg/l for a 28-day experiment (Long et al., 2008, cited in Song et al., 2024).

Ehrnsten et al. (2019a) described Macoma balthica (as Limecola) as being less sensitive to hypoxia compared to other deposit feeders and predators based on two models. However, its biomass increased once oxygen levels increased. Field evidence in the Baltic Sea supported this observation, as Macoma balthica was present at sites described as intermittently hypoxic, but occurred in higher abundances at sites which remained normoxic all year round (Gammal et al., 2017). However, Macoma balthica was absent from benthic macrofauna communities at sites that experienced prolonged seasonal hypoxia (0.0 mg/l and 0.2 mg/l), suggesting that prolonged hypoxia can reduce local communities (Gammal et al., 2017).

Sub-lethal behaviour and physiological effects of hypoxia have been identified and observed (Villnas et al., 2019). Brafield & Newell (1961) frequently observed that in conditions of oxygen deficiency (e.g. less than 1 mg O2/l) Macoma balthica moved upwards to fully expose itself on the surface of the sand. Specimens lay on their side with the foot and siphons retracted, but with valves gaping slightly, allowing the mantle edge to be brought into full contact with the more oxygenated surface water lying between sand ripples. In addition, Macoma balthica was observed under laboratory conditions to extend its siphons upwards out of the sand into the overlying water when water was slowly deoxygenated with a stream of nitrogen. The lower the oxygen concentration became, the further the siphons extended (Brafield & Newell, 1961; Long et al., 2014; Villnas et al., 2019; Song et al., 2024). Migration to the surface due to hypoxia may leave it at greater risk of predation (Long et al., 2008; Long et al., 2014; Villnas et al., 2019; Song et al., 2024). In one study, these behaviours were observed after short-term (3-day) recurring hypoxic disturbance, but recovered quickly by reburying once oxic conditions were re-established (Villnas et al., 2019). Under recurring hypoxic disturbance, impacts were observed to affect juveniles before adults. However, long-term (30 days) uninterrupted hypoxia increasingly reduced the bivalve population, and the majority of Macoma balthica individuals exposed did not rebury at all (Villnas et al., 2019). Long et al. (2014) also provided evidence that hypoxia decreases individual Macoma balthica growth, causes local extinction of populations and reduces egg production by 40%, while increasing protein investment per egg (cited in Song et al., 2024).

This behaviour, an initial increase in activity stimulated by oxygen deficiency, is of interest because the activity of lamellibranchs is generally inhibited by oxygen-deficient conditions (Brafield & Newell, 1961). Dries & Theede (1974) reported the following LT50 values for Macoma balthica maintained in anoxic conditions: 50 to 70 days at 5°C, 30 days at 10°C, 25 days at 15°C and 11 days at 20°C. Theede (1984) reported that Macoma balthica's ability to resist extreme oxygen deficiency was mainly due to anaerobic metabolism. Macoma balthica is therefore very tolerant of hypoxia, although it may react by reducing metabolic activity, and predation risk may increase. Metabolic function should quickly return to normal when oxygen levels are resumed, so recovery is expected.

Streblospio shrubsolii characterizes 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 benchmark level. 

Nutrient enrichment

Benchmark. Increased levels of the elements nitrogen, phosphorus, silicon, and iron in the marine environment compared to background concentrations (Nutrient enrichment pressure definition).

Evidence

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. 

Al et al. (2022) reported that bivalves, amphipods, and polychaetes, including Nephtys hombergii, are potential bioindicators of nutrient enrichment associated with mangrove forests, aquaculture effluent and sewage. However, Martinez-Garcia et al. (2019) found that beneath Mediterranean fish farms, where sediments had high organic matter and sulphide concentrations, Nephtys cirrosa was absent or declined in abundance, and was more abundant in reference sites more than 1 km away from the farms (Table 2 in Martinez-Garcia et al., 2019). De Jong et al. (2015a) reported deposit feeding polychaetes Nephtys cirrosa was one of the most abundant species at sites near the sediment disposal site in the Port of Rotterdam, where bed shear stress was high and organic matter in sediment was low (0.4 to 0.5% sediment organic matter).

Dauvin et al. (2022) investigated the effects of dredged sediment disposal from the ports of Le Havre and Rouen on macrobenthic communities in the eastern Bay of Seine. The dumped sediment was largely composed of fine mud, sand and gravel with an elevated total organic carbon of around 1.2% on average, at impacted sites studied. Nephtys hombergii was amongst the ten dominant benthic species in the Bay of Seine. Dauvin et al. (2022) found negative correlations between disposed volume and both taxonomic richness and abundance of the whole community, and noted seasonal recruitment with rapid community recovery after disturbance.

Macoma balthica is reported as an important bioturbator and key bio-irrigator species that contributes significantly to nutrient cycling, sediment oxygenation and bioremediation of nutrients, making it a major contributor to benthic ecosystem functioning (Fang et al., 2021; Ehrnsten et al., 2019b; Gammal et al., 2025). Singer et al. (2023) found that long-term de-eutrophication in the East-Frisian Wadden Sea reduced nutrient inputs, primary production, and the supply of particulate organic matter to the seabed, leading to a substantial decline in benthic food resources. This decrease in organic enrichment was linked to a decrease in the abundance and biomass of Macoma balthica (reported as Limecola). This shows that Macoma balthica is adapted to organically enriched environments and relies on the organic matter supply.

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 above evidence suggests that Macoma balthica and Nepthys hombergii increase in abundance in nutrient and organic-enriched conditions, and may decrease in abundance when nutrient enrichment is removed. Therefore, resistance to nutrient enrichment is assessed as ‘High’ based on the fact that Nepthys and Macoma can benefit from nutrient or organic enrichment, but with ‘Low’ confidence due to limited evidence available. Hence, resilience is assessed as 'High' (by default), so that the biotope is assessed as 'Not sensitive'.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr (Organic enrichment pressure definition).

Evidence

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. 

Al et al. (2022) reported that bivalves, amphipods, and polychaetes, including Nephtys hombergii, were potential bioindicators of nutrient enrichment associated with mangrove forests, aquaculture effluent and sewage. However, Martinez-Garcia et al. (2019) found that beneath Mediterranean fish farms, where sediments had high organic matter and sulphide concentrations, Nephtys cirrosa was absent or declined in abundance, and was more abundant in reference sites more than 1 km away from the farms (Table 2 in Martinez-Garcia et al., 2019). De Jong et al. (2015a) reported that the deposit-feeding polychaete Nephtys cirrosa was one of the most abundant species at sites near the sediment disposal site in the Port of Rotterdam, where bed high bed shear stress was high and organic matter in sediment was low (0.4 to 0.5% sediment organic matter).

Dauvin et al. (2022) investigated the effects of dredged sediment disposal from the ports of Le Havre and Rouen on macrobenthic communities in the eastern Bay of Seine. The dumped sediment is largely composed of fine mud, sand and gravel with an elevated total organic carbon of around 1.2% on average, at impacted sites studied. Nephtys hombergii was amongst the ten dominant benthic species in the Bay of Seine. Dauvin et al. (2022) found negative correlations between disposed volume and both taxonomic richness and abundance of the whole community, and noted seasonal recruitment with rapid community recovery after disturbance.

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 to 1973 (Gray, 1976). In Ria de Averio, western Portugal, Streblospio shrubsolii and Tubificoides benedii were characteristic species of communities further upstream in estuarine sample sites, at sites with increased organic matter (Rodrigues et al., 2011). Streblospio shrubsolii is also considered a characteristic species of communities in polluted environments, suggesting the species is likely to be resistant to increased organic enrichment (Cooksey & Hyland, 2007).

In the Marine Biotic index, Macoma balthica was assigned as ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’ (Borja et al., 2000). However, case studies display the resilience of Macoma balthica populations to enrichment. Macoma balthica has been shown experimentally to resist periods of up to nine weeks under algal clover, their long siphon allowing them to reach oxygenated water, although other bivalves decreased in abundance (Thiel et al., 1998). Organic enrichment from wastewater discharge in the Dutch Wadden Sea resulted in positive effects on Macoma balthica abundance, biomass, shell growth and production. These effects were suggested to be due to increased food supply (Madsen & Jensen, 1987). Tubificoides benedii and other oligochaetes are very tolerant of high levels of organic enrichment and often dominate sediments where sewage has been discharged, or other forms of organic enrichment have occurred (Pearson & Rosenberg, 1978; Gray, 1971; McLusky et al., 1980). In Jade Bay (Wadden Sea), Macoma balthica is recorded in macrofauna communities associated with total organic carbon content between 0.82% to 1.77% (Schückel et al., 2015).

Singer et al. (2023) found that long-term de-eutrophication in the East-Frisian Wadden Sea reduced nutrient inputs, primary production, and the supply of particulate organic matter to the seabed, leading to a substantial decline in benthic food resources. This decrease in organic enrichment was linked to a decrease in the abundance and biomass of Macoma balthica (reported as Limecola). This shows that Macoma balthica is adapted to organically enriched environments and relies on the organic matter supply.

Gittenberger & Van Loon (2011) assigned Macoma balthica and Streblospio shrubsolii to their Ecological Group III "Species tolerant to excess organic matter enrichment. These species may occur under normal conditions, but their populations are stimulated by organic enrichment (slightly unbalanced situations). They are surface deposit-feeding species, as tubicolous spionids". Nepthys hombergii was assigned to Ecological Group II "Species indifferent to enrichment, always present in low densities with non-significant variations with time (from initial state, to slight unbalance)". These include suspension feeders, less selective carnivores and scavengers.

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 (Physical loss pressure definition). 

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 (Physical change in subtratum type pressure definition).

Evidence

This biotope is only found in sediment, in particular, sandy mud, and the burrowing organisms, Nephtys hombergii, Macoma balthica, Streblospio shrubsolii and Tubificoides benedii would not be able to survive if the substratum type were changed to soft or hard rock or artificial type. Consequently, the biotope would be lost altogether if such a change occurred. 

Sensitivity assessment. Biotope resistance is assessed as 'None' as a change at the pressure benchmark would result in loss of the biotope. Resilience is assessed as 'Very low' as a change at the pressure benchmark is permanent.  Sensitivity within the direct spatial footprint of this pressure is, therefore, assessed as ‘High’.  Although no specific evidence is described, confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification) (Physical change in sediment type pressure definition). 

Evidence

The change in one Folk class is considered to relate to a change in the sediment 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, ii) sand or iii) mixed sediments.

High densities of Nephtys hombergii were found in substrata of 0.3% particles >0.25 mm and 5.8% <0.125 mm in diameter, but the worm tolerated up to 3.8% 0.25 mm and 2.2 to 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 the species was absent in grain sizes over 0.5 mm in the Belgian part of the North Sea.

De Jong et al. (2015a) studied the distribution patterns of macrozoobenthic assemblages in the Dutch coastal zone in front of the Port of Rotterdam, an area largely affected by human activities, including a deepened shipping lane, sediment dredging and disposal, high intensity fishing and sewage effluent discharge. Results found that Nephtys cirrosa was among the most abundant species at sites near the sediment disposal site, where bed high bed shear stress was high and organic matter in sediment was low (0.4 to 0.5% sediment organic matter).

Pezy et al. (2017) investigated the impacts of dumping muddy fine sand dredged material in the Machu site in the Seine estuary. Before dumping, the site was a fine to medium sand habitat characterized by an Ophelia borealis and Nephtys cirrosa community. Following one year of dumping of muddy fine sand, the sediment composition changed, and there was an increase in Abra alba and a shift toward a mix of Nephtys cirrosa and Abra alba community of medium and muddy fine sand. Despite this change, the original Nephtys cirrosa community still appeared resilient and showed a strong capacity for recovery through recruitment and recolonization.

This was supported by long-term evidence from Raoux et al. (2020), which reported that Nephtys hombergii was amongst the dominant species at the Octeville dumping site in the Bay of Seine, remaining stable after 70 years of fine sand deposition. However, significant differences in species richness, abundance and diversity were found between the impacted Machu and Octeville dumping sites, near the deposition sites and non-impacted sites in the Bay of Seine (Pezy et al., 2018; Raoux et al., 2020). In these comparison studies, the abundance and biomass of Nephtys cirrosa were higher in the non-impacted site compared to both dumping sites (Pezy et al., 2018; Raoux et al., 2020). After a short one-year dumping phase in the Machu site, there was a higher abundance of Nephtys cirrosa in sites near the dumping sites compared to directly impacted and non-impacted sites (Pezy et al., 2018). This evidence shows that the fine to medium sand habitat in the Seine estuary, which undergoes regular natural physical disturbance, has high resilience after short and long-term dumping periods (Pezy et al., 2017; Raoux et al., 2020), and highlights that Nephtys cirrosa has a high tolerance to changes in sediment composition.

Jammar et al. (2025) reported that the installation of offshore wind farms (OWF) in soft sandy medium to coarse sediment habitats in the Southern North Sea altered the seabed and shifted the microbenthic community structure. The OWF foundations provided new hard substrata, which increased the surface available for fouling organisms, and sediment near turbines became finer and organically enriched due to the increase in faecal pellets and detritus from fouling organisms. This resulted in a shift from a soft sediment Nepthys cirrosa community to a more diverse “intermediate community”, characterized by higher abundances of species associated with finer sediment, such as those typical of the Abra alba community (Jammar et al., 2025). Nepthys sp., Bathyporeia sp., Spiophanes bombyx and Ophelia borealis were amongst the abundant species recorded at the two studied OWF in the North Sea (Jammar et al., 2025).

Macoma balthica is likely to tolerate increased gravel content, as sediment was not shown to affect burrowing (Tallqvist, 2001). However, growth, shell size and body mass were greatest in higher sand content sediments and lower in higher gravel content sediments (Azouzi et al., 2002), suggesting long-term health and abundance may be affected by a permanent increase in gravel content.

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

Sensitivity assessment. The characteristic species are resistant to increases in mud content. An increase in sand content may have a greater impact on the biotope and lead to the replacement of Nephtys hombergii by Nephtys cirrosa, which is characteristic of cleaner, fairly coarse sand. An increase in the relative content of sand or gravel would result in a change to muddy sand or fine sand, or to a range of mixed sediments, resulting in the loss or reclassification of this biotope and its associated community. Therefore, resistance is assessed as 'None', resilience is 'Very low' as the change at the pressure benchmark is permanent, and sensitvity 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) (Removal of substratum pressure definition). 

Evidence

The substratum of this biotope consists of soft wet mud with a fine sand fraction (Conner et al., 2004). The characterizing species, Nephtys hombergii and Macoma balthica, burrow into the sediment to depths not exceeding 30 cm. 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 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). 

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 on 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.

Sensitivity assessment. Resistance to the pressure is assessed as ‘None’, and resilience as ‘High’ based on the presence of a suitable substratum. Hence, biotope sensitivity has been assessed as ‘Medium’. (It should be noted that recovery could be longer and sensitivity greater, where remaining sediments are unsuitable).

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat) (Surface abrasion/disturbance pressure definition).

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 was also demonstrated to 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 the suspension of fine sediments by the 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 on this biotope than other intertidal biotopes.

Collie et al. (2000) found that Nephtys hombergii displayed a negative effect on abundance as a result of fishing activities, and the mean response of infauna and epifauna communities to fishing activities was much more negative in mud and sand communities than in 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).

However, in a study from the Belgian part of the North Sea, where the prohibition of beam trawl fisheries was implemented due to the construction of offshore wind farms, Coates et al. (2016) found the first signs of recovery in soft sediment macrofaunal communities three years after disturbance stopped. The polychaete Nephtys cirrosa remained dominant in the site before and after the exclusion of trawling. A decrease in average abundance and biomass was observed. Despite subtle changes observed, none of the differences was significant.

Sciberras et al. (2017) examined sandy sediments collected from the Isle of Man, with different histories of bottom fishing disturbance, described as ‘Low’ and ‘High’ fishing frequency. Results found that the history of fishing activity influenced the community composition in the collected sediments. In sediments from sites which had a high frequency of fishing, there was a higher abundance of disturbance-tolerant suspension feeders, including Nephtys sp. (Sciberras et al. 2017). In addition, other studies have reported tube-dwelling polychaetes associated with low trawling frequencies (Beauchard et al., 2023). Model results have suggested that low levels of trawling (once or twice a year) may increase the productivity of small polychaetes, but higher trawling frequency lowers benthic production across all taxa (Hiddink et al., 2008, cited in Collie et al., 2017).

Clarke et al. (2018) examined pump-scoop dredging in Poole Harbour using a Before-After-Control-Impact (BACI) design with three study sites: a long-term heavily dredged area, a newly opened dredged area and a control with no dredging. Their results found that Streblospio shrubsolii density dramatically increased at the newly dredged site compared to other sites (Clarke et al., 2018).

Long-term monitoring of the Wadden Sea found that cockle dredging had no significant effect on the recruitment of Macoma balthica (reported as Limecola balthica), and survival of recruits did not differ between fishing and non-fishing years (Beukema & Dekker, 2018; 2020; Van Der Meer & Folmer, 2023).

Sensitivity assessment. Evidence on the distribution of Nephtys spp. and Streblospio shrubsolii associated with physical fishing disturbance has suggested these species may be able to tolerate the effects of physical disturbance. The characterizing species are burrowing infauna and likely to be relatively protected from a single event of abrasion at the surface. Therefore, resistance is assessed as 'Medium' and resilience as 'High', so sensitivity is assessed as 'Low', with Low confidence due to the limited amount of evidence. 

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat) (Sub-surface penetration pressure definition).

Evidence

 As the characterizing species are burrowing species, the impact from damage to the sub-surface seabed could be greater than damage to the seabed surface (see abrasion pressure). Collie et al. (2000) found that the abundance of Nephtys hombergii declined as a result of fishing activities, and the mean response of infauna and epifauna communities to fishing activities was much more negative in mud and sand communities than in 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).

In a study from the Belgian part of the North Sea, where the prohibition of beam trawl fisheries was implemented due to the construction of offshore wind farms, Coates et al. (2016) found the first signs of recovery in soft sediment macrofaunal communities three years after disturbance stopped. The polychaete Nephtys cirrosa remained dominant in the site before and after the exclusion of trawling; a decrease in average abundance and biomass was observed. Despite subtle changes observed, none of the differences was significant.

Sciberras et al. (2017) examined sandy sediments collected from the Isle of Man, with different histories of bottom fishing disturbance, described as ‘Low’ and ‘High’ fishing frequency. Results found that the history of fishing activity influenced the community composition in the collected sediments. In sediments from sites which had a high frequency of fishing, there was a higher abundance of disturbance-tolerant suspension feeders, including Nephtys sp. (Sciberras et al., 2017). In addition, other studies have reported tube-dwelling polychaetes associated with low trawling frequencies (Beauchard et al., 2023). Model results have suggested that low levels of trawling (once or twice a year) may increase the productivity of small polychaetes, but higher trawling frequency lowers benthic production across all taxa (Hiddink et al., 2008, cited in Collie et al., 2017).

Clarke et al. (2018) examined pump-scoop dredging in Poole Harbour using a Before-After-Control-Impact (BACI) design with three study sites: a long-term heavily dredged area, a newly opened dredged area and a control with no dredging. They reported that Streblospio shrubsolii density also dramatically increased at the newly dredged site compared to other sites (Clarke et al., 2018).

Long-term monitoring of the Wadden Sea found that cockle dredging had no significant effect on the recruitment of Macoma balthica (reported as Limecola balthica), and survival of recruits did not differ between fishing and non-fishing years (Beukema & Dekker, 2018; 2020; Van Der Meer & Folmer, 2023).

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 (Suspended sediment pressure definition).

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 the characterizing species. Evidence has shown that suspended particulate organic matter (SPOM) can be an important food source, supporting growth in estuarine bivalves (Jung et al., 2019). For example, in the western Wadden Sea, Macoma balthica relies on SPOM in March, but shows seasonal flexibility and can shift feeding modes in summer months when freshwater runoff, providing the SPOM, is low (Jung et al., 2019).

Increases in turbidity may reduce benthic diatom productivity and the 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 limit prey resources, where these are suspension feeders relying on organic solids or phytoplankton.

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 assessment. Resistance is ‘High’ as no significant negative effects are identified and potential benefits from increased food resources may occur, based on expert judgement.  Resilience is also ‘High’ as no recovery is required under the likely impacts. Therefore, the sensitivity of the biotope is 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 (Smothering pressure definition).

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.  

Powilleit et al. (2009) studied the response of the polychaete Nephtys hombergii to smothering. This species successfully migrated to the surface of a 32 to 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. 

In the eastern Bay of Seine, Northern France, Nephtys hombergii was consistently amongst the most abundant taxa over a monitoring period (1988 to 2016), despite an increase in siltation from around 2006 (Bacouillard et al., 2020). It was suggested that the siltation increased due to changes in morpho-sedimentary dynamics and large inputs of dredged sediments from the extension of the Le Havre harbour (Bacouillard et al., 2020).  

Macoma balthica can burrow both vertically and horizontally through the substratum. Macoma balthica is probably not sensitive to smothering by a layer of sediment 5 cm thick, as it is a mobile species able to burrow upwards and surface from a depth of 5 to 6 cm (Brafield & Newell, 1961; Brafield, 1963; Stekoll et al., 1980). Turk and Risk (1981) investigated the effect of experimentally induced sedimentation (through fences and boxes that induced sediment deposition on intertidal mudflats in the Bay of Fundy) of 1 to 3.5 cm at a rate of 1.9 to 10.2 cm/month. The results showed that Macoma balthica was generally unaffected. 

Ehrnsten et al. (2019a) reported that increased organic matter sedimentation, increased food availability and generally increased Macoma balthica (reported as Limecola balthica) biomass. However, this positive effect was counteracted by higher bottom temperatures, because increased temperatures increased degradation of the food available and metabolic cost, reducing biomass in response to sedimentation. 

Sensitivity assessment. Based on the available evidence, resistance of the characterizing species Nephtys hombergii and Macoma balthica is likely to be 'High', and biotope resistance is assessed as 'High', although some short-term changes in sediments may occur.  Hence, resilience is also ‘High’, and sensitivity is assessed as ‘Not Sensitive’ at the benchmark level. 

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Limited evidence was found on responses of the characterizing species to a deposition of up to 30 cm 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 90 cm of burial in response to nourishment (Speybroek et al., 2007). Peterson et al. (2000) found that the dominant macrofauna were reduced by 86 to 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. Some impacts on some characterizing species may occur, and it is considered unlikely that significant numbers of the population could reposition, based on Bolam (2011). Polychaete species have been reported to migrate through depositions of sediment greater than the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Placement of the deposit will, therefore, result in a defaunated habitat until the deposit is recolonized.

Powilleit et al. (2009) studied the response of the polychaete Nephtys hombergii to smothering. This species successfully migrated to the surface through between 32 and 41 cm of 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.

In the eastern Bay of Seine, Northern France, Nephtys hombergii was consistently amongst the most abundant taxa over a monitoring period (1988 to 2016), despite an increase in siltation from around 2006 (Bacouillard et al., 2020). It was suggested that the siltation increased due to changes in morpho-sedimentary dynamics and large inputs of dredged sediments from the extension of the Le Havre harbour (Bacouillard et al., 2020).

Macoma balthica is able to burrow both vertically and horizontally through the substratum. Macoma balthica is probably not sensitive to smothering by a layer of sediment 5 cm thick, as it is a mobile species able to burrow upwards and surface from a depth of 5 to 6 cm up to 10 cm (Brafield & Newell, 1961; Brafield, 1963; Stekoll et al., 1980; Bhuiyan et al., 2025). Turk & Risk (1981) investigated the effect of experimentally induced sedimentation (through fences and boxes that induced sediment deposition on intertidal mudflats in the Bay of Fundy), of 1 to 3.5 cm at a rate of 1.9 to 10.2 cm/month. The results showed that Macoma balthica was generally unaffected.

Ehrnsten et al. (2019a) found that increased organic matter sedimentation and increased food availability generally increased Macoma balthica (reported as Limecola balthica) biomass. However, this positive effect was counteracted by higher bottom temperatures, because increased temperatures increased degradation of the food available and metabolic cost, reducing biomass in response to sedimentation.

Bolam (2011) showed that Streblospio shrubsolii vertical migration capability was reduced by deposition of just 6 cm of 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.

Sensitivity assessment. Deposition of up to 30 cm of fine material is likely to provide different impacts for the different species that characterize the biotope. Nephtys hombergii is likely to burrow and reposition through a fine sediment overburden at the pressure benchmark (30 cm), but other species, such as Steblospio shrubsolii and Macoma balthica, may be smothered. Therefore, resistance is assessed as ‘Low’, and resilience is assessed as ‘High’, so sensitivity is assessed as 'Low'.

Litter

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

Evidence

No evidence was found on the impact of litter on the characteristic species for this biotope, although studies show impacts from ingestion of microplastics 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, was shown to have an impact on ragworms. Hediste diversicolor showed increased burrowing times, 30% weight loss and a greater than 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 the impacts of litter on infauna, and this UK study suggests the health of infauna populations is 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 on. Wright et al. (2013) reported that the 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 colonization of sediment by other species, reducing diversity in the biotopes the species occurs within. 

Sensitivity assessment. Evidence and confidence in the assessment are limited, and this pressure is 'Not assessed'.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT (Electromagnetic pressure definition).

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. No studies examining the effect of EMFs on macroalgae were found. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms, and there is a lack of evidence on the effects of electric and magnetic fields on many of the characteristic species within this biotope.

Laboratory evidence found that 12-day exposure of 1 mT (50 Hz) EMF to Hediste diversicolor and Macoma balthica (reported as Limecola) caused a significant increase in the frequencies of genotoxicity and cytotoxicity endpoints, which can potentially affect the integrity of genetic material and cause a variety of diseases (Stankevičiūte et al., 2019). The strongest responses were seen in Macoma balthica, as six out of eight endpoints had increased, and Stankevičiūte et al. (2019) concluded that this bivalve species could be the most suitable bioindicator for EMF-induced effects. However, the survival of Hediste diversicolor and Macoma balthica in EMF-exposed treatments was 100% and 92%, respectively, with individuals spending most of the exposure time buried in the sediment (Stankevičiūte et al., 2019).

Field measurements of electric fields at North Hoyle wind farm, North Wales, recorded 110 µV/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 potential predators of Hediste diversicolor, such as the thornback ray Raja clavata and attraction to the source in catshark Scyliorhinus canicula (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 a 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. Recent evidence has suggested that after short-term exposure to EMF, the species characteristic of this biotope were able to survive (Stankevičiūte et al., 2019). However, low frequency EMF could cause sublethal cytotoxic and genotoxic effects, although at levels a hundred-fold higher than the benchmark. At present, there is ‘Insufficient evidence’ on which to base an assessment. 

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. Wang et al. (2022)’s recent laboratory experiments demonstrated that low-frequency noise (10 to 500 Hz of added noise) had no significant effect on the burial depth or bioturbation rates of Macoma balthica (reported as Limecola balthica), but anti-burrowing behaviour was observed which indicates potential signs of a stress response which could reduce its bioturbation potential (Wang et al., 2022).

Sensitivity assessment. However, at the benchmark level the community is unlikely to be sensitive to noise and given the lack of data and evidence. Sensitivity is therefore recorded as ‘Insufficient evidence’.

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means (Introduced light or shade pressure definition).

Evidence

Since 2016, research on artificial light at night (ALAN) has expanded considerably in the marine and coastal environment. Light was previously assumed to be of low ecological significance in subtidal and intertidal habitats, but there is now evidence that ALAN is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50 m (Davies et al., 2020; Smyth et al., 2021). ALAN can alter biological processes across taxa and at multiple levels of organization. Documented responses include disruption of diel and circalunar rhythms, changes in activity and foraging, altered predator–prey interactions, shifts in community composition, and impacts on algal growth and phenology (Davies et al., 2014, 2015; Gaston et al., 2017; Tidau et al., 2021; Lynn et al., 2022; Marangoni et al., 2022; Miller & Rice, 2023; Ferretti et al., 2025). Evidence for benthic habitats and assemblages specifically is beginning to emerge (e.g. Trethewy et al., 2023; Schaefer et al., 2025), but remains limited and fragmented, often focusing on single taxa or short-term experiments. Mortality thresholds, long-term consequences, and responses at the biotope scale are rarely addressed, and there are major gaps around indirect effects such as trophic cascades or habitat modification.

Garratt et al. (2019) was one of the first studies to focus on the consequences of artificial light exposure on intertidal organisms (Saenz-Arias et al. 2024). The study mapped ALAN from a High Pressure Sodium promenade lighting across Llandudno West Shore Beach, North Wales, and sampled macroinvertebrates at 54 stations along an illumination gradient (5.12 lux to 0.005 lux) at three shore heights: high shore (1 to 1.5 m), middle shore (-0.25 to 0.25 m), and low shore (-1.5 to -1 m). They found that the community composition shifted with the degree of artificial light exposure, even after accounting for other environmental variables (shore height, particle size and organic matter). Overall, species richness and biomass increased with increasing illuminance, a relationship which was enhanced with increasing organic enrichment. Nephthys spp. significantly decreased in abundance or probability of occurrence with increasing illuminance (Garratt et al., 2019). Garratt et al. (2019) suggested that the ALAN may directly disrupt reproductive light cues in many macroinvertebrates and increase predation risk on species that aggregate around illuminated areas. 

Sensitivity assessment. Artificial light is unlikely to affect any but the shallowest biotopes. Given the rapid expansion of the evidence base but the continuing lack of data at the level of individual biotopes, resistance and resilience cannot be robustly assessed. Therefore, sensitivity is recorded as ‘Insufficient evidence’.

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 (Barrier to species movement pressure definition).

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 (Adamset 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 (Death for collision pressure definition).

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 (Visual disturbance pressure definition). 

Evidence

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

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species may result in changes in the genetic structure of local populations, hybridization, or a change in community structure (Translocation pressure definition).

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 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) (pathogen or disease pressure definition).

Evidence

No evidence on the effect of microbial pathogens or metazoan disease vectors was found for Nephtys hombergii. Macoma balthica in Delaware Bay, north-east USA, was found to host Perkinsus genus pathogens Perkinsus andrewsi and Perkinsus marinus (Lindsay et al., 2007).

Dairain et al. (2020) reported that disseminated neoplasia, a leukaemia-like disease common amongst marine bivalves, was highly prevalent (21 to 89%) in Macoma balthica (recorded as Limecola balthica) in the Dutch Wadden Sea, and the local prevalence of the disease increased with higher Macoma balthica density. This high disease prevalence may affect population dynamics by increasing mortality and could explain the decline in adult survival in the European Wadden Sea over the past two decades. Further evidence is required to conclude that disseminated neoplasia causes mortality in Macoma balthica, but current literature shows that the disease usually increases mortality amongst affected species. Cerastoderma edule has been reported to host approximately 50 viruses, bacteria and fungi, including turbellaria, digeneans and cestodes (Longshaw & Malham, 2013).

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. Hence, 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 (targeted removal pressure definition).

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). The recovery of Nephtys hombergii has been assessed to be high, as repopulation could 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 assessed as ‘Low’ due to direct removal of a characteristic species, which 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, so sensitivity is assessed as 'Low'. It is important to consider that the spatial extent and duration of harvesting are 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 (non-targeted removed pressure definition).

Evidence

Hiddink (2003) showed that the density of Macoma balthica (as Limcola 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 characteristic 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 method and spatial extent.

McLusky et al. (1983) found that Macoma balthica (as Limcola balthica) populations were unaffected in dug areas, following bait digging for lugworms, suggesting the biotope would recover from this impact if it occurred over a limited spatial scale. 

Incidental removal of the characteristic 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, and their loss could alter the provision of food to these species.

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 assessed as ‘Low’.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and on the Essex coast from 1887 to 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, which 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 the intertidal zone 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 to 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 to 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 to 64% mortality of intertidal Crepidula and up to 97% on one mussel bed, compared to only 11 to 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.

Sensitivity assessment. 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 larval settlement (Tillin et al., 2020). Despite the 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. 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 there is a lack of direct evidence of Crepidula being reported to occur in the biotope.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024).

In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Michin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localised populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks, transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged from 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

While natural fragmentation of tendrils is thought to allow Didemnum vexillum to invade longer distances and increase its dispersal potential, Stefaniak & Whitlatch (2014) found that only one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight hours) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-mediated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffiths et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009).

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables and dredging (Holt, 2024). Recent evidence from genetic material suggests human-mediated dispersal, between marinas and shellfish culture sites, is the most likely pathway for connectivity of Didemnum vexillum populations throughout Ireland and Britain (Prentice et al., 2021; Holt, 2024). Didemnum vexillum can disperse away from artificial substrata, invading and colonizing natural substrata in surrounding areas (Tillin et al., 2020). Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. The current evidence of Didemnum vexillum’s ability to spread on natural habitats in this area is sparse and often conflicting, complicated by genetics, its apparent variable habitat preferences and tolerances and its variable ability to adapt to ‘new’ conditions (Holt 2024).

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July), colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September, sometimes continuing into December. Colonies begin to decline in health and ‘die off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold winter months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009; Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold winter (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in the Sandwich tide pool (temperature range from -1°C to 24°C, with daily fluctuations), probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4°C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a).  Larval release and recruitment typically occur between 14 and 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; McKenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C), and the lowest were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and can establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders (Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020). 

There are few observations of Didemnum vexillum on soft bottom habitats as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found established in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; McKenzie et al., 2017; Tillin et al., 2020). For example, at Georges Bank, USA, the Didemnum vexillum mats were limited to gravelly areas and unable to colonize the sand ridges that bounded the site, which have a mobile surface that is moved daily by the strong tidal currents (Valentine et al., 2007b). In addition, evidence found that the species can also not survive being buried or smothered by coarse or fine-grained sediment. Furthermore, in Holyhead Marina, Didemnum vexillum colonies were contained in the harbour and established on artificial pontoons, and they were not present on the natural seabed under the pontoon, which is composed of silty mud or on deeper sections of mooring chains that are immersed in mud at low spring tides (Griffiths et al., 2009). 

However, some studies on Georges Bank, USA, and Sandwich, Massachusetts, observed colonies were able to survive partial covering by sand (Bullard et al., 2007; Valentine et al., 2007a). Gittenberger et al. (2015) reported that Didemnum vexillum was able to overgrow the sandy bottom (cited Gittenberger, 2007). In the Netherlands, the coastal zone is composed of mud and sand, with only shells as hard substrata. Didemnum sp. remained rare until 1996, when populations quickly expanded, and it became a dominant invasive species because of an increase in available hard substrata for colonization after a cold winter between 1995 and 1996 caused a decrease in the abundance of many marine animals (Gittenberger, 2007). Thus, Didemnum vexillum was able to colonize and establish in mud and sand habitats where hard substrata were present.  

In contrast to Didemnum vexillum’s preference for sheltered conditions, established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al. (2007b) describe the substratum as immobile, presumably consolidated gravel, cobbles and pebbles. Kleeman (2009) stated that the presence of a consistent mild wave action or ‘swash zone’ appears to favour Didemnum sp. establishment in the intertidal. Although some evidence suggests that waves and currents can facilitate the fragmentation and spread of Didemnum vexillum (Mckenzie et al., 2017), the tidal current velocities at some sites where Didemnum vexillum has been reported (for example, New England, where current velocities reach up to around 3 m/s) is lower than the current velocity required for the dislodgement of Didemnum vexillum fragments (around 7.6 m/s) (Reinhardt et al., 2012). This suggests that not all tidal currents are likely to dislodge Didemnum vexillum fragments. When on boat hulls, the species can experience higher current velocities, which are enough to cause dislodgement (Reinhardt et al., 2012).   

Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020). It is abundant across various shore heights, thriving in both nearshore and offshore sites, particularly in subtidal areas. For example, colonies of Didemnum vexillum were dominant at depths between 45 and 60 m, occupying 50 to 90% of available space in two gravelly areas (more than 230 km²) composed of immobile pebble and cobble pavement on Georges Bank fishing ground, USA (Bullard et al., 2007; Valentine et al., 2007b; Lengyel et al., 2009). In addition, patchy mats have been observed covering approximately 1 to 1.5 km² of the pebble cobble seabed, which is interspersed with large boulders and 30 m deep in Long Island Sound, USA (Mercer et al., 2009). In an offshore scallop dredge survey, Didemnum sp. was found attached to cobbles and boulders at 10 to 34 m (Vercaemer et al., 2015). 

Sensitivity assessment. This biotope is likely to be unsuitable for the colonization of Didemnum vexillum due to the lack of gravel, shells, or any other hard substrata used for larval settlement. Despite the sheltered to extremely sheltered conditions of the habitat that would otherwise be suitable for Didemnum, the mobility of the sediment is unsuitable and makes it unlikely for Didemnum to become established. Therefore, resistance is assessed as ‘High’, albeit with low confidence due to no direct evidence of colonization in this biotope. Hence, resilience is assessed as ‘High’, and sensitivity is assessed as ‘Not sensitive’.

The Pacific oyster, Magallana gigas

Evidence

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 its 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./m^2, little of the underlying substratum remains visible (Herbert et al., 2016). These reefs can stabilise 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 composed mainly of 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 more quickly 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 substrata (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 that 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 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).

In the Wadden Sea, the Pacific oyster Magallana gigas has colonized intertidal flats (Smaal et al., 2005). This species consumes pelagic larvae, reducing recruitment (Smaal et al., 2005). 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 of bivalves such as Macoma balthica and polychaetes such as Steblospio shrubsolii. Burrowing infauna such as Nephtys hombergii and oligochaetes may persist within sediments, but the overall character of the mixed sediment biotope would be altered. Tillin et al. (2020) suggested that Magallana gigas could significantly alter intertidal sediments where adequate hard substrata or shell debris were present, and the conditions allow reefs of the oyster to form, based on its occurrence in Poole Harbour, and reefs on littoral sediments in southern England, Ireland, France and the Wadden Sea (Herbert et al., 2016b).  

Sensitivity assessment. 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). Although Magallana gigas has been found in intertidal mudflats and sandflats, the Pacific oysters were absent from sandflat areas in Poole Harbour (McKinstry & Jensen, 2013). The distribution of Magallana gigas can overlap with Macoma balthica and other native bivalve species, which occur in the mid to upper shores (Troost, 2010; Herbert et al., 2012, 2016). The mid-shore extent of this biotope is 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). Therefore, resistance to colonization by Magallana gigas is assessed as 'High' due to the lack of hard substrata in this biotope. Hence, resilience is assessed as 'High', so this biotope is assessed as 'Not sensitive'. 

Wireweed, Sargassum muticum

Evidence

Wireweed, Sargassum muticum, is a circumglobal invasive species (Engelen et al., 2015). Sargassum muticum is known to grow in the shallow subtidal around the UK, usually in areas sheltered from wave action. Its distribution is limited by the availability of hard substratum (e.g. stones >10 cm) and light (Staeher et al., 2000; Strong & Dring, 2011; Engelen et al., 2015).  It is most abundant between 1 and 3 m below mean water (Engelen et al., 2015).

Sensitivity assessment. This biotope is likely to be unsuitable for the colonization of Sargassum muticum due to the lack of hard substrata used for settlement. Despite the sheltered to extremely sheltered wave exposure conditions that would otherwise be suitable for Sargassum, this biotope may be too shallow for colonization. Therefore, resistance is assessed as ‘High’, albeit with low confidence due to no direct evidence of colonization in this biotope. Hence, resilience is assessed as ‘High’, and sensitivity is assessed as ‘Not sensitive’.

Wakame, Undaria pinnatifida

Evidence

Wakame, Undaria pinnatifida is a large brown seaweed and an Invasive Non-Indigenous Species (INIS) that could out-compete native UK kelp species (see Farrell & Fletcher, 2006; Thompson & Schiel, 2012; Brodie et al., 2014; Hieser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017; Epstein & Smale, 2018; Kraan, 2017; Epstein et al., 2019a,b; Tidbury, 2020).  Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016). It has since proliferated along UK coastlines. One year after its discovery at the Queen Anne Battery marina, Plymouth, it had become a major fouling plant on pontoons (Minchin & Nunn, 2014).  Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound.

Undaria pinnatifida seems to settle better on artificial substrata (e.g. floats, marinas or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018).  James (2017) suggested that Undaria pinnatifida could out-compete native species on artificial substrata (such as marinas and wharf structures). 

Undaria pinnatifida prefers sites sheltered with low wave exposure and weak tidal streams (Heiser et al., 2014; Epstein & Smale, 2018). In natural habitats, Undaria pinnatifida was not recorded if the wave fetch was greater than 642 km, but increased in abundance and cover in very sheltered sites (Epstein & Smale, 2018).

Sensitivity assessment. This biotope is likely to be unsuitable for the colonization of Undaria pinnatifida due to the lack of hard substrata used for settlement. Despite the low shore and wave sheltered to extremely sheltered conditions that would otherwise be suitable, the mobility of the sediment is unsuitable and makes it unlikely for Undaria to become established. Therefore, resistance is assessed as ‘High’, albeit with low confidence due to no direct evidence of colonization in this biotope. Hence, resilience is assessed as ‘High’, and sensitivity is assessed as ‘Not sensitive’.

Other INIS

Evidence

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). However, 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), but they could become significant predators of Cerastoderma edule and other species associated with the biotope in the future.

Evidence from the Balthica sea has shown that Macoma balthica can coexist in soft-sediment communities with invasive clam Rangia cuneata (Möller & Kotta, 2017; Karlson et al., 2024; Dziaduch et al., 2025). Macoma balthica is forced to switch from suspension feeding to deposit feeding in the presence of Rangia cuneata (Möller & Kotta, 2017).

Sensitivity assessment. The above evidence suggests that the characteristic species in the biotope have not yet been adversely affected by the INIS listed. Therefore, 'Insufficient evidence' is recorded until further evidence becomes available.