The Marine Life Information Network

Temperature increase (local)

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

Evidence

There was limited evidence available on the thermal thresholds and thermal ranges for the characterizing species recorded in this biotope, and there is no direct evidence on effects of temperature to these species. Very few laboratory studies have been carried out on the characterizing species, and the assessment relies on information on larvae in the plankton or monitoring of settlement and records of species distribution. Species from different areas may be acclimated to prevailing conditions and life histories may vary, e.g. Chamelea gallina longevity varies between populations (Gaspar et al., 2004) as does the longevity of Amphipholis squamata in different locations and habitats (Emson et al., 1989).

The characterizing bivalve Abra prismatica is widespread in the North Sea, around the coasts of Britain, less common on the western coast of Ireland to Norway, but also occurs down to the Mediterranean and northwest Africa. It is also found along the south and west coasts of Iceland and the Faroes. It has been recorded in temperatures ranging from 0 to 25°C (OBIS, 2024). Abra prismatica is similar to Abra alba, which also has a relatively wide temperature range of 9 to 23°C (Rombouts et al., 2012). Abra alba has been recorded in areas exposed to temperatures of 16.9 to 19.9°C in the Moroccan Atlantic coast (El Asri et al., 2015; 2022) and high annual temperatures of 21.8 to 23.8 °C in the Mediterranean (Fersi et al., 2023).

In a study modelling the effects of climate change on the distribution of benthic species in the Eastern Mediterranean, Moraitis et al. (2019) defined Abra prismatica as a tolerant species, but their model suggested the habitat suitability of the species was expected to decline as temperatures increase under predicted climate change (including scenario RCP 8.5) conditions by 2100. In a study on the changes in species occurrence as temperature increased in the English Channel from 1985 to 2012, Gaudin et al. (2018) observed extensions in the distribution range of Abra pristmatica and a more than 50% increase in its spatial occurrence in response to an increase in temperature over the time period. 

Bernard et al. (2016) experimentally tested how temperature and food availability affect Abra alba, a bioturbating species through its feeding and burrowing activity. The authors found that Abra alba showed much greater particle mixing at warmer ‘summer’ temperatures (around 18 to 22°C), with more frequent and longer sediment movements and higher calculated biodiffusion rates. In contrast, mixing activity was very low at cooler ‘autumn’ temperatures (around 14 to 16°C), suggesting that bioturbation by Abra alba is strongly temperature dependent and likely to decrease in colder conditions (Bernard et al., 2016).

Masanja et al. (2023) reported that bivalves were highly sensitive to temperature changes, and even minor deviations from their ideal temperature range could have an adverse impact on their physiology and behaviour. Despite some species acclimating over a long period of time, acute and rapid temperature increases may cause a more substantial stress (Masanja et al., 2023). Successful reproduction and bivalve growth depend on the preferred temperature range of 17 to 24°C (Masanja et al., 2023). Heat stress has been shown to cause cellular damage, oxidative stress, reduced growth and avoidance and deep burrowing behaviour.

Echinocyamus pusillus is widely distributed in the North East Atlantic Ocean, common in the North Sea and all around British and Irish coastlines. Its range extends from Iceland, Northern Norway and Denmark southwards to the Azores and the Mediterranean (OBIS, 2024). Richter & Bruckschen, (1988) found the variability in annual average temperatures experienced by Echinocyamus pusillus in 15 European locations (Baltic Sea, North Sea, Atlantic Ocean and Mediterranean Sea) was between 9.5 and 20°C. In their study, they found that the Mg content of the high–magnesium calcite skeleton was strongly related to and mainly controlled by the average annual water temperature (Richter & Bruckschen, 1988, cited by Asnaghi et al., 2014).  OBIS (2024) has recorded the species from 5 to 25°C, with the majority of records occurring between 10 to 15°C. Echinocyamus pusillus breeding season occurs in summer months, and planktonic larval development occurs from summer to autumn (Warwick & Pearce, 2020).

One of the characterizing polychaetes Ophelia borealis is widespread around the British Isles and the North Sea but less common on the Irish coasts. OBIS (2024) recorded this species from 0 to 20°C, with the majority of records occurring between 10 to 15°C. Similarly, Rombouts et al., (2012) defined the optimal temperature preferences of Ophelia borealis as narrow, between 9 and 16°C. The predominantly northern distribution of Ophelia borealis could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species. Their model suggested that Ophelia borealis would retreat from the Western English Channel to cooler northern regions of the North Sea due to warming by the end of the century (Rombouts et al., 2012). Ophelia borealis was observed to decrease in abundance between 1986 and 2000 in the North Sea,  largely associated with an increase in sea surface temperature (Kröncke et al., 2011; cited by Gaudin et al., 2018).

It has been reported that the polychaete Nephtys spp. can withstand summer temperatures of 30 to 35°C (Emery et al.  1957), so is likely to withstand the benchmark acute temperature increase. An acute increase in temperature at the benchmark level may result in physiological stress endured by the infaunal species but is unlikely to lead to mortality. Nephtys cirrosa is a temperate species, with a southern distribution and reaches the northern limit of its range in the north of the British Isles. Nepthys spp. is common in the North East Atlantic, occurring in the English Channel, the North Sea, the Baltic, the Barents Sea and to the Mediterranean. Nephtys cirrosa has been found in the Black Sea down to a depth of 45 m (Rainer, 1991). Nephtys hombergii has been reported from as far south as South Africa, suggesting the species can tolerate temperatures above even a 5°C increase in UK and Irish coasts.

Most species of Bathyporeia (e.g. Bathyporeia pelagica, Bathyporeia elegans, Bathyporeia sarsi) have a limited distribution, being primarily found around  the UK, and from the coast of Norway down to the French coast of the Bay of Biscay, and are abundant in the North Sea (Künitzer et al., 1992). The characterizing amphipod Bathyporeia elegans has been recorded in temperatures ranging from 5 to 20°C, with the majority of records occurring between 10 to 15°C (OBIS, 2025). Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in the Bandirma Gulf, Turkey, in temperatures, which range from 6.6°C to 27°C (Mülayim et al., 2015) and Bathyporeia sp. has been recorded in abundance and high frequency in temperatures ranging from 19.81 to 21.40°C in Portugal (Leitão et al., 2015).

Preece (1971) tested temperature tolerances of Bathyporeia pilosa in the laboratory. Individuals acclimated to 15°C for 24 hours were exposed to temperature increases (water temperature raised by 0.2°C/minute). As test temperature were reached individuals were removed, placed in seawater at 4°C and allowed to recover for 24 hours at which point mortalities were tested. Amphipods were also allowed to bury into sediments and held at test temperatures for 24 hours of 32.5°C, 31.8°C and 29.5°C before being allowed to recover in fresh seawater at 15°C for a further 24 hours, before mortalities were assessed. Upper lethal temperatures (LC50) for adult males and gravid females of Bathyporeia pilosa were 39.4°C. These tests measured short-term exposure only and species had lower tolerance for longer-term (24 hour exposure). No mortality occurred for Bathyporeia pilosa individuals held at 29.5°C and 30.8°C. However, 15% of individuals exposed to water temperatures of 31.8°C and 96% at 32.5°C died. Therefore, Bathyporeia spp. have high upper lethal temperature limits (24-hour LC50s) of 37.5°C for Bathyporeia pilosa and 33.4°C for Bathyporeia pelagica (Preece, 1971). The ability to withstand these high temperatures may be because they can be found in the intertidal, where temperatures fluctuate much more than the subtidal. While they can withstand a short-term, sharp temperature increase, their ability to withstand long-term changes in temperature is more difficult to discern.

Kröncke et al. (1998) examined long-term changes in the macrofauna in the subtidal zone off Norderney, one of the East Frisian barrier islands. Their analysis suggested that macrofauna were severely affected by cold winters whereas storms and hot summers have no impact on the benthos. A long-term increase in temperature might cause a shift in species composition. Long‐term analysis of the North Sea pelagic system has identified yearly variations in larval abundance of Echinodermata, Arthropoda, and Mollusca larvae that correlate with sea surface temperatures. Larvae of benthic echinoderms and decapod crustaceans increased after the mid‐1980s, coincident with a rise in North Sea sea surface temperature, whereas bivalve larvae underwent a reduction (Kirby et al., 2008). An increase in temperature may alter larval supply and in the long-term, and over large spatial scales, may result in changes in community composition.

Temperature cues influence the timing of gametogenesis and spawning in several species present in the biotope. Seasonal variations in reproductive cycle of Spisula solida were studied at a site off Vilamoura, southern Portugal. The onset of spawning took place in February when the seawater temperature began to increase and spawning ended in May. It is possible that Spisula solida does not spawn at a definite temperature, rather responding to the increase in seawater temperature (Gaspar & Monteiro, 1999). Many polychaete species, including Mediomastus fragilis, Owenia fusiformis and Protodorvillea kefersteini also show spring/early summer recruitment (Sardá et al., 1999).

Sensitivity assessment. UK assemblages in fine sands and muddy sands share many of the characterizing species that occur in the Mediterranean (see Sardá et al., 1999; Sardá et al., 2000), where temperatures are higher than experienced in the UK. It is considered likely, therefore, that a chronic change in temperature at the pressure benchmark would be tolerated by species with a wide distribution or that some species or groups of species would be resistant, such as characteristic polychaete or amphipod species (Emery et al., 1957; Preece, 1971). An acute change may exceed thermal tolerances or lead to spawning or other biological effects. These effects may be sub-lethal or result in the removal of only a proportion of less tolerant species. Biotope resistance is therefore assessed as ‘Medium’, and resilience is assessed as ‘High’. Biotope sensitivity is therefore assessed as ‘Low’.

Temperature decrease (local)

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

Evidence

Long‐term analysis of the North Sea pelagic system has identified yearly variations in larval abundance of Echinodermata, Arthropoda, and Mollusca larvae that correlate with sea surface temperatures. Larvae of benthic echinoderms and decapod crustaceans increased after the mid‐1980s, coincident with a rise in North Sea sea surface temperature, whereas bivalve larvae underwent a reduction (Kirby et al., 2008). A decrease in temperature may alter larval supply and, in the long term, over large spatial scales, may result in changes in community composition.

There was limited evidence available on the thermal thresholds and thermal ranges for the characterizing species recorded in this biotope, and there is no direct evidence on effects of temperature to these species. Very few laboratory studies have been carried out on the characterizing species, and the assessment relies on information on larvae in the plankton or monitoring of settlement and records of species distribution. Species from different areas may be acclimated to prevailing conditions and life histories may vary, e.g. Chamelea gallina longevity varies between populations (Gaspar et al., 2004) as does the longevity of Amphipholis squamata in different locations and habitats (Emson et al., 1989).

The characterizing bivalve Abra prismatica is widespread in the North Sea, around the coasts of Britain, less common on the western coast of Ireland to Norway, but also occurs down to the Mediterranean and northwest Africa. It is also found along the south and west coasts of Iceland and the Faroes. It has been recorded in temperatures ranging from 0 to 25°C (OBIS, 2024). Abra prismatica is similar to Abra alba, which also has a relatively wide temperature range of 9 to 23°C (Rombouts et al., 2012). Abra alba has been recorded in areas exposed to temperatures of 16.9 to 19.9°C in the Moroccan Atlantic coast (El Asri et al., 2015; 2022) and high annual temperatures of 21.8 to 23.8 °C in the Mediterranean (Fersi et al., 2023).

Bernard et al. (2016) experimentally tested how temperature and food availability affect Abra alba, a bioturbating species through its feeding and burrowing activity. The authors found that Abra alba showed much greater particle mixing at warmer ‘summer’ temperatures (around 18 to 22°C), with more frequent and longer sediment movements and higher calculated biodiffusion rates. In contrast, mixing activity was very low at cooler ‘autumn’ temperatures (around 14 to 16°C), suggesting that bioturbation by Abra alba is strongly temperature dependent and likely to decrease in colder conditions (Bernard et al., 2016).

Masanja et al. (2023) reported that bivalves were highly sensitive to temperature changes, and even minor deviations from their ideal temperature range could have an adverse impact on their physiology and behaviour. Despite some species acclimating over a long period of time, acute and rapid temperature increases may cause a more substantial stress (Masanja et al., 2023). Successful reproduction and bivalve growth depend on the preferred temperature range of 17 to 24°C (Masanja et al., 2023). Heat stress has been shown to cause cellular damage, oxidative stress, reduced growth and avoidance and deep burrowing behaviour.

Echinocyamus pusillus is widely distributed in the North East Atlantic Ocean, common in the North Sea and all around British and Irish coastlines. Its range extends from Iceland, Northern Norway and Denmark southwards to the Azores and the Mediterranean (OBIS, 2024). Richter & Bruckschen, (1988) found the variability in annual average temperatures experienced by Echinocyamus pusillus in 15 European locations (Baltic Sea, North Sea, Atlantic Ocean and Mediterranean Sea) was between 9.5 and 20°C. In their study, they found that the Mg content of the high–magnesium calcite skeleton was strongly related to and mainly controlled by the average annual water temperature (Richter & Bruckschen, 1988, cited by Asnaghi et al., 2014).  OBIS (2024) has recorded the species from 5 to 25°C, with the majority of records occurring between 10 to 15°C. Echinocyamus pusillus breeding season occurs in summer months, and planktonic larval development occurs from summer to autumn (Warwick & Pearce, 2020).

One of the characterizing polychaetes Ophelia borealis is widespread around the British Isles and the North Sea but less common on the Irish coasts. OBIS (2024) recorded this species from 0 to 20°C, with the majority of records occurring between 10 to 15°C. Similarly, Rombouts et al., (2012) defined the optimal temperature preferences of Ophelia borealis as narrow, between 9 and 16°C. The predominantly northern distribution of Ophelia borealis could suggest it is likely to move further northward as the climate changes and temperature shifts out of its preferred range, particularly as it is a cold temperate species. Their model suggested that Ophelia borealis would retreat from the Western English Channel to cooler northern regions of the North Sea due to warming by the end of the century (Rombouts et al., 2012). Ophelia borealis was observed to decrease in abundance between 1986 and 2000 in the North Sea,  largely associated with an increase in sea surface temperature (Kröncke et al., 2011; cited by Gaudin et al., 2018).

It has been reported that the polychaete Nephtys spp. can withstand summer temperatures of 30 to 35°C (Emery et al.  1957), so is likely to withstand the benchmark acute temperature increase. An acute increase in temperature at the benchmark level may result in physiological stress endured by the infaunal species but is unlikely to lead to mortality. Nephtys cirrosa is a temperate species, with a southern distribution and reaches the northern limit of its range in the north of the British Isles. Nepthys spp. is common in the North East Atlantic, occurring in the English Channel, the North Sea, the Baltic, the Barents Sea and to the Mediterranean. Nephtys cirrosa has been found in the Black Sea down to a depth of 45 m (Rainer, 1991). Nephtys hombergii has been reported from as far south as South Africa, suggesting the species can tolerate temperatures above even a 5°C increase in UK and Irish coasts.

Most species of Bathyporeia (e.g. Bathyporeia pelagica, Bathyporeia elegans, Bathyporeia sarsi) have a limited distribution, being primarily found around  the UK, and from the coast of Norway down to the French coast of the Bay of Biscay, and are abundant in the North Sea (Künitzer et al., 1992). The characterizing amphipod Bathyporeia elegans has been recorded in temperatures ranging from 5 to 20°C, with the majority of records occurring between 10 to 15°C (OBIS, 2025). Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in the Bandirma Gulf, Turkey, in temperatures, which range from 6.6°C to 27°C (Mülayim et al., 2015) and Bathyporeia sp. has been recorded in abundance and high frequency in temperatures ranging from 19.81 to 21.40°C in Portugal (Leitão et al., 2015).

It has been reported that the polychaete Nephtys spp. can withstand summer temperatures of 30 to 35°C (Emery et al.  1957), so is likely to withstand the benchmark acute temperature increase. An acute increase in temperature at the benchmark level may result in physiological stress endured by the infaunal species but is unlikely to lead to mortality. Nephtys cirrosa is a temperate species, with a southern distribution and reaches its northern limit of its range in the north of the British Isles. Nepthys spp. are common in the North East Atlantic, occurring in the English Channel, the North Sea, the Baltic, the Barents sea and to the Mediterranean. Nephtys cirrosa has been found in the Black Sea down to a depth of 45 m (Rainer, 1991). Nephtys hombergii has been reported from as far south as South Africa, suggesting the species can tolerate temperatures above even a 5°C increase in UK and Irish coasts.

Most species of Bathyporeia (e.g. Bathyporeia pelagica, Bathyporeia elegans, Bathyporeia sarsi) have a limited distribution, being primarily found around the UK, and from the coast of Norway down to the French coast of the Bay of Biscay, and are abundant in the North Sea (Künitzer et al., 1992). The characterizing amphipod Bathyporeia elegans has been recorded in temperatures ranging from 5 to 20°C, with the majority of records occurring between 10 to 15°C (OBIS, 2025). Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in the Bandirma Gulf, Turkey, in temperatures which range from 6.6 °C to 27 °C (Mülayim et al., 2015) and Bathyporeia sp. has been recorded in abundance and high frequency in temperatures ranging from 19.81 to 21.40 °C in Portugal (Leitão et al., 2015). The presence of Bathyporeia in the intertidal zone suggests that it is likely to be resistant to changes in temperature, including decreases in temperature and occasional winter frosts.

Kröncke et al. (1998) examined long-term changes in the macrofauna in the subtidal zone off Norderney, one of the East Frisian barrier islands. The analysis suggested that macrofauna were severely affected by cold winters whereas storms and hot summers have no impact on the benthos. A long-term increase in temperature might cause a shift in species composition. Long‐term analysis of the North Sea pelagic system has identified yearly variations in larval abundance of Echinodermata, Arthropoda, and Mollusca larvae that correlate with sea surface temperatures. Larvae of benthic echinoderms and decapod crustaceans increased after the mid‐1980s, coincident with a rise in North Sea sea surface temperature, whereas bivalve larvae underwent a reduction (Kirby et al., 2008).

Sensitivity assessment. Many of the characterizing species are found in more northern waters than the UK and may be adapted to colder temperatures, particularly the polychaete Ophelia borealis, as this is a cold-temperate species. Plankton studies suggest that colder waters may favour bivalve larvae. An acute change may exceed thermal tolerances or lead to spawning or other biological effects. The wide temperature ranges tolerated by characterizing species suggest some resistance to decreasing temperatures, but preferred temperatures for reproduction and growth is 17 to 24°C (Masanja et al., 2023). These effects may be sub-lethal or remove only a proportion of less tolerant species. Biotope resistance is therefore assessed as ‘Medium’ and resilience is assessed as ‘High’. Biotope sensitivity is therefore assessed as ‘Low’. 

Salinity increase (local)

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

Evidence

The assessed biotope occurs in full salinity (30 to 35 ppt) (JNCC, 2015, 2022). A change at the pressure benchmark, therefore refers to a change to hypersaline conditions (>40 psu). No directly relevant evidence was found to assess this pressure, but assessments can be made based on species distributions.

A study from the Canary Islands indicates that exposure to high salinity effluents (47 to 50 psu) from desalination plants alter the structure of biological assemblages, reducing species richness and abundance (Riera et al., 2012). Bivalves and amphipods appear to be less tolerant of increased salinity than polychaetes and were largely absent at the point of discharge. Polychaetes, including species or genera that occur in this biotope, such as Spio filicornis, Glycera spp. and Lumbrineris sp., were present at the discharge point (Riera et al., 2012). The ophiuroid Amphipholis squamata has been recorded in areas of high salinity (52 to 55 ppt) in the Arabian Gulf (Price, 1982), indicating local acclimation may be possible.

Abra prismatica has been recorded in sea surface salinities ranging from 0 to 40 psu, with the majority of records occurring between 30 to 40 psu (OBIS, 2025). Abra alba, a bivalve species similar to Abra prismatica, has also been recorded in sea surface salinities ranging from 0 to 40 psu, with the majority of records occurring between 30 to 35 psu (OBIS, 2025). Abra alba has been identified as a dominant species in tidal channels in the Mediterranean, exposed to salinities 36 to 47 (Fersi et al., 2023), and also in the Oualidia lagoon on the Moroccan Atlantic coast, exposed to salinities 10.1 to 39.5 ‰ (El Asri et al., 2015; 2022).

Polychaete can be highly abundant in estuarine conditions, making them able to tolerate changes in water salinity (Dauvin et al., 2017; Castellano et al., 2020) but their tolerance is species dependant. For example, Nephtys fluviatilis is an estuarine oligohaline and has been recorded preferring low salinity conditions between 4 to 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). Nephtys hombergii was recorded as one of the dominant species in the Bay of Seine, a complex estuarine environment with varying salinity and positively correlated with the euhaline zone (more than 30). Nepthy cirrosa and Nepthys assimilis were also abundant in the Bay of Seine, occurring in medium to fine sand and muddy assemblages, respectively. 

Nephtys hombergii is considered to be a brackish water species (Barnes, 1994) but where the species occurs in open coastal locations the species would have to tolerate salinities of 25 psu and above. Within a few months of the closure of a dam across the Krammer-Volkerak estuary in the Netherlands, Wolff (1971) observed that species with pelagic larvae or a free-swimming phase, expanded rapidly with a concomitant increase of salinity to 9-15 psu everywhere. Prior to the closure of the dam the estuary demonstrated characteristics of a typical 'salt-wedge' estuary with a salinity gradient from 0.3 to 15 psu. Hence, Nephtys hombergii is likely to survive increases in salinity within estuarine environments. In the estuarine Bay of Seine, Nephtys hombergii positively correlated with euhaline environmental conditions (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.

Gerbruk et al. (2023) identified polychaete Nepthys longoestosa as a dominant species at the marine end of a salinity gradient in Pechora Bay, where salinity increased from estuarine to near euhaline conditions (from around 18 to 26.3 psu surface salinity and around 26.5 to 29.4 psu near bottom salinity). Ophelia limacina and Scoloplos armiger were also present in the same marine microbenthic assemblage. These three polychaete species were described as stenohaline species (Gerbruk et al., 2023).

Bathyporeia elegans has been recorded in sea surface salinities ranging from 5 to 40 psu, with the majority of records occurring between 30 to 35 psu (OBIS, 2025). Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in a salinity range of 21.32 to 36.03 psu in the Bandirma Gulf, Turkey (Mülayim et al., 2015). Speybroeck et al. (2008) noted that Bathyporeia pilosa tends to occur subtidally in estuarine and brackish conditions. Bathyporeia pilosa is tolerant of low salinities, and it is capable of reproducing at salinities as low as 2 (Khayrallah, 1977). Populations of Bathyporeia pilosa within the upper reaches of the Severn Estuary experience wide fluctuations in salinity ranging from 1 to 22, depending on the season and tidal cycle (Mettam, 1989). The physiological stress resulting from this environment, however, affects size and reproduction (Mettam, 1989).

Sensitivity assessment. High saline effluents alter the structure of biological assemblages (Roberts et al., 2010b; Riera et al., 2012). Some of the characteristic polychaete species may be more tolerant than bivalves and amphipods, so that an increase in salinity may lead to a shift in community composition. Roberts et al. (2010b) reported that hypersaline effluents tend to disperse within ca 10 m of the discharge point but also sink to the seabed. This biotope is unlikely to be exposed to hypersaline effluents from coastal installations due to its depth, but might be exposed to saline seeps that arise at depth. However unlikely, exposure to hypersaline effluent would probably be detrimental to many of the characteristic species. Hence, resistance is assessed as ‘Low’ and resilience as ‘Medium’, as bivalve recovery may depend on episodic recruitment. Biotope sensitivity is assessed as ‘Medium’.

Salinity decrease (local)

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

Evidence

The assessed biotope occurs in full salinity (30 to 35 ppt) (JNCC, 2015). A change at the pressure benchmark, therefore refers to a change from full salinity to reduced. No directly relevant evidence was found to assess this pressure, but assessments can be made based on species distributions.

Abra prismatica has been recorded in sea surface salinities ranging from 0 to 40 psu, with the majority of records occurring between 30 to 40 psu (OBIS, 2025). Abra alba, a bivalve species similar to Abra prismatica, has also been recorded in sea surface salinities ranging from 0 to 40 psu, with the majority of records occurring between 30 to 35 psu (OBIS, 2025). Abra alba has been identified as a dominant species in tidal channels in the Mediterranean, exposed to salinities 36 to 47 (Fersi et al., 2023), and also in the Oualidia lagoon on the Moroccan Atlantic coast, exposed to salinities 10.1 to 39.5 ‰ (El Asri et al., 2015; 2022).

Polychaete can be highly abundant in estuarine conditions, making them able to tolerate changes in water salinity (Dauvin et al., 2017; Castellano et al., 2020) but their tolerance is species dependant. For example, Nephtys fluviatilis is an estuarine oligohaline and has been recorded preferring low salinity conditions between 4 to 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). Nephtys hombergii was recorded as one of the dominant species in the Bay of Seine, a complex estuarine environment with varying salinity and positively correlated with the euhaline zone (more than 30). Nepthy cirrosa and Nepthys assimilis were also abundant in the Bay of Seine, occurring in medium to fine sand and muddy assemblages, respectively. 

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. Prior to the closure of the dam the estuary demonstrated characteristics of a typical 'salt-wedge' estuary with a salinity gradient from 0.3 to 15 psu. Hence, Nephtys hombergii is likely to survive increases in salinity within estuarine environments. In the estuarine Bay of Seine, Nephtys hombergii positively correlated with euhaline environmental conditions (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.

Gerbruk et al. (2023) identified polychaete Nepthys longoestosa as a dominant species at the marine end of a salinity gradient in Pechora Bay, where salinity increased from estuarine to near euhaline conditions (from around 18 to 26.3 psu surface salinity and around 26.5 to 29.4 psu near bottom salinity). Ophelia limacina and Scoloplos armiger were also present in the same marine microbenthic assemblage. These three polychaete species were described as stenohaline species (Gerbruk et al., 2023).

Bathyporeia elegans has been recorded in sea surface salinities ranging from 5 to 40 psu, with the majority of records occurring between 30 to 35 psu (OBIS, 2025). Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in a salinity range of 21.32 to 36.03 psu in the Bandirma Gulf, Turkey (Mülayim et al., 2015). Speybroeck et al. (2008) noted that Bathyporeia pilosa tends to occur subtidally in estuarine and brackish conditions. Bathyporeia pilosa is tolerant of low salinities and it is capable of reproducing at salinities as low as 2 (Khayrallah, 1977). Populations of Bathyporeia pilosa within the upper reaches of the Severn Estuary experience wide fluctuations in salinity ranging from 1 to 22, depending on the season and tidal cycle (Mettam, 1989). The physiological stress resulting from this environment, however, affects size and reproduction (Mettam, 1989). Bathyporeia sp. was abundant and most frequently recorded in the periphery and surrounding areas of a submarine groundwater discharge seep, where the average salinity ranged from around 28.93 to 37.60 (Leitão et al., 2015). Bathyporeia sp. was absent or less abundant at the seep, where salinity was highly variable (ranging from 4.94 to 35.60) and considerably lower than the surrounding areas, but overall, the differences in salinities in all sites were less pronounced (Leitão et al., 2015). This suggests some tolerance to variable or reduced salinity, but may avoid low salinity.

Sensitivity assessment. Species tolerances to decreases in salinity are likely to vary, depending on species, but changes in sensitivity are likely to result in changes in species richness and abundance, with some shift in species composition. Distribution evidence suggests that some polychaete species (for example Nepthys spp.) can occur in estuarine conditions and therefore may be more resistant to a decrease in salinity.  However other stenohaline, fully marine species characterizing this biotope (for example Abra spp., Nepthys longoestosa, Scoloplos armiger, Ophelia limacine) are likely to be more sensitive to salinity decrease. Therefore, a decrease in salinity for a year can reduce the abundance of characterizing species and cause a shift in species composition to more tolerant taxa. Biotope resistance is therefore assessed as ‘Low’ and resilience as ‘Medium’, as bivalve recovery may depend on episodic recruitment. Biotope sensitivity is assessed as ‘Medium’.

Water flow (tidal current) changes (local)

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

Evidence

There is no direct evidence on the characterizing species’ tolerance to wave flow or wave exposure changes, but assessments can be made based on species distribution.

Abra alba has been reported to be highly abundant and dominant in environments associated with strong tidal currents and high wave energy (Foulquier et al., 2020; El Asri et al., 2022). Dauvin et al. (2017) reported that the Abra alba community in the Bay of Seine remained relatively stable and resilient over decades despite strong hydrodynamic conditions (regular swell and tidal currents), river discharge, sediment changes and anthropogenic impacts from dredging and port construction. The top eight dominant species (including Abra alba, Owenia fusiformis and Nephtys hombergii had remained the same over 25 years (Dauvin et al., 2017). The study also found that Abra alba and Spiophanes bombyx positively correlated with periods of low current speed (Dauvin et al., 2017). Similarly, Baffreau et al., (2017) noted that Abra alba muddy fine sand communities (described as EUNIS code A5.244) in the Bay of Seine typically occurs in the western and eastern subtidal zone exposed to weaker tidal currents (less than 1.5 knots, ca 0.77 m/s), while Ophelia borealis, Nepthys cirrosa, Spiophanes bombyx and Echinocyamus pusillus fine and medium clean sand communities (described as EUNIS code A5.251) occur further offshore and exposed to stronger tidal currents (maximum speed around 3 knots, ca 1.5 m/s). This suggests that the species characterizing this biotope can tolerate different tidal currents, and therefore may tolerate changes in water flow.

Kröncke et al. (2018) reported Spiophanes bombyx and Bathyporeia elegans were amongst the characterizing species in macrofaunal communities in the near shore along a beach system (German North Sea). Bathyporeia elegans characterized the inner nearshore exposed to high hydrodynamic energy with current velocity between 0.24 to 0.35 m/s, while Spiophanes bombyx was more characteristic of the outer nearshore exposed to a more stable current velocity between 0.25 to 0.41 m/s (Kröncke et al., 2018).

Foulquier et al. (2020) reported that Owenia fusiformis and Nepthys cirrosa remained 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 1000 m³/s during floods, and significant wave heights ranging from 0.2 m to 5.2 m.In Jade Bay, German Wadden Sea Nepthys 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).

This biotope is recorded in areas where tidal flow varies between weak (>0.5 m/s) and negligible water flow with moderate exposed to or very sheltered from wave action (JNCC, 2015). Sands are less cohesive than mud sediments and a change in water flow at the pressure benchmark may alter sediment transport patterns within the biotope. Hjulström (1939), concluded that fine sand (particle diameter of 0.3 to 0.6 mm) was easiest to erode and required a mean velocity of 0.2 m/s. Erosion and deposition of particles greater than 0.5 mm require a velocity >0.2 m/s to alter the habitat. The topography of this habitat is shaped by currents and wave action that influence the formation of ripples in the sediment. Specific fauna may be associated with troughs and crests of these bedforms may form following an increase in water flow or disappear following a reduction in flow.

Sensitivity assessment. This biotope occurs in areas subject to weak water flows or wave action sufficient to maintaining the clean sand habitat with low silt content (<5%) (JNCC, 2015, 2022). Changes in water flow may alter the topography of the habitat and may cause some shifts in abundance. However, a change at the pressure benchmark (increase or decrease) is unlikely to affect biotopes that occur in mid-range flows and biotope resistance is therefore assessed as ‘High’, and resilience is assessed as ‘High,’ so that the biotope is considered to be ‘Not sensitive’.

Emergence regime changes

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

Evidence

Changes in emergence are 'Not relevant' to this biotope which is restricted to fully subtidal habitats. 

Wave exposure changes (local)

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

Evidence

There is no direct evidence on the characterizing species’ tolerance to wave flow or wave exposure changes, but assessments can be made based on species distribution.

Abra alba has been reported to be highly abundant and dominant in environments associated with strong tidal currents and high wave energy (Foulquier et al., 2020; El Asri et al., 2022). Dauvin et al. (2017) reported that the Abra alba community in the Bay of Seine remained relatively stable and resilient over decades despite strong hydrodynamic conditions (regular swell and tidal currents), river discharge, sediment changes and anthropogenic impacts from dredging and port construction. The top eight dominant species (including Abra alba, Owenia fusiformis and Nephtys hombergii had remained the same over 25 years (Dauvin et al., 2017). The study also found that Abra alba and Spiophanes bombyx positively correlated with periods of low current speed (Dauvin et al., 2017). Similarly, Baffreau et al., (2017) noted that Abra alba muddy fine sand communities (described as EUNIS code A5.244) in the Bay of Seine typically occurs in the western and eastern subtidal zone exposed to weaker tidal currents (less than 1.5 knots, ca 0.77 m/s), while Ophelia borealis, Nepthys cirrosa, Spiophanes bombyx and Echinocyamus pusillus fine and medium clean sand communities (described as EUNIS code A5.251) occur further offshore and exposed to stronger tidal currents (maximum speed around 3 knots, ca 1.5 m/s). This suggests that the species characterizing this biotope can tolerate different tidal currents, and therefore may tolerate changes in water flow.

Kröncke et al. (2018) reported Spiophanes bombyx and Bathyporeia elegans were amongst the characterizing species in macrofaunal communities in the near shore along a beach system (German North Sea). Bathyporeia elegans characterized the inner nearshore exposed to high hydrodynamic energy with current velocity between 0.24 to 0.35 m/s, while Spiophanes bombyx was more characteristic of the outer nearshore exposed to a more stable current velocity between 0.25 to 0.41 m/s (Kröncke et al., 2018).

Foulquier et al. (2020) reported that Owenia fusiformis and Nepthys cirrosa remained 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 1000 m³/s during floods, and significant wave heights ranging from 0.2 m to 5.2 m.In Jade Bay, German Wadden Sea Nepthys 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).

As this biotope occurs in sublittoral habitats, it is not directly exposed to the action of breaking waves. Associated polychaete species that burrow are protected within the sediment but the characterizing bivalves would be exposed to oscillatory water flows at the seabed. They and other associated species may be indirectly affected by changes in water movement, where these impact the supply of food or larvae or other processes. No specific evidence was found to assess this pressure.

Sensitivity assessment. The range of wave exposures experienced by this biotope and similar infralittoral and circalittoral biotopes is considered to indicate, by proxy, that the biotope would have ‘High’ resistance and by default ‘High’ resilience to a change in significant wave height at the pressure benchmark. The biotope is therefore classed as ‘Not sensitive’.

Transition elements & organo-metal contamination

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

Evidence

The capacity of bivalves to accumulate heavy metals in their tissues, far in excess of environmental levels, is well known. Reactions to sub-lethal levels of heavy metal stressors include siphon retraction, valve closure, inhibition of byssal thread production, disruption of burrowing behaviour, inhibition of respiration, inhibition of filtration rate, inhibition of protein synthesis and suppressed growth (see review by Aberkali & Trueman, 1985). Stirling (1975) investigated the effect of exposure to copper on Tellina tenuis. The 96-hour LC₅₀ for Cu was 1,000 µg/l. Exposure to Cu concentrations of 250 µg/l and above inhibited burrowing behaviour and would presumably result in greater vulnerability to predators. Similarly, burial of the venerid bivalve, Venerupis senegalensis, was inhibited by copper-spiked sediments, and at very high concentrations, clams closed up and did not bury at all (Kaschl & Carballeira, 1999). The copper 10-day LC₅₀ for Venerupis senegalensis was found to be 88 µg/l in sandy sediments (Kaschl & Carballeira, 1999).

Nephtys hombergii from the middle and lower reaches of Restronguet Creek contained appreciably higher concentrations of Cu (2,227 µg/g dry wt), Fe and Zn than comparable specimens of Hediste diversicolor (as Nereis diversicolor). However, amongst polychaetes within the creek, and 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. McQuillan et al. (2014) suggested that some species (Nephtys hombergii) had developed metal-resistant populations as a functional genetic trait to copper homeostasis.

Khayrallah (1985) examined the effect of mercuric chloride and ethyl mercuric chloride on Bathyporeia pilosa, a sand-dwelling amphipod, through toxicity tests conducted in the Tay Estuary, Scotland. The study used 108 factorial combinations, testing three salinities (10, 20, 35%), three temperatures (0, 10, 20°C), and six mercury concentrations (up to 0.75 mg/l). Toxicity was directly related to concentration and temperature but inversely related to salinity. Mortality increased with higher mercury exposure, especially at lower salinities and higher temperatures. The mean survival time (MST) for Bathyporeia pilosa exposed to 0.75 mg/l mercuric chloride at 35% salinity was 57.6 hours, while for ethyl mercuric chloride, it ranged from 6 to 24 hours. The organic mercury compound was more lethal and was absorbed nearly twice as fast as the inorganic form, though lethal concentrations were similar (~3.8 µg/g mercury in tissue). Khayrallah (1985) concluded that mercury pollution, particularly in estuarine environments with fluctuating salinities and temperatures, could significantly impact Bathyporeia pilosa populations, with potential ecological consequences for its predators and the wider ecosystem.

Strode & Balode (2013) examined the effect of heavy metal exposure on Baltic amphipods, including Bathyporeia pilosa, in Latvian territorial waters of the open Baltic Sea and the Gulf of Riga using laboratory-based acute toxicity tests. Juvenile Bathyporeia pilosa were exposed to cadmium (CdCl₂), copper (CuSO₄), and zinc (ZnSO₄·7H₂O) in controlled conditions for 48-hour and 96-hour periods. The determined 48-hour LC50 values for Bathyporeia pilosa were 1.46 mg/l for cadmium, 1.74 mg/l for copper, and 3.53 mg/l for zinc and the 96-hour LC50s were 0.35 mg/l for cadmium, 1.07 mg/l for copper, and 1.15 mg/l for zinc. The study found that Bathyporeia pilosa exhibited moderate resistance compared to other tested amphipods, with the freshwater species Gammarus pulex being among the most sensitive. Strode & Balode (2013) concluded that sensitivity varied significantly between species (p<0.05), with cadmium being the most toxic that was tested. The findings highlight the need for species specific ecotoxicological assessments in brackish environment and using native species for more accurate results, aligning with region specific environmental conditions.

Strode et al. (2017) examined the effect of metal-contaminated sediment on Bathyporeia pilosa and Corophium volutator. Bathyporeia pilosa exhibited survival rates ranging from 38% to 100%, while Corophium volutator had a survival range of 70 to 95%, indicating varied sensitivity between species, although the sediment was described as low toxicity.

Echinoderms are also regarded as being intolerant of heavy metals (e.g. Bryan, 1984; Kinne, 1984) while polychaetes are tolerant (Bryan, 1984).

Sensitivity assessment. The above evidence reports significant (25 to 75%) mortality due to heavy metal exposure in some of the characteristic bivalves, amphipods and polychaetes, depending on the metal, its concentration and environmental conditions (e.g. salinity and temperature). Therefore, the worst-case resistance to heavy metal contamination is assessed as ‘Low’, albeit with ‘Low’ confidence due to variation in response between species and metals. Hence, resilience is assessed as ‘High’ and sensitivity as ‘Low’.  Additional evidence on selected polychaetes, molluscs and amphipods is available in the relevant contaminant evidence reviews (https://www.marlin.ac.uk/sensitivity/contaminants).

Hydrocarbon & PAH contamination

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

Evidence

Suchanek (1993) reviewed the effects of oil on bivalves. Generally, contact with oil causes an increase in energy expenditure and a decrease in feeding rate, resulting in less energy available for growth and reproduction. Sublethal concentrations of hydrocarbons also reduce byssal thread production (thus weakening attachment) and infaunal burrowing rates. Conan (1982) investigated the long-term effects of the Amoco Cadiz oil spill at St Efflam beach in France. It was estimated that the delayed mortality effects on sand and mud biotas were 1.4 times as large as the immediate effects. Fabulina fabula (studied as Tellina fabula) started to disappear from the intertidal zone a few months after the spill and from then on was restricted to subtidal levels. In the following 2 years, recruitment of Fabulina fabula was very much reduced. The author commented that, in the long-term, the biotas most severely affected by oil spills are low energy sandy and muddy shores, bays and estuaries. In such places, populations of species with long and short-term life expectancies (e.g. Fabulina fabula, Echinocardium cordatum and Ampelisca sp.) either vanished or displayed long-term decline following the Amoco Cadiz oil spill. Polychaetes, however, including Nephtys hombergii, cirratulids and capitellids were largely unaffected.

Nephtys hombergii and other polychaetes were reported to be unaffected after the Amoco Cadiz spill while the West Falmouth spill eradicated the benthos. 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).

Dauvin (1987) examined the effect of the Amoco Cadiz oil spill on benthic amphipod populations, in particular Ampelisca sarsi, Ampelisca tenuicornis, Bathyporeia elegans, and Corophium crassicorne, in a long-term study conducted in the Pierre Noire area of the Bay of Morlaix, France. Dauvin (1987) monitored amphipod populations from 1978 to 1986, assessing mortality, recolonization, and population recovery following oil contamination. Ampelisca sarsi and Ampelisca tenuicornis were the most severely impacted, with Ampelisca populations suffering a 99% decline, and five out of six species disappearing completely. Ampelisca sarsi initially showed 99.4% population reduction, and while Ampelisca spinipes reappeared within one year, Ampelisca tenuicornis was not observed again until 1985, nearly eight years later, with total Ampelisca densities still 90% lower than pre-spill levels. Bathyporeia elegans also experienced severe declines, nearly disappearing from contaminated sediments. However, due to its greater mobility, recolonization occurred within three to five years and populations recovered faster than Ampelisca species. Corophium crassicorne, a burrowing species, was eliminated from affected areas and was not observed again by the end of the study, indicating the habitat was unsuitable after the spill. Other benthic species, such as polychaetes and the bivalve Abra alba, showed higher resilience, with Abra alba recovering within two to three years. Dauvin (1987) concluded that Ampelisca, Bathyporeia, and Corophium species were highly vulnerable to oil pollution due to their sediment-dwelling habits and limited dispersal abilities.

Dauvin (1998) examined the effect of the Amoco Cadiz oil spill on benthic invertebrate communities, with a focus on Ampelisca sp. but also Bathyporeia sp., in a long-term study conducted in the English Channel along the northern coast of France. The study analysed the impact of oil contamination on amphipod populations and compared recovery patterns with other benthic species, including Abra alba and polychaete worms. Ampelisca populations were severely affected, with some areas reporting complete disappearance in heavily contaminated sediments. Recovery was slow, with significant recolonization not occurring until five to seven years post-spill, due to the persistence of hydrocarbons in sediments. Other benthic species, such as Abra alba, showed greater resilience, with populations recovering within two to three years. Dauvin (1998) concluded that Ampelisca species were highly sensitive to oil pollution, making them valuable bioindicators of long-term ecological damage, with populations recovering 15 years after the initial spill. However, he noted that recovery rates varied depending on sediment type, hydrocarbon degradation, and larval dispersal dynamics, which influenced the duration for full ecosystem restoration.

Rostron (1998) examined the effects of the Sea Empress oil spill in the Milford Haven Waterway, Pembrokeshire, in 1996 on infauna committees, in particular amphipod populations including Ampelisca, Bathyporeia, and Corophium. Rostron (1998) concluded that amphipod species, especially Ampelisca were highly sensitive to oil pollution leading to mortalities amongst populations, compared to other benthic communities such as polychaetes which were less sensitive to oil. Rostron (1998) cited data from Chasse & Morvan (1978) of different infauna species following the Amoco Cadiz spill. After the Amoco Cadiz spill Ampelisca brevicornis had the lowest survival rate at 0%. Both Bathyporeia pilosa and Bathyporeia sarsi had survival rates of 10%, and Corophium volutator had the highest resistance to the oil pollution with 20% survival reported.

Nikitik & Robinson (2003) examined the impact of the Sea Empress (1996) oil spill on Ampelisca, Corophium, and Bathyporeia populations in the Milford Haven Waterway (Wales). They compared populations pre spill (1993) to post spill (2000). The spill caused a sharp decline in amphipod populations, particularly Ampelisca, while polychaete populations increased, reflecting shifts in community structure. Recovery was evident at all survey sites by 1998, with Ampelisca reaching pre-spill levels in some areas by 2000, though recovery remained incomplete in the middle Haven. Nikitik & Robinson concluded like other oil spill papers that amphipods (Ampelisca in particular) are highly sensitive to oil contamination, making them effective indicators of ecosystem disturbance following oil spills. The authors highlighted the polychaete/amphipod ratio as a potential bioindicator for oil pollution impacts and suggested that other amphipod groups, such as Harpinia spp. and Isaeidae, could be valuable indicators in future assessments. Nikitik & Robinson overall emphasized the need for long-term monitoring to capture sites specific variability in amphipod responses to oil spills.

Echinoderms also seem to be especially intolerant of the toxic effects of oil, probably because of the large amount of exposed epidermis (Suchanek, 1993). The high intolerance of Echinocardium cordatum to hydrocarbons was seen by the mass mortality of animals, down to about 20m depth, shortly after the Amoco Cadiz oil spill (Cabioch et al., 1978).

Dauvin (2000) The muddy fine sand Abra alba-Melinna palmata community from the Bay of Morlaix (western English Channel) was strongly polluted by hydrocarbons from the Amoco Cadiz oil spill in April 1978. Long-term changes of this community (1977-1996) showed that it was weakly affected by the spill. This was due to the low number and low abundance of sensitive species present on the community in normal conditions. Polychaetes, such as Chaetozone setosa dominated the community, supporting high levels of organic matter. Only two opportunistic polychaetes Mediomastus fragilis and Tharyx marioni increased in abundance just after the spill.

Sensitivity assessment. The above evidence reports some or significant (0 to 75%) mortality due to hydrocarbon exposure in some of the characteristic bivalves and some polychaetes, and up to severe mortality (>75%) in amphipods depending on the species, especially after oil spills. Amphipods, in particular, were severely affected by oil spills and recovery was protracted while Abra recovered within a few years.  Therefore, the worst-case resistance to hydrocarbon contamination is assessed as ‘Low’, albeit with ‘Low’ confidence due to variation in response between species.   Hence, resilience is assessed as ‘High’ and sensitivity as ‘Low’.  Additional evidence on selected polychaetes, molluscs and amphipods is available in the relevant contaminant evidence reviews (https://www.marlin.ac.uk/sensitivity/contaminants).

Synthetic compound contamination

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

Evidence

Stirling (1975) investigated the effects of phenol, a non-persistent, semi-synthetic organic pollutant, on Tellina tenuis. Exposure to phenol produced a measurable effect on burrowing at all concentrations tested, i.e. 50 mg/l and stronger. Sub-lethal effects of exposure to phenol included delayed burrowing and valve adduction to exclude the pollutant from the mantle cavity. After exposure to 100 mg/l for 24 hours, the majority of animals were extended from their shells and unresponsive to tactile stimulation. Following replacement of the phenol solution with clean seawater, good recovery was exhibited after 2 days for animals exposed to 50 mg/l and some recovery occurred after 4 days for animals exposed to 100 mg/l.

Collier & Pinn (1998) investigated the effect on the benthos of ivermectin, a feed additive treatment for infestations of sea-lice on farmed salmonids. The polychaete Hediste diversicolor was particularly susceptible, exhibiting 100% mortality within 14 days when exposed to 8 mg/m² of ivermectin in a microcosm. Arenicola marina was also intolerant of ivermectin through the ingestion of contaminated sediment (Thain et al., 1998; cited in Collier & Pinn, 1998) and it was suggested that deposit feeding was an important route for exposure to toxins. Beaumont et al. (1989) investigated the effects of tri-butyl tin (TBT) on benthic organisms. At concentrations of 1 to 3 µg/l there was no significant effect on the abundance of Hediste diversicolor after 9 weeks in a microcosm. However, no juvenile polychaetes were retrieved from the substratum and hence there is some evidence that TBT had an effect on the larval and/or juvenile stages.

Scanes et al. (1993) observed the effects of a spillage of the pesticide Aldrin on the biota of an estuarine beach, after an industrial accident in Hardys Bay, New South Wales. Water and sediment samples were taken three weeks following the spill, to measure the level of contamination caused by the spill. In addition, the abundance of some intertidal biota was determined at the site of the spillage and in other uncontaminated areas. The samples of water taken at the spill site were not contaminated by Aldrin three weeks post-spill; however, the sediment samples contained Aldrin. The abundance of polychaetes (including Nepthys spp.) was not significantly different when compared to the uncontaminated sites, but the abundance of Crustacea was greatly reduced.

Detergents used to disperse oil from the Torrey Canyon oil spill caused mass mortalities of Echinocardium cordatum (Smith, 1968) and its intolerance to TBT is similar to that of other benthic organisms with LC₅₀ values of 222ng Sn/l in pore water and 1594 ng Sn/g dry weight of sediment (Stronkhorst et al., 1999). Gammaridean amphipods have also been reported to be intolerant of TBT with 10-day LC₅₀ values of 1 to 48 ng/l (Meador et al., 1993).

Sensitivity assessment. The above evidence suggests that bivalves, polychaetes and echinoderms are probably sensitive to exposure to synthetic contaminants. However, the evidence presented is limited to only a few proxy species and examples of chemicals. Therefore, there is 'Insufficient evidence' to form the basis of an assessment at present.

Radionuclide contamination

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

Evidence

No evidence.

Introduction of other substances

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

Evidence

This pressure is Not assessed.

De-oxygenation

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

Evidence

Riedel et al. (2012) assessed the response of benthic macrofauna to hypoxia advancing to anoxia in the Mediterranean. Hypoxic and anoxic conditions were created for 3 to 4 days in a box that enclosed in-situ sediments. In general, molluscs were more resistant than polychaetes, with 90% surviving hypoxia and anoxia, whereas only 10% of polychaetes survived. Exposed individual Timoclea ovata and Tellina serrata survived the experiment but the exposed Glycera spp. died. In general, epifauna were more sensitive than infauna, mobile species more sensitive than sedentary species and predatory species more sensitive than suspension and deposit feeders. The test conditions did not lead to the production of hydrogen sulphide, which may have reduced mortalities compared to some observations.

Further evidence of sensitivity was available for some of the polychaete species associated with this biotope group. Rabalais et al. (2001) observed that hypoxic conditions in the north coast of the Gulf of Mexico (oxygen concentrations from 1.5 to 1 mg/l (1 to 0.7 ml/l) led to Lumbrineris sp. to leave the substratum and then lie motionless on the surface. Glycera alba was found to be able to tolerate periods of anoxia resulting from inputs of organic rich material from a wood pulp and paper mill in Loch Eil (Scotland) (Blackstock & Barnes, 1982). Niermann et al. (1990) reported changes in a fine sand community for the German Bight in an area with regular seasonal hypoxia. In 1983, oxygen levels were exceptionally low (<3 mg O₂/l) in large areas and <1 mg O₂/l in some areas. Species richness decreased by 30 to 50% and overall biomass fell. Owenia fusiformis were reduced in abundance significantly by the hypoxia Spiophanes bombyx was found in small numbers at some, but not all areas, during the period of hypoxia. Once oxygen levels returned to normal Spiophanes bombyx increased in abundance; the evidence suggests that at least some individuals would survive hypoxic conditions. 

Abra alba is typically found in organically enriched sediments where it may be present in high densities (Dauvin & Gentil, 1989; Khedhri et al., 2016; Sciberras et al., 2017; Foulquier et al., 2020; Dilmi et al., 2024). Experimental examination of the interactions between eutrophication and oxygen deficiency (2.4 to 3.5 mg O₂/l over a 93-day experimental period) revealed that Abra alba became inefficient in its use of the available organic matter under prolonged conditions of hypoxia (Hylland et al., 1996). Abra alba was also reported to be sensitive to lowered oxygen concentrations off the Swedish west coast (Rosenberg & Loo, 1988; Weigelt & Rumohr, 1986, both cited in Rees & Dare, 1993).

Benthic communities containing Abra alba, have been shown to influence chemical, physical and biological processes in marine sediments through bioturbation. Rius et al. (2018) showed evidence that muddy fine sand Abra alba communities in the English Channel persist and thrive in naturally oxygenated conditions, and drive high rates of sediment oxygen uptake, with oxygen saturation in the bottom waters between 90 to 100 % (9.0 mg/l to 10.0 mg/l) (Rius et al., 2018). This suggests that Abra alba communities contribute to sediment oxygen consumption and are likely to be sensitive to declines in oxygen.

Information concerning the reduced oxygen tolerance of Nephtys cirrosa was not found, but evidence (Alheit, 1978; Arndt & Schiedek, 1997; Fallesen & Jørgensen, 1991) indicated a similar species, Nephtys hombergii, to be very tolerant of episodic oxygen deficiency and at the benchmark duration of one week.

Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in the Bandirma Gulf, Turkey, in 4.04 to 11.26 mg/l dissolved oxygen concentrations (Mülayim et al., 2015). Laboratory studies by Khayrallah (1977) on Bathyporeia pilosa, indicated that it has a relatively poor resistance to conditions of hypoxia in comparison to other interstitial animals. However, Mettam (1989) and Sandberg (1997) suggest that Bathyporeia pilosa can survive short-term hypoxia.

Sensitivity assessment. Riedel et al. (2012) provide evidence on general sensitivity trends. The characterizing bivalves are likely to survive hypoxia at the pressure benchmark although Bathyporeia spp., and the polychaetes present, particularly the mobile predatory species such as Glycera and Nephtys may be less tolerant. As the biotope is characterized by bivalves and polychaetes, resistance is assessed as ‘Low’ and resilience as ‘High’ based on migration, water transport of adults and recolonization by pelagic larvae. Biotope sensitivity is assessed as ‘Low’.

Nutrient enrichment

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

Evidence

Bivalves, polychaetes and other invertebrate species are unlikely to be directly affected by changes in nutrient enrichment, although excessive enrichment could alter the community composition.

Abra alba has been described as an indicator of increased organic matter (Dilmi et al., 2024) and has been frequently recorded in benthic assemblages, commonly associated with organic enrichment (Dauvin & Gentil, 1989; Khedhri et al., 2016; Sciberras et al., 2017; Foulquier et al., 2020; Dilmi et al., 2024). Multiple studies have reported Abra alba being highly abundant and/or a dominant species at sites with elevated sediment organic matter and in close proximity to organically enriched industrial discharge and sewage outfalls.

For example, Abra alba has been recorded as the most dominant species at a shellfish farm in the Mediterranean, amongst species tolerant of organic enrichment (Dilmi et al., 2024), and is dominant in organically enriched sediment within tidal channels in the Mediterranean where detritus accumulates (Fersi et al., 2023). This evidence is consistent with findings from Boughrara Lagoon, in the Southwest Mediterranean, where the tolerant Abra alba increased in abundance despite improvements in measured nutrient concentrations following enlargement of the El-Kantra channel (Khedhri et al., 2016). The lagoon was described as an almost-closed, fine-sediment system impacted by increased pollution (such as aquaculture effluent, sewage outfalls, fishing ports and industrial discharges) and had high levels of nitrate, nitrite, phosphate and ammonium. These nutrient and organic levels significantly decreased after the expansion of the El-Kantra channel (e.g. nitrites 9.17 µg/l decreased to 0.20 µg/l; nitrates 55.53 µg/l to 2.46 µg/l; phosphates 61.07 µg/l to 0.44 µg/l; ammonia 14.23 µg/l to 4.07 µg/l), which allowed increased boat activities and increased water exchange into the lagoon (Khedhri et al., 2016). Khedhri et al., (2016) concluded that although physico-chemical conditions may have improved, the lagoon is still impacted by human pressures. The results found that species richness and abundance of macrofauna in the lagoon decreased, and  tolerant species, including Abra alba and Cerastoderma edule, increased in abundance.

Abra alba has been identified as one of the most abundant and dominant species in multiple benthic assemblages in the Oualidia lagoon, Moroccan Atlantic coast, exposed to both silty sediment with high organic content and coarser sand with low organic matter content and subject to strong tidal currents (El Asri et al., 2015; 2022). The Oualidia lagoon organic matter content ranged from 1.94 to 31.97 % (El Asri et al., 2015).

De Jong et al. (2015a) reported deposit feeders, Abra alba and Owenia fusiformis, dominated sites in the deepened shipping land and disposal sites in the Port of Rotterdam, which had the lowest mean bed shear stress, significantly higher sediment organic matter (2.1% mean sediment organic matter) and supported the highest biomass and species richness (De Jong et al. 2015a). In addition, Abra alba increased in abundance and became dominant following a summer flash flood in the Adour estuary coastal zone, off the French Basque coast, which caused a widespread fine sediment deposition (Foulquier et al., 2020). This site is a high energy and is naturally physically stressed and recovered rapidly within four months. Abra alba density decreased, and the benthic assemblage shifted to a more stable community as fine particles decreased (Foulquier et al., 2020). Owenia fusiformis and Nepthys cirrosa remained consistent components of the fauna at the site studied, characteristic of disturbed sandy mud habitats throughout the study period (Foulquier et al., 2020).

Evidence has suggested that many polychaete species can tolerate organic and nutrient enrichment. Dunlop et al. (2024) reported that open-net salmon farms create a clear organic enrichment gradient that drives increases in opportunistic polychaetes close to cages and shifts the benthic community structure. Opportunistic polychaetes (e.g. Capitella sp., Ophryotrocha sp., and Arenicola marina) are dominant infaunal communities close to farms (50 and 250 m from cages), while ophiuroids are more abundant farther away from the cages (360 m from cages) (Dunlop et al., 2024). The abundances decreased further away from the fish farms, as organic enrichment decreased, resulting in an organic enrichment gradient. Ophiuroids were abundant in farms away from the farms, where organic enrichment in sediments declines. 

For example, in oyster farms in the Bay of Veys, France, Scoloplos armiger increased in abundance following organic enrichment from decomposing oyster mortalities (Vanhuysse et al., 2021). Scoloplos armiger became one of the dominant species at the study sites, as more sensitive taxa declined, resulting in a shift towards more opportunistic and tolerant species (Vanhuysse et al., 2021). Al et al. (2022) reported that bivalves, amphipods, and polychaetes, including Nepthys 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, characterizing polychaete species Chaetozone setosa, Spiophanes bombyx, Scoloplos armiger, Nephtys cirrosa, and Owenia fusiformis, were absent or declined in abundance, and were more abundant in reference sites more than 1 km away from the farms (from table 2 in Martinez-Garcia et al., 2019). De Jong et al. (2015a) reported deposit feeding polychaetes Nepthys cirrosa and Spiophanes bombyx were 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. Owenia fusiformis, Abra alba, and Nephtys hombergii were among the ten dominant benthic species in the Bay of Seine. However, abundances of Owenia fusiformis and Abra alba declined at some impacted disposal sites. 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.

Sciberras et al. (2017) examined the chronic impacts of fishing frequency and organic matter enrichment on benthic communities and nitrogen cycling in sandy sediments from the Isle of Man. In laboratory conditions, sediment cores were enriched with a moderate dose of microalga, Isochrysis galbana, to simulate natural organic enrichment from an algal bloom. Results found that after one month, enrichment caused little difference in community composition, total density and species richness compared to non-enriched sediment. Sciberras et al. (2017) found that the macroinvertebrate community composition (including Abra prismtica, Bathyporeia gracilis, Scoloplos armiger, Spiophanes bombyx, and Echinocyamus pusillus) was dependent on the fishing history of the sediment collected rather than organic enrichment. Overall, enrichment acted as a food source for the sand-associated community, and nutrient cycling remained stable and efficient.

There is limited evidence on Bathypoeria spp. tolerance to organic and nutrient enrichment. However, Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in the Bandirma Gulf, Turkey in conditions with total organic carbon values between 0.07 to 4.42 %, total calcium carbonate between 0.88 to 84.82%, total phosphorus between 609 to 12740 μg/g (Mülayim et al., 2015).

Sensitivity assessment. The evidence presented shows that the characteristic species within this biotope have a strong tolerance to organic and nutrient enrichment. Long-term data from Swansea Bay shows that despite being impacted by sewage outfalls and industrial discharge between 1984 and 2014, species such as Bathyporeia pelagica, Nephtys spp., and Spiophanes bombyx persisted over the 30-year period (Callaway, 2016). Therefore, the biotope is assessed as ‘High’ resistance to this pressure and ‘High’ resilience, (by default) and is assessed as ‘Not sensitive’.

Organic enrichment

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

Evidence

At the pressure benchmark, organic inputs are likely to represent a food subsidy for the associated deposit-feeding species and are unlikely to significantly affect the structure of the biological assemblage or impact the physical habitat.

Abra alba has been described as an indicator of increased organic matter (Dilmi et al., 2024) and has been frequently recorded in benthic assemblages, commonly associated with organic enrichment (Dauvin & Gentil, 1989; Khedhri et al., 2016; Sciberras et al., 2017; Foulquier et al., 2020; Dilmi et al., 2024). Multiple studies have reported Abra alba being highly abundant and/or a dominant species at sites with elevated sediment organic matter and in close proximity to organically enriched industrial discharge and sewage outfalls.

For example, Abra alba has been recorded as the most dominant species at a shellfish farm in the Mediterranean, amongst species tolerant of organic enrichment (Dilmi et al., 2024), and is dominant in organically enriched sediment within tidal channels in the Mediterranean where detritus accumulates (Fersi et al., 2023). This evidence is consistent with findings from Boughrara Lagoon, in the Southwest Mediterranean, where the tolerant Abra alba increased in abundance despite improvements in measured nutrient concentrations following enlargement of the El-Kantra channel (Khedhri et al., 2016). The lagoon was described as an almost-closed, fine-sediment system impacted by increased pollution (such as aquaculture effluent, sewage outfalls, fishing ports and industrial discharges) and had high levels of nitrate, nitrite, phosphate and ammonium. These nutrient and organic levels significantly decreased after the expansion of the El-Kantra channel (e.g. nitrites 9.17 µg/l decreased to 0.20 µg/l; nitrates 55.53 µg/l to 2.46 µg/l; phosphates 61.07 µg/l to 0.44 µg/l; ammonia 14.23 µg/l to 4.07 µg/l), which allowed increased boat activities and increased water exchange into the lagoon (Khedhri et al., 2016). Khedhri et al., (2016) concluded that although physico-chemical conditions may have improved, the lagoon is still impacted by human pressures. The results found that species richness and abundance of macrofauna in the lagoon decreased, and  tolerant species, including Abra alba and Cerastoderma edule, increased in abundance.

Abra alba has been identified as one of the most abundant and dominant species in multiple benthic assemblages in the Oualidia lagoon, Moroccan Atlantic coast, exposed to both silty sediment with high organic content and coarser sand with low organic matter content and subject to strong tidal currents (El Asri et al., 2015; 2022). The Oualidia lagoon organic matter content ranged from 1.94 to 31.97 % (El Asri et al., 2015).

De Jong et al. (2015a) reported deposit feeders, Abra alba and Owenia fusiformis, dominated sites in the deepened shipping land and disposal sites in the Port of Rotterdam, which had the lowest mean bed shear stress, significantly higher sediment organic matter (2.1% mean sediment organic matter) and supported the highest biomass and species richness (De Jong et al. 2015a). In addition, Abra alba increased in abundance and became dominant following a summer flash flood in the Adour estuary coastal zone, off the French Basque coast, which caused a widespread fine sediment deposition (Foulquier et al., 2020). This site is a high energy and is naturally physically stressed and recovered rapidly within four months. Abra alba density decreased, and the benthic assemblage shifted to a more stable community as fine particles decreased (Foulquier et al., 2020). Owenia fusiformis and Nepthys cirrosa remained consistent components of the fauna at the site studied, characteristic of disturbed sandy mud habitats throughout the study period (Foulquier et al., 2020).

Evidence has suggested that many polychaete species can tolerate organic and nutrient enrichment. Dunlop et al. (2024) reported that open-net salmon farms create a clear organic enrichment gradient that drives increases in opportunistic polychaetes close to cages and shifts the benthic community structure. Opportunistic polychaetes (e.g. Capitella sp., Ophryotrocha sp., and Arenicola marina) are dominant infaunal communities close to farms (50 and 250 m from cages), while ophiuroids are more abundant farther away from the cages (360 m from cages) (Dunlop et al., 2024). The abundances decreased further away from the fish farms, as organic enrichment decreased, resulting in an organic enrichment gradient. Ophiuroids were abundant in farms away from the farms, where organic enrichment in sediments declines. 

For example, in oyster farms in the Bay of Veys, France, Scoloplos armiger increased in abundance following organic enrichment from decomposing oyster mortalities (Vanhuysse et al., 2021). Scoloplos armiger became one of the dominant species at the study sites, as more sensitive taxa declined, resulting in a shift towards more opportunistic and tolerant species (Vanhuysse et al., 2021). Al et al. (2022) reported that bivalves, amphipods, and polychaetes, including Nepthys 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, characterizing polychaete species Chaetozone setosa, Spiophanes bombyx, Scoloplos armiger, Nephtys cirrosa, and Owenia fusiformis, were absent or declined in abundance, and were more abundant in reference sites more than 1 km away from the farms (from table 2 in Martinez-Garcia et al., 2019). De Jong et al. (2015a) reported deposit feeding polychaetes Nepthys cirrosa and Spiophanes bombyx were 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. Owenia fusiformis, Abra alba, and Nephtys hombergii were among the ten dominant benthic species in the Bay of Seine. However, abundances of Owenia fusiformis and Abra alba declined at some impacted disposal sites. 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.

Sciberras et al. (2017) examined the chronic impacts of fishing frequency and organic matter enrichment on benthic communities and nitrogen cycling in sandy sediments from the Isle of Man. In laboratory conditions, sediment cores were enriched with a moderate dose of microalga, Isochrysis galbana, to simulate natural organic enrichment from an algal bloom. Results found that after one month, enrichment caused little difference in community composition, total density and species richness compared to non-enriched sediment. Sciberras et al. (2017) found that the macroinvertebrate community composition (including Abra prismtica, Bathyporeia gracilis, Scoloplos armiger, Spiophanes bombyx, and Echinocyamus pusillus) was dependent on the fishing history of the sediment collected rather than organic enrichment. Overall, enrichment acted as a food source for the sand-associated community, and nutrient cycling remained stable and efficient.

There is limited evidence on Bathypoeria spp. tolerance to organic and nutrient enrichment. However, Bathyporeia elegans and Bathyporeia guilliamsoniana have been recorded in the Bandirma Gulf, Turkey in conditions with total organic carbon values between 0.07 to 4.42 %, total calcium carbonate between 0.88 to 84.82%, total phosphorus between 609 to 12740 μg/g (Mülayim et al., 2015).

Sensitivity assessment. The evidence presented shows that the characteristic species within this biotope have a strong tolerance to organic and nutrient enrichment. Long-term data from Swansea Bay shows that despite being impacted by sewage outfalls and industrial discharge between 1984 and 2014, species such as Bathyporeia pelagica, Nephtys spp., and Spiophanes bombyx persisted over the 30-year period (Callaway, 2016). Therefore, the biotope is assessed as ‘High’ resistance to this pressure and ‘High’ resilience, (by default) and is assessed as ‘Not sensitive’.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

The biotope is characterized by the sedimentary habitat (JNCC, 2015), so a change to an artificial or rock substratum would alter the character of the biotope leading to reclassification and the loss of the sedimentary community including the characterizing bivalves, polychaetes and echinoderms that live buried within the sediment.

Jammar et al. (2025) reported that the installation of offshore wind farms (OWF) in soft sandy medium to coarse sediment habitats the Southern North Sea altered the seabed and shifted the microbenthic community structure. The OWF foundations provided new hard substrata, which increased the surfaces 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).

Sensitivity assessment. Based on the loss of the biotope, resistance is assessed as ‘None’, recovery is assessed as ‘Very Low’ (as the change at the pressure benchmark is permanent), and sensitivity is assessed as ‘High’.

Physical change (to another sediment type)

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

Evidence

This biotope is found in medium to very fine sand with some silt (JNCC, 2015, 2022). The change referred to at the pressure benchmark is a change in sediment classification (based on Long, 2006) rather than a change in the finer-scale original Folk categories (Folk, 1954).  For sand sediments, resistance is assessed based on a change to either mixed sediments or mud and sandy muds. 

Sediment type is a key factor structuring the biological assemblage present in the biotope. Surveys over sediment gradients and before-and-after impact studies from aggregate extraction sites where sediments have been altered indicate patterns in change. The biotope classification (JNCC, 2015, 2022) provides information on the sediment types where biotopes are found and indicates likely patterns in change if the sediment were to alter.  

Differences in biotope assemblages in areas of different sediment type are likely to be driven by pre and post recruitment processes. Sediment selectivity by larvae will influence levels of settlement and distribution patterns. Snelgrove et al. (1999) demonstrated that Spisula solidissima, selected coarse sand over muddy sand, and capitellid polychaetes selected muddy sand over coarse sand, regardless of site. Both larvae selected sediments typical of adult habitats, however, some species were nonselective (Snelgrove et al., 1999) and presumably in unfavourable habitats post recruitment, mortality will result for species that occur in a restricted range of habitats. Some species may, however, be present in a range of sediments. Post-settlement migration and selectivity also occurred on small scales (Snelgrove et al., 1999).

Cooper et al. (2011) found that characterizing species from sand dominated sediments were equally likely to be found in gravel dominated sediments, and an increase in sediment coarseness may not result in loss of characterizing species but biotope classification may revert to the biotope SS.SCS.CCS.MedLumVen, which occurs in gravels (JNCC, 2015).

Desprez (2000) found that a change of habitat to fine sands from coarse sands and gravels (from deposition of screened sand following aggregate extraction) changed the biological communities present. Tellina pygmaea and Nephtys cirrosa dominated the fine sand community. Dominant species of coarse sands, Echinocyamus pusillus and Amphipholis squamata, were poorly represented and the characteristic species of gravels and shingles were absent (Desprez, 2000).

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 deposit feeding polychaetes Nepthys cirrosa and Spiophanes bombyx were 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). Whereas deposit feeders, Abra alba and Owenia fusiformis, dominated sites in the deepened shipping land and disposal sites, which had the lowest mean bed shear stress, significantly higher sediment organic matter (2.1% mean sediment organic matter) and supported the highest biomass and species richness (De Jong et al. 2015a). Follow-up research by De Jong et al. 2015b) in a nearby 20 m deep burrow pit, supported this evidence and found that two years after cessation of sand extraction, macrozoobenthic biomass significantly increased fivefold in the deepest areas, and Abra alba was the most abundant species in the burrow pit. In the burrow pit and deepened shipping lane, there was a shift in sediment characteristics from fine to medium sand, to muddier fine sand (De Jong et al. 2015a;b). This supported a shift in community structure.

Dauvin et al. (2017) reported that the Abra alba community in the Bay of Seine remained relatively stable and resilient over decades despite strong sediment changes, hydrodynamic conditions, river discharge, and anthropogenic impacts from dredging and port construction. This suggested that it could tolerate changes in sediment.

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 environment characterized by an Ophelia borealis and Nepthys 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 Nepthys cirrosa and Abra alba community of medium and muddy fine sand. Despite this change, the original Nepthys 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 Abra alba, Owenia fusiformis and Nephtys hombergii were 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 Bathyporeia elegans, Nepthys cirrosa, Spiophanes bombyx and Abra alba was 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 higher abundance of Bathyporeia elegans, Nepthys cirrosa, Spiophanes bombyx and Abra alba 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 a short and long-term dumping periods (Pezy et al., 2017; Raoux et al., 2020). Also highlighting that Abra alba and Nephtys cirrosa have high tolerance to changes in sediment composition, particularly to increases in fine material.

Jammar et al. (2025) reported that the installation of offshore wind farms (OWF) in soft sandy medium to coarse sediment habitats the Southern North Sea altered the seabed and shifted the microbenthic community structure. The OWF foundations provided new hard substrata, which increased the surfaces 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).

Sensitivity assessment. A change to finer, muddy and mixed gravel sediments is likely to reduce the abundance of the characterizing Tellina spp., venerid bivalves and other bivalves such as Spisula solida, and favour polychaetes. Such changes would lead to biotope reclassification. However, evidence from opportunistic species Abra alba and Nepthys cirrosa, suggests some tolerance to a change to finer muddier sediment. This biotope (SS.SSa.CFiSa.ApriBatPo) is charactertic of medium to fine sands. It is one of a number of similar biotopes with similar infaunal communities (SS.SSa.IMuSa.FfabMag; SS.SSa.IMuSa.SsubNhom; SS.SSa.CFiSa.EpusOborApri; SS.SSa.CFiSa.ApriBatPo and SS.SSa.CMuSa.AalbNuc) that grade from fine sands to muddy sands with or without some mixed sediment.  Therefore, changes in sediment type would probably result in transition to another biotope, resulting in loss of the biotope under assessment. Hence, resistance is assessed as ‘Low’ (as some species may remain), resilience is ‘Very low’ (the pressure is a permanent change) and sensitivity is assessed as ‘High’.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Most of the animals that occur in this biotope are shallowly buried and extraction of the sediment will remove the biological assemblage. For example, Abra alba is a shallow burrower, positioned 0 to 5 cm in the seabed (Tebble, 1976; van Denderen et al., 2015). Bathyporeia pilosa burrows between 0-10 cm in sediments and is reported living in the uppermost 3 cm of sandy substratum (Nicolaisen & Kanneworff, 1969; Fish, 1970).

Following sediment extraction, any remaining species, given their new position at the sediment / water interface, may be exposed to unsuitable conditions. Newell et al. (1998) state that removal of 0.5 m depth of sediment is likely to eliminate benthos from the affected area. Therefore, removal of 30 cm of sediment will remove species that occur at the surface and within the upper layers of sediment, such as the characterizing species of these biotopes. Although, some epifaunal and swimming species may be able to avoid this pressure.

In terms of sand extraction, the recovery time for species assemblages, richness and biomass to return to pre-dredged conditions has been reported to be within 4 to 6 years (Van Dalfsen et al., 2000; Boyd et al., 2005; De Jong et al., 2015b). Boyd et al. (2005) found that in a site subject to long-term extraction (25 years), extraction scars were still visible after six years and sediment characteristics were still altered in comparison with reference areas, with ongoing effects on the biota.

In an area that had been subjected to intensive aggregate extraction for 30 years, the abundance of juvenile and adults Nephtys cirrosa had greatly increased three years after extraction had stopped (Mouleaert & Hostens, 2007). Boyd et al. (2005) found that in a site subject to long-term extraction (25 years), extraction scars were still visible after six years and sediment characteristics were still altered in comparison with reference areas, with ongoing effects on the biota. Therefore, polychaete and amphipods are likely to recover more rapidly than the characterizing bivalves and the biotope classification may revert, during recovery, to a polychaete-dominated biotope.

Evidence from De Jong et al., (2015a;b) demonstrated that deep sand extraction caused a community shift, and two years after cessation of sand extraction, macrozoobenthic biomass significantly increased fivefold in the deepest areas, and Abra alba was the most abundant species in the burrow pit. Similarly, Pezy et al. (2017; 2018) reported that Abra alba recolonised dredged and disposal sites in the Bay of Seine, after the dumping of muddy fine sands.

Sensitivity assessment. Resistance is assessed as ‘None’ as the extraction of the sediment will remove the characterizing and associated species present. Resilience is assessed as ‘Medium’ as some species may require longer than two years to re-establish (see resilience section), and sediments may need to recover (where exposed layers are different). Biotope sensitivity is therefore assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

The epifauna and infaunal assemblages of both stable and dynamic fine sands are susceptible to direct physical disturbance from fishery bottom trawling. Evidence has shown that sublittoral sand biotopes (SS.SSa EUNIS code A5.2) including circalittoral fine sands (SS.SSa.CFiSa) are amongst the most extensively trawled habitats in the Northeast Atlantic, North Sea and Mediterranean Sea (Eigaard et al., 2017; Collie et al., 2017; Eggleton et al., 2018).The impacts of bottom trawling vary amongst benthic species, fishing gear type, and fishing intensity and frequency (van Denderen et al., 2015; Sciberras et al. 2017; Hiddink et al., 2017; Bradshaw et al., 2024). In general, short-lived opportunistic species and deep burrowing species appear to be less affected by severe disturbance by bottom trawling, often rapidly colonizing disturbed habitats (Pearson & Rosenberg, 1978, Tillin et al., 2006; van Denderen et al., 2015; Beauchard et al., 2023; Bradshaw et al., 2024). Surface-dwelling and mobile swimmers and crawlers (such as Bathyporeia spp. and Nephtys spp.) may also be less affected as they can quickly repopulate trawled areas after disturbance (van Denderen et al., 2015).

In contrast, long-lived, hard-bodied, sessile organisms and suspension feeders show the larger declines due to trawling disturbance (Tillin et al., 2006; van Denderen et al., 2015; Bradshaw et al., 2024). The most adverse effects of trawling occur on shallow burrowing species, typical of biotope due to towed demersal gears and dredges which penetrate and disturb the sediment (see Penetration pressure).

Abra alba is a shallow burrower with a fragile shell (Tebble, 1976), and has been considered amongst the list of bivalve species most vulnerable to trawling (Bergman & Van Santbrink, 2000) who reported between <0.5% and 18% mortality of Abra alba due to trawling in the southern North Sea. However, the small size of Abra alba relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993).

In the North Sea, the biomass of small polychaetes (unidentified species) did not change after chronic beam trawling (Jennings et al., 2002, cited in Collie et al., 2017; Eggleton et al., 2018), whereas other studies reported reduced polychaete biomass and abundance in trawled areas (Loret et al., 2007, cited in Collie et al., 2017). Both Eggleton et al. (2018) and Collie et al. (2017) note that although small benthic species may increase in relative abundance after trawling, the average abundance of all taxa decreases.

Collie et al. (2000) found that Nephtys hombergii displayed a negative effect on abundance as a result of fishing activities and mean response of infauna and epifauna communities to fishing activities was much more negative in mud and sand communities than other habitats. Nephtys hombergii abundance also significantly decreased in areas of the Solent, UK, where bait digging (primarily for Nereis virens) had occurred (Watson et al. 2007).

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. Echinocyamus pusillus increased in abundance within the exclusion no-fishery zone, suggesting these species were sensitive to trawling activities (from table 3, Coates et al., 2016). Coates et al. (2016) noted the increase in Echinocyamus pusillus, which has a one-year reproductive maturity age, could illustrate the first signs of recovery in fragile echinoderms. Bathyporeia guilliamsonana and Bathyporeia elegans also showed signs of an increase in abundance, but trends were variable and not consistent (from table 3, Coates et al., 2016). Despite this Bathyporiea spp. remained dominant over the study period. The polychaete Nepthys cirrosa also 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 were significantly different. This suggests that full recovery will likely take longer than three years, and three years is possibly too short to determine significant changes in the community (Coates et al., 2016). This reinforces that species sensitive to trawling (Echinocyamus pusillus and Ophelia borealis) may only re-establish once fishing stops, while opportunistic and resilient species remain stable and constant.

Variation in fishing intensity may influence the impacts of disturbance on sedimentary benthic communities. 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 previously experienced a low frequency of fishing, there was a higher abundance of deposit feeders characteristic of this biotope (including Abra prismtica, Bathyporeia gracilis, Scoloplos armiger, Spiophanes bombyx, and Echinocyamus pusillus) than suspension feeders. However, in sediments from sites which had a high frequency of fishing, there was a higher abundance of disturbance-tolerant suspension feeders, Abra alba, Owenia fusiformis, Chaetozone sp., and Nepthys 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).

Benthic responses to trawling are smaller or absent in subtidal sand habitats subject to high natural disturbance by periodic wave or current disturbance and/or tidal-bed shear stress (van Denderen et al. 2015; Cantrell et al., 2023). Mobile sediments such as sand and mud, appear to be more resilient (Cantrell et al., 2023). In naturally disturbed areas communities tend to be dominated by more tolerant small-sized, deposit-feeding animals or mobile predators and scavengers (van Denderen et al. 2015). Short-term carrion exposure caused by trawling can also promote scavenging behaviour by these more tolerant taxa, such as amphipods and some polychaetes (Beauchard et al., 2023).

Sensitivity assessment. Abrasion is likely to damage epifauna and flora and may damage a proportion of the characterizing species, depending on the type of gear used and fishing frequency. Nevertheless, the evidence above suggests that the important characteristic species Abra spp. and Bathyporeia spp. are probably amongst the most resistant of the effects of physical disturbance. Therefore, resistance is assessed as ‘Medium’. Resilience is assessed as ‘High’ as opportunistic species are likely to recruit rapidly and some damaged characterizing species may recover or recolonize. Biotope sensitivity is assessed as ‘Low’.

Penetration or disturbance of the substratum subsurface

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

Evidence

The epifauna and infaunal assemblages of both stable and dynamic fine sands are susceptible to direct physical disturbance from towed demersal gears and dredges which penetrate and disturb the sediment (e.g. Eleftheriou & Robertson 1992; Kaiser et al., 1998; Robinson & Richardson, 1998; Schwinghamer et al., 1996; Freese et al., 1999; Prena et al,. 1999; Bergman & Van Santbrick 2000a,b; Tuck et al., 2000; Kenchington et al., 2001; Gilkinson et al., 2005). In general, fishing using towed gears results in the mortality of non-target organisms either through physical damage inflicted by the passage of the trawl or indirectly by disturbance, damage, exposure and subsequent predation. Beam trawling, for example, decreases the density of common echinoderms, polychaetes and molluscs (Bergman & Hup, 1992) and decreases the density and diversity of epifauna in stable sand habitats (Kaiser & Spencer, 1996).    

Evidence has shown that sublittoral sand biotopes (SS.SSa EUNIS code A5.2) including circalittoral fine sands (SS.SSa.CFiSa) are amongst the most extensively trawled habitats in the Northeast Atlantic, North Sea and Mediterranean Sea (Eigaard et al., 2017; Collie et al., 2017; Eggleton et al., 2018).

Gilkinson et al. (1998) simulated the physical interaction of otter trawl doors with the seabed in a laboratory test tank using a full-scale otter trawl door model. Between 58% and 70% of the bivalves in the scour path that were originally buried were completely or partially exposed at the test bed surface. However, only two out of a total of 42 specimens showed major damage. The pressure wave associated with the otter door pushes small bivalves out of the way without damaging them. Where species can rapidly burrow and reposition (typically within species occurring in unstable habitats) before predation mortality rates will be relatively low. These experimental observations are supported by diver observations of fauna dislodged by a hydraulic dredge used to catch Ensis spp. Small bivalves were found in the trawl tracks that had been dislodged from the sediments, including the venerid bivalves Dosinia exoleta, Chamelea striatula and the hatchet shell Lucinoma borealis. These were usually intact (Hauton et al., 2003) and could potentially reburrow.

Larger, fragile species, typical of this biotope are more likely to be damaged by sediment penetration and disturbance than smaller species (Tillin et al., 2006). Bottom trawling, that penetrates the seabed, causes benthic mortality and declines in infaunal abundance and biomass (Hiddink et al., 2017; Eggleton et al., 2018). Bergman & van Santbrink (2000) suggested that the megafauna were amongst the species most vulnerable to direct mortality due to bottom trawling in sandy sediments. Stomach analysis of fish caught scavenging in the tracks of beam trawls found parts of Spatangus purpureus and Ensis spp. indicating that these had been damaged and exposed by the trawl (Kaiser & Spencer, 1994a). Capasso et al. (2010) compared benthic survey datasets from 1895 and 2007 for an area in the English Channel. Although methodological differences limit direct comparison, the datasets appear to show that large, fragile urchin species including Echinus esculentus, Spatangus purpureus and Psammechinus miliaris and larger bivalves had decreased in abundance. Small, mobile species such as amphipods and small errant and predatory polychaetes (Nephtys, Glycera, Lumbrineris) appeared to have increased (Capasso et al., 2010). The area is subject to beam trawling and scallop dredging and the observed species changes would correspond with predicted changes following physical disturbance. Two small species: Timoclea ovata and Echinocyamus pusillus had increased in abundance between the two periods.

Deep burrowing species are generally more resistant to bottom trawling, particularly at high trawling frequencies (Beauchard et al., 2023). Evidence found that trawling had most adverse effects on infaunal organisms (including Abra spp. and other bivalve species) positioned 0 to 5 cm in the seabed, but species positioned deeper in the sediment and species living on the seabed surface were less affected by trawling due to mobile swimmers or crawlers which may repopulate trawled grounds easily after disturbance (van Denderen et al., 2015).

Bergman and Van Santbrink (2000) found that direct mortality of gammarid amphipods, following a single passage of a beam trawl (in silty sediments where penetration is greater) was 28%. Similar results were reported from experiments in shallow, wave disturbed areas, using a toothed, clam dredge. Bathyporeia spp. experienced a reduction of 25% abundance in samples immediately after intense clam dredging, abundance recovered after 1 day (Constantino et al., 2009). Experimental hydraulic dredging for razor clams resulted in  no statistically significant differences in Bathyporeia elegans abundances between treatments after 1 or 40 days (Hall et al., 1990), suggesting that recovery from effects was very rapid. Ferns et al. (2000) examined the effects of a tractor-towed cockle harvester on benthic invertebrates and predators in intertidal plots of muddy and clean sand. Harvesting resulted in the loss of a significant proportion of the most common invertebrates from both areas. In the muddy sand, the population of Bathyporeia pilosa remained significantly depleted for more than 50 days, whilst the population in clean sand recovered more quickly. These results agree with other experimental studies that clean sands tend to recover more quickly that other habitat types with higher proportions of fine sediment (Dernie et al., 2003).

Sensitivity assessment. The trawling studies and the comparative study by Capasso et al. (2010) suggest that the biological assemblage present in this biotope is characterized by species that are relatively tolerant of penetration and disturbance of the sediments. Either species are robust or buried within sediments or are adapted to habitats with frequent disturbance (natural or anthropogenic) and recover quickly. The results suggest that a reduction in physical disturbance may lead to the development of a community with larger, more fragile species including large bivalves. Biotope resistance is assessed as ‘Medium’ as some species will be displaced and may be predated or injured and killed. Biotope resilience is assessed as ‘High’ as most species will recover rapidly and the biotope is likely to still be classified as the same type following disturbance. Biotope sensitivity is therefore assessed as ‘Low’.

Changes in suspended solids (water clarity)

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

Evidence

A change in turbidity at the pressure benchmark is assessed as an increase from intermediate 10-100 mg/l to medium (100-300 mg/l) and a change to clear (<10 mg/l). An increase or decrease in turbidity may affect primary production in the water column and indirectly alter the availability of phytoplankton food available to species in filter feeding mode. However, phytoplankton will also be transported from distant areas and so the effect of increased turbidity may be mitigated to some extent.  According to Widdows et al. (1979), growth of filter-feeding bivalves may be impaired at suspended particulate matter (SPM) concentrations >250 mg/l.

Sensitivity assessment. No direct evidence was found to assess impacts on the characterizing and associated species. The characterizing, suspension feeding bivalves  are not predicted to be sensitive to decreases in turbidity and may be exposed to, and tolerant of, short-term increases in turbidity following sediment mobilization by storms and other events. An increase in suspended solids, at the pressure benchmark may have negative impacts on growth and fecundity by reducing filter feeding efficiency and imposing costs on clearing. Biotope resistance is assessed as ‘Medium’ as there may be some shift in the structure of the biological assemblage and resilience is assessed as ‘High’ (following restoration of typical conditions). Biotope sensitivity is assessed as ‘Low’.

Smothering and siltation rate changes (light)

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

Evidence

The addition of fine material could alter the character of this habitat by covering it with a layer of dissimilar sediment and will reduce suitability for the species associated with this feature. Recovery will depend on the rate of sediment mixing or removal of the overburden, either naturally or through human activities. Recovery to a recognisable form of the original biotope will not take place until this has happened. In areas where the local hydrodynamic conditions are unaffected, fine particles will be removed by wave action, moderating the impact of this pressure. The rate of habitat restoration would be site-specific and would be influenced by the type of siltation and the rate. Long-term or permanent addition of fine particles would lead to re-classification of this biotope type (see physical change pressures). For example, the addition of silts to a Spisula solida bed in Waterford Harbour (Republic of Ireland) from earthworks further upstream reduced the extent of the bed (Fahy et al., 2003). No information was provided on the depth of any deposits.

Most bivalve species are capable of burrowing through sediment to feed, e.g. Abra alba are capable of upwardly migrating if lightly buried by additional sediment (Schafer, 1972). There may be an energetic cost expended by species to either re-establish burrow openings, to self-clean feeding apparatus or to move up through the sediment, though this is not likely to be significant. Most animals will be able to reburrow or move up through the sediment within hours or days. Bijkerk (1988, results cited from Essink, 1999) indicated that the maximal overburden through which small bivalves could migrate was 20 cm in sand for Donax and approximately 40 cm in mud for Tellina sp. and approximately 50 cm in sand.  No further information was available on the rates of survivorship or the time taken to reach the surface. Little direct evidence was found to assess the impact of this pressure at the benchmark level. Powilleit et al. (2009) studied the response of the polychaete Nephtys hombergii to smothering. This species successfully migrated to the surface of 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, the Abra alba – Lagis koreni muddy fine sand community remained largely persistent in its species composition and densities of dominant species 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). Owenia fusiformis, Nephtys hombergii and Abra alba were consistently amongst the most abundant taxa recorded over the study period. Bacouillard et al. (2020) concluded that despite its exposure to multiple stressors, the Abra alba - Lagis koreni muddy fine sand community is dominated by highly resilient species capable of quickly rebuilding their populations (Bacouillard et al., 2020).

Similar evidence was found by Dauvin et al. (2017), who reported that the Abra alba community in the Bay of Seine remained relatively stable and resilient over decades despite strong sediment changes, hydrodynamic conditions, river discharge, and anthropogenic impacts from dredging and port construction. This suggests that it could tolerate changes in sediment.

Sensitivity assessment. Bivalves and polychaetes, and other species are likely to be able to survive short periods under sediments and to reposition. However, as the pressure benchmark refers to fine material, this may be cohesive, and species characteristic of sandy habitats may be less adapted to move through this than sands. Hence, resistance is assessed as 'Medium' as some mortality of characterizing and associated species may occur. Biotope resilience is assessed as 'High', and biotope sensitivity is assessed as 'Low'.

Smothering and siltation rate changes (heavy)

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

Evidence

The addition of fine material could alter the character of this habitat by covering it with a layer of dissimilar sediment and will reduce suitability for the species associated with this feature. Recovery will depend on the rate of sediment mixing or removal of the overburden, either naturally or through human activities. Recovery to a recognisable form of the original biotope will not take place until this has happened. In areas where the local hydrodynamic conditions are unaffected, fine particles will be removed by wave action, moderating the impact of this pressure. The rate of habitat restoration would be site-specific and would be influenced by the type of siltation and the rate. Long-term or permanent addition of fine particles would lead to re-classification of this biotope type (see physical change pressures). For example, the addition of silts to a Spisula solida bed in Waterford Harbour (Republic of Ireland) from earthworks further upstream reduced the extent of the bed (Fahy et al., 2003). No information was provided on the depth of any deposits.

Most bivalve species are capable of burrowing through sediment to feed, e.g. Abra alba are capable of upwardly migrating if lightly buried by additional sediment (Schafer, 1972). There may be an energetic cost expended by species to either re-establish burrow openings, to self-clean feeding apparatus or to move up through the sediment, though this is not likely to be significant. Most animals will be able to reburrow or move up through the sediment within hours or days. Bijkerk (1988, results cited from Essink, 1999) indicated that the maximal overburden through which small bivalves could migrate was 20 cm in sand for Donax and approximately 40 cm in mud for Tellina sp. and approximately 50 cm in sand.  No further information was available on the rates of survivorship or the time taken to reach the surface. Little direct evidence was found to assess the impact of this pressure at the benchmark level. Powilleit et al. (2009) studied the response of the polychaete Nephtys hombergii to smothering. This species successfully migrated to the surface of 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, the Abra alba – Lagis koreni muddy fine sand community remained largely persistent in its species composition and densities of dominant species 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). Owenia fusiformis, Nephtys hombergii and Abra alba were consistently amongst the most abundant taxa recorded over the study period. Bacouillard et al. (2020) concluded that despite its exposure to multiple stressors, the Abra alba - Lagis koreni muddy fine sand community is dominated by highly resilient species capable of quickly rebuilding their populations (Bacouillard et al., 2020).

Similar evidence was found by Dauvin et al. (2017), who reported that the Abra alba community in the Bay of Seine remained relatively stable and resilient over decades despite strong sediment changes, hydrodynamic conditions, river discharge, and anthropogenic impacts from dredging and port construction. This suggests that it could tolerate changes in sediment.

Sensitivity assessment. The character of the overburden is an important factor determining the degree of vertical migration of buried bivalves. Individuals are more likely to escape from a covering similar to the sediments in which the species is found than a different type. Resistance is assessed as ‘Low’ as few individuals are likely to reposition. Resilience is assessed as ‘Medium’ and sensitivity is assessed as ‘Medium’.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

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.  

Sensitivity assessment. Given the lack of data at the level of individual biotopes, resistance and resilience to EMFs cannot be robustly assessed. Sensitivity is therefore recorded as 'Insufficient evidence'. 

Underwater noise changes

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

Evidence

Not relevant. No information was found concerning the intolerance of the biotope or the characterizing species to noise. The siphons of bivalves and palps of polychaetes are likely to detect vibrations and are probably withdrawn as a predator avoidance mechanism. However, it is unlikely that the biotope will be affected by noise or vibrations caused by noise at the level of the benchmark.

Introduction of light or shading

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

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 50m (Davies et al., 2020; Smyth et al., 2021). ALAN can alter biological processes across taxa and at multiple levels of organisation. 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. 

Saenz-Arias et al. (2024) investigated the effects of ALAN on macroinvertebrate communities using light traps on non-illuminated fine sands and coarse sand-gravel beaches along the south coast of Spain. The amphipod Bathyporeia guilliamsoniana was one of the most abundant species caught in the light traps, particularly at night, showing a clear attraction to the artificial light. Sanez-Arias et al. (2024) concluded that species with migratory or emergent behaviours, such as Bathyporeia spp. are the taxa most likely to be influenced by ALAN, as they emerge from sediment to perform vertical migrations. Polychaetes (Sphaerosyllis bulbosa, Parapionosyllis elegans and Sphaerosyllis taylori) were mostly abundant in the sediment collected from the sites and scarcely abundant in the light traps; there was not enough evidence to consider this an impact of ALAN (Sanez-Arias et al. 2024).

Garratt et al. (2019) was one of the first studies to focus on the consequences of artificial light exposure on intertidal organisms (Sanez-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 1.5m), middle shore (-0.25 to 0.25), 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. However, species-specific relationships showed Bathyporeia elegans, Nephthys spp., Lanice conchilega, and Haustoris arenarius significantly decreased in abundance and/or probability of occurrence with increasing illuminance, while Arenicola marina, Tellinidae spp. and Nereididae spp. significantly increased 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. 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. Sensitivity is therefore 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. Further detail

Evidence

Not relevant.

Death or injury by collision

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

Evidence

Not relevant’ to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion.

Visual disturbance

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

Evidence

Not relevant.

Genetic modification & translocation of indigenous species

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

Evidence

Key characterizing species within this biotope are not cultivated or translocated. This pressure is therefore considered ‘Not relevant’ to this biotope group.

Introduction of microbial pathogens

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

Evidence

No evidence was found for the characterizing polychaete species. Populations of bivalve species may be subject to a variety of diseases and parasites but evidence for the characterizing bivalves is limited. Berilli et al. (2000) conducted a parasitological survey of the bivalve Chamelea gallina in natural beds of the Adriatic Sea, where anomalous mortalities had been observed in 1997-1999. The occurrence of protozoans belonging to the families Porosporidae, Hemispeiridae and Trichodinidae was recorded. Porosporidae of the genus Nematopsis, present with 4 species, showed a prevalence of 100%. The results suggested that severe infections of protozoans of the genus Nematopsis could cause a not negligible respiratory sufferance, with a possible role in the decline of the natural banks of Chamelea gallina (Berilli et al., 2000).

Bacterial diseases are frequently found in molluscs during their larval stages, but seem to be relatively insignificant in populations of adult animals (Lόpez-Flores et al., 2004). This may be due to the primary defence mechanisms of molluscs, phagocytosis and encapsulation, which fight against small-sized pathogens, and whose resistance may be age related (Sindermann, 1990; Lόpez-Flores et al., 2004).

Individuals of Fabulina fabula from Boulogne-sur-Mer (studied as Angulus fabula) were infected with the trematode parasite Gymnophallus strigatus, causing erosion of the shell (Giard, 1897, cited in Kinne, 1983).

Sensitivity assessments. Pathogens may cause mortality and there may be a minor decline in species richness or abundance in the biotope. As there is no evidence for mass mortalities of characterizing species that would alter biotope classification biotope resistance is assessed as ‘Medium’. Biotope resilience is assessed as ‘High’ as changes may fall within natural population variability and a recognizable biotope is likely to be present after two years. Biotope sensitivity is therefore assessed as ‘Low’.

Removal of target species

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

Evidence

A number of the larger bivalve species that may be associated with this biotope group are targeted by commercial fishers in some parts of their range. These include Chamelea gallina (Ballarin et al., 2003); Spisula solida (Fahy et al., 2003; Joaquim et al., 2008); Glycymeris glycymeris and Paphia spp. (Savina & Pouvreau, 2004); Ensis spp., Donax spp. and Pharus spp. (Chícharo et al., 2002). In targeted areas, the populations of fished bivalves may be depleted, for example, fishing has led to declines in Spisula solida (Joaquim et al., 2008; Fahy et al., 2003).

Sensitivity assessment. In general dredges that are used to target bivalves are likely to be efficient at removing targeted species. Removal of commercially targeted bivalves may lead to biotope reclassification based on the dominance of polychaetes to a similar biotope Biotope resistance, based on the characterizing bivalves is assessed as ‘Low’. Undersized juveniles may be returned and can re-burrow but are likely to suffer from stress. Targeted removal of adult bivalves within the biotope may allow successful recruitment of juveniles where intra-specific competition for space and food and possibly consumption of larvae has prevented successful spatfall. Some species such as Glycymeris glycymeris are characteristic of habitats with low levels of competition and may benefit from removal of other species. Biotope resilience is assessed as ‘Medium’, as recruitment in many bivalve species is episodic and unpredictable. Biotope sensitivity is therefore assessed as ‘Medium’.

Removal of non-target species

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

Evidence

Species within the biotope are not functionally dependent on each other, although biological interactions will play a role in structuring the biological assemblage through predation and competition. Removal of adults may support recruitment of juvenile bivalves by reducing competition for space and consumption of larvae. 

Removal of species would also reduce the ecological services provided by these species such as secondary production and nutrient cycling.

Sensitivity assessment. Species within the biotope are relatively sedentary or slow moving, although the infaunal position may protect some burrowing species from removal. Biotope resistance is therefore assessed as ‘Low’ and resilience as ‘High’, as the habitat is likely to be directly affected by removal and some species will recolonize rapidly, biotiope sensitivity is therefore assessed as Low'. Some variability in species recruitment, abundance and composition is natural and therefore a return to a recognizable biotope should occur within 2 years. Repeated chronic removal would, however, impact recovery.

The American slipper limpet, Crepidula fornicata

Evidence

Few invasive non-indigenous species may be able to colonize mobile sands, due to the high levels of sediment disturbance. The slipper limpet Crepidula fornicata may settle on stones in substrates and hard surfaces such as bivalve shells and can sometimes form dense carpets which can smother bivalves and alter the seabed, making the habitat unsuitable for larval settlement. Dense aggregations trap suspended silt, faeces and pseudofaeces altering the benthic habitat. Where slipper limpet stacks are abundant, few other bivalves can live amongst them (Fretter & Graham, 1981; Blanchard, 1997). Muddy and mixed sediments in wave sheltered areas are probably optimal but Crepidula fornicata has been recorded from a wide variety of habitats including clean sands and areas subject to moderately strong tidal streams (Blanchard, 1997; De Montaudouin & Sauriau, 1999).  Bohn et al. (2015) report that in the Milford Haven Waterway (MHW) in south-west Wales, UK, subtidally, highest densities were found in areas of high gravel content (grain sizes 16 to 256 mm), suggesting that the availability of this substrata type is beneficial for its establishment.

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

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile or otherwise unsuitable for most of the recorded invasive non-indigenous species currently recorded in the UK. The presence of fine sand and lack of hard substrata in this biotope may be unsuitable for the colonization by Crepidula (Tillin et al., 2020). Therefore, resistance is assessed as ‘High’, resilience as ’High’, and sensitivity is assessed as ‘Not sensitive’.

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 localized 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 a one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight hour) 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-meditated transport via aquaculture facilities, boat hulls, commercial fishing vessels, 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; Dijstra 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 and 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 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 to 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 highest average temperatures were recorded in February (18 to 22°C) and the lowest average temperatures 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 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 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.  

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 to 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. The sediments characterizing this biotope are likely to too mobile or otherwise unsuitable for most of the recorded invasive non-indigenous species currently recorded in the UK. Therefore, this biotope is likely to be unsuitable for the colonization of Didemnum vexillum due to the presence of fine sands, unstable substrata, risk of smothering by fine sediment and the lack of hard substrata 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’.

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; GBNNSIP, 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; GBNNSIP, 2011, 2012; Humphreys et al., 2014 cited in Alves et al., 2021; Hansen et al., 2023).  

Since introduction, the species has invaded and established self-sustaining natural populations throughout Europe from the North Sea, Wadden Sea and Scandinavian coastlines to the Atlantic coastlines of Spain and Portugal, as well as the Mediterranean and Adriatic Sea (Wrange et al., 2010; GBNNSIP, 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 (GBNNSIP, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).  

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann et al., 2012, 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020) because its larvae require hard substrata for successful settlement and development (Mckinstry & Jensen, 2013; 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). Therefore, fine mud sediments without hard substrata (such as small stones, gravel, and shell) are unlikely to be suitable (Tillin et al., 2020).

The majority of the evidence indicates that infralittoral rock and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). However, in suitable situations (e.g. Oosterschelde) it may form beds down to 42 m.  

Sensitivity assessment. The sediments characterizing this biotope are likely to too mobile or otherwise unsuitable for most of the recorded invasive non-indigenous species currently recorded in the UK. Also, this biotope is likely to be unsuitable for the colonization of Magallana gigas due to the presence of fine sands and the lack of any hard substrata suitable for colonization. The depth range of SS.SSa.CFiSa.ApriBatPo (>25), is likely too deep for the colonization of Magallana gigas, as the majority of evidence indicates that habitats that occur at depths more than 10 m are unlikely to be suitable 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’.

Wireweed, Sargassum muticum

Evidence

The depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum. 

Wakame, Undaria pinnatifida

Evidence

The depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria. 

Other INIS

Evidence

Although not currently established in UK waters, the whelk Rapana venosa, may spread to UK habitats from Europe. Both Rapana venosa and the introduced oyster drill Urosalpinx cinerea both predate on bivalves and could therefore negatively affect the characterizing bivalve species.

Tillin et al. (2020) reported that circalittoral fine sands provide suitable habitats for the colonization of the American jack knife clam, Ensis leei. Ensis leei can inhabit low shore levels and a variety of substrata from fine sand to coarse sand (Beukema & Dekker, 1995; Palmer, 2003; cited in Tillin et al., 2020). In local Abra alba communities along the Belgian coast, Ensis leei is the most common species (Gollash et al., 2015; cited in Tillin et al., 2020). 

Sensitvity assessment. No evidence of direct effects of the above species on the species that characterize this biotope was found. Hence, the evidence is insufficient to for the basis of an assessment at present.