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

Davenport & Davenport (2005) demonstrated that the limits of thermal tolerance to high and low temperatures relate to the distribution of intertidal macroinvertebrate species. Species that occur highest on the shore are more tolerant of a wider range of temperatures than species that occurred low on the shore or subtidally. As subtidal biotopes are less exposed to temperature fluctuations, the characterizing species may be less able to tolerate temperature fluctuations.

No direct evidence was found to support assessment of this pressure. Very few laboratory studies have been carried out on the monitoring of settlement and recruitment and records of species distribution. 

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. However, 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. Many polychaete species including Mediomastus fragilis recruit in Spring/early Summer recruitment (Sardá et al., 1999).

The characterizing bivalve Timoclea ovata has a wide distribution from northern Norway and Iceland south to west Africa.  It is also recorded  from  the  Canary  Islands,  the  Azores and  the  Mediterranean  and  Black  Sea (Morton, 2009). Goodallia triangularis also has a widespread distribution in the Atlantic coasts of Europe to the Mediterranean and north-western Africa (Giribet & Peňas, 1999). Polychaetes and other species associated with the biotope may also have wide global distributions. Mediomastus fragilis has been recorded throughout the British Isles (NBN, 2015) and in the Mediterranean (Serrano et al., 2011). Glycera lapidum is found in the north-eastern Atlantic, Mediterranean, North Sea, Skagerrak and Kattegat (Marine Species Identification Portal).

Sensitivity assessment. Little evidence was available to assess this pressure.  Assemblages in fine sands that contain many of the characterizing species occur in the Mediterranean (see resilience section 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. 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

Davenport & Davenport (2005) demonstrated that the limits of thermal tolerance to high and low temperatures reflect the distribution of intertidal macroinvertebrate species. Species that occur highest on the shore are more tolerant of a wider range of temperatures than species that occurred low on the shore or subtidally. As subtidal biotopes are less exposed to temperature fluctuations  the characterizing species may be less able to tolerate temperature fluctuations.

The characterizing bivalve Timoclea ovata has a wide distribution from northern Norway and Iceland south to west Africa.  It is also recorded  from  the  Canary  Islands,  the  Azores and  the  Mediterranean  and  Black  Sea (Morton, 2009). Goodallia triangularis also has a widespread distribution in the Atlantic coasts of Europe to the Mediterranean and north-western Africa (Giribet & Peňas, 1999). Polychaetes and other species associated with the biotope may also have wide global distributions. Mediomastus fragilis has been recorded throughout the British Isles (NBN, 2015) and in the Mediterranean (Serrano et al., 2011). Glycera lapidum is found in the north-eastern Atlantic, Mediterranean, North Sea, Skagerrak and Kattegat (Marine Species Identification Portal, http://species-identification.org/).

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, and over large spatial scales, may result in changes in community composition.

Sensitivity assessment. Many of the characterizing species are found in more northern waters than the UK and may be adapted to colder temperatures. 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. 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

This biotope occurs in full salinity. A study from the Canary Islands indicates that exposure to high salinity effluents (47- 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 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 (52-55 ppt) in the Arabian Gulf (Price, 1982), indicating local acclimation may be possible.

Sensitivity assessment. High saline effluents alter the structure of biological assemblages. Polychaete species may be more tolerant than bivalves but an increase in salinity is likely to result in declines in species richness and abundance based on Riera et al., (2012). Biotope 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

No evidence was found to assess the sensitivity of key characterizing species.

Sensitivity assessment.  A reduction in salinity may result in changes in biotope composition as some sensitive species are lost and replaced by typical estuarine species more tolerant of the changed conditions, such as Nephtys cirrosa, Macoma balthica, and Bathyporeia spp. so that the biotope may be reclassified as SS.SSa.SSaVS.NcirLim. 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

This biotope is recorded in areas where tidal flows are weak (>0.5 m/s) (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 resulting in some changes in sediment composition. Hjulström (1939) concluded that fine sand (particle diameter of 0.3-0.6 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. As this biotope is found in a range of sediment types; from gravelly sand to muddy mixed sediment (JNCC,2015) changes in sediment type and some alteration in the identity or abundance of characterizing species may occur but the biotope may still be present. 

Many of the species occur in a range of sediment types, which, given the link between hydrodynamics and sediment type, suggests that these species are not sensitive to changes in water flow at the pressure benchmark. Timoclea ovata occur in muddy sands in areas that are sheltered and where fine sediments are deposited. Glycera spp. are found in areas with strong tidal streams where sediments are mobile (Roche et al., 2007) and in extremely sheltered areas (Connor et al., 2004).

Sensitivity assessment. Changes in water flow may result in sediment changes in the habitat and may cause some shifts in abundance. However, a change at the pressure benchmark (increase or decrease)  is unlikely to affect biotope classification  and biotope sensitivity is therefore assessed as ‘High’ and resilience is assessed as ‘High’, so 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

As this biotope occurs in circalittoral 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 could 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. As the biotope occurs in habitats that are sheltered and moderately exposed to wave action (JNCC, 2015)  it is considered that the weak tidal currents and substratum, rather than wave action, are significant factors determining species composition

Sensitivity assessment. The range of wave exposures experienced by SS.SMx.OMx.PoVen 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

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

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 1000 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).

Rygg (1985) classified Lumbrineris spp. as non-tolerant of Cu (species only occasionally found at stations in Norwegian fjords where Cu concentrations were >200 ppm (mg/kg)).

Hydrocarbon & PAH contamination

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

Evidence

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

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. 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. Mediomastus fragilis increased in abundance (Dauvin, 2000). Other studies support the conclusion that polychaetes are generally a tolerant taxa. Hiscock et al. (2004; 2005, based on Levell et al. 1989) described Glycera spp. as a very tolerant taxa, found in enhanced abundances in the transitional zone along hydrocarbon contamination gradients surrounding oil platforms.

The amphipods, Ampelisca sp. are also very intolerant of oil contamination and the recovery of the Ampelisca populations in the fine sand community in the Bay of Morlaix took up to 15 years following the Amoco Cadiz oil spill (Poggiale & Dauvin, 2001).

Synthetic compound contamination

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

Evidence

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

The anti-parasite compound Ivermectin is highly toxic to benthic polychaetes and crustaceans (Black et al., 1997; Collier & Pinn, 1998; Grant & Briggs, 1998, cited in Wildling & Hughes, 2010). OSPAR (2000) stated that, at that time, Ivermectin was not licensed for use in mariculture but was incorporated into the feed as a treatment against sea lice at some farms. Ivermectin has the potential to persist in sediments, particularly fine-grained sediments at sheltered sites. Data from a farm in Galway, Ireland indicated that Ivermectin was detectable in sediments adjacent to the farm at concentrations up to 6.8 μm/kg and to a depth of 9 cm (reported in OSPAR, 2000). Infaunal polychaetes have been affected by deposition rates of 78-780 mg Ivermectin/m².

Radionuclide contamination

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

Evidence

No evidence was found to support an assessment at the pressure benchmark. Following the Fukushima Dai-ichi nuclear power plant accident in August 2013, radioactive cesium concentrations in invertebrates collected from the seabed were assessed. Concentrations in bivalves and gastropods were lower than in polychaetes (Sohtome et al., 2014). The data does not indicate that there were mortalities.

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. The hypoxic and anoxic conditions were created for 3-4 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. Generally, 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 (Riedel et al., 2012).

Further evidence of sensitivity was available for some of the polychaete species associated with this biotope. 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^␣1) led to the emergence of  Lumbrineris sp. from the substrate these 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). 

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 the polychaetes present, particularly the mobile predatory species such as Glycera and Nephtys may be less tolerant.  As the biotope is characterized by 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

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. The pressure benchmark is set at compliance with Water Framework Directive (WFD) criteria for good status, based on nitrogen concentration (UKTAG, 2014).  

The bivalves, polychaetes and other associated invertebrate species are unlikely to be directly affected by changes in nutrient enrichment. The biotope is found in the circalittoral zone (JNCC, 2015) where light penetration is limited. 

Sensitivity assessment. As this biotope is structured by the sediments and water flow rather than nutrient enrichment, the biotope is considered to have ‘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

The biotope occurs in mobile sand sediments where sediment disturbance leads to particle sorting, and in-situ primary production is restricted to microphytobenthos and some macroalgae (JNCC, 2015). An input of organic matter would provide a food subsidy to the deposit feeding polychaetes and may be utilized by amphipods.

Borja et al. (2000) and Gittenberger & Van Loon (2011) assigned Glycera alba and Glycera lapidum to their AMBI Group III, defined as: ‘Species tolerant to excess organic matter enrichment. These species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)’. Lumbrineris latreilli was characterized as AMBI Group II- 'Species indifferent to enrichment, always present in low densities with non-significant variations with time (from initial state, to slight unbalance)' (Borja et al., 2000, Gittenberger & Van Loon, 2011).

Simboura & Zenetos (2002) assigned Timoclea ovata to their Ecological Group II (GII) category for the biotic index that they developed, called BENTIX. Ecological Group II is defined as: ‘Species tolerant to disturbance or stress whose populations may respond to enrichment or other source of pollution by an increase of densities (slight unbalanced situations)’.

Sensitivity assessment.  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. Biotope sensitivity is therefore assessed as ‘High’ and resilience as ‘High’ (by default), and the biotope is therefore considered to be ‘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.

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 muddy mixed sediments and gravelly sand (JNCC, 2015). 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).  Based on the range of habitats that this biotope occurs in, a change between coarse sediments and mixed sediments is not assessed. 

Surveys over sediment gradients and before-and-after impact studies from aggregate extraction sites where sediments have been altered indicate patterns in change. Long-term alteration of sediment type to finer more unstable sediments was observed six years after aggregate dredging at moderate energy sites (Boyd et al., 2005). The on-going sediment instability was reflected in a biological assemblage composed largely of juveniles (Boyd et al., 2005). Holme (1966), observed that Glycymeris glycymeris was absent from areas of the English Channel with finer sediments but was abundant in tidally-swept coarse areas. Some species may, however, be present in a range of sediments. 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). 

Sensitivity assessment.  Changes in the sediment type may lead to biotope reclassification. Biotope resistance is, therefore, assessed as ‘Low’ (as some species may remain), as resilience is Very low (the pressure is a permanent change), and sensitivity is, therefore, '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

A number of studies assess the impacts of aggregate extraction on sand and gravel habitats. Most of the animals that occur in this biotope are shallowly buried, for example, Glycymerids occur at the surface with the mantle margins exposed at the surface (Thomas, 1975).

Recovery of sediments will be site-specific and will be influenced by currents, wave action and sediment availability (Desprez, 2000). Except in areas of mobile sands, the process tends to be slow (Kenny & Rees, 1996; Desprez, 2000 and references therein).  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. The strongest currents are unable to transport gravel. A further implication of the formation of these depressions is a local drop in current strength associated with the increased water depth, resulting in deposition of finer sediments than those of the surrounding substrate (Desprez et al., 2000 and references therein). See the physical change pressure for assessment

Sensitivity assessment. Resistance is assessed as ‘None’ as extraction of the sediment swill 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

Comparative studies between disturbed and undisturbed areas indicate that abrasion and disturbance from bottom trawling on  gravels and sands reduce abundance of organisms, biomass and species diversity (Collie et al., 1997). Undisturbed sites contain more calcareous tube worms, bryozoans and hydroids and small fragile polychaetes and brittle stars. Thick-shelled bivalves, hermit crabs and gastropods appeared unaffected by dredging. Glycymeris is a mobile burrower (Thomas, 1975). Venerid bivalves, such as the characterizing species Timoclea ovata, live close to the surface (Morton, 2009). Burrowing species such as Glycera lapidum and Lumbrineris latreilli may be unaffected by surface abrasion. Lumbrineris latreilli was characterized as AMBI Fisheries Review Group III-Species insensitive to fisheries in which the bottom is disturbed. Their populations do not show a significant decline or increase (Gittenberger & Van Loon, 2011).

Sensitivity assessment. Abrasion is likely to damage epifauna and may damage a proportion of the characterizing species, biotope resistance is therefore 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

Comparative studies between disturbed and undisturbed areas indicate that abrasion and disturbance from bottom trawling on coarse gravels and sands, reduce abundance of organisms, biomass and species diversity (Collie et al., 1997). Undisturbed sites contain more calcareous tube worms, bryozoans and hydroids and small fragile polychaetes and brittle stars.

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) and the small bivalve Timoclea ovata 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. 

Experiments in shallow, wave disturbed areas, using a toothed, clam dredge, found that deposit feeding polychaetes were more impacted than carnivorous species. Dredging resulted in reductions of >90% of Spiophanes bombyx  immediately post dredging compared with before impact samples and the population reduction persisting for 90 days (although results may be confounded by storm events within the monitoring period which caused sediment mobility). Some predatory polychaete taxa were enhanced by fishing. Protodorvillea kefersteini was one of these: large increases in abundance in samples were detected post dredging and persisting over 90 days. The passage of the dredge across the sediment floor will have killed or injured some organisms that will then be exposed to potential predators/scavengers (Frid et al., 2000; Veale et al., 2000) providing a food source to mobile scavengers including these species. Protodorvillia kefersteini  also showed a rapid increase in abundance at 21 days after sediment disturbance (Thrush, 1986).

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., 2003a) and could potentially reburrow.

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. 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 SS.SMx.OMx.PoVen 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.

The venerid bivalves are active suspension feeders, trapping food particles on their gill filaments (ctenidia). An increase in suspended sediment is, therefore, likely to affect both feeding and respiration by potentially clogging the ctenidia. The characterizing species Timoclea ovata, generally occurs in areas with low suspended solids and has ‘ tiny' palps and a short, narrow, mid-gut, as there is little need for particle sorting (Morton, 2009). This suggests this species and other venerids may have difficulty sorting organic materials in high levels of suspended sediment.Glycymeris glycymeris is intolerant of turbidity as the palps are very simple and fine sediments or inorganic solids are not tolerated (Thomas, 1975). Changes in turbidity and seston are not predicted to directly affect Glycera spp. and Lumbrineris latreilli which live within sediments.

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 although the biotope uis likely to still be characterized as SS.SMx.OMx.PoVen. Biotope resilience is assessed as ‘High’ (following restoration of typical conditions) and 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

Addition of fine material will 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 rate. Long-term or permanent addition of fine particles would lead to re-classification of this biotope type (see physical change pressures). The additions of silts to a Spisula solida bed in Waterford Harbour (Republic of Ireland) from earthworks further upstream, for example, reduced the extent of the bed (Fahy et al., 2003). No information was provided on the depth of any deposits.

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 32-41 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.

The venerid bivalves are shallow burrowing infauna and active suspension feeders and therefore require their siphons to be above the sediment surface in order to maintain a feeding and respiration current. Kranz (1972, cited in Maurer et al., 1986) reported that shallow burying siphonate suspension feeders are typically able to escape smothering with 10-50 cm of their native sediment and relocate to their preferred depth by burrowing. Smothering will result in temporary cessation of feeding and respiration. The energetic cost may impair growth and reproduction but is unlikely to cause mortality.

The characterizing bivalve Tellina pygmaea was characterized by Gittenberger & Van Loon (2011) in their index of sedimentation tolerance as  Group IV species: ‘Although they are sensitive to strong fluctuations in sedimentation, their populations recover relatively quickly and even benefit. This causes their population sizes to increase significantly in areas after a strong fluctuation in sedimentation’ (Gittenberger & Van Loon, 2011).  Lumbrineris latreilli was characterized as AMBI sedimentation Group III: 'Species insensitive to higher amounts of sedimentation, but don’t easily recover from strong fluctuations in sedimentation' (Gittenberger & Van Loon, 2011). Glycera alba and Glycera lapidum were categorized as AMBI sedimentation Group II: 'Species sensitive to high sedimentation. They prefer to live in areas with some sedimentation, but don’t easily recover from strong fluctuations in sedimentation' (Gittenberger & Van Loon, 2011).

Sensitivity assessment.  Bivalves and polychaetes 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. Biotope 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

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.

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

No 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.

Introduction of light or shading

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

Evidence

Invertebrate species such as the bivalves and polychaetes may possess rudimentary eyes and be able to perceive light and dark. Changes in light levels are not considered likely to affect adult stages, although little evidence is available to support this conclusion. This pressure is therefore assessed as ‘Not relevant’. 

Barrier to species movement

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

Evidence

Not relevant to off-shore habitats.

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 or spread of invasive non-indigenous species

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

Evidence

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

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

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020, Fitzgerald, 2007, Blanchard, 2009, Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015, Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets, that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy species between or within shells, resulting in a homogeneous Crepidula dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m². Where slipper limpet stacks are abundant, few other bivalves can live amongst them (Fretter & Graham, 1981; Blanchard, 1997). 

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

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 predate on bivalves and could therefore negatively affect bivalve species.

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile or otherwise unsuitable for most of the invasive non-indigenous species currently recorded in the UK.  Non-native predatory gastropods may also emerge as a threat to this biotope.  However, the above evidence suggests that Crepidula could colonize offshore mixed sediment habitats, typical of this biotope, due to the presence of gravel, shells, or any other hard substrata embedded in the substratum that can be used for larvae settlement (Tillin et al., 2020). Bohn et al. (2015) demonstrated that Crepidula had a preference for gravelly habitats, while De Montaudouin & Sauriau (1999) and Bohn et al. (2015) noted that Crepidula densities were low in intertidal coarse sediments. This is a moderately wave exposed to sheltered habitat, so storms may mobilise the sediment (JNCC, 2022), which may also mitigate or prevent colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011).  Therefore, Crepidula has the potential to colonize, and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020), as occurs in maerl gravels (Grall & Hall-Spencer, 2003), resulting in the loss of the biotope.However, the habitat may be most suitable for Crepidula in wave sheltered examples of the biotope and where water movement is mediated by tidal flow rather than wave action, e.g., the deeper examples of the biotope.

Therefore, resistance is assessed as 'Medium' in examples subject to wave action and possibly storms but 'Low' in wave sheltered areas dominated by tidal flow. Resilience is assessed as 'Very low' as it would require the removal of Crepidula, probably by artificial means. Hence, sensitivity is assessed as 'High' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope so the confidence in the assessment is 'Low' and further evidence is required.

Introduction of microbial pathogens

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

Evidence

No evidence 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. Bacterial diseases are frequently found in molluscs during their larval stages, but seem to be relatively insignificant in populations of adult animals (Lόpez 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, from Lόpez-Flores et al., 2004).

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 recognisable 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

The characterizing polychaete species are not directly targeted by fishers. However, the small venerid bivalve Timoclea ovata may be fished. The direct physical effects of species removal is assessed through the abrasion and penetration and abrasion pressures. As the removal of the bivalves is unlikely to have direct or indirect ecological effects on the other characterizing species or to alter biotope classification biotope resilience is assessed as ‘High’, resilience is assessed as ‘High’ (by default) and the biotope is considered to be ‘Not sensitive’.

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. 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.