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

Species are widely distributed. Kurtiella bidentata is widespread around the British Isles and its distribution ranges from Norway to west Africa and the Mediterranean (Carter, 2008). Scoloplos armiger is a species complex as is Chaetozone setosa. Both are widely distributed.  Until recently Chaetozone setosa was considered cosmopolitan with records world-wide, from the intertidal zone to the deep sea.  It is now known that there are several species of eyeless Chaetozone in the north-east Atlantic and the worldwide distribution is unclear. Chambers et al., (2007) assessed numerous records of Chaetozone setosa in the north-east Atlantic. The species is frequently found in habitats where the mean minimum winter bottom temperature is 5-10 degrees and the summer maximum is >10^oC.

Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007).

No specific information concerning temperature tolerances of the biotope and the characterizing species was found, but inferences may be made. For example, Kurtiella bidentata (studied as Mysella bidentata) was recorded in Kinsale Harbour at temperatures ranging from 7.7-18.8°C (O’Brien & Keegan, 2006), and Künitzer (1989) reported that the main factor affecting the growth rate of Kurtiella bidentata (studied as Mysella bidentata) was temperature.

Sensitivity assessment. The characterizing species of the biotopes are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4-19°C (Huthnance, 2010). No information was found on the maximum temperature tolerated by the characterizing species. Elevated temperatures may affect growth of some of the characterizing species, but no mortality is expected. It is, therefore, likely that Kurtiella bidentata and Thyasira spp. are able to resist a long-term increase in temperature of 2°C. However, Thyasira spp. may suffer some mortality as a result of an acute increase in temperature so resistance is therefore assessed as ‘Medium’ (loss <25%). Resilience is likely to be ‘High’ so the biotopes are considered to have ‘Low’ sensitivity to an increase in temperature at the pressure benchmark.  

Temperature decrease (local)

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

Evidence

There is no information on the response of the biotopes to a decrease in temperature. Species are widely distributed. Kurtiella bidentata ranges from Norway to west Africa and the Mediterranean (Carter, 2008). Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007). No specific information on temperature tolerances of Thyasira spp. was found.

Kurtiella bidentata (studied as Mysella bidentata) was among the species that suffered high losses that could be related to low temperatures in the Wadden Sea area in 1979, where temperature was 3 degrees below average for 3 months (Beukema, 1979).

Scoloplos armiger is a species complex as is Chaetozone setosa. Both are widely distributed.  Until recently Chaetozone setosa was considered cosmopolitan with records world-wide, from the intertidal zone to the deep sea.  It is now known that there are several species of eyeless Chaetozone spp. in the north-east Atlantic and the worldwide distribution is unclear. Chambers et al., (2007) assessed numerous records of Chaetozone setosa in the north-east Atlantic. The species is frequently found in habitats where the mean minimum winter bottom temperature is 5-10 ^oC and the summer maximum is  >10 ^oC.

Sensitivity assessment. The characterizing species of the biotope are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4-19°C (Huthnance, 2010). Although no information was found on the minimum temperature tolerated by the characterizing species, it is likely that Kurtiella bidentata and Thyasira spp. are able to resist a long-term decrease in temperature of 2°C. However, the characterizing species Kurtiella bidentata may suffer some mortality as a result of an acute decrease in temperature so resistance is, therefore, assessed as ‘Low’ (25-75% loss) and resilience is likely to be ‘High' so the biotopes are considered to have ‘Low’ sensitivity to a decrease in temperature at the pressure benchmark level.

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 (JNCC, 2015). No directly relevant evidence was found to assess this pressure. 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 the genera Lumbrineris spp. and Scoloplos armiger that occur in this biotope, were present at the discharge point (Riera et al., 2012). However, in the western Baltic Sea, Scoloplos armiger abundance was greatest between 12 psu and 17 psu and reduced abundance with increasing salinity was observed (Gogina et al., 2010). As Scoloplos armiger is a species complex and is not a cosmopolitan species, there may be differences in tolerances between populations.

Sensitivity assessment. It is likely that key components of the biotopes communities would not be resistant of an increase in salinity to >40 psu. Resistance is therefore assessed as ‘Low’ (loss of 25-75%) but with low confidence. Once normal conditions are resumed, resilience is probably ‘High’ so that sensitivity is therefore assessed as ‘Low’.

Salinity decrease (local)

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

Evidence

According to OBIS data (OBIS, 2014), the minimum and maximum range of salinities for the characterizing species are 18.6 - 38.6 pps for Kurtiella bidentata. This data suggests Kurtiella bidentata are able to tolerate wider salinity ranges, which confirm records of Kurtiella bidentata (studied as Mysella bidentata) in Kinsale Harbour at salinities ranging from 19.3-35 (O’Brien & Keegan, 2006). However, Gogina et al. (2010a) reported that Kurtiella bidentata (studied as Mysella bidentata) showed a strong positive correlation with salinity varying at a factor of 8.30-27.10 psu.

Thyasira spp. inhabit waters of reduced salinity with 25-30 psu being optimal. However, adults exposed to lower than optimal salinities produced non-viable or slow developing eggs (Jackson, 2007). No evidence for adult salinity tolerance was found.

Sensitivity assessment. The evidence presented suggests that Thyasira flexuosa are unlikely to tolerate a decrease in salinity at the pressure benchmark level. Resistance is therefore assessed as 'Low' (loss of 25-75%) but with low confidence. Once normal conditions are resumed, resilience is probably 'High' so that sensitivity is therefore assessed as 'Low'.

Water flow (tidal current) changes (local)

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

Evidence

This biotope is recorded in areas where tidal flow varies between moderately strong (0.5-1.5 m/s) and weak (>0.5 m/s) (JNCC, 2015).

Sensitivity assessment. This biotope occurs in areas subject to moderately strong and weak water flows, 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’;  resilience is assessed as ‘High’ (by default) and 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

'Not relevant' to sublittoral biotopes.

Wave exposure changes (local)

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

Evidence

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 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 SS.SMx.CMx.MysThyMx 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.

No evidence was found for the key bivalve species. 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). Bryan (1984) states that Hg is the most toxic metal to bivalve molluscs while Cu, Cd and Zn seem to be most problematic in the field. In bivalve molluscs, Hg was reported to have the highest toxicity, mortalities occurring above 0.1-1 g/l after 4-14 days exposure (Crompton, 1997), toxicity decreasing from Hg > Cu and Cd > Zn > Pb and As > Cr ( in bivalve larvae, Hg and Cu > Zn > Cd, Pb, As, and Ni > to Cr).

Experimental studies with various species suggests that polychaete worms are generally quite tolerant of heavy metals (Bryan, 1984). Rygg (1985) classified Prionospio cirrifera as non-tolerant of Cu (species only occasionally found at stations in Norwegian fjords where copper concentrations were >200 ppm (mg/kg). Total faunal abundance and the density of the polychaete Prionospio cirrifera also decreased significantly at experimentally enriched sediment Cu concentrations of 300 mg/kg (Olsgard, 1999). However, Prionospio malmgreni appeared to be moderately tolerant and were present at some stations in Norwegian fjords with Cu concentrations >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 spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected. Sub-lethal concentrations may produce substantially reduced feeding rates and/or food detection ability, probably due to ciliary inhibition. Respiration rates may increase at low concentrations and decrease at high concentrations. 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. However, the Abra alba population affected by the 1978 Amoco Cadiz benefited from the nutrient enrichment caused by the oil pollution. The biomass of the fine-sand community remained low in 1979, a year after the spill, owing to the decimation of the Ampelisca amphipod population, but the biomass then doubled as a result of an increase in Abra alba abundance in 1980 and Abra alba remained a dominant species over the 20 year duration over which recovery of the community was monitored (Dauvin, 1998).

After a major spill of fuel oil in West Virginia Capitella increased dramatically alongside large increases in Polydora ligni and Prionospio sp. (Sanders et al. 1972, cited in Gray 1979). Prionospio fallax is characteristic of sediments enriched with hydrocarbons (May & Pearson, 1975; cited in Hiscock et al., 2004).

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.

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

A number of animals have behavioural strategies to survive periodic events of reduced dissolved oxygen. These include shell closure and reduced metabolic rate in bivalve molluscs and either decreased burrowing depth or emergence from burrows for sediment dwelling crustaceans, molluscs and annelids.

At oxygen concentrations below ca 0.4 mg O₂/l, Kurtiella bidentata eventually emerged from the substratum (Ockelmann & Muus, 1978). Nilsson & Rosenberg (1994) investigated hypoxic responses of benthic communities and reported Kurtiella bidentata (studied as Mysella bidentata) leaving the sediment at oxygen concentrations of 1.7 mg/l. According to the authors, this is a behaviour that occurs at hypoxic oxygen concentrations slightly higher than those causing mortality, suggesting high levels of stress caused to the organisms. For Kurtiella bidentata (studied as Mysella bidentata), the median sub-lethal oxygen concentrations reported in experimental assessments was 1 mg/l, and for Abra spp. was 0.57 mg/l (Vaquer-Sunyer & Duarte, 2008, references therein).

López-Jamar et al. (1987) stated that Thyasira flexuosa is adapted to living in reduced sediments and also is found in organically enriched sediments. However, Dando & Spiro (1993, cited in Riley, 2008) found that numbers of the congeners Thyasira equalis and Thyasira sarsi decreased rapidly following the de-oxygenation of bottom water in the deep basin of the Gullmar fjord in 1979-80.

Rosenberg et al. (1991) exposed benthic species from the north east Atlantic to oxygen concentrations of around 1 mg/l for several weeks, including species of small bivalves. After 11 days in hypoxic conditions, bivalve individuals were still alive, although individuals showed increased stretching of siphons out of the sediment. In a meta-analysis study of hypoxia, median sub-lethal oxygen concentrations were reported from experimental assessments of tolerance. Although no specific data was reported for all the characterizing species of these biotopes, the thresholds of hypoxia for different benthic groups was LC50 1.42 mg/l for bivalves, and SLC50 of 1.20 mg/l for annelids (Vaquer-Sunyer & Duarte, 2008, references therein).

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) led to the emergence of  Lumbrineris sp. from the substrate these then lie motionless on the surface. Prionospio spp. are often dominant in areas subject to hypoxia/anoxia (Gooday et al., 2009; Ingole, 2010).

Sensitivity assessment: Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Different species in the biotope will have varying responses to de-oxygenation. Based on the evidence presented, the characterizing species are likely to only be affected by severe de-oxygenation episodes. Resistance to de-oxygenation at the pressure benchmark level is likely to be 'High'. Resilience is assessed as 'High' (by default) and the biotope is therefore considered 'Not sensitive' to exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for 1 week.

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).  Increased nutrients are most likely to affect abundance of phytoplankton which may include toxic algae (OSPAR, 2009b). This primary effect resulting from elevated nutrients will impact upon other biological elements or features (e.g. toxins produced by phytoplankton blooms or de-oxygenation of sediments) and may lead to ‘undesirable disturbance’ to the structure and functioning of the ecosystem. With enhanced primary productivity in the water column, organic detritus that falls to the seabed may also be enhanced, which may be utilized by the deposit feeders in the community.

In a report to identify seabed indicator species to support implementation of the EU habitats and water framework directives Kurtiella bidentata, have been reported as likely to be favoured by nutrient enrichment.

Sensitivity assessment. At the pressure benchmark, 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

Many of the species present are deposit feeders characteristic of organically enriched areas. An input of organic matter at the pressure benchmark is likely to provide a food subsidy to these species.

Thyasira spp. are characteristic of organically enriched offshore sediments with Capitella capitata (Connor et al., 2004) and have been identified as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975, cited in Gray, 1979). In the development of the AMBI index to assess disturbance (including organic enrichment), both Borja et al. (2000) and Gittenberger & Van Loon (2011) assigned Thyasira flexuosa to their Ecological 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)’. Kurtiella bidentata (referred to as Mysella bidnetata) was characterized as AMBI Group I - 'Species very sensitive to organic enrichment and present under unpolluted conditions (initial state)' (Gittenberger & Van Loon, 2011). The evidence for the assessments of Kurtiella bidentata was not identified.

Spionid polychaetes are surface deposit feeders and an increase in food at the pressure benchmark could lead to an increase in abundance. Pronospio spp. has been identified as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975, cited in Gray, 1979). Hiscock et al. (2004) also identified Prionospio spp. as increasing under conditions of organic enrichment based on Pearson (1975), Beneath fishfarms, Prionospio spp. were among the most abundant species and in analysis were responsible for differences between those stations and the control condition (Terlizzi et al., 2010), i.e. populations were enhanced beneath stations. Elias et al. (2005) identified Prionospio spp. as an indicator species of organic enrichment in samples collected during the summer season off Mar del Plata City in Argentina. Weston (1990) found high densities of Prionospio cirrifera within organically enriched sediments directly beneath a large aquaculture (although higher densities were found at stations at greater distancesfrom the farm with lower enrichment levels). The Pearson and Black (2001) model of benthic faunal succession from sedimentary loadings (following cessation of fish farming) indicate that Prionospio fallax  would be expected to be found in moderately enriched sediments (after about 9 months).

Chaetezone setosa and cumaceans were typical of enriched sites off the coast of Barcelona that were subject to effluents and sludge disposal from treatment plants (Corbera & Cardell, 1995).

Borja et al. (2000) assessed relative sensitivity of Scoloplos armiger as an ABMI Ecological Group II species (indifferent/tolerant to enrichment). Identified as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975, cited in Gray, 1979).

At high levels of organic input,decreases in abundance of more sensitive species such as Kurtiella bidentata may lead to shifts in community composition towards a biotope dominated by tolerant species, such as polychaete worms (Pearson & Rosenberg, 1978). This could lead to biotope reclassification to the enriched SS.SMu.OMu.CapThy. However, this is likely to occur at levels greater than the pressure benchmark. The Marine Ecosystems Research Laboratory studied the fate and effects of sewage solids added to mesocosms.  Organic loading rates less than 36 gC/m²/yr had little effect, rates between 36 and 365 gC/m²/yr  enriched the sediment community, and a loading over 548 gC/m²/yr  produced degraded conditions (Kelly & Nixon, 1984; Frithsen et al., 1987; Oviatt et al., 1987; Maughan & Oviatt, 1993, cited from Cromey et al., 1998). 

Sensitivity assessment. The evidence presented suggests that the majority of the characterizing and associated species in the biotopes are likely to be able to utilize additional organic load as food and are present in enriched habitats. Biotope resistance is therefore assessed as ‘High’ and resilience as ‘high’ so that the biotope is assessed as ‘Not sensitive’.

Physical loss (to land or freshwater habitat)

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

Evidence

All marine habitats and benthic species are considered to have a resistance of None to this pressure and to be unable to recover from a permanent loss of habitat (resilience is 'Very low'). Sensitivity 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

If the sediment that characterizes the biotopes were replaced with rock or artificial hard substrata, this would represent a fundamental change to the physical character of the biotopes. The characterizing species would no longer be supported and the biotope would be lost and/or re-classified.

Sensitivity assessment. Resistance to the pressure is considered to be 'None', and resilience 'Very low', as the change at the pressure benchmark is permanent. Sensitivity has been assessed as 'High'. Although no specific evidence is described, confidence in this assessment is ‘High’ due to the incontrovertible nature of this pressure and the agreement between classification schemes on the substratum type.

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 sand and gravel (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).  For mixed sediments, resistance is assessed based on a change to either coarse sediments or sand and muddy sands or mud and sandy muds. The characterizing species within these biotopes have wide ranges of sediment preferences. Kurtiella bidentata lives in muddy sand or fine gravel (Carter, 2008), while Thyasira spp. prefer fine sediments including mud, muddy sand and sandy mud (Jackson, 2007).

Sensitivity assessment:  A change in Folk class from mixed sediments to mud and sandy mud to sand or muddy sand would probably not eliminate the characterizing species which all have habitats preferences that would fall within this range. However, a change in one Folk class to coarse or fully mud sediments is likely to result in loss of some of the characterizing species due to habitat unsuitability and increased competition with species more suited to the changed habitat. A change in sediment type will result in biotope reclassification. Resistance is therefore assessed as ‘Low’ (loss of 25-75%) and resilience is considered 'Very low’ given the permanent nature of this pressure. Sensitivity is therefore 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

Sedimentary communities are likely to be ‘Highly' intolerant of substratum removal, which will lead to partial or complete defaunation, expose underlying sediment which may be anoxic and/or of a different character and lead to changes in the topography of the area (Dernie et al., 2003). 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.

Recovery of the sedimentary habitat would occur via infilling, although some recovery of the biological assemblage may take place before the original topography is restored, if the exposed, underlying sediments are similar to those that were removed. Newell et al. (1998) indicate that local hydrodynamics (currents and wave action) and sediment characteristics (mobility and supply) strongly influence the recovery of soft sediment habitats.

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotopes so resistance is assessed as ‘None’, resilience is therefore judged as ‘Medium’ based on sediment and species recovery. Sensitivity has been assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

The characterizing species are infaunal and hence have some protection against surface disturbance. Bivalves and other species require contact with the surface for respiration and feeding. Siphons and delicate polychaete feeding structures may be damaged or withdrawn as a result of surface disturbance, resulting in loss of feeding opportunities and compromised growth.

Sensitivity assessment. Some soft-bodied organisms and a proportion of the characterizing bivalves are likely to be damaged and removed by abrasion. Resistance to abrasion is therefore considered ‘Medium’ (loss <25%). Resilience of the biotopes is likely to be ‘High’. The biotope is therefore considered to have ‘Low’ sensitivity to abrasion or disturbance of the surface of the seabed.

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

Activities that disturb the surface and penetrate below the surface would remove /damage infaunal species such as the characterizing species within the direct area of impact. The footprint of the impact will depend on the type of gear used (Hall et al., 2008).

Bergman & van Santbrink (2000a) estimated the direct mortality of benthic macrofauna caused by the single pass of commercial beam and otter trawls. The results showed that a single pass of a 4 m or 12 m beam trawl or an otter trawl, in shallow sandy areas and deep silty sand areas (with 3-10% silt) in the North Sea caused a mortality of 20-65% of bivalves and 5-40% of gastropods, starfish, small-medium sized crustaceans and annelid worms. The mortality of Kurtiella bidentata (studied as Mysella bidentata) was reported as 4%. Some mortality was not caused directly by the passage of the trawl, but instead by disturbance, exposure and subsequent predation. Ball et al. (2000a) reported on the short-term effects of fishing on benthos from a mud patch in the northwestern part of the Irish Sea investigated in 1994–1996 by means of samples taken both before and shortly after (ca. 24 hr) fishing activity. Kurtiella bidentata (studied as Mysella bidentata) was one of the species that was common at the inshore site and for which estimates of mortality were calculated and was uncommon or totally absent on the offshore fishing ground. Direct mortality from passage of an otter trawl was estimated as 72%. The direct mortality (percentage of initial density) of Thyasira flexuosa was estimated as 0-28%, based on samples taken with a Day grab before and 24 hours after trawling (Ball et al., 2000b). 

The shells of Thyasira spp. are thin and fragile and penetration and disturbance of the sediment is likely to lead to damage and mortality within the population. Sparks-McConkey & Watling (2001) found that trawler disturbance resulted in a decline of Thyasira flexuosa in Penobscot Bay, Maine. However, the population recovered after 3.5 months. 

Ferns et al. (2000) investigated the effect of tractor dredging for cockles on an intertidal muddy sand at Burry Inlet, South Wales mechanical cockle harvesting. A decline of 31% in populations of Scoloplos armiger (initial density 120 m²) was recorded  in muddy sands, Scoloplos armiger demonstrated recovery >50 days after harvesting (Ferns et al., 2000).

Tuck et al. (1998) found that following trawl disturbance, abundances of Chaetozone setosa had recovered and became greater at treatment sites than undisturbed sites 10 months after disturbance. Scoloplos armiger, however, had declined at disturbed sites.

Sensitivity assessment. A proportion of the characterizing species in these biotopes is likely to be lost or severely damaged, depending on the scale of the activity (see abrasion pressure). Therefore, a resistance of ‘Low’ (>75% loss) is suggested based on Mysella bidentata. Muddy sand habitats have been reported as having the longest recovery times, whilst mud habitats had an ‘intermediate’ recovery time (compared to clean sand communities which had the most rapid recovery rate) (Dernie et al., 2003). Resilience is probably ‘High’, and therefore the biotopes’ sensitivity to this pressure if likely to be ‘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). The venerid bivalves are active suspension feeders, trapping food particles on their gill filaments (ctenidia).

Lethal effects on Kurtiella bidentata are considered unlikely given the occurrence of this species in estuaries where turbidity is frequently ‘High’ from suspended organic and inorganic matter.  As Thyasira flexuosa are buried within the sediment and are fed by symbiotic bacteria they are considered insensitive to a change in suspended solids. 

Sensitivity assessment. No direct evidence was found to assess impacts on the characterizing bivalves and associated polychaete species. Based on infaunal position and the dominance of deposit feeders biotope resistance is likely to be 'High’. Resilience is assessed as 'High' (by default) and the biotope is therefore considered 'Not sensitive'.

Smothering and siltation rate changes (light)

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

Evidence

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. This suggests that characterizing species such as Kurtiella bidentata and Thyasira spp. may be able to burrow through similar overburdens. Thyasira flexuosa have ‘highly extensible feet’ (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Sensitivity assessment. Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). However, the biotopes are likely to resist smothering at the benchmark level since the majority of associated fauna are burrowing infauna. Resistance is therefore assessed as ‘High’, and resilience is also ‘High’ (by default) so that the biotopes are considered 'Not Sensitive' to a ‘light’ deposition of up to 5 cm of fine material added to the seabed in a single, discrete event.  

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. This suggests that characterizing species Kurtiella bidentata and Thyasira spp. may be able to burrow through similar overburdens, although sudden smothering with 30 cm of sediment would temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure. Thyasira flexuosa have ‘highly extensible feet’ (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Bijkerk (1988, results cited from Essink 1999) indicated that the maximal overburden through which Scoloplos could migrate was 50 cm in sand and mud.  No further information was available on the rates of survivorship or the time taken to reach the surface. Warner (1971) simulated the effects of dredge disposal of different thicknesses on animals in aquaria or plastic cores for 2 weeks.  In core experiments at temperatures ranging from 14 to 18°C and 20 to 21°C, there was a relationship between vertical migration distance and sediment depth for the congener Scoloplos fragilis.  This species could vertically migrate through 30 cm of sand.  In other core experiments in silt-clay at temperatures of 17^oC to 18^oC, there was a suggestion of reduced efficiency of burrowing in finer grained sediment where even the smallest amount of silt-clay proportion tested (20%) affected the burrowing ability of this species.

Sensitivity assessment: Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). Bivalve and polychaete species have been reported to migrate through depositions of sediment greater that the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Bijkerk, 1988; Powilleit et al., 2009; Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Resistance is therefore assessed as ‘Low’ (25-75% loss) and resilience as ‘High’ and the biotopes are considered to have ‘Low’ sensitivity to a ‘heavy’ deposition of up to 30 cm of fine material in a single discrete event.

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

 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

This pressure is considered to be 'Not relevant' to biotopes that occur only in open waters, rather than coastal bays and estuaries.

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

The characterizing species of the biotopes live infaunally, so are likely to have poor or no visual perception and unlikely to be affected by visual disturbance such as shading.

Sensitivity assessment. The characterizing species are likely to be tolerant of visual disturbance. Resistance and resilience are therefore assessed as 'High' and the biotopes judged as 'Not sensitive' to visual disturbance.

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). 

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile and unsuitable for most of the invasive non-indigenous species currently recorded in the UK. However, the above evidence suggests that Crepidula could colonize mixed sediment habitats in the subtidal, typical of this biotope, due to the presence of gravel, shells, cobbles, 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. 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. This is a moderately 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 it has been recorded from areas of strong tidal streams (Hinz et al., 2011). Therefore, the habitat may be more suitable for Crepidula in wave sheltered areas 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 where wave action is high and subject to 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

More than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996).

Little information specifically concerning the effects of microbial pathogens and parasites on the viability of the characterizing species was found. A viral infection of the mutualist bacterium living on the gills of Thyasira gouldii has been suggested as the reason for a major decline in the Loch Etive population (Jackson, 2007, references therein),

Sensitivity assessment. No direct evidence of the biotopes being affected by the introduction of microbial pathogens was found to assess this pressure.

Removal of target species

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

Evidence

No species within the biotope are targeted by commercial or recreational fishers or harvesters. This pressure is therefore considered ‘Not relevant’.

Removal of non-target species

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

Evidence

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Species in these biotopes, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Loss of these species would alter the character of the biotope resulting in re-classification, and would alter the physical structure of the habitat resulting in the loss of the ecosystem functions such as secondary production performed by these species.

Sensitivity assessment. Removal of the characterizing species would result in the biotopes being lost or re-classified. As many species are relatively small and the bivalves may be displaced by sediment disturbance but survive (see physical damage pressures), the biotope is considered to have a resistance of ‘Low’ to this pressure and to have ‘High’ resilience, so that sensitivity is assessed as ‘Low’.