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

Limited information on the thermal tolerance of Lanice conchilega was found. Lanice conchilega occurs in temperate regions all around UK and Irish shores in the intertidal and shallow subtidal, populations also occur in the Mediterranean, in the Arabian Gulf and the Pacific and along the south east of the North Sea, but the species is absent from Arctic waters (Ropert & Dauvin, 2000; Connor et al., 2004; Degraer et al., 2008). The geographical range suggests that the species is adapted to variable temperature conditions but local populations are likely to be acclimated to prevailing temperatures.

Magelona mirabilis are widespread on the coasts of Britain and Northern Ireland (Richards, 2007) and their distribution extends to north-west Europe and the Mediterranean (Hayward & Ryland, 1995b). Scoloplos armiger is a species complex as is Chaetozone setosa. Both are widely distributed but populations may be sibling species and exhibit different tolerances. 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 but the worldwide distribution is unclear. Chambers et al. (2007) assessed numerous records of Chaetozone setosa in the north-east Atlantic, and identified habitat preferences Chatezone setosa  was frequently found in habitats where the mean minimum winter bottom temperature is 5-10°C and the summer maximum is >10°C (Chambers et al., 2007). 

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4-19°C (Huthnance, 2010). The associated species are considered likely to be tolerant of acute and chronic increases in temperature at the pressure benchmark. Lanice conchilega and the other characterizing polychaetes have a wide geographic range. The lack of evidence for mass mortalities of intertidal populations in very hot summers (compared with reports for low winter temperatures), suggest that the characterizing and associated species are likely to tolerate a chronic or acute increase 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

Infaunal species such  Lanice conchilega may be protected to some extent for short periods by the ability to burrow deeper within the sediment. Prolonged freezing temperatures may, however, result in mortalities. In the German coastal area during 1978-79 winter, water temperature remained below 0°C on 45 successive days and resulted in the eradication of Lanice conchilega. Up to July 1980 no recolonization of the area had taken place (Buhr, 1981). An intertidal population of Lanice conchilega, in the northern Wadden Sea, was wiped out during the severe winter of 1995/96 (Strasser & Pielouth, 2001), and Crisp (1964) described mortality of Lanice conchilega between the tidemarks but not at lower levels during the severe winter of 1962/63. These severe winters probably exceed the pressure benchmark. Beukema (1990) had noted that the recovery of most species including Lanice conchilega, was rapid (within 1 and 2 years) and total-biomass value increased faster as a consequence of generally high successful recruitment in the Wadden Sea. Whilst Heuers et al. (1998) observed Lanice conchilega re-established abundance within 3 to 4 years in the Spiekeroog area of the Wadden Sea following severe freezing.

Scoloplos armiger is a species complex as is Chaetozone setosa. Both are widely distributed but populations may be sibling species and exhibit different tolerances. 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°C and the summer maximum is >10°C.

Sensitivity assessment. Biotope resistance has been assessed to be ‘Low’ as evidence suggests that populations of the key characterizing species Lanice conchilega are likely to be vulnerable to acute decreases in temperature during winter, resilience is assessed as ‘High’ where some adults remain and sensitivity is 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 (30-35 ppt) salinity (JNCC, 2015). At the pressure benchmark, an increase from full to hypersaline (>40 ppt) was considered but no direct evidence was found to assess the sensitivity of this biotope to a long-term increase.

Salinities within tide swept habitats vary due to weather conditions such as rainfall at low tide and evaporation causing hypersaline conditions on hot days. Intertidal shores within estuarine environments can also experience considerable short-term changes in salinities. During summer, the Atlantic French coast rapidly increased in salinity and reached hypersaline conditions (48 ppt) in the upper sediment layer within 1.5 hrs (Juneau et al., 2015).  Lanice conchilega as a tube dwelling polychaete may retract deeper into the sediment to avoid unfavourable conditions but some physiological tolerance for short-term increases in salinity is suggested by its presence in the intertidal.

No direct evidence was found to assess sensitivities of the other characterizing polychaete species. Scoloplos armiger was found at low abundances at the discharge point the discharge point of brine effluents at 47-50 psu in the Canary Islands (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. Species within the biotope may tolerate short periods of high salinity, however, prolonged exposure to hypersalinity may lead to changes in species richness, abundance and biomass and loss of characterizing and associated species. Sensitivity to this pressure is not assessed due to lack of evidence.

Salinity decrease (local)

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

Evidence

The subtidal biotope is found in full salinity (JNCC, 2015), but the intertidal biotope LS.LSa.MuSa.Lan is found in both full and variable salinity. It is therefore considered that a decrease in salinity at the pressure benchmark, from full to variable (18-35 ppt), would not result in loss of Lanice conchilega as this falls within the natural habitat distribution.

Scoloplos armiger shows a lower salinity limit of 10.5 psu (Gogina et al., 2010), suggesting the species is tolerant of a decrease from full to reduced salinity and even the low salinity category in the MNCR scale. Magelona mirabilis occurs in the Baltic Sea (Fiege et al., 2000) where salinity is typically lower than in the open ocean. It is likely that some populations of Magelona mirabilis are adapted to reduced salinity habitats, however, no information on the effects of an overall decrease in salinity were found.

Sensitivity assessment. Little empirical evidence was found to assess sensitivity of Lanice conchilega and other characterizing polychaetes to this pressure at the benchmark level and therefore the assessment was made based on the intertidal biotope. As a reduction may lead to some mortalities and a reduction in growth and reproductive success of the key characterizing species, biotope resistance is assessed as ‘Medium’, recovery is assessed as ‘High’ (following restoration of typical conditions) and resilience is ‘High’. Biotope 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 found in areas exposed to tidal streams varying from strong (1.5-3 m/s) to negligible (JNCC, 2015).

There is limited evidence to assess changes in water flow rates for >1 year at the benchmark level. Laboratory flume studies have showed that Lanice conchilega feeding rates on diatoms alter according to flow (Denis et al., 2007). Feeding rates were optimal at 0.15 m/s and lower at flow speeds of 0.04 m/s and 0.27 m/s (Denis et al., 2007). Changes in current velocity at the pressure benchmark are relevant to feeding rates, however, impacts are likely to depend on food supply, population density and proportion of inorganic particles.

The motion of the water is the dominant factor in the transportation of sediment, facilitating erosion and redistribution of materials (Hjulström, 1939). This may change the sediment structure and have associated effects on the intertidal community. Lanice conchilega alter between passive and active feeding depending on the tidal regimes, i.e. in still water, they move their tentacles or in high velocities the fringed tentacles are supported. A switch from active to passive feeding behaviour was observed during the experiments between 4 and 8 cm/s (Denis et al., 2007). Moderate to high velocities of water flow have been reported to enhance settlement of Lanice conchilega larvae (Harvey & Bourget, 1997), increasing average particle size in favour of gravels and pebbles.

Reduced water flow is a factor that has been identified as affecting the density of Lanice conchilega, as recruitment to the benthos is reduced (Harvey & Bourget, 1995). The average grain size of the sediment would also be reduced and probably favour deposit feeders and detritivores, to the detriment of the suspension feeders.

Sensitivity assessment.  A change in water flow rate at the pressure benchmark level of 0.1-0.2 m/s is considered to fall within the range of flow speeds experienced by the biotope. Resistance and resilience are assessed as ‘High’ and the biotope considered ‘Not sensitive’ to a change in water flow at the pressure benchmark level.

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

The biotope occurs from the shallow sublittoral (0-5 m) down to a depth of 20 m, therefore it does not normally experience periods of emergence. An increase in emergence may affect the uppermost part of it. However, on certain shores the LS.LSa.MuSa.Lan biotope may be a littoral extension of this biotope. An increase in emergence may lead to an increase in predation (of the relatively small proportion of the biotope affected) by wading sea birds. However, on balance the major extent of the biotope would be unaffected and biotope resistance is assessed as ‘High’, resilience as ‘High’ and the biotope is considered ‘Not sensitive’.

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

The biotope is recorded from locations that have variable wave exposures (moderately exposed to sheltered) (Connor et al., 2004). The degree of wave exposure influences wave height, as in more exposed areas with a longer fetch waves would be predicted to be higher. 

Sensitivity assessment. As this biotope is found across a range of wave exposed shores, resistance is assessed as ‘High’ and resilience as ‘High’ to a 3 to 5% change in wave height at the pressure benchmark. By default the biotope is considered ‘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.

Contamination at levels greater than the pressure benchmark may adversely affect the biotope. Bryan (1984) reported that short-term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb with Cd, Ni, Co and Se being the least toxic. It was recorded that polychaetes have a range of tolerances to heavy metal levels of Cu, Zn, As and Sn being in the order of 1500-3500 µg/g.

Boilly & Richard (1978) stated that the presence of Magelona mirabilis is indicative of sediments which have been contaminated with iron. Studies on a dredge spoil disposal site in the harbours of Boulogne and Dunkerque in France (Bourgain et al., 1988) found higher densities of Magleona mirabilis three months after the dumping of dredge spoil than after five months, when the metal contamination of the sediments was higher. No information regarding the effect of other metals on this species was found.

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.

Contamination at levels greater than the pressure benchmark may adversely influences the biotope. Suchanek (1993) reviewed the effects of oil spills and concluded that soft sediment polychaetes, bivalves and amphipods were particularly sensitive. Dauvin (2000) noted that 20 years after the Amoco Cadiz oil spill in 1978, Lanice conchilega was re-established between 1978 and 1984 but disappeared after 1985. A similar delayed response was observed by Sanders et al. (1980) and Kingston et al. (1995) as a result of the Florida oil spill and the Braer oil spill (Gómez Gesteira & Dauvin, 2000).

Gray et al. (1990) found that Scoloplos armiger were a dominant species in uncontaminated soft sediments at a case study site adjacent to the Ekofisk oil field but were not present at contaminated sites.

Synthetic compound contamination

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

Evidence

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

No evidence concerning specific effects of synthetic chemical contaminants on Lanice conchilega was found.

Polychaetes vary greatly in their tolerance of chemical contamination. The persistence of these chemical residues is highly dependent on the matrix and ambient environmental conditions. Generally, residues in water are less likely to be a long-term concern because of photodegendation and dilution to below biological significant concentrations. However, TBT has a high binding affinity to sediments and residues incorporated into the sediment tend to persist for longer periods (Austen & McEvoy, 1997; Huntington et al., 2006).

Radionuclide contamination

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

Evidence

No evidence was found

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

Niermann et al. (1990) reported changes in a fine sand community for the German Bight in an area with regular seasonal hypoxia. In 1983, oxygen levels were exceptionally low (<3 mg O₂/l) in large areas and <1 mg O₂/l in some areas. Species richness decreased by 30-50% and overall biomass fell. The abundance of Lanice conchilega was significantly reduced during the period of hypoxia (1-3 mg/O₂/dm³). However, the period of hypoxia lasted for about a month and, therefore, exceeds the pressure benchmark. Magelona sp. remained abundant during the hypoxic period and decreased slightly in abundance on resumption of normoxia. Spiophanes bombyx was found in small numbers at some, but not all areas, during the period of hypoxia. Once oxygen levels returned to normal Spiophanes bombyx increased in abundance; the evidence suggests that at least some individuals would survive hypoxic conditions (Niermann et al., 1990)

In laboratory conditions, Scoloplos armiger survived low oxygen conditions for 40 hours (Schöttler & Grieshaber, 1988). No evidence was returned by searches on extended exposure to low levels of dissolved oxygen.

Sensitivity assessment. Based on the available evidence biotope resistance is assessed as ‘Low’ (based on Lanice conchilega) and resilience is assessed as 'High' (following restoration of typical habitat conditions). Biotope resistance is therefore assessed as ‘Low’. Due to the lack of evidence for Lanice conchilega, at the pressure benchmark confidence in applicability 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 benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014). 

Sensitivity assessment. 'Not sensitive' at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Organic enrichment

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

Evidence

The polychaete species characterizing this biotope are all deposit feeders that may benefit from an input of organic material and are likely to be resistant to this pressure at the benchmark. Chaetezone setosa were  typical of enriched sites off the coast of Barcelona that were subject to effluents and sludge disposal from treatment plants (Corbera & Cardell, 1995) and other studies suggest the characterizing polychaetes are unlikely to be affected by organic enrichment at the pressure benchmark. Borja et al. (2000) assessed relative sensitivity of Scoloplos armiger as an ABMI Ecological Group II species (indifferent/tolerant to enrichment). Field studies have also identified Scoloplos armiger as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975 cited in Gray, 1979).

Lanice conchilega was categorised by Borja et al. (2000) 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)'. This assessment was reviewed by Gittenberger & Van Loon (2011) and changed to AMBI Group III - '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)'. 

Sensitivity assessment. There is little empirical evidence to quantify the effect of organic enrichment deposits of 100 g C/m^²/yr on Lanice conchilega. Based on the ranges presented by Cromey et al. (1998), this benchmark pressure is unlikely to have an influence upon Lanice conchilega and associated species. Biotope resistance is assessed as ‘High’ and resilience as ‘High’ (by default), and the biotope is categorized as ‘Not sensitive’.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

The species characterizing this biotope occur within the upper layers of the soft sediment (JNCC, 2015). This biotope does not occur on artificial substrata or rock habitats, although patches of sediment may support some of the species associated with this biotope. Any substratum other than the sediments on which this biotope is found would therefore lead to a loss of this biotope. Consequently, biotope resistance is assessed as 'None’, resilience is assessed as ‘Very Low’ (as the change, at the benchmark is permanent), and biotope sensitivity is assessed as ‘High’. 

Physical change (to another sediment type)

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

Evidence

This biotope is found in medium to very fine gravelly sands  (Connor et al., 2004). The biotope would not be considered sensitive to a change to mixed sediments for muddy sands, as the sediment would still be likely to support the biotope without a fine sand fraction could, however, lead to a reduction in habitat suitability for both Lanice conchilega and the associated burrowing polychaetes.

An assessment of distribution records of Chaetozone setosa in the North Sea concluded that the species is usually associated with fine sediments (Chambers et al., 2007). Magelona spp. occur in coarse clean sand and fine clean sand (Rayment, 2007b). The polychaete Scoloplos armiger has relatively broad sediment preferences. Changes in sediment composition that alter the grade of sediment this species must move through can affect the suitability of the habitat. An increase in coarse composition to gravels would be expected to negatively impact this burrowing species. 

Sensitivity assessment. Changes in sediment that lead to increased finer sediments are likely to be tolerated, however, a change in the sediment Folk class classification at the pressure benchmark to gravel is likely to alter the abundance of Lanice conchilega and the composition of the species assemblage, resulting in loss of this biotope. Resistance is therefore assessed as 'None' and resilience as 'Very Low' (as the change at the pressure benchmark is permanent). The biotope is considered to have 'High' sensitivity to this pressure.

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, exposure of underlying sediment which may be anoxic and/or of a different character or bedrock 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 conditions to which they are not suited, i.e. unfavourable conditions. Newell et al. (1998) state that removal of 0.5 m depth of sediment is likely to eliminate benthos from the affected area.

Removal of 30 cm of sediment will remove species that occur at the surface and within the upper layers of sediment. The extraction of sediment would remove the characterizing species Lanice conchilega, and the associated species present, including Cerastoderma edule, which is found to a depth of 5 cm and Nephtyid species and other polychaetes, such as Scoloplos armiger and Pygospio elegans that burrow between 5 and 20 cm into the sediment (Schӧttler, 1982; Pedersen, 1991; Kruse et al., 2004).   

Hydrodynamics and sedimentology (mobility and supply) influence the recovery of soft sediment habitats (Van Hoey et al., 2008). Recovery of the sedimentary habitat would occur via infilling, 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.

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotope. Resistance is assessed as ‘None’ and biotope resilience is assessed as ‘Medium’ as some sediment recovery may be required before recovery processes begin. Biotope sensitivity is therefore assessed as ‘Medium’. 

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

The key characterizing species of this biotope (Lanice conchilega) has robust, flexible tubes and may retract below the surface. They are also able to rapidly rebuild or repair tubes (Nickolaidou, 2003). These characteristics reduce exposure to this pressure and enhance recovery. Rabaut et al. (2008) studied fisheries impacts at the species level in temperate sandy bottom areas. A controlled field manipulation experiment was designed focusing on areas with high densities of Lanice conchilega (i.e. Lanice conchilega reefs). A treatment zone was exposed to a one-off experimental trawling and the impact on, and recovery of the associated fauna was investigated for a period of 9 days post-impact. Community analysis showed a clear impact on associated species such as Eumida sanguinea, followed by a relatively quick recovery. The passage of a single beam trawl did not significantly alter the density of Lanice conchilega (Rabaut et al., 2008).

The characterizing polychaetes Magelona mirabilis, Chaetozone setosa and Spiophanes bombyx are soft bodied organism which will be exposed to abrasion while feeding at the sediment surface while feeding. Abrasion would be likely to cause physical damage to these species.

Juveniles and adults of Scoloplos armiger stay permanently below the sediment surface, and freely move without establishing burrows. While juveniles are only found a few millimetres below the sediment surface, adults may retreat to 10 cm depth or more (Reise, 1979; Kruse et al., 2004). The egg cocoons are laid on the surface and hatching time is 2-3 weeks during which these are vulnerable to surface abrasion.

Sensitivity assessment. The experiments by Rabaut et al. (2008) suggest that Lanice conchilega has ‘High’ resistance to abrasion, however, other associated species may be more impacted but species may be able to repair and recover from damage. Biotope resistance to a single abrasion event is assessed as ‘High’ based on the key characterizing species Lanice conchilega. There may be some damage to Lanice conchilega tubes and some reduction in abundances of the associated polychaete species but this unlikely to significantly alter the character of the biotope. Recovery from impacts of associated species is predicted to be ‘High’ and the biotope is assessed as ‘Not sensitive’.  

Penetration or disturbance of the substratum subsurface

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

Evidence

The characterizing species of this biotope (Lanice conchilega) has robust, flexible tubes and may retract below the surface. They are also able to rapidly rebuild or repair tubes (Nickolaidou, 2003). These characteristics reduce exposure to this pressure and enhance recovery.

Rabaut et al. (2008) studied fisheries impacts at the species level in temperate sandy bottom areas. A controlled field manipulation experiment was designed focusing on areas with high densities of Lanice conchilega (i.e. Lanice conchilega reefs). A treatment zone was exposed to a one-off experimental trawling, and the impact on and recovery of the associated fauna was investigated for a period of 9 days post-impact. Community analysis showed a clear impact followed by a relatively quick recovery. The passage of a single beam trawl did not significantly alter the density of Lanice conchilega. Rabaut et al. (2009) also studied the direct mortality of Lanice conchilega as a consequence of sustained physical disturbance at varying frequencies to reflect the effect of beam trawl fisheries. Research was based on a laboratory experiment in which four different disturbance regimes were applied (disturbance every other 12, 24 and 48 h and no fishing disturbance as a control). Survival dropped significantly after 10 and 18 days (with a disturbance frequency of every 12 and 24 h, respectively). The results indicate that Lanice conchilega is relatively resistant to physical disturbance but that reef systems can potentially collapse under continuous high frequency disturbance.

Sparks-McConkey & Watling (2001) identified Chaetozone setosa as a common species that declined in abundance in response to experimental trawling. 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.

Direct mortality (percentage of initial density) of Scoloplos armiger from a single pass of a beam trawl was estimated from experimental studies on sandy and silty grounds as 22% and 28% respectively. Mortality of Magelona sp. was estimated as 30% (Bergman & Van Santbrink, 2000a). 

Experimental intertidal dredging for cockles reduced the abundance of Scoloplos armiger in disturbed plots compared to control sites. These differences persisted for 56 days (Hall & Harding, 1997). Ferns et al. (2000) reported a decline of 31% in intertidal populations of Scoloplos armiger  in muddy sands when a mechanical tractor towed harvester was used (in a cockle fishery) (surpassing the study monitoring timeline). Scoloplos armiger demonstrated recovery >50 days after harvesting in muddy sands. 

Sensitivity assessment. The experiments by Rabaut et al. (2008, 2009) suggest that Lanice conchilega has ‘High’ resistance to single abrasion and penetration events. Other polychaetes within the biotope may be more impacted. Biotope resistance to a single event is assessed as ‘Medium’ based on the key characterizing species Lanice conchilega and associated characterizing species.  There may be some damage to tubes but this is expected to be sub-lethal and tubes are likely to be rapidly repaired. Recovery from impacts of associated species is predicted to be ‘High’ and the biotope is assessed as ‘Not sensitive’.  

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

Production within the biotope is predominantly secondary, derived from detritus and to some extent phytoplanktonic production. Characteristic infauna do not require light and therefore the effects of increased turbidity on light attenuation are not directly relevant. However, an increase in turbidity may affect primary production in the water column and therefore reduce the availability of phytoplankton as food but phytoplankton would also be transported in to the biotope from distant areas, so the effect of increased turbidity may be mitigated. As soon as light levels return to normal, phytoplanktonic primary production would increase, the species would resume optimal feeding, so recoverability has been assessed to be very high. A decrease in suspended organic particles may reduce food supply to Lanice conchilega, however, this species can switch from suspension to deposit feeding and vice versa (Buhr & Winter, 1977). Supply of materials needed to build the tubes may be affected with a decrease in suspended solids but wave action would be likely to continue to re-suspend and transport sediments.

Sensitivity assessment. A decrease in turbidity and hence increased light penetration may result in increased phytoplankton production and hence increased food availability for suspension feeders, including Lanice conchilega and Cerastoderma edule. Therefore, reduced turbidity may be beneficial. In areas of high suspended sediment, a decrease may result in improved condition and recruitment due to a reduction in the clogging of filtration apparatus of suspension feeders and an increase in the relative proportion of organic particulates. However, a decrease in suspended organic particles in some areas may reduce food availability for deposit feeders resulting in lower growth or reduced energy for reproduction. 

Where increased turbidity results from organic particles then subsequent deposition may enhance food supply for suspension and deposit feeders within the biotope. Alternatively, if turbidity results from an increase in suspended inorganic particles then energetic costs may be imposed on these species as sorting and feeding becomes less efficient, reducing growth rates and reproductive success. Lethal effects are considered unlikely given the occurrence of Lanice conchilega and Cerastoderma edule and other associated species in estuaries where turbidity is frequently high from suspended organic and inorganic matter. Resistance and resilience are therefore assessed as 'High' and the biotope is considered to be 'Not sensitive'.

Smothering and siltation rate changes (light)

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

Evidence

The tube of Lanice conchilega protrudes above the sediment by 1–4 cm, an adaptation to trap suspended particles (Zühlke, 2001). Furthermore, Ziegelmeier (1952) showed that the tubes of Lanice conchilega increased the height with increasing sedimentation so that it could continue feeding and metabolizing aerobically. This enables Lanice conchilega to switch between deposit and suspension feeding (Buhr, 1976). Therefore, Lanice conchilega is suitably adapted to discrete events of sedimentation, characteristic of sandy and muddy sand intertidal flats wher epopulations of this species occur. Lanice conchilega  has dominated areas of Manila clam cultivation where protective netting had led to an increase in sedimentation (Spencer et al., 1998) and in areas where oysters were cultivated and sedimentation increased (Sylvand, 1995).

Chaetezone setosa occurs in areas subject to high natural rates of sedimentation where other benthic macrofauna were excluded (Wlodarska-Kowalczuk et al., 2007) although this is likely to be due to rapid recolonization rather than survival. In general, the surface deposit feeders, Chaetozone setosa, Magelona mirabilis and Spiophanes bombyx will be smothered by a depoist at the benchmark and prevented from feeding while this remains in place. No evidence was found on the ability of these species to reposition within fine sediment deposits or survive burial.

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°C to 18°C, 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. Based upon the ability of Lanice conchilega to increase tube elevation above the sediment surface when deposition has increased, it is likely that Lanice conchilega is resistant to the effects of a single discrete event of deposition of 5 cm. Although Scoloplos armiger is considered to be able to migrate vertically, this may be limited where the overburden consists of fine sediments (based on Maurer et al., 1978). Sensitivity to continuous events will depend on tidal hydrodynamics, and spawning and recruitment, prior to the event (see recover/resilience rates). As the benchmark refers to a single discrete event, biotope resistance is assessed as ‘High’ based on Lanice conchilega and resilience is ‘High’, therefore this biotope is ‘Not sensitive’.

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

No evidence was found to assess sensitivity of Lanice conchilega to heavy sedimentation. It is likely that intertidal flats may sometimes be subject to sedimentation following storm events but no species-specific examples were found. Ziegelmeier (1952) showed that Lanice conchilega increased the height of its tube top with increasing sedimentation so that it could continue feeding and respire. However, heavy sedimentation of 30 cm during a single event may have a severe effect on Lanice conchilega and the associated species.

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°C to 18°C, 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. Although Scoloplos armiger is considered to be able to migrate vertically, this may be limited where the overburden consists of fine sediments (based on Maurer et al., 1978). The addition of 30 cm of sediment would prevent the surface deposit feeders from extending palps to the surface. It is unlikely that these species could emerge from a 30 cm thick deposit of fine materials., although some individuals may survive and sediment may be rapidly removed by tide and wave action. It is likely, however, that there would be considerable mortality of characterizing species, including Lanice conchilega. Resistance is assessed as ‘Low’ and resilience as ‘High’. Sensitivity is therefore assessed as ‘Low’. 

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

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

Introduction of light or shading

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

Evidence

No evidence. As this feature is not characterized by the presence of primary producers, it is not considered that shading would alter the character of the habitat directly.

Barrier to species movement

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

Evidence

Barriers that reduce the degree of tidal excursion may alter larval supply to suitable habitats from source populations. Conversely, the presence of barriers at brackish waters may enhance local population supply by preventing the loss of larvae from enclosed habitats to environments, which are unfavourable, reducing settlement outside of the population. Resistance to this pressure is assessed as 'High' and resilience as 'High' by default. This biotope is therefore considered to be 'Not sensitive'.

Death or injury by collision

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

Evidence

‘Not relevant’ to seabed habitats. NB. Collision by 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

Characterizing and associated species are not cultivated or transplanted and this pressures is considered 'Not relevant'.

Introduction or spread of invasive non-indigenous species

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

Evidence

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; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

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

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

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

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Baron, 2010; GBNNSS, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Baron, 2010; Padilla, 2010). Magallana gigas is recognised as a beneficial and important species in aquaculture worldwide (Padilla, 2010). It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSS, 2011, 2012; Humphreys et al., 2014 cited in Alves et al., 2021; Hansen et al., 2023).

Since introduction, the species has invaded and established self-sustaining natural populations throughout Europe from the North Sea, Wadden Sea and Scandinavian coastlines to the Atlantic coastlines of Spain and Portugal, as well as the Mediterranean and Adriatic Sea (Wrange et al., 2010; GBNNSS, 2011, 2012; Ezgeta-Balic et al., 2019; Spagnolo et al., 2019; Bergstrom et al., 2021; Hansen et al., 2023). In the UK, the species predominantly occurs around the southern and western coastlines (OBIS, 2024; NBN, 2024). Shipping activity has also been associated with the introduction of Magallana gigas in the northeastern Adriatic Sea, where it was not introduced for aquaculture (Ezgeta-Balic et al., 2019). It was also suggested that some Magallana gigas populations were established in southwest England from France possibly via fouling on ships (GBNNSS, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).

Magallana gigas has a high fecundity, a long-lived pelagic larval phase (2 to 4 weeks) and can produce up to 200 million eggs during spawning (Herbert et al., 2012, 2016; Alves et al., 2021; Wood et al., 2021; Hansen et al., 2023). Hence, as a broadcast spawner, it has a high dispersal potential of more than 1000 km (Padilla, 2010; Wood et al., 2021). Larval mortality can be as large as 99%, as larvae are sensitive to environmental conditions (Alves et al., 2021). However, adults are long-lived so populations can survive with infrequent recruitment (Padilla, 2010). Larval dispersal and mass spawning events have facilitated the settlement and establishment of Pacific oysters, as seen in the Oosterschelde estuary, Netherlands (Hansen et al., 2023). It has been suggested that the spread of the Pacific oyster in Scandinavia is due to northward larval drift on tidal and wind-driven currents (Hansen et al., 2023). Wood et al. (2021) suggested that larval dispersal of the Pacific oyster from populations within and outside the UK was possible via unaided (passive) transport by currents, but that aquaculture and offshore structures (e.g. windfarms) increased the risk of the invasive species spreading and the geographical extent of spread.

Magallana gigas is an ecosystem engineer and can dramatically change habitat structure when it invades. Once successfully settled, groups of Pacific oysters may form dense aggregations, potentially forming a reef, which in some regions can reach densities of 700 individuals m² (Herbert et al., 2012, 2016). Once, the density of live or dead Pacific oysters reaches or exceeds 200 ind./m² little of the underlying substratum remains visible (Herbert et al., 2016). These reefs can stabilize the sediment surface locally (Troost, 2010). When such reefs are formed or, particularly when the species colonizes soft sediments such as mud or sand, it can change and affect local communities, by creating hard substrata for mobile species, which might not otherwise be present before the invasion (Padilla, 2010). However, Hansen et al. (2023) suggested that no immediate ecosystem risk is observed where the Pacific oyster occurs sporadically.

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann, 2012; Kochmann et al., 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). It also prefers mudflats with mixed sediment composed of shingle and sand, attaching to whatever hard substrata are available within otherwise unsuitable fine muddy sediment (Spencer et al., 1994; Mckinstry & Jensen, 2013; Tillin et al., 2020). Magallana gigas has been reported from estuaries growing on intertidal mudflats, sandflats, and other soft sediments (Padilla, 2010; Herbert et al., 2016; Cabral et al., 2020). The settlement of spat on hard substrata within sediments has been observed in the estuaries of the River Dart, Exe, Fal, Fowey, Tamar, Teign, and Yealm in Devon and Cornwall, the Menai Straits, Wales and large estuaries of Lough Swilly, Lough Foyle and the Shannon in Ireland, and the Tagus Estuary in Portugal (Spencer et al., 1994; Kochmann, 2012; Kochmann et al., 2013; Cabral et al., 2020). In Lough Swilly, Lough Foyle and the Shannon, the Pacific oyster was often associated with intertidal mud or sandflats (Kochmann et al., 2013). In contrast, the Pacific oysters were absent from sandflat areas in Poole Harbour (Mckinstry & Jensens, 2013).

Although shorelines comprised of mainly mud were suggested to be unsuitable for spat settlement (Spencer et al., 1994), the presence of smaller hard substrata, such as shells or pebbles, can enable larvae to settle (Tillin et al., 2020). For example, in the River Teign estuary, Pacific oyster settlement was observed on shell-covered ground mainly attached to mussel shells, and occasionally attached to cockles, stones and common periwinkle (Littorina littorea) shells on a mud flat in the estuarine intertidal zone otherwise mainly comprised of sand and mud (Spencer et al., 1994). In addition, the Blue Lagoon on the north shore of Poole Harbour had the highest abundance of oysters on mud mixed with shingle and shell (Mckinstry & Jensen, 2013). Outside of the Blue Lagoon, oysters were also recorded on mixed substrata composed of mud, gravel, and shell (McKinstry & Jensen, 2013). Tillin et al. (2020) concluded that while successful invasions occurred on mudflats, Magallana gigas prefers mixed substrata. Fine mud sediments without hard substrata (such as small stones, gravel, and shell) are unlikely to be suitable (Tillin et al., 2020). The speed of Magallana gigas reef formation on soft substrata seems to be dependent on the amount of hard substrata present, developing quicker once there is a sufficient amount (Troost, 2010). Bergstrom et al. (2021) reported that the presence of Magallana gigas was partially dependent on increasing gravel content up to 15% but remained stable with increasing percentages (measured up to 80%). 

The oyster reefs, in the Wadden Sea and Brittany, on littoral muddy and sandy habitats formed predominantly at lower tidal levels from Mean Low Water levels to the shallow subtidal (Herbert et al., 2012, 2016). Pacific oyster spatfall was recorded in the estuarine intertidal zone on areas with hard substrata of stone and shell, particularly between the low water of spring tides and high water of neap tides, such as in the Menai Strait (Spencer et al., 1994). At high densities the Pacific oyster reef smothers sediment, provides hard substrata in an otherwise sedimentary environment with additional niches for colonization by other species that require hard substratum (e.g. barnacles), and changes surface roughness and local hydrography (Troost, 2010; Herbert et al., 2012, 2016; Tillin et al., 2020). Lejart & Hily (2011) found the surface available for epibenthic species in the Bay of Brest, increased 4-fold when oysters were present on mud, for every 1 m² of colonized substrata the oyster reef added 3.87 m² of surface area on mud sediment. An increase in available settlement substrata, free of epibiota, could be the reason oyster reefs see an increase in macrofaunal abundance. This can change the community composition and habitat structure in reefs on soft mud sediments, creating new habitats for an increasing abundance of infaunal and epibenthic mobile species (Kochmann et al., 2008; Lejart & Hily, 2011; Zwerschke et al., 2018). Results have shown 38% of species present in the oyster reefs on mud were characteristic of rocky substratum habitats (Lejart & Hily, 2011).

In the Bay of Brest, Pacific oyster reefs had a higher diversity and species richness than surrounding mud habitats, including the mud underneath the reefs, where the population was dominated by carnivores rather than suspension the feeders found on the mudflats (Lejart & Hily, 2011; Herbert et al., 2012). In addition, in muddy habitats around the UK, Ireland and Northern France, macrofaunal diversity increased as Pacific oyster density increased but epifaunal diversity decreased as oyster densities increased (Zwerschke et al., 2018). It was suggested that the decrease in epifaunal diversity was due to a decrease in settlement space and a increase in habitat fragmentation because of dense oyster assemblages (Zwerschke et al., 2018).

Green & Crowe (2014) examined the effects of Magallana gigas density in experimental plots (0.25 m²) in Lough Swilly and Lough Foyle, Ireland. The number of species and species diversity increased with oyster cover on mudflats, depending on site and duration. The assemblage also changed due to the increased abundance of barnacles and bryozoans on the oyster shells and polychaetes within the sediment (Green & Crowe, 2014). Zwerschke et al. (2020) suggested that Pacific oyster beds could replace the ecosystem services provided by native oysters, in areas where native oysters had been lost. Morgan et al. (2021) suggested that the smothering of sediment habitats could prevent fish and bird species from feeding on infauna like worms, molluscs, and crustaceans. Also, the development of tidepools within mixed Pacific oyster and blue mussel reefs in soft sediment intertidal sites has been observed in the Wadden Sea, which can create new microhabitats within the reefs (Weniger et al., 2022).

Pacific oysters have been found to reduce the proportion of fine particles and increase the proportion of large particles in the mud under the reef (Lejart & Hily, 2011). The evidence suggests that Pacific oyster reefs change sediment characteristics, by affecting nutrient cycling and increasing the organic content of sediment, sand-to-silt ratio and levels of porewater ammonium (Kochmann et al., 2008; Padilla, 2010; Wagner et al., 2012 cited in Tillin et al., 2020; Green & Crowe, 2014; Herbert et al., 2012, 2016; Zwerschke et al., 2020; Hansen et al., 2023).  Zwerschke et al. (2020) found no significant differences in nutrient cycling rates of native oyster beds or Magallana gigas beds or their associated benthic communities, in experimental plots in Ireland. Persistent changes in the rates of nutrient cycling were driven by the density and presence of oysters (Zwerschke et al., 2020).

The deposition of faeces and pseudo-faeces by Magallana gigas can increase the toxic levels of sulphide in sediments and associated hypoxic sediment conditions, which can reduce photosynthesis and growth in eelgrass (Kelly & Volpe, 2007). Faecal deposition and hypoxia have also been suggested to explain a reduction in species diversity in the sediment underlying high density oyster reefs (Green & Crowe, 2013, 2014; Herbert et al., 2016). However, Lejart & Hily (2011) observed no organic or silt enrichment by Pacific oysters in mud beneath oyster reefs in the Bay of Brest, and no significant difference in the amount of organic matter found in the mud underneath oyster reefs and on bare mud not colonized by the oyster. The biodeposits excreted by the oyster may be washed away by powerful tides and currents seen in the Bay of Brest and the effects of organic enrichment at oyster reefs might be minimal due to wave action (Lejart & Hily, 2011).

Magallana gigas also colonizes littoral intertidal biogenic reefs formed by the blue mussel Mytilus edulis or honeycomb worm Sabellaria alveolata (GBNNSS, 2011, 2012; Kochmann, 2012; Kochmann et al., 2013; Herbert et al., 2016; Tillin et al., 2020). In chalk reefs in Kent with 50% cover of Lanice conchilega, the worm has been partially displaced by Magallana gigas, which has reached maximum densities of 14 oyster individuals per m² (Mcknight, 2011, 2012 cited in Herbert et al., 2012, 2016). 

The Pacific oyster may be extending its invasive range subtidally (Tillin et al., 2020), with evidence suggesting it can occupy the shallow subtidal on subtidal sediments, as it does in its native habitat (Padilla, 2010, Herbert et al., 2012). For example, it has been reported at 10 m depth in the Oosterschelde, Netherlands and in the Wadden Sea on subtidal sediments, 1 m to 9 m on subtidal sediments Sweden, and at least 2 m to 3 m below Chart Datum in the Thames estuary and Essex and Kent area on subtidal sediments (Herbert et al., 2012, 2016; Tillin et al., 2020). In Lough Swilly, Ireland Kochmann et al. (2013) found Pacific oysters on a shallow subtidal mussel bed during an exceptionally low spring tide. High densities of Magallana gigas were observed in shallow subtidal areas around the Irish coastline, for example, the oyster is found in Lough Foyle at sites predominately composed of mussels, mud and sand and in Lough Swilly, at sites composed of mussels, boulders or cobbles and mud (Kochmann, 2012). The Pacific oyster has also been reported at depths up to 40 m (Wrange et al., 2010; Ezgeta-Balic et al., 2019, Herbert et al., 2016; Wood et al., 2021). Smaal et al. (2009) recorded 700 ha of Pacific oyster beds at all depths, down to 42 m, in deep gulleys of the intertidal flats of the Oosterschelde. The density of the beds was not reported but they were visible in side-scan sonar.

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. However, the above evidence suggests that Crepidula fornicata could colonize coarse sediment habitats in the subtidal, typical of this biotope. 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 wave exposed to wave sheltered energy habitat, in which wave action and storms may mobilise the sediment (JNCC, 2022), which may 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 meditated by tidal flow rather than wave action, e.g., the deeper examples of the biotope. However, Crepidula reduced the density of suspension feeders and mobile Crustacea in coarse sediment even at low densities (De Montaudouin & Sauriau, 1999). Therefore, resistance to colonization by Crepidula is assessed as 'Medium' in examples where wave action is high 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. 

The above evidence suggests that this biotope could be suitable for the colonization of Magallana gigas due to the presence of mixed sediment including gravel, shells, or other hard substrata required for successful settlement and establishment (Kochmann, 2012; Kochmann et al., 2013; Mckinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). This biotope also occurs in the shallow sublittoral (0-20 m) and Magallana gigas populations have a preference for the shallow sublittoral (ca 10 m) but occur down to 42 m in suitable conditions.  Magallana gigas populations may be limited to low densities in wave exposed to moderately wave exposed conditions (Teschke et al., 2020) typical of most (ca 50%) records of this biotope. However, Magallana gigas has been observed, in one study, to partially displace Lanice conchilega cover in intertidal chalk reefs, suggesting that Lanice conchilega dominated habitats could be invaded by the Pacific oyster (Mcknight, 2011, 2012 cited in Herbert et al., 2012, 2016). Therefore, resistance to colonization by Magallana is assessed as 'Low' as a precaution. Resilience is likely to be ‘Very low’ as the Magallana gigas population would need to be removed for recovery to occur. Therefore, sensitivity is assessed as ‘High’ for Lanice dominated sediments The confidence in the assessment is 'Low' because the assessment is based on one observation of Magallana gigas invading a Lanice conchilega habitat on soft rock, and no evidence from sedimentary habitats was found. 

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 microbial pathogens, parasites or diseases of Lanice conchilega or other characterizing polychaetes. 

Removal of target species

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

Evidence

The direct, physical impacts of removal are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of target species on this biotope. No characterizing or associated benthic species are targeted by recreational or commercial fishers and this pressure is considered to be '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

Species within this biotope may be removed or damaged by static or mobile gears that are targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope.

Lanice conchilega (sand mason worm) is the dominant polychaete in this biotope. It qualifies as an ‘ecosystem engineer’ in that it changes and/or creates a habitat, which affects the abundance of other species (Jones et al., 1994, 1997). The tube which Lanice conchilega builds provides structure and stabilizes the sediment (Jones & Jago, 1993). The tubes also obstruct the activities of predatory burrowers enabling other sedentary animals to establish themselves (Wood, 1987). The burrows and tubes allow oxygenated water to penetrate into the sediment, the oxic upper layer of sediment to support shallow burrowing species such as amphipods (Ampelisca spp., Bathyporeia spp. & Gammarus spp.) and small Crustacea. Lanice conchilega may therefore increase species richness and abundance through habitat modification (Rabaut et al., 2007; Zuhlke, 2001). Dense aggregates of Lanice conchilega have the potential to enhance food supply to birds and fish due to increased biodiversity (Rabaut et al., 2010; Godet et al., 2008; Van Hoey et al., 2008; Callaway et al., 2010; De Smet et al., 2013).

Sensitivity assessment. The removal of non-target species may result in changes to the biological community and hence the classification of the assemblage type as assessed in the biotope. Incidental removal of Lanice conchilega would alter the biotope classification and most likely result in changes to the habitat and associated species. As Lanice conchilega is sedentary and present at the surface, resistance is ‘Low’ and resilience is ‘High’. Biotope sensitivity is therefore assessed as ‘Low’. This assessment considers the ecological effect of removal, the abrasion pressure assessment provides evidence for the robustness of tubes and the species may be resistant to removal by bottom trawls deployed at low intensity (e.g. single passes).