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

The infaunal communities of pockmarks in shallow-water (<200 m) are drawn from the surrounding area. Most pockmarks are recorded from north of 56° in the North Sea with high densities in the Witch Ground and Norwegian Trench (Dando 2001; Judd & Hovland, 2007) but are also recorded in western mud belt in the Irish Sea. Judd & Holvand (2007) suggested that seeps (and pockmarks) are likely to be more widespread in the North Sea.

The majority of the species recorded in UK pockmarks are widely distributed around the coasts of the British Isles and to the north and south of the British Isles.  Similarly, epifaunal communities on boulders and outcrops of MDAC (methane-derived authigenic carbonate) in pockmarks and extensive areas of MDAC in 'bubbling reefs' are opportunistic species drawn from the surrounding area. 

Therefore, the exact infaunal communities present in pockmarks or epifaunal communities present on MDAC (e.g. from the North Sea and the Irish Sea) may vary with latitude but also local hydrography, depth, sediment type and scour. The majority of the species present are widely distributed to the north and south of the British Isles and are unlikely to be affected by chronic changes in temperature, e.g. a change of 2°C. A short-term change of 5°C may interrupt breeding or alter growth rates (depending on season). An increase in temperature may favour more southerly species, while a decrease may favour more northerly species but the character of the habitat is unlikely to be changed and a resistance of ‘High’ is recorded. Hence, resilience is also ‘High’ and the habitat is probably ‘Not sensitive’ at the benchmark level.

Temperature decrease (local)

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

Evidence

The infaunal communities of pockmarks in shallow-water (<200 m) are drawn from the surrounding area. Most pockmarks are recorded from north of 56° in the North Sea with high densities in the Witch Ground and Norwegian Trench (Dando 2001; Judd & Hovland, 2007) but are also recorded in western mud belt in the Irish Sea. Judd & Holvand (2007) suggested that seeps (and pockmarks) are likely to be more widespread in the North Sea.

The majority of the species recorded in UK pockmarks are widely distributed around the coasts of the British Isles and to the north and south of the British Isles. Similarly, epifaunal communities on boulders and outcrops of MDAC (methane-derived authigenic carbonate) in pockmarks and extensive areas of MDAC in 'bubbling reefs' are opportunistic species drawn from the surrounding area. 

Therefore, the exact infaunal communities present in pockmarks or epifaunal communities present on MDAC (e.g. from the North Sea and the Irish Sea) may vary with latitude but also local hydrography, depth, sediment type and scour. The majority of the species present are widely distributed to the north and south of the British Isles and are unlikely to be affected by chronic changes in temperature, e.g. a change of 2°C. A short-term change of 5°C may interrupt breeding or alter growth rates (depending on season). An increase in temperature may favour more southerly species, while a decrease may favour more northerly species but the character of the habitat is unlikely to be changed and a resistance of ‘High’ is recorded. Hence, resilience is also ‘High’ and the habitat is probably ‘Not sensitive’ at the 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

Pockmarks occur globally between a depth of 6 and 4800 m (Fader, 1991; cited in Webb et al., 2009b). In the North Sea they occur in deep soft sediments (<200 m) in stable fully saline conditions. Reefs of MDAC are recorded from a range of depths, for example from the intertidal and shallow subtidal in the Kattegat (Dando et al., 1994); at 10-12 m in the Kattegat (Jensen et al., 1992); at 6.2-9.5 m at Holden's Reef (Irving et al., 2007); at 60-75 m in the Texel 11 Reefs, and at 50-120 m in the Codling Fault Zone in the Irish Sea (Judd et al., 2007; O’Reilly et al., 2014). Therefore, the resident infauna and epifauna is likely to be adapted to the salinity regime in which the habitat (pockmark or MDAC reef) occurs.

An increase in salinity at the benchmark level would result in a salinity of >40 psu, and as hypersaline water is likely to sink to the seabed, the biotope may be affected by hypersaline effluents. Ruso et al. (2007) reported changes in the community structure of soft sediment communities due to desalinisation plant effluent in Alicante, Spain. In particular, in close vicinity to the effluent, where the salinity reached 39 psu, the community of polychaetes, crustaceans and molluscs was lost and replaced by one dominated by nematodes. Roberts et al. (2010b) suggested that hypersaline effluent dispersed quickly but was more of a concern at the seabed and in areas of low energy where widespread alternations in the community of soft sediments were observed. In several studies, echinoderms and ascidians were amongst the most sensitive groups examined (Roberts et al., 2010b).

Sensitivity assessment. Hypersaline effluents are likely to be localised but dispersed quickly in areas of strong currents, e.g. Texel 11 in the Irish Sea (Judd, 2005). Therefore, in areas of strong tidal streams or wave action, hypersaline effluents may not have an adverse effect. However, in low energy environments, hypersaline effluents may be detrimental and result in reduced diversity and an impoverished infauna and epifauna in the area of effect.  Therefore, a resistance of ‘Low’ is suggested. Colonization from the surrounding area is probably rapid but the community may take two or more years to recover and a resilience of ‘Medium’ is recorded. Hence, sensitivity is assessed as of ‘Medium’.

Salinity decrease (local)

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

Evidence

Pockmarks occur globally between a depth of 6 and 4800 m (Fader, 1991; cited in Webb et al., 2009b). In the North Sea they occur in deep soft sediments (<200 m) in stable fully saline conditions. Reefs of MDAC are recorded from a range of depths, for example from the intertidal and shallow subtidal in the Kattegat (Dando et al., 1994); from 10-12 m in the Kattegat (Jensen et al., 1992); at 6.2-9.5 m at Holden's Reef (Irving et al., 2007); at 60-75 m in the Texel 11 Reefs, and at 50-120 m in the Codling Fault Zone, in the Irish Sea (Judd et al., 2007; O’Reilly et al., 2014). Therefore, the resident infauna and epifaunal is likely to be adapted to the salinity regime in which the habitat (pockmark or MDAC reef) occurs.

A decrease in salinity at the benchmark level, e.g. from full to reduced salinity for a year would probably result in a change in the resident community and a reduction in diversity.  If the habitat occurred in variable salinity environments then the community is likely to be more resistant. However, in relatively stenohaline circalittoral habitats then the community is likely to be adversely affected.  Therefore, a resistance of ‘Low’ is suggested. Colonization from the surrounding area is probably rapid but the community may take two or more years to recover and a resilience of ‘Medium’ is recorded. Hence, sensitivity is assessed as ‘Medium’.

Water flow (tidal current) changes (local)

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

Evidence

Pockmark morphology may alter water flow and currents. Increased water flow could lead to increased numbers of suspension feeders while reduced flow increases deposition and hence the abundance of deposit feeders (Dando, 2001; Webb et al., 2009b). Deep pockmarks probably reduce flow and increase the suitability of the habitat to deposit feeders (Dando, 2001). For example, the Scanner pockmark reduces bottom flow within the pockmark, which enhances fill by fine sediment and also retain colder more saline water than the surrounding seabed (Dando, 2001).  Visibility in deep pockmarks is often reduced due to high concentrations of suspended particulates that attract swarms of shrimps and euphausids. But in wide and shallow pockmarks the bottom currents may not be altered (Dando, 2001). However, Dando (2001) noted that limited information on flow rates within pockmarks was available.

The epifauna of hard substrata are primarily suspension feeder’s dependent water flow to bring food particulates within reach. Webb et al. (2009) suggested that the large gorgonian corals found on MDAC with the pockmarks within the Norwegian Channel and abundant sea anemones and sea pens on the pockmark slopes benefited from a rich food supply generated by increased currents and re-suspension of particulates by turbulent currents and/or by active gas seepage.

In the Irish Sea, the MDAC reefs in Texel 11 occur in areas of strong tidal currents and mobile sands (Judd et al., 2007; Whomersely et al., 2010). The effect of burrowing species and currents are causing erosion of the reef into boulders, cobbles and sands. The resident epifaunal community is typical of sand scoured rocky habitats, dominated by Flustra foliacea with abundant Sabellaria spinulosa (Whomersely et al., 2007).

Sensitivity assessment. The effect of changes in water flow will depend on the habitat. However, there is little information on the water flow regime on which to base an assessment. Decreased water flow is probably detrimental to suspension feeding epifauna that depend on water flow to supply food, where water flow is more important than flow and circulation created by gas seepage. In areas of strong tidal streams and scour, a reduction in flow may reduce scour and allow a more diverse epifaunal community to develop.  Therefore, in areas of strong tidal flow, a change in water flow at the benchmark level (0.1-0.2 m/s change for a year) is probably not significant. Therefore, resistance and resilience are considered ‘High’ and the biotope assessed as ‘Not sensitive’, at the benchmark level.

However, in soft muddy habitats, a change in water flow at the benchmark level (0.1-0.2 m/s change for a year) may modify and mobilise the fine sediment. A significant increase in water flow may disturb bottom sediments and modify the shape of or fill pockmarks. Croker et al. (2005) noted that pockmarks will not remain if seabed erosion occurs. However, the pockmark may be recreated due to continued or episodic gas seepage. Therefore, a resistance of ‘Medium’ is recorded to represent the potential change in sediment and the infaunal community. Hence, resilience is probably ‘High’ and sensitivity is assessed as ‘Low’, but with ‘Low confidence.

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 habitats. Emergence is relevant to intertidal examples of MDAC reefs but they are excluded from the definition of this biotope (A5.71). 

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

In the North Sea, pockmarks were recorded from 120 -249 m (Hovland & Judd, 1988) and 50-100 m in the Irish Sea (Judd, 2005).  Reefs of MDAC are recorded from a range of depths, for example from the intertidal and shallow subtidal in the Kattegat (Dando et al., 1994); at 10-12 m in the Kattegat (Jensen et al., 1992); at 6.2-9.5 m at Holden's Reef (Irving et al., 2007); at 60-75 m in the Texel 11 Reefs, and at 50-120 m in the Codling Fault Zone in the Irish Sea (Judd et al., 2007; O’Reilly et al., 2014).

In UK waters the habitats occur at depths where water movement is determined by tidal currents rather than wave action. Therefore, a change of <5% in significant wave height is unlikely to create a significant change in water movement at the seabed. Therefore, resistance and resilience are considered ‘High’ and the habitat is assessed as ‘Not sensitive’ at the benchmark level.

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.

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.

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

Pockmarks are naturally organic-rich habitats dominated by infauna adapted to low oxygen, sulphide-rich conditions. Infauna closer to the gas outlet(s) exhibit species exclusion due to sulphide toxicity, lower oxygen penetration, modification of the sediment, together with limited burrowing depth due to sub-bottom concretions (Dando, 2010).  Infauna close to gas outlets is dominated by sulphide tolerant species, e.g. Capitella spp., and Thyasira spp. Thyasira sarsi hosts endosymbiotic bacteria from which it derives nutrition. Bacterial mats of sulphur bacteria (Beggiatoa, Thiothrix and Thioplaca spp.) are typical around the gas outlets (Dando & Hovland, 1992; Dando, 2010).  The infauna becomes more similar to that of the surrounding sediment with increasing distance from the gas outlet(s).  Dando (2001; 2010) also noted that pockmarks act as sediment traps so that even inactive pockmarks will have a higher organic content, and potentially lower oxygen levels than the surrounding sediment.

Epifaunal communities within pockmarks colonize boulders or outcrops of MDAC above the sediment, and in areas of water flow, although the exact water flow regime will vary with the size and depth of the pockmarks and the area of seabed in which it is found.  Most epifauna are suspension feeders adapted to water flow, although their tolerance or requirement for water flow varies with species or taxonomic group. For example, Gray et al. (2002) concluded that fish were more sensitive to hypoxia that crustacean and echinoderms, which in turn were more sensitive than annelids with molluscs the most tolerant.  Riedel et al. (2010) noted that infauna were generally more tolerant than epifauna.  They also noted that decapods, echinoderms, and polychaetes showed lower tolerance while ascidians and anthozoans showed higher tolerance to hypoxia and anoxia (Riedel et al. 2010). Mobile species, such as fish, would probably move away from the affected area.

Sensitivity assessment. The infaunal and microbial communities within pockmarks are adapted to low oxygen conditions and are unlikely to be adversely affected by a reduction in oxygen levels at the benchmark levels.  The infauna on the slopes and the surrounding sediment in silty sedimentary areas are also probably adapted to low oxygen conditions.  However, epifauna dependent on the MDAC within pockmarks may be adversely affected in areas of low water movement, depending on the species present.

Similarly, epifauna dominating reefs of MDAC in areas of low water movement may be adversely affected. Therefore, it is possible that a proportion of the epifaunal community could be lost due to deoxygenation at the benchmark level and a resistance of ‘Medium’ is suggested. Hence, resilience is assessed as ‘High’ and sensitivity as ‘Low’ but with ‘Low’ confidence.

Epifaunal communities that occur on exposed MDAC reefs in areas strong water movement (e.g. in the mid-Irish Sea) are unlikely to be adversely affected at the benchmark level.  Strong water movement would prevent anything but transient decreases in oxygenation.  Therefore, in these habitats, the epifauna communities are considered to have a ‘High’ resistance to deoxygenation at the benchmark level.  Hence, resilience is also ‘High’ and the habitat is likely to be ‘Not sensitive’.

Nutrient enrichment

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

Evidence

Nutrient input from the sediment is significant at shallow-water (<200 m) cold seep habitats (Dando, 2010).  Near the gas outlets, shallow sediment re-circulation of water causes organic enrichment by particulate organic carbon (POC) in a zone around the outlet (O’Hara et al., 1995); Dando, 2010) which supports deposit and filter feeders, and dilutes the sulphide concentrations.  Algal species indicative of eutrophication can be found at shallow examples of seeps (Dando, 2010). Anoxic conditions are typical in the sediment, especially close to the gas outlet(s). In addition, the majority of the shallow-water seep fauna derive their nutrition from the photosynthetic food chain and the fossil carbon (via chemosynthesis) makes a limited contribution to the food chain (Dando, 2010). 

Epifauna can live closer to the gas outlet(s) due to the rapid dilution of the seep gases.  Epifauna probably benefit from the increased POC and possibly the bacterial biomass resulting from organic enrichment in active seeps.  However, evidence on the direct effect of organic or nutrient enrichment on epifauna is limited and effects are probably due to hypoxia and smothering rather than the enrichment itself.  

However, nutrient enrichment may have adverse effects on epifaunal communities. Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected six marine habitats (including subtidal reefs).  A 30-50% reduction in species diversity and richness were identified from all habitats exposed to the contaminant types.  

Sensitivity assessment. Pockmarks are nutrient enriched habitats. Active ‘bubbling reefs’ are also likely to exhibit nutrient enrichment in the vicinity of the gas outlets. Therefore, the effect of additional nutrient enrichment will probably depend on the type of habitat (pockmark vs. MDAC) the degree of gas seepage and distance from the gas outlet(s).  Extensive reefs that host a diverse epifaunal community may see an increase in green algae in shallow waters or a decrease in species richness.  However, the pressure benchmark assumes compliance with good status as defined by the WFD. Therefore, resistance and resilience are assessed as ‘High’, and the habitat is considered to be ‘Not sensitive’ at the benchmark level.

Organic enrichment

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

Evidence

Nutrient input from the sediment is significant at shallow-water (<200 m) cold seep habitats (Dando, 2010).  Near the gas outlets, shallow sediment re-circulation of water causes organic enrichment by particulate organic carbon (POC) in a zone around the outlet (O’Hara et al., 1995; Dando, 2010) which supports deposit and filter feeders, and dilutes the sulphide concentrations.  Shallow-water seeps communities are typical of reducing sediments rich in organic matter.  POC inputs can come from chemosynthetic bacteria, heterotrophic prokaryotes, cyanobacteria, benthic diatoms, photosynthetic plankton and marine and terrestrial plants (Dando, 2010).  However, infauna closer to the gas outlet(s) exhibit species exclusion due to sulphide toxicity, lower oxygen penetration, modification of the sediment, together with limited burrowing depth due to sub-bottom concretions (Dando, 2010).  Dando (2001; 2010) also noted that pockmarks act as sediment traps so that even inactive pockmarks will have a higher organic content than the surrounding sediment.

Bacterial mats of sulphur bacteria (Beggiatoa, Thiothrix and Thioplaca spp.) are typical around the gas outlets (Dando & Hovland, 1992; Dando, 2010).  Infauna close to gas outlets is dominated by sulphide tolerant species, e.g. Capitella spp., and Thyasira spp. Thyasira sarsi hosts endosymbiotic bacteria from which it derives nutrition. Echiurans may also feed on bacterial mats (Dando, 2010). Dando (2010) suggested that chaetopterid polychaetes thrived at vent and seep sites due to the high bacterial biomass they could filter from the water column while avoiding high sulphide concentrations diluted by the water flow through their tubes. 

Epifauna can live closer to the gas outlet(s) due to the rapid dilution of the seep gases.  Epifauna probably benefit from the increased POC and possibly the bacterial biomass resulting from organic enrichment in active seeps.  However, evidence on the direct effect of organic or nutrient enrichment on epifauna is limited and effects are probably due to hypoxia and smothering rather than the enrichment itself.

For example, O’Dea & Okamura (2000) found that annual growth of Flustra foliacea in western Europe has substantially increased since 1970. They suggested that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998). Koopmans & Wijffels (2008) found no correlation between the growth rate of Halilcona oculata and dissolved organic carbon, suggesting that Haliclona oculata is more dependent on particulate organic carbon.  Rose & Risk (1985) described an increase in abundance of the sponge Cliona delitrix in an organically polluted section of Grand Cayman fringing reef affected by the discharge of untreated faecal sewage.  Mayer-Pinto & Junqueira (2003) studied the effects of organic pollution on fouling communities in Brazil and found that tolerance of polluted/unpolluted artificial reefs varied among bryozoan species. It should be noted that Bugula spp. preferred the polluted sites.

In addition, Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected six marine habitats (including subtidal reefs).  A 30-50% reduction in species diversity and richness were identified from all habitats exposed to the contaminant types.  

Sensitivity assessment. Pockmarks are naturally organic-rich habitats dominated by infauna adapted to low oxygen, sulphide-rich conditions.  Therefore, infauna within the pockmarks and its slopes are probably unlikely to be affected by organic enrichment at the benchmark level.  Hence, resistance is assessed as ‘High’, resilience as ‘High’ and sensitivity is assessed as ‘Not sensitive’ at the benchmark level.

Epifaunal communities, either on boulders and outcrops within pockmarks or on reefs of MDAC are probably more sensitive to increases in nutrient and organic enrichment. However, the extent of the effect likely depends on the hydrography, the species present, and the background levels of nutrients within that area. Active ‘bubbling reefs’ are probably organic and nutrient enriched due to the gas seepages.  Therefore, a resistance of ‘Low’ is suggested to represent the potential loss of species diversity but with ‘Low’ confidence.  Hence, resilience is assessed as ‘Medium’ and sensitivity assessed as ‘Medium’.

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

This pressure represents a permanent change in seabed type from sedimentary to hard or hard to sedimentary, e.g. by the addition of rock armour or artificial reefs. A pockmark 'filled' by hard substrata would be lost, although it is assumed that gas could find another route to the surface in the surrounding area. Reefs of MDAC (methane-derived authigenic carbonate) that are replaced with sediment (e.g via prolonged spoil dumping or physical removal) would be lost together with the epifaunal and interstitial community they host. MDAC deposits probably taker 100s or 1000s of years to develop.  Therefore, resistance is likely to be 'None' and permanent loss of habitat means that resilience is 'Very low'  by definition. Hence, sensitivity is assessed as 'High'. Pockmarks may be lost but remerge in the vicinity if seeping gases find another route through the sediment, however, the pockmark(s) of interest will be lost.

Physical change (to another sediment type)

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

Evidence

This pressure represents a permanent change in sediment type.  Pockmarks characterize soft fine sediments (Dando & Hovland, 1992; Dando, 2001).  In sandy sediments (e.g. fine to medium sands and silty sand of the Tommeliten field) seeps resulted in small funnels or small eye pockmarks (3-5 m across and 0.5 m deep) (Hovland & Judd, 1988).  Therefore, a change in sediment type from mud and sandy muds to muddy sands and sands may result in smaller pockmarks but a change in sediment to coarser gravels dominated sediments may allow gas to escape without forming a pockmark. However, consolidated mixed ‘lag’ deposit may allow pockmarks to form. Therefore, a change in sediment type may result in loss of pockmark, depending on the sediment present in the affected area, and a resistance of ‘Low’ is suggested, albeit with ‘Low’ confidence.

MDAC forms within the sediment and is exposed in pockmarks or as extensive reefs.  Whomersely et al. (2010) suggested that the action of boring organisms and currents resulted in the breakdown of the MDAC reefs in the mid-Irish Sea to boulders and eventually sand. The sediment type surrounding refs of MDAC affect the rate of erosion and affect the epifaunal community due to scour. However, a change in sediment type’ is technically ‘Not relevant’ on hard substrata such as MDAC.

Sensitivity assessment. A change is sediment type from fine silty and sandy sediment to coarse sediments may result in loss of the pockmarks and prevent their recovery. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Very low’ (a permanent change) and sensitivity is assessed as ‘High’ but with ‘Low’ confidence.

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

The physical removal of the sediment within and around a pockmark(s) would result in loss of the pockmark itself and its associated infaunal and epifaunal community. Small MDAC boulders are also likely to be removed.  Therefore, resistance is assessed as ‘None’. In sedimentary habitats, pockmarks are like to reform and the community recolonize quickly once the sediment returns, which will depend on the nature of the sediment and the hydrographic regime.  Therefore, resilience is assessed as ‘Medium’ (2-10 years). Hence, sensitivity is assessed as ‘Medium’.

This pressure is considered ‘Not relevant’ on hard substrata. However, MDAC is considered a ‘soft rock’ (Whomersley et al., 2010) as it is easily bored by sponges and piddocks.  Therefore, it may be possible for large sections of a reef of MDAC to be removed by extraction, together with its resident epifaunal and interstitial community. Therefore, resistance is assessed as ‘None’ within the affected area.  However, as MDAC forms very slowly (over 100s or 1000s of years; Bowden et al., 2013), resilience is assessed as ‘Very low’ and sensitivity as ‘High’.

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

Dando (2001) suggested that the bottom of pockmarks may be less likely to be damaged by trawling, depending on their size, shape and depth. Webb et al. (2009b) examined the epifaunal community on MDAC within a complex of pockmarks between 160-235 m diameter and around 10 m deep, in the Norwegian Chanel, North Sea.  They suggested that the depth of the pockmark protected the epifaunal community (on MDAC) within the pockmark from the effects of trawling.  This was because trawl tracks were visible across the top of and near the pockmarks, fishing data indicated that the area was subject to high intensity fishing, and yet the diverse epifauna boasted large, long-lived, slow-growing gorgonian corals (Paragongia arborea) (Webb et al., 2009b).

In deep-water (>200 m) cold-seep communities of the New Zealand Hikurangi Margin, the seep megafauna (mussels, clams and tubeworms) were restricted to depressions and crevices in carbonates at several sites (Baco et al., 2010).  Coral or vesicomyid shell debris, lost trawl gear, or trawl marks in adjacent sediments, accompanied these observations.  Baco et al. (2010) suggested that trawling had disturbed the seep communities at most of the seep locations examined.  In subsequent studies, increased trawling density was associated with reduced occurrence of live chemosynthetic fauna at sites in the Rock Garden region of the Hikurangi Margin.  In addition, shallower sites had fewer live fauna and greater fishing intensity (Bowden et al., 2013).

Physical disturbance by fishing gear has been shown to adversely affect emergent epifaunal communities with hydroid and bryozoan matrices reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998).  For example, drop-down video surveys of Scottish reefs exposed to trawling showed that visual evidence of damage to bryozoans and hydroids on rock surfaces was generally limited and restricted to scrape scars on boulders (Boulcott & Howell, 2011).  The study showed that damage was incremental with damage increasing with the frequency of trawls rather than a blanket effect occurring on the pass of the first trawls. The level of impact may be mediated by the rugosity of the attachment, surfaces with greater damage occurring over smooth terrains where the fishing gear can move unimpeded across a flat surface.  Therefore, MDAC reef communities may provide refuges for some species within burrows, under overhangs, or in crevices. Veale et al. (2000) reported that the abundance, biomass, and production of epifaunal assemblages decreased with increasing fishing effort.  Re-sampling of grounds that were historically studied (from the 1930s) indicates that some upright species have increased in areas subject to scallop fishing (Bradshaw et al. 2002).  This study also found increases in the tough stemmed hydroids including Nemertesia spp., whose morphology may prevent excessive damage. Bradshaw et al. (2002) suggested that as well as having high resistance to abrasion pressures, Nemertesia spp. have benthic larvae that could rapidly colonize disturbed areas with newly exposed substrata close to the adult.  Similarly, increases in the abundance of the soft coral Alcyonium digitatum were found, although its increase was probably due to its ability to recover from fragments.

Re-sampling of grounds that were historically studied (from the 1930s) indicates that Ophiothrix fragilis has declined in areas subject to scallop fishing (Bradshaw et al., 2002).  Examination of historical and recent samples suggest that the spatial presence of Ophiothrix fragilis and Amphiura spp. in the North Sea has more than halved in comparison with the number of ICES rectangles in which they were sampled at the beginning of the century, apparently in response to fishing effort (Callaway et al., 2007).

Rock burrowing fauna may receive protection by nature of their habit.  For example, Hiatella arctica burrow depths were approximately 2 cm (mean length of Hiatella arctica individuals was 1-1.2 cm) with a maximum depth of 4 cm on limestone shores off the coast of Ireland (Trudgill & Crabtree, 1987). Clearly, the surface epifauna and flora are more susceptible to damage and removal by surface abrasion.  However, burrows may weaken the MDAC and make it susceptible to damage by abrading activities, especially mechanical gear. Seffel (2010) reported that fishing gear “like bottom trawling nets are known to tear pieces off the carbonate structures”, although no direct evidence was provided.

Sensitivity assessment.  Shallow pockmarks may be disturbed by physical abrasion while deep pockmarks may provide protection to infauna and epifaunal at the bottom of the pockmark.  Resistance is probably related to the depth and size of the pockmarks in the affected area.  Therefore, resistance is probably ‘Low’ except for deep pockmarks where the infauna and epifauna within the pockmark are protected from bottom gear. Hence, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’ but with ‘Low’ confidence.

Where present, MDAC boulders and reefs are probably more susceptible to physical abrasion, especially from mechanical gear that may also damage the carbonate matrix itself. Therefore, resistance is also probably ‘Low’. However, MDAC deposits may take 100s to 1000s of years to develop.  The removal of small boulders is probably permanent.  Similarly, any activity that broke up and removed a proportion of the MDAC ‘reef’ is likely to be permanent.  Hence, resilience would be considered ‘Very low’ and sensitivity assessed as 'High'.

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

Dando (2001) suggested that the bottom of pockmarks may be less likely to be damaged by trawling, depending on their size, shape and depth.  Webb et al. (2009b) examined the epifaunal community on MDAC within a complex of pockmarks between 160-235 m diameter and around 10 m deep, in the Norwegian Chanel, North Sea.  They suggested that the depth of the pockmark protected the epifaunal community (on MDAC) within the pockmark from the effects of trawling.  This was because trawl tracks were visible across the top of and near the pockmarks, fishing data indicated that the area was subject to high intensity fishing, and yet the diverse epifauna boasted large, long-lived, slow-growing gorgonian corals (Paragongia arborea) (Webb et al., 2009b).

In deep-water (>200 m) cold-seep communities of the New Zealand Hikurangi Margin, the seep megafauna (mussels, clams and tubeworms) were restricted to depressions and crevices in carbonates at several sites (Baco et al., 2010).  Coral or vesicomyid shell debris, lost trawl gear, or trawl marks in adjacent sediments, accompanied these observations.  Baco et al. (2010) suggested that trawling had disturbed the seep communities at most of the seep locations examined.  In subsequent studies, increased trawling density was associated with reduced occurrence of live chemosynthetic fauna at sites in the Rock Garden region of the Hikurangi Margin.  In addition, shallower sites had fewer live fauna and greater fishing intensity (Bowden et al., 2013).

Physical disturbance by fishing gear has been shown to adversely affect emergent epifaunal communities with hydroid and bryozoan matrices reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998).  For example, drop down video surveys of Scottish reefs exposed to trawling showed that visual evidence of damage to bryozoans and hydroids on rock surfaces was generally limited and restricted to scrape scars on boulders (Boulcott & Howell, 2011).  The study showed that damage was incremental with damage increasing with the frequency of trawls rather than a blanket effect occurring on the pass of the first trawls. The level of impact may be mediated by the rugosity of the attachment, surfaces with greater damage occurring over smooth terrains where the fishing gear can move unimpeded across a flat surface.  Therefore, MDAC reef communities may provide refuges for some species within burrows, under overhangs, or in crevices. Veale et al. (2000) reported that the abundance, biomass, and production of epifaunal assemblages decreased with increasing fishing effort.  Re-sampling of grounds that were historically studied (from the 1930s) indicates that some upright species have increased in areas subject to scallop fishing (Bradshaw et al. 2002).  This study also found increases in the tough stemmed hydroids including Nemertesia spp., whose morphology may prevent excessive damage. Bradshaw et al. (2002) suggested that as well as having high resistance to abrasion pressures, Nemertesia spp. have benthic larvae that could rapidly colonize disturbed areas with newly exposed substrata close to the adult.  Similarly, increases in the abundance of the soft coral Alcyonium digitatum were found, although its increase was probably due to its ability to recover from fragments.

Re-sampling of grounds that were historically studied (from the 1930s) indicates that Ophiothrix fragilis has declined in areas subject to scallop fishing (Bradshaw et al., 2002).  Examination of historical and recent samples suggest that the spatial presence of Ophiothrix fragilis and Amphiura spp. in the North Sea has more than halved in comparison with the number of ICES rectangles in which they were sampled at the beginning of the century, apparently in response to fishing effort (Callaway et al., 2007).

Rock burrowing fauna may receive protection by nature of their habit.  For example, Hiatella arctica burrow depths were approximately 2 cm (mean length of Hiatella arctica individuals was 1-1.2 cm) with a maximum depth of 4 cm on limestone shores off the coast of Ireland (Trudgill & Crabtree, 1987). Clearly, the surface epifauna and flora are more susceptible to damage and removal by surface abrasion.  However, burrows may weaken the MDAC and make it susceptible to damage by abrading activities, especially mechanical gear. Seffel (2010) reported that fishing gear “like bottom trawling nets are known to tear pieces off the carbonate structures”, although no direct evidence was provided.

Sensitivity assessment.  Shallow pockmarks may be disturbed by physical disturbance while deep pockmarks may provide protection to infauna and epifaunal at the bottom of the pockmark.  Resistance is probably related to the depth and size of the pockmarks in the affected area.  It is assumed that penetrative gear would have a greater impact in soft sediments and on soft rocks but in the absence of any direct evidence, it is assumed to have at least the same potential impact as surface abrasion.  Therefore, resistance is probably related to the depth and size of the pockmarks in the affected area.  Therefore, resistance is probably ‘Low’ except for deep pockmarks where the infauna and epifauna within the pockmark are protected from bottom gear. Hence, resilience is assessed as ‘Medium’ and sensitivity as ‘Medium’ but with ‘Low’ confidence.

Where present, MDAC boulders and reefs are probably more susceptible to physical abrasion, especially from mechanical gear that may also damage the carbonate matrix itself. Therefore, resistance is also probably ‘Low’. However, MDAC deposits may take 100s to 1000s of years to develop.  The removal of small boulders is probably permanent.  Similarly, any activity that broke up and removed a proportion of the MDAC ‘reef’ is likely to be permanent.  Hence, resilience would be considered ‘Very low’ and sensitivity assessed as 'High'.

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

Pockmarks occur in silty soft sediments and, therefore, depositional habitats.  The rate of sedimentation and suspended sediment concentrations probably vary seasonally, and are probably higher in stormy winter months than relatively calm summer months.  Gas seepage winnows away fines and creates the pockmark.  The rate of seepage or periods of episodic seepage probably compensates for sedimentation.  Dando (2001) noted that the rate of gas seepage was higher in shallow-water (<200 m) than deep-water (>200 m) because of the increased temperature and lower water pressure.  Inactive pockmarks can act as sediment traps (and larval traps) (Dando, 2001).  Croker et al. (2005) suggested that pockmarks could become covered with sediment in the event of high sedimentation rates and that buried pockmarks were not uncommon.  However, visibility in deep pockmarks is often reduced due to high concentrations of suspended particulates that attract swarms of shrimps and euphausids (Dando, 2001).  Therefore, an increase in suspended sediment (e.g. from intermediate to medium or high turbidity) may result in sedimentation and possibly the loss of inactive pockmarks.  A similar increase may also infill activate pockmarks, although they will recover rapidly depending on the rates (and periodicity) of gas seepage.  Hence, resistance is assessed as ‘Medium’ and resilience is probably ‘High’ for active seeps.

Where MDAC outcrops and boulder hosts epifaunal communities in pockmarks, then suspension feeders may be adversely affected by increases in suspended sediment. The presence of hard rock epifauna, as well as sediment epifauna (e.g. sea pens), suggests that pockmarks and their surrounds are rich feeding areas, an increase in suspended sediment may result in loss of some species depending on their individual tolerance to suspended sediments. 

In most pockmarks and areas of carbonate reef, images show that the carbonate is surrounded by sediment, and is often partially covered by sediment.  For example, In the Irish Sea, the MDAC reefs in Texel 11 occur in areas of strong tidal currents and mobile sands (Judd et al., 2007; Whomersely et al., 2010).  The resident epifaunal community is typical of sand scoured rocky habitats, dominated by Flustra foliacea with abundant Sabellaria spinulosa (Whomersely et al., 2007). Flustra foliacea is probably tolerant of suspended sediment based on its occurrence in areas of high suspended sediment e.g. the turbid, fast flowing waters of the Menai Straits (Moore, 1977) and on tide-swept seabed, exposed to high levels of suspended sediment and sediment scour in the English Channel subject to sediment transport (mainly sand) and periodic, temporary, submergence by thin layers of sand (ca <5 cm) (Holme & Wilson 1985). The tide swept seabed communities in the English Channel were also characterized by the presence of Urticina spp. (Holme & Wilson, 1985).  Sabellaria spinulosa is also associated with areas of high suspended sediment (Davies, et al., 2009; Last et al., 2011).  Similarly, MDAC at the Croker Fault Zone was dominated by sparse Nemertesia sp. and Sabellaria spinulosa (O’Reilly et al., 2014).  Alcyonium digitatum has been shown to be tolerant of high levels of suspended sediment. Hill et al. (1997) demonstrated that Alcyonium digitatum sloughed off settled particles with a large amount of mucous. Alcyonium digitatum is also known to inhabit the entrances to sea lochs (Budd, 2008) or the entrances to estuaries (Braber & Borghouts, 1977) where water clarity is likely to be highly variable.

Many encrusting sponges appear to be able to survive in highly sedimented conditions, and many species prefer such habitats (Bell & Barnes 2001; Bell & Smith 2004).  Storr (1976) observed the sponge Sphecispongia vesparium backwashing to eject sediment and noted that other sponges (such as Condrilla nucula) use secretions to remove settled material.  Raspailia ramosa and Stelligera stuposa have a reduced maximum size in areas of high sedimentation (Bell et al., 2002).  Tjensvoll et al. (2013) found that Geodia barretti physiologically shuts down when exposed to sediment concentrations of 100 mg /l (86% reduction).  Rapid recovery to initial respiration levels directly after the exposure indicated that Geodia barretti can cope with a single short exposure to elevated sediment concentrations.  Schönberg (2015) reviewed and observed the interactions between sediments and marine sponges and described the lack of research on Porifera, with most studies grouping them together when looking at sediment effects.  Her findings were that whilst many sponges are disadvantaged by sedimentation, many examples exist of sponges adapting to sediment presence, including through sediment incorporation, sediment encrusting, soft sediment anchoring using spicules and living, at least partially, embedded within the sediment.  Schönberg (2015) found that Polymastiida interacted with sediment in 18.9% of observations (primarily through spicules), Clionaida had a highly variable interaction with sediment, with 5.7±11.4 %, Tethyida interacted in 13.1±21.1%.  De Kluijver & Leewis (1994) monitored the marine species before and two years after construction of a storm barrier in the Oosterschelde Estuary.  The barrier resulted in lower tidal flow, higher sedimentation and increased Haliclona oculata abundance. 

Increased siltation can cause clogging of ascidians respiratory organs (Bakus, 1968). Clavelina lepadiformis has relatively wide apertures that help prevent clogging from particles (Naranjo et al., 1996).  The simplistic structure of its branchial sac (Fiala-Medioni, 1978) may be less efficient in expelling particles, and more likely to suffer from clogging of feeding apparatus than other forms of sea squirts, such as Ciona intestinalis.  Clavelina lepadiformis was found to dominate Spanish harbours and nearby zones with a low rate of water renewal, excess silting and suspended matter and the species was described as biofouling and opportunist (Naranjo et al., 1996).

Sensitivity assessment.  The effect of increased suspended sediment will depend on the epifaunal community present.  As shown above, many epifauna (Hydrozoa, Anthozoa, Bryozoa, Ascidiacea, and Porifera) are tolerant of high levels of suspended sediment and scour.  In areas subject to high sediment load or scour, resistance is probably ‘High’. However, in areas of particular species richness (e.g. the Bubbling reefs of the Kattegat, Jensen et al., 1992) an increase in suspended sediment may result in loss of some members of the epifaunal community, especially in areas of strong water movement and/or low suspended sediment loads.  Therefore, a resistance of ‘Medium’ is suggested.  Resilience is probably ‘High’.  Overall, sensitivity is assessed as ‘Low’ but with ‘Low confidence.

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

This pressure represents a single deposit of fine sediment to a depth of 5 cm.  Pockmarks occur in silty soft sediments and, therefore, depositional habitats.  The rate of sedimentation and suspended sediment concentrations probably vary seasonally, and are probably higher in stormy winter months than relatively calm summer months.  Gas seepage winnows away fines and creates the pockmark.  The rate of seepage or the periods of episodic seepage probably compensates for sedimentation.  Dando (2001) noted that the rate of gas seepage was higher in shallow-water (<200 m) than deep-water (>200 m) because of the increased temperature and lower water pressure.  Inactive pockmarks can act as sediment traps (and larval traps) (Dando, 2001).  Croker et al. (2005) suggested that pockmarks could become covered with sediment in the event of high sedimentation rates and that buried pockmarks were not uncommon.

The effect of the deposit of 5 cm of fine sediment in a pockmark will depend on its size and depth.   In the North Sea they are typically 50-100 m in diameter and 1-3 m deep, but small unitary pockmarks are very small (< 5m) while giant pockmarks may be 500 m across and >20 m deep and small funnels or small eye pockmarks are 3-5 m across and 0.5 m deep (Hovland & Judd, 1988; Judd & Hovland, 2007, Webb et al., 2009).  Therefore, a deposit of 5cm of fine sediment is unlikely to have an adverse effect, especially in active seeps.  Hence, resistance and resilience are assessed a ‘High’, and the pockmark habitat is considered to be ‘Not sensitive’ at the benchmark level.

In most pockmarks and areas of carbonate reef, images show that the carbonate is surrounded by sediment, and is often partially covered by sediment (Judd, 2005; Judd et al., 2007; Whomersley et al., 2010; O’Reilly et al., 2014).  For example, In the Irish Sea, the MDAC reefs in Texel 11 occur in areas of strong tidal currents and mobile sands (Judd et al., 2007; Whomersley et al., 2010) and the carbonate reefs were usually only 40% uncovered.  Whomersley et al. (2010) described three epifaunal communities depending on their height above the sediment.  MDAC at least 20 cm above the sediment included a diverse community of epifauna, while MDAC lower than 20 cm was characterized by scour resistant epifauna (e.g. Flustra foliacea) and low relief MDAC was un-colonized by epifauna.  It is probable that the reef undergoes regular periods of smothering and uncovering due to movement of sediment.  Dando et al. (1994) noted that the intertidal and shallow subtidal MDAC reef the Kattegat was exposed, colonized by barnacles, mytilids etc. and eroded before being buried again.  In the Codling Fault Zone, the MDAC forms mounds ca 5-10 m above the seabed and in Holden Reefs, the MDAC stands up to 1.5 m above the seabed (Judd et al., 2007).

Overall, the effect of a deposit of 5 cm on MDAC reefs depends on their size and height above the seabed.  In pockmarks, small boulders may be affected slightly, and even on large areas of MDAC (e.g. the carbonate reefs of Texel and Holden Reefs) may be covered on the surface.  However, many of the species present are probably adapted to sedimentation (see suspended sediment above).  In addition, the action of gas seepage or strong water movement will probably remove the deposit rapidly. Hence, resistance and resilience are assessed a ‘High’, and the habitat is considered to be ‘Not sensitive’ at the benchmark level but with ‘Low confidence.

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

This pressure represents a single deposit of fine sediment to a depth of 30 cm.  Pockmarks occur in silty soft sediments and, therefore, depositional habitats.  The rate of sedimentation and suspended sediment concentrations probably vary seasonally, and are probably higher in stormy winter months than relatively calm summer months.  Gas seepage winnows away fines and creates the pockmark.  The rate of seepage or the periods of episodic seepage probably compensates for sedimentation.  Dando (2001) noted that the rate of gas seepage was higher in shallow-water (<200 m) than deep-water (>200 m) because of the increased temperature and lower water pressure.  Inactive pockmarks can act as sediment traps (and larval traps) (Dando, 2001).  Croker et al. (2005) suggested that pockmarks could become covered with sediment in the event of high sedimentation rates and that buried pockmarks were not uncommon.

The effect of the deposit of 30 cm of fine sediment in a pockmark will depend on its size and depth.  In the North Sea they are typically 50-100 m in diameter and 1-3 m deep, but small unitary pockmarks are very small (< 5m) while giant pockmarks may be 500 m across and >20 m deep and small funnels or small eye pockmarks are 3-5 m across and 0.5 m deep (Hovland & Judd, 1988; Judd & Hovland, 2007, Webb et al., 2009).  Therefore, while large and deep pockmarks are probably not affected, a deposit of 30 cm of fine sediment may affect small pockmarks and result in a short-term alteration in the infaunal community. However, where the pockmarks are active, the deposited material may be removed rapidly.  Hence, the resistance of pockmarks is assessed as ‘Medium’, resilience as ‘High’ and the sensitivity is assessed as ‘Low’ at the benchmark level.

In most pockmarks and areas of carbonate reef, images show that the carbonate is surrounded by sediment, and is often partially covered by sediment (Judd, 2005; Judd et al., 2007; Whomersley et al., 2010; O’Reilly et al., 2014).  For example, In the Irish Sea, the MDAC reefs in Texel 11 occur in areas of strong tidal currents and mobile sands (Judd et al., 2007; Whomersley et al., 2010) and the carbonate reefs were usually only 40% uncovered.  Whomersley et al. (2010) described three epifaunal communities depending on their height above the sediment.  MDAC at least 20 cm above the sediment included a diverse community of epifauna, while MDAC lower than 20 cm was characterized by scour resistant epifauna (e.g. Flustra foliacea) and low relief MDAC was un-colonized by epifauna.  It is probable that the reef undergoes regular periods of smothering and uncovering due to movement of sediment.  Dando et al. (1994) noted that the intertidal and shallow subtidal MDAC reef the Kattegat was exposed, colonized by barnacles, mytilids etc. and eroded before being buried again.  In the Codling Fault Zone, the MDAC forms mounds ca 5-10 m above the seabed and in Holden Reefs, MDAC stands up to 1.5 m above the seabed (Judd et al., 2007).

Overall, the effect of a deposit of 30 cm on MDAC reefs depends on their size and height above the seabed.  In pockmarks, small boulders may be buried and the epifauna adversely affected, depending on the time taken for the deposit to be removed by active gas seepage or water flow.  Large areas of MDAC (e.g. the carbonate refs of Texel and Holden Reefs) may be covered on the surface.  In the Texel 11 reefs (Whomersley et al., 2010) a difference in height above the sediment of ca 20 cm made a significant difference to the epifaunal community.  Therefore, a deposit of 30 cm may adversely affect the epifaunal community.  Nevertheless, in areas of strong currents (e.g. Texel 11) the deposited sediment may not remain for more than one tidal cycle, and any effect would be minimal. In addition, many of the species present are probably adapted to sedimentation (see suspended sediment above).

Therefore, a deposit of 30 cm of fine sediment may adversely affect the epifaunal community and reduce species richness where the MDAC reef is adjacent to the sediment, and/or the MDAC is of low relief.  Hence, the resistance of MDAC structures is assessed a ‘Medium’, resilience as ‘High’ and the sensitivity is assessed as ‘Low’ at the benchmark level but with ‘Low confidence.  However, in areas of strong water flow, the effect is probably minimal and the MDAC reef is probably ‘Not sensitive’ at the benchmark level (resistance and resilience are assessed a ‘High’).

Litter

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

Evidence

Litter is common on the seabed of the Greater North Sea, Celtic Seas and Eastern Bay of Biscay (OSPAR, 2017) and commonly reported in the deep sea (Ramirez-Llodra et al., 2011). Litter includes hard plastics but also glass, clinker, barrels, and nets (Ramirez-Llodra et al., 2011; OSPAR, 2017).  Discarded or lost nets and lines may continue to ‘ghost fish’, while hard plastics, moved by currents may result in physical disturbance and abrasion, may provide additional hard substrata for colonization by epifauna, or may introduce non-native species or adsorb contaminants.  Micro-plastics also adsorb contaminants and may be ingested by marine fauna (Wright et al., 2013).  However, information on the direct effects of litter on benthic fauna, especially micro-plastics, are limited. Hence, ‘no assessment’ of sensitivity has been recorded.

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

Stanley et al. (2014) studied the effects of vessel noise on fouling communities and found that the bryozoans Bugula neritina, Watersipora arcuate and Watersipora subtorquata responded positively.  More than twice as many bryozoans settled and established on surfaces with vessel noise (128 dB in the 30–10,000 Hz range) compared to those in silent conditions.  Growth was also significantly higher in bryozoans exposed to noise, with 20% higher growth rate in encrusting and 35% higher growth rate in branching species. However, the majority of epifaunal and infaunal species are unlikely to respond directly to underwater noise. Fish associated with MDAC reefs or pockmarks may exhibit avoidance responses to underwater noise but are considered to be vagrants that use the habitat for shelter, rather than characterizing species.

Sensitivity assessment. Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is therefore considered to be 'Not sensitive' at the benchmark level.

Introduction of light or shading

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

Evidence

The introduction of artificial light is unlikely to be relevant to seeps (pockmarks or reefs of MDAC) unless they occur in the shallow subtidal and even then the effects are probably limited to slight changes in the timing of reproduction due to changes in circadian rhythms, although there is no evidence to substantiate this idea. Shading is also only likely to affect the depth to which macroalgae penetrate the water column and it only likely to affect MDAC reefs that are shallow enough to support macroalgae, e.g. Holdens Reefs that support filamentous red and brown algae on the upper surfaces.  Shading has the potential to reduce the limit of macroalgae and reduce their diversity but long-term or permanent artificial structures are unlikely to occur outside inshore waters. Therefore, this pressure is probably 'Not relevant' in this habitat.

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 and changes in tidal excursion are 'Not relevant' to biotopes restricted to open waters.

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

There is no evidence to suggest that the dominant species within the infauna of pockmarks or epifauna on MDAC (methane-derived authigenic carbonate) are subject to genetic modification of translocation.

Introduction or spread of invasive non-indigenous species

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

Evidence

Didemnum vexillum is an invasive colonial sea squirt native to Asia which was first recorded in the UK in Darthaven Marina, Dartmouth in 2005. Didemnum vexillum can form extensive mats over the substrata it colonizes, binding boulders, cobbles and altering the host habitat (Griffith et al., 2009). Didemnum vexillum can also grow over and smother the resident biological community.  Recent surveys within Holyhead Marina, North Wales have found Didemnum vexillum growing on and smothering native tunicate communities, including Ciona intestinalis (Griffith et al., 2009). Due to the rapid-re-colonization of Didemnum vexillum eradication attempts have to date failed. 

Didemnum vexillum is isolated to several sheltered locations in the UK (GBNNSIP, 2014; NBN, 2015), however, Didemnum vexillum successfully colonized offshore in Georges Bank, USA (Lengyel et al., 2009), which is more exposed than the locations that Didemnum vexillum has colonized in the UK.  It is, therefore, possible that Didemnum vexillum could pose a threat to epifaunal communities on refs of MDAC (methane-derived authigenic carbonate).

A number of invasive bryozoans are of concern including Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013b).  Tricellaria inopinata has been reported to colonize the byssal threads of the mussel Mytilus galloprovincialis, Hymeniacidon perleve and the ascidian Styela plicata (Dyrynda et al., 2000). Tricellaria inopinata dominated the fouling community in the Lagoon of Venice, within seven years of being introduced (Ambrogi, 2000).

The red king crab, Paralithodes camtschaticus was introduced into the East Barents Sea in the 1960s and has spread east along the Kola Peninsula and westwards into Norwegian waters (GBNNSIP, 2011). It is not recorded in UK waters at present.  It is a voracious omnivore and scallop and flatfish populations have been reduced in Norway (GBNNSIP, 2011).  If it reaches UK waters, it has the potential to adversely affect epifaunal communities.

However, there is ‘insufficient evidence’ at present that this habitat has been affected by introduced non-native invasive species.  Due to the constant risk of new invasive species, the literature on this pressure should be revisited.

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

Numerous species of epifauna harbour parasites (e.g. bryozoans) and all species are probably targeted by parasites and pathogens.  However, no evidence of disease induced mortality in the species typical of pockmark or MDAC reef communities was found.

Removal of target species

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

Evidence

Several fish species that are found associated with pockmarks (e.g. cod, ling) are targeted by commercial fisheries. Similarly, the sediment in pockmarks may also harbour Nephrops. Judd & Hovland (2007) suggested that the majority of scavengers and predators were ‘vagrants’. Mobile scavengers (e.g. starfish and crustaceans) probably utilise the available habitat.  However, they would move to other habitats if the pockmark or MDAC was lost or covered. The fish most likely use pockmarks and MDAC structures for shelter and forage elsewhere, as fish are also attracted to artificial structures and even to depressions on the seabed (Dando, 2001, 2010).  Therefore, loss of these species will probably not have a significant effect on the ecology of the pockmark infauna or the epifauna of MDAC in pockmarks or on reefs.  Therefore, a resistance of 'High' is suggested.  Hence, resilience is also 'High' and the habitat is assessed as 'Not sensitive'.  Please note, the physical effects of commercial harvesting (e.g. trawling) are addressed under the ‘physical’ pressures above (see abrasion’). 

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

Pockmarks may be affected by trawling or bottom gear depending on their size and depth.  It is possible that shallow pockmarks could be disturbed by bottom gear and species removed from the pockmark or its slopes (see 'abrasion' above).  However, removal of emergent infauna (e.g. sea pens or sea anemones) from the slopes of pockmarks or removal of a proportion of the macro-infauna as by-catch may not adversely affect the character of the pockmark community as the microbial and meiofaunal communities will probably remain.  Therefore, resistance is probably 'Medium' and resilience 'High', and sensitivity is assessed as 'Low' but with 'Low' confidence.  Note, the physical effects of commercial harvesting (e.g. trawling) are addressed under the ‘physical’ pressures above (see abrasion’) and only the possible biological interactions are discussed here.   

Epifaunal species present on boulders and outcrops of MDAC in pockmarks or on reefs probably compete for space and overgrow each other.  Space cleared by a physical pressure is likely to be rapidly colonized by hydroids and tubeworms with subsequent succession from hydroids to erect bryozoan and anemones (see Sebens, 1985, 1986). By-catch (e.g. due to passing bottom gears) could potentially remove a proportion of the epifaunal community, and a resistance of ‘Low’ is suggested.  Therefore, resilience is probably ‘Medium’ and a sensitivity is assessed a ‘Medium’.