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

All characterizing sponges (Cliona celata, Halichondria bowerbanki, Halichondria panacea, Hymeniacidon perleve, Leucosolenia botryoides,  Suberites ficus, Raspailia ramosa, Amphilectus fucorum) are widely distributed across the coasts of the British Isles and are all found from the Channel Isles to Northern Scotland (NBN, 2015).

Berman et al. (2013) monitored sponge communities off Skomer Island, UK over three years with all characterizing sponges for this biotope assessed.  Seawater temperature, turbidity, photosynthetically active radiation and wind speed were all recorded during the study. It was concluded that, despite changes in species composition, primarily driven by the non-characterizing Hymeraphia, Stellifera and Halicnemia patera, no significant difference in sponge density was recorded in all sites studied.  Morphological changes most strongly correlated with a mixture of water visibility and temperature.  Lemoine et al. (2007) studied the effects of thermal stress on the holobiont of the sponge Halichondria bowerbanki collected from Virginia, USA.   Whilst no apparent change in density or diversity of symbionts was detected over the range of temperatures (29°C, 30°C and 31°C), the presence of particular symbionts was temperature dependent.  Barthel (1986) reported that reproduction and growth in Halichondria panicea in the Kiel Bight were primarily driven by temperature, with higher temperatures corresponding with the highest growth. 

Cantero et al. (2002) described the presence and year-round fertility of Obelia dichotoma, Kirchenpaureria pinnata, Nemertesia ramosa and Halecium spp.in the Mediterranean, indicating probable tolerance to temperature increases at the benchmark figure.  

For the ascidian Dendrodoa grossularia, gamete release occurs from spring-autumn, with peaks in early spring and another in late summer. Gamete release is reduced at temperatures above 15°C and totally suppressed above ca 20°C (Miller, 1954). No information was found on the upper temperature threshold of mature Dendrodoa grossularia. Whilst widespread throughout the British Isles (NBN, 2015),  a dramatic increase in temperature may cause mortality. 

Sensitivity assessment: The important characterizing species are distributed to the north and south of the British Isles and unlikely to be sensitivity to change at the benchmark level. Resistance is, therefore ‘High’, resilience is ‘High’ and the biotope is ‘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

All characterizing sponges (Cliona celata, Halichondria bowerbanki Halichondria panacea, Hymeniacidon perleve, Leucosolenia botryoides, Suberites ficus, Raspailia ramosa, Amphilectus fucorum) are widely distributed across the coasts of the British Isles and are all found from the Channel Isles to Northern Scotland (NBN, 2015). Berman et al. (2013) monitored sponge communities off Skomer Island, UK over three years with all characterizing sponges for this biotope assessed.  seawater temperature, turbidity, photosynthetically active radiation and wind speed were all recorded during the study.  It was concluded that, despite changes in species composition, primarily driven by the non-characterizing Hymeraphia Stellifera and Halicnemia patera, no significant difference in sponge density was recorded in all sites studied.  Morphological changes most strongly correlated with a mixture of visibility and temperature.

Some sponges do exhibit morphological strategies to cope with winter temperatures e.g.  Halichondria bowerbanki goes into a dormant state below 4°C, characterized by major disintegration and loss of choanocyte chambers with many sponges surviving mild winters in more protected areas from where it can recolonize (Vethaak et al., 1982).

Crisp (1964) observed the effects of an unusually cold winter (1962-63) on the marine life in Britain, including Porifera in North Wales.   Whilst difficulty in distinguishing between mortality and delayed development was noted, Crisp found that Pachymastia johnstonia and Halichondria panicea were wholly or partly killed by frost and several species appeared to be missing including Amphilectus fucorum. Others, including Hymeniacidon perleve were unusually rare and a few species, including Polymastia boletiformis, were not seriously affected.  Barthel (1986) reported that Halichondria panicea in the Kiel Bight degenerated and disintegrated after reproduction before winter, however, young colonies were observed from September.

Palerud et al. (2004) described the presence of the characterizing hydroids Halecium halecinum and Nemertesia sp. in Svalbard, suggesting that these hydroids are probably tolerant of the lowest temperatures they are likely to encounter in Britain and Ireland of ca 4°C (Beszczynska-Möller & Dye, 2013). Dendrodoa grossularia has been recorded as an abundant component of benthic fauna in Nottinghambukta, Svalbard (Różycki & Gruszczyński, 1991).

Sensitivity assessment: There is evidence of sponge mortality at extremely low temperatures in the British Isles.  Given this evidence, it is likely that a cooling of 5°C for a month could potentially affect the characterizing sponges, and resistance has been assessed as ‘Medium’ with a resilience of ‘High’.  Sensitivity has, therefore, been 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

Marin et al. (1998) described the presence of Dysidea fragilis in a hypersaline coastal lagoon (42-47 g/l) in El Mar Menor, Spain.  No evidence could be found for characterizing sponges. Studies on hydroids have found that prey capture rates may be affected by salinity and temperature (Gili & Hughes, 1995) although no evidence was found for species that characterize this biotope. This biotope occurs in Full salinity (Connor et al., 2004). An increase to hypersaline conditions is likely to affect the characterizing species, but ‘No evidence’ to support an assessment could be found.

Salinity decrease (local)

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

Evidence

Castric-Fey & Chassé (1991) conducted a factorial analysis of the subtidal rocky ecology near Brest, France and rated the distribution of species from estuarine to offshore conditions.  Dysidea fragilis and Raspailia ramosa were rated as unaffected by salinity at this range.  Cliona celata and Pachymatisma johnstonia had a slight preference for more estuarine conditions. Mean salinity difference across the study was low (35.1 and 33.8 ‰ respectively) but with a greater range being experienced in the Inner Rade (± 2.4‰ compared with± 0.1‰).  It should be noted that the range of salinities identified in this study does not reach the lower benchmark level.   Some of the characterizing species occur in harbours and estuaries, including Halichondria spp. and Hymeniacidon perleve (Ackers et al., 1992).

Little evidence for the characterizing hydroids could be found.  Stebbing (1981a) found that, for the hydroid Campanularia flexuosa, growth was inhibited in 70% seawater (ca 25‰) and that exposure to below 30% seawater (ca 10‰) was lethal after 3 days.  Nemertesia spp. and Halecium halecinum were recorded as occurring in variable to full salinity biotopes (18-35 ‰). Dendrodoa grossularia has been recorded in biotopes occurring from full to low salinity regimes (<18-35 ‰).

Sensitivity assessment. This biotope occurs in Full salinity.  All characterizing species also appear in similar biotopes with lower salinities (Connor et al., 2004) and mortality is unlikely at the benchmark level, however, should a reduction in salinity be permanent, this may result in reclassification of the biotope.  Resistance is ‘High’, resilience is ‘High’ and the biotope is ‘Not sensitive’ at the benchmark level.

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

Riisgard et al. (1993) discussed the low energy cost of filtration for sponges and concluded that passive current-induced filtration may be insignificant for sponges. Pumping and filtering occurs in choanocyte cells that generate water currents in sponges using flagella (De Vos et al., 1991).

Whilst little evidence for the characterizing sponges could be found, the important characterizing hydroids are typically found in places of low to moderate water movement, although Hayward & Ryland (1995b) note that abundant communities occur in narrow straits and headlands which may experience high levels of water flow. Hydroids can bend passively with water flow to reduce drag forces to prevent detachment and enhance feeding (Gili & Hughes, 1995). The hydroid growth form also varies to adapt to prevailing conditions, allowing species to occur in a variety of habitats (Gili & Hughes, 1995). In general, flow rates are an important factor for feeding in hydroids and prey capture appears to be higher in more turbulent conditions that prevent self-shading by the colony (Gili & Hughes, 1995). The capture rate of zooplankton by hydroids is correlated with prey abundance (Gili & Hughes, 1995), thus prey availability can compensate for sub-optimal flow rates. Water movements are also important to hydroids to prevent siltation which can cause death (Round et al., 1961). Tillin & Tyler-Walters (2014) suggest that the range of flow speeds experienced by biotopes in which hydroids are found indicate that a change (increase or decrease) in the maximum water flow experienced by mid-range populations for the short periods of peak spring tide flow would not have negative effects on this hydroids . 

As sessile filter feeders, ascidians generally require a reasonable water flow rate in order to ensure sufficient food availability.  It was shown that in stagnant water, phytoplankton density became reduced in a 20-30 cm layer immediately above a dense colony of Ciona intestinalis (Riisgård et al., 1996).  If dislodged, juvenile and adult ascidians have a limited capability to re-attach, given calm conditions and prolonged contact with the new substrata (Carver et al., 2006; Millar, 1971).

Sensitivity assessment. The CR.MCR.CFaVS biotope group occurs in moderate water flow and low wave exposure.  A significant decrease in water flow could result in a change to the CR.LCR.BrAs group of biotopes, whereas a significant increase may result in a change to CR.HCR.FaT.  However, change at the benchmark level of 0.1-0.2 m/s is unlikely to be significant and resistance is therefore assessed as ‘High’, resilience is ‘High’ and the biotope is ‘Not sensitive’ at the 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

Changes in emergence are not relevant to this biotope as it is restricted to fully subtidal/circalittoral conditions - the pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

Wave exposure changes (local)

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

Evidence

Roberts et al. (2006) studied deep sponge reef communities (18-20 m) in sheltered and exposed locations in Australia. They reported greater diversity and cover (>40% cover) of sponges in wave-sheltered areas compared with a sparser and more temporal cover in exposed sites (25% cover).  Erect sponges dominated the sheltered sites while encrusting sponges dominated in exposed locations.  Jackson (2004) reported that Nemertesia ramosa was intolerant of high wave exposure because it was only  found in sheltered areas.   Faucci et al. (2000) recorded hydroid communities at two sites of different wave exposure and recorded the presence of Obelia dichotoma and Halecium spp. in both the exposed and sheltered sites, but only found Kirchenpaueria sp. in the sheltered site.  High energy wave action can be detrimental to ascidian populations. This is mainly through physical damage to the sea squirts and through the abrasive action of suspended sediment (Jackson, 2008).  Ciona intestinalis is often dominant in highly sheltered areas such as harbours (Carver et al., 2006). Decreases in wave exposure are unlikely to have any effect.  If dislodged, juvenile and adult Ciona intestinalis have a limited capability to re-attach, given calm conditions and prolonged contact with the new substratum (Carver et al., 2006; Jackson 2008; Millar, 1971) but increases in wave exposure above moderately exposed are likely to cause a proportion of the population to die, especially in the shallower examples of the biotope if the cobbles and pebbles on which the biotope occurs are mobilized by wave action.  Ascidia mentula has rarely been recorded at depths shallower than 15 m (Svane, 1984), it is possible that damage could occur if subjected to increased wave exposure.

Sensitivity Assessment. The CR.MCR.CFaVS biotope group occurs in moderate water flow and low wave exposure.  Decrease in wave exposure is therefore not relevant, however, an increase in wave exposure is likely to result in fundamental change.  Nevertheless, change at the benchmark level of 3-5% in wave height is unlikely to be significant and resistance is therefore assessed as ‘High’, resilience is ‘High’ and the biotope is ‘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.

While some sponges, such as Cliona spp. have been used to monitor heavy metals by looking at the associated bacterial community (Marques et al., 2007; Bauvais et al., 2015), no literature on the effects of transition element or organo-metal pollutants on the characterizing sponges could be found.

Although no information on the effects of heavy metals on the assessed hydroids was found, evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination. Various heavy metals have been shown to have sublethal effects on growth in the few hydroids studied experimentally (Bryan, 1984). Stebbing (1981a) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1976) reported that 1 µg/l Hg²⁺ was stimulatory, although the effect was transitory, exposure resulting in reduced growth towards the end of his 11 day experiments. Cadmium (Cd) was reported to cause irreversible retraction of 50% of hydranths in Laomedea loveni after 7 days exposure at concentrations between 3 µg/l (at 17.5 °C and 10 ppt salinity) and 80 µg/l (at 7.5 °C and 25 ppt salinity) (Theede et al., 1979). Laomedea loveni was more tolerant of Cd exposure at low temperatures and low salinities. Karbe (1972, summary only) examined the effects of heavy metals on the hydroid Eirene viridula (Campanulidae). He noted that Cd and Hg caused cumulative effects and morphological changes. Mercury (Hg) caused irreversible damage at concentrations as low as 0.02 ppm. He reported threshold levels of heavy metals for acute effects in Eirene viridula of 1.5-3 ppm Zn, 1-3 ppm Pb, 0.1-0.3 ppm Cd, 0.03-0.06 ppm Cu and 0.001-0.003 ppm Hg. Karbe (1972, summary only) suggested that Eirene viridula was a sensitive test organism when compared to other organisms.

Trace metals (particularly mercury and copper) have been found to affect embryogenesis and larval settlement in Ciona intestinalis (Bellas et al., 2004). Whilst there are extensive studies of larval intolerance to TBT (Mansueto et al., 1993, Pellerito et al., 1996, Bellas, 2005) and zinc pyrithione (Bellas, 2005), data appears non-existent for the adult stage. Chesher (1971) found that Ascidia niagra was surprisingly intolerant of desalination effluent (50% mortality in 5.8% effluent solution after 96 hours), far less tolerant than the other species included in the study (echinoids, crabs and gorgonians). Whilst presence of copper was considered the most deleterious factor across the study, the increased sensitivity of the ascidians was attributed to synergistic copper and temperature effects, although the presence of other contaminants (e.g. nickel) could not be ruled out (Chesher, 1971).

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.

Oil pollution is mainly a surface phenomenon its impact upon circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sublittoral habitats (Castège et al., 2014).

Filter feeders are highly sensitive to oil pollution, particularly those inhabiting the tidal zones which experience high exposure and show correspondingly high mortality, as are bottom dwelling organisms in areas where oil components are deposited by sedimentation (Zahn et al., 1981). Tethya lyncurium concentrated BaP (benzo[a ]pyrene) to 40 times the external concentration and no significant repair of DNA was observed in the sponges, which, in higher animals, would likely lead to cancers. As sponge cells are not organized into organs the long-term effects are uncertain (Zahn et al., 1981). Ignatiades & Becacos-Kontos (1970) found that Ciona intestinalis can resist the toxicity of oil polluted water and ascidians are frequently found in polluted habitats such as marinas and harbours, etc. (Carver et al., 2006) as well as Ascidia mentula (Aneiros et al., 2015).

Synthetic compound contamination

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

Evidence

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

The species richness of hydroid communities decreases with increasing pollution but hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1981a) cited reports of growth stimulation in Obelia geniculata caused by methyl cholanthrene and dibenzanthrene. Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l. No information concerning the resistance of the characteristic hydroids was found. However, the above evidence suggests that several species of hydroid exhibit sublethal effects due to synthetic chemical contamination and lethal effects due to TBT contamination.

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

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates, this concentration is about 2 ml/l (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Cole et al. (1999) suggested possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l.

Hiscock & Hoare (1975) reported an oxycline forming in the summer months (Jun-Sep) in a quarry lake (Abereiddy, Pembrokeshire) from close to full oxygen saturation at the surface to <5% saturation  (ca 0.5 mg/l) below ca 10 m.  Despite the presence of Tethya aurantia, Kirchenpaueria pinnata, Hymeniacidon pereleve, Polymastia boletiformis or Ascidia mentula in shallower, no sponges or ascidians were recorded at depths below the oxycline at 10 - 11 m. 

Demosponges maintained under laboratory conditions can tolerate hypoxic conditions for brief periods. (Gunda & Janapala, 2009) investigated the effects of variable dissolved oxygen (DO) levels on the survival of the marine sponge, Haliclona pigmentifera. Under hypoxic conditions (1.5-2.0 ppm DO), Haliclona pigmentifera with intact ectodermal layers and subtle oscula survived for 42 ± 3 days.  Sponges with prominent oscula, foreign material, and damaged pinacoderm exhibited poor survival (of 1-9 days) under similar conditions. Complete mortality of the sponges occurred within 2 days under anoxic conditions of <0.3 ppm DO (ca 0.3 mg/l).

Hydroids mainly inhabit environments in which the oxygen concentration exceeds 5 ml/l (ca 7 mg/l) (Gili & Hughes, 1995). Although no information was found on oxygen consumption for the characterizing hydroids, Sagasti et al. (2000) reported that epifaunal species, including several hydroids and Obelia bidentata (as bicuspidata) in the York River, Chesapeake Bay, tolerated summer hypoxic episodes of between 0.5 and 2 mg O₂/l (0.36 and 1.4 ml/l) for 5-7 days at a time, with few changes in abundance or species composition.

The ability of solitary ascidians to withstand decreasing oxygen levels has not been well documented. Mazouni et al. (2001) noted that whilst oysters (Magallana gigas) can survive short-term exposure to periods of anoxia (Thau Lagoon, France), the associated biofouling community dominated by Ciona intestinalis suffered heavy mortality.  It should be noted, however, that Ciona intestinalis is frequently found in areas with restricted water renewal where oxygen concentrations may drop (Carver et al., 2006). Whilst adverse conditions could affect health, feeding, reproductive capability and could eventually lead to mortality, recovery should be rapid.

Sensitivity assessment. Resistance is recorded as ‘Low’, with a resilience of ‘Medium’ and sensitivity is classed as 'Medium'. Due to the lack of specific data for these species, confidence is recorded as ‘Low’.   

Nutrient enrichment

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

Evidence

Gochfeld et al. (2012) studied the effect of nutrient enrichment (≤0.05 to 0.07 μM for nitrate and ≤0.5 μM for phosphate)  as a potential stressor in Aplysina caulifornis and its bacterial symbionts and found that nutrient enrichment had no effects on sponge or symbiont physiology when compared to control conditions.  This study does contradict findings in Gochfeld et al. (2007) in which Aplysina spp. were virtually absent from a site of anthropogenic stress in Bocas del Toro, Panama, which experienced high rainfall and terrestrial runoff.  The author suggested that whilst this site did include elevated nutrient concentrations, other pressures and stresses could be contributing to its absence.

Rose & Risk (1985) described an increase in abundance of Cliona delitrix in organically polluted section of Grand Cayman fringing reef  affected by the discharge of untreated faecal sewage.  Ward-Paige et al. (2005) described the greatest size and biomass of clionids corresponded with the highest nitrogen, ammonia and ɗ¹⁵N levels.  Witt et al. (2004) found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with control reference areas.  Ascidia mentula has been reported in Iberian bays subject to both nutrient-rich upwelling events and anthropogenic pollution (Aneiros et al., 2015).  There is some suggestion that there are possible benefits to ascidians from increased organic content of water; ascidian ‘richness’ in Algeciras Bay was found to increase in higher concentrations of suspended organic matter (Naranjo et al. 1996).

Nevertheless, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.

Organic enrichment

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

Evidence

Some of the characterizing sponges occur in harbours and estuaries, including Halichondria spp. and Hymeniacidon perleve (Ackers et al., 1992) and may, therefore, tolerate high levels of organic carbon. Fu et al. (2007) described Hymeniacidon perleve in aquaculture ecosystems in sterilized natural seawater with different concentrations of total organic carbon (TOC), at several concentrations between 52.9 and 335.13 mg/L).   Hymeniacidon perleve removed 44–61% TOC during 24 h, with retention rates of ca. 0.19–1.06 mg/hr ·g-fresh sponge. Hymeniacidon perleve removed organic carbon excreted by Fugu rubripes with similar retention rates of ca. 0.15 mg/h · g-fresh sponge, and the sponge biomass increased by 22.8%. 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.

Witt et al. (2004) found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany), which experienced sedimentation of 1 cm for more than 25 days.  However, another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.   As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids.

There is some suggestion that there are possible benefits to the ascidians from increased organic content of water; Ascidian ‘richness’ in Algeciras Bay was found to increase in higher concentrations of suspended organic matter (Naranjo et al., 1996).  Kocak & Kucuksezgin (2000) noted that Ciona intestinalis was one of the rapid breeding opportunistic species that tended to be dominant in Turkish harbours enriched by organic pollutants and was frequently found in polluted environments (Carver et al., 2006).  Ascidia mentula has been reported in Iberian bays subject to both nutrient-rich upwelling events and anthropogenic organic pollution (Aneiros et al., 2015).

Sensitivity assessment: The above evidence suggests that resistance to this pressure is s 'High'.  Therefore, resilience is assessed as 'High' and the biotope is therefore considered to be 'Not sensitive'.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

If rock were replaced with sediment, this would represent a fundamental change to the physical character of the biotope and the species would be unlikely to recover. The biotope would be lost.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very low’. Sensitivity has been 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

‘Not relevant’ to biotopes occurring on bedrock.

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 species characterizing this biotope are epifauna or epiflora occurring on rock and would be sensitive to the removal of the habitat. However, extraction of rock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

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

Van Dolah et al. (1987) studied the effects on sponges and corals of one trawl event over a low-relief hard bottom habitat off Georgia, US.  The densities of individuals taller than 10 cm of three species of sponges in the trawl path and in adjacent control area were assessed by divers and were compared before, immediately after and 12 months after trawling.  Of the total number of sponges remaining in in the trawled area, 32% were damaged.  Most of the affected sponges were the barrel sponges Cliona spp., whereas other sponges including Haliclona oculta and Ircina campana were not significantly affected, the abundance of sponges had increased to pre-trawl densities or greater12 months after trawling.

Tilmant (1979) found that, following a shrimp trawl in Florida, US, over 50% of sponges, including Neopetrosia, Spheciospongia, Spongia and Hippiospongia, were torn loose from the bottom.  The highest damage incidence occurred to the finger sponge Neopetrosia longleyi. Size did not appear to be important in determining whether a sponge was affected by the trawl.  Recovery was ongoing, but not complete 11 months after the trawl, although no specific data was provided.

Boulcott & Howell (2011) conducted experimental Newhaven scallop dredging over a circalittoral rock habitat in the sound of Jura, Scotland and recorded the damage to the resident community. The results the sponge Pachymatisma johnstoni was highly damaged by the experimental trawl.  However, other members of the faunal turf community were not as vulnerable to damage through trawling as sedimentary fauna and whilst damage to circalittoral rock fauna did occur, it was of an incremental nature, with the loss of faunal turf communities increasing with repeated trawls.

The ascidians are epifaunal and physical disturbance is likely to cause damage with mortality likely.  Emergent epifauna are generally very intolerant of disturbance from fishing gear (Jennings & Kaiser, 1998).  However, studies have shown Ascidia spp. to become more abundant following disturbance events (Bradshaw et al., 2000). 

Resampling of grounds that were historically studied (from the 1930s) indicates that some species have increased in areas subject to scallop fishing (Bradshaw et al., 2002).  This study also found a (unquantified) increase in abundance of tough stemmed hydroids including Nemertesia  pp., 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. Hydroids may also recover rapidly as the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of colonies may be able to develop into new colonies. Hydroid colonies were still present in the heavily fished area, albeit at lower densities than in the closed area. This may largely be because the Isle of Man scallop fishery is closed from 1^st June to 31^st October (Andrews et al., 2011), so at the time the samples were taken for the study in question, the seabed had been undredged for at least 3.5 months. The summer period is also the peak growing/breeding season for many marine species. (Bradshaw et al., 2003).

Sensitivity assessment. Whilst a large proportion of the sponge community is likely to be affected by abrasion events, there is some debate as it the level of effects depending on the size of the sponge and the type of abrasion effect (Coleman et al., 2013).  The majority of the literature agrees that damage would fall within the ‘Low’ bracket of 25-75% reduction.  Ascidians are also likely to be significantly affected, although, given their high resilience, they are likely to recover quickly.  The assessment is, therefore, based on the sponge component of the biotope. Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’ and a sensitivity of ‘Medium’ is recorded.

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 species characterizing this biotope group are epifauna or epiflora occurring on rock which is resistant to subsurface penetration.  The assessment for abrasion at the surface only is therefore considered to equally represent sensitivity to this pressure. This pressure is thought ‘Not relevant’ to hard rock biotopes.

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

Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig, 1979), many encrusting sponges appear to be able to survive in highly sedimented conditions and, in fact, many species prefer such habitats (Schönberg, 2015; Bell & Barnes, 2000; Bell & Smith, 2004).  Castric-Fey & Chassé (1991) conducted a factorial analysis of the subtidal rocky ecology near Brest, France and rated the distribution of species in varying turbidity (corroborated by the depth at which laminarians disappeared).  Cliona celata and Stelligera rigida were classed as indifferent to turbidity, Tethya aurantium, Pachymatisma johnstonia and Polymastia boletiformis (as Polymastia robusta) had a slight preference for clearer water, while Dysidea fragilis, Polymastia mamillaris, and Raspailia ramosa had a strong preference for turbid water.   Some of the characterizing sponges occur in harbours and estuaries, including Halichondria spp. and Hymeniacidon perleve (Ackers et al., 1992).  Storr (1976) observed the sponge Sphecispongia vesparium ‘back washing’ 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 (86% reduction in respiration) when exposed to sediment concentrations of 100 mg /l.  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.  It should be noted that many of the characterizing sponges have been recorded in the turbid biotope CR.MCR.CFaVS.CuSpH.Vs (Connor et al., 2004) and resistance to suspended solids in these species are likely to be high.

CR.MCR.CFaVS.CuSpH.Vs  occurs in estuarine waters and therefore the species in the biotope are likely to be well adapted to turbid conditions.  Wass et al. (1997) described suspended sediment maxima for ‘medium’ sized rivers as rarely exceeding 500 mg/l, with a few rivers (including the Don and the Swale) experiencing concentrations in excess of 1000 mg/l.  Langston et al. (2003) described annual mean suspended sediment concentrations in the Tamar as varying from 61 mg/l to 1039 mg/l in the upper estuary, 6 to 18 mg/l in the outer estuary and 2 to 9 mg/l beyond.  It should be noted that the values quoted are mean annual concentrations and the same report states that conditions could be ‘very turbid’ in the outer estuary. The estuarine turbidity maximum (the point at which highest turbidity is experienced) can be highly variable and has been reported to move by ca 12 km down-estuary during the transition from neap to spring tides in the Humber estuary (Uncles et al., 2006).  Estuarine environments are likely to experience variable turbidity and the species present are probably tolerant of significant short-term changes in suspended solid concentrations.

Long-term increase in turbidity may affect primary production in the water column and, therefore, reduce the availability of diatom food, both for suspension feeders and deposit feeders. In addition, primary production by the microphytobenthos on the sediment surface may be reduced, further decreasing food availability for deposit feeders. However, primary production is probably not a major source of nutrient input into the system and, furthermore, phytoplankton will also immigrate from distant areas and so the effect may be decreased.

Sensitivity assessment. This biotope occurs in moderate flow in variable salinity.  A large proportion of the characterizing species have been recorded in the turbid biotope CR.MCR.CFaVS.CuSpH.Vs (Connor et al., 2004).  Mortality at the benchmark level is therefore considered unlikely and resistance is ‘High’, resilience is ‘High’ and the biotope is ‘Not sensitive’. The lack of evidence for characterizing species, as well as a lack of consensus in the literature result in a ‘Low’ quality confidence score.

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

Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig, 1979), many encrusting sponges appear to be able to survive in highly sedimented conditions and, in fact, many species prefer such habitats (Schönberg, 2015; Bell & Barnes 2000; Bell & Smith, 2004). However, Wulff (2006) described mortality in three sponge groups after four weeks of complete burial under sediment;  16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina respectively. Ackers et al. (1992) reported that Halichondria bowerbanki can be partly embedded in mud. 

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea have been linked to environmental changes including siltation.  Round et al. (1961) reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected. Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover some individuals, while others may just have the lower section of the main stem covered (Hayward & Ryland, 1994). Halecium halecinum can grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994).  Some of the community is, therefore, likely to survive smothering by 5 cm.

The solitary ascidians considered in this report are permanently attached to the substratum and are active suspension feeders. The complete disappearance of the sea squirt Ascidiella aspera biocoenosis and ‘associated sponges’ in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev, 2008 cited in Tillin & Tyler-Walters, 2014).   Dendrodoa grossularia is a small ascidian, capable of reaching a size of approx 8.5 mm (Millar, 1954) and is, therefore, likely to be inundated by deposition of 5 cm of sediment.  

Sensitivity assessment. Smothering by 5 cm of sediment is likely to impact hydroids, ascidian and sponge species, however, it is likely that enough of the population would survive to recover quite rapidly should the thin layer of sediment be removed.  Resistance has been assessed as ‘Medium’, resilience as ‘High’ and sensitivity has been assessed as ‘Low’ at the benchmark level.

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

Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig, 1979), many encrusting sponges appear to be able to survive in highly sedimented conditions and, in fact, many species prefer such habitats (Schönberg, 2015; Bell & Barnes 2000; Bell & Smith 2004). However, Wulff (2006) described mortality in three sponge groups following four weeks of complete burial under sediment.  16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina respectively. In 30 cm of deposition, the majority of sponges are likely to be buried, unless the topography of the biotope includes many vertical surfaces.

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea have been linked to environmental changes including siltation.  Round et al., 1961 reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible (Hayward & Ryland, 1994). Monosipphonic Obelia dichotoma stems grow to 5 cm, polysiphonic structures can reach up to 35 cm in height, Halecium halecinumcan grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994).  Smothering by 30 cm of material is likely to cover almost all the hydroids.  Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered but, although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected.  Therefore, if the deposition is removed fairly rapidly, the impact may be limited.

The solitary ascidians considered in this report are permanently attached to the substratum and are active suspension feeder. The complete disappearance of the sea squirt Ascidiella aspera biocoenosis and associated sponges in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev 2008 cited in Tillin & Tyler-Walters, 2014). Dendrodoa grossularia is a small ascidian, capable of reaching a size of approx 8.5 mm (Millar, 1954) and is, therefore, likely to be inundated by deposition of 30 cm of sediment.

Sensitivity assessment. Smothering by 30 cm of sediment is likely to cause mortality amongst the majority of characterizing and important species of these biotopes and impact recovery unless the sediment is removed.  However, vertical surfaces may protect a proportion of the population, so that the effects will depend on the topography of the substratum and sediment is likely to be removed rapidly. Resistance at the benchmark has been assessed as ‘Low’.  Resilience is assessed as ‘Medium’ and sensitivity has been assessed as ‘Medium’. 

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

‘No evidence’ was found to assess this pressure.

Underwater noise changes

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

Evidence

Whilst no evidence could be found for the effect of noise or vibrations on the characterizing species of these biotopes, it is unlikely that these species have the facility for detecting or noise vibrations.

Sensitivity assessment. The characterizing sponges are unlikely to respond to noise or vibrations and resistance is, therefore, assessed as ‘High’, resilience as ‘High’ and sensitivity as ‘Not Sensitive’.

Introduction of light or shading

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

Evidence

Jones et al. (2012) compiled a report on the monitoring of sponges around Skomer Island and found that many sponges, particularly encrusting species, preferred vertical or shaded bedrock to open, light surfaces, presumably due to a decrease in competition from algae. Whilst no evidence could be found for the effect of light on the characterizing species of these biotopes, it is unlikely that these species would be impacted.

Gili & Hughes (1995) reviewed the effect of light on a number of hydroids and found that there is a general tendency for most hydroids to be less abundant in well-lit situations.  Whilst hydroid larvae can be positively or negatively photoactic, the planulae of Nemertesia antennina show no response to light. (Hughes, 1977)

In vitro studies of solitary ascidians indicate that both spawning and settlement are controlled by light, however Ciona intestinalis in vivo has been observed to spawn and settle at any time of the day.  (Svane & Havenhand, 1993).

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

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

'No evidence' was found. 

Introduction or spread of invasive non-indigenous species

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

Evidence

This biotope is classified as circalittoral and therefore no algal species have been considered. Crepidula fornicata 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 from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’ it preferred shell, especially that of conspecifics. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1-0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space and many are fast growing and opportunistic, so may out-compete Crepidula for space even if it gained a foothold in the community. 

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 and 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.  Presently Didemnum vexillum is isolated to several sheltered locations in the UK (NBN, 2015), however, Didemnum vexillum has successfully colonized the offshore location of the Georges Bank, USA (Lengyel et al., 2009) which is more exposed than the locations which Didemnum vexillum have colonized in the UK. It is, therefore, possible that Didemnum vexillum could colonize more exposed locations within the UK and could, therefore, pose a threat to these biotopes.

Sensitivity assessment. The circalittoral rock characterizing this biotope could be suitable for colonization by Crepidula fornicata due to the sheltered to extremely sheltered wave conditions, where water is mediated by tidal flow rather than wave actions. The presence of cobble (JNCC, 2015), in some examples of the biotope, could provide suitable hard substrata for the colonization by Crepidula however, there is no evidence to suggest Crepidula would establish. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or infralittoral or circalittoral rock habitats. Didemnum vexillum could pose a potential threat. At present, there is 'Insufficient evidence' to suggest that the circalittoral rock biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; further evidence is required. 

Introduction of microbial pathogens

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

Evidence

Gochfeld et al. (2012) found that diseased sponges hosted significantly different bacterial assemblages compared to healthy sponges, with diseased sponges also exhibiting a significant decline in sponge mass and protein content.  Sponge disease epidemics can have serious long-term effects on sponge populations, especially in long-lived, slow-growing species (Webster, 2007).  Numerous sponge populations have been brought to the brink of extinction including cases in the Caribbean (with 70-95% disappearance of sponge specimens) (Galstoff, 1942) and  the Mediterranean (Vacelet, 1994; Gaino et al., 1992).  Decaying patches and white bacterial film were reported in Haliclona oculata and Halichondria panicea in North Wales, 1988-89 (Webster, 2007).  Specimens of Cliona spp. exhibited blackened damage since 2013 in Skomer. Preliminary results have shown that clean, fouled and blackened Cliona all have very different bacterial communities. The blackened Cliona are effectively dead and have a bacterial community similar to marine sediments. The fouled Cliona have a very distinct bacterial community that may suggest a specific pathogen caused the effect (Burton, pers comm; Preston & Burton, 2015).  Hydroids exhibit astonishing regeneration and rapid recovery from injury (Sparks, 1972) and the only inflammatory response is active phagocytosis (Tokin & Yaricheva, 1959;1961, as cited in Sparks, 1972).  No record of diseases in the characterizing hydroids could be found.  There appears to be little research into ascidian diseases, particularly in the Atlantic.  The parasite Lankesteria ascidiae targets the digestive tubes and can cause ‘long faeces syndrome’ in Ciona intestinalis (although it has also been recorded in other species). Mortality occurs in severely affected individuals within about a week following first symptoms. (Mita et al., 2012).

Sensitivity assessment. Sponge diseases have caused limited mortality in some species in the British Isles, although mass mortality and even extinction have been reported further afield.  However, ‘No evidence’ of mortality due to disease could be found for the important  characterizing species of this biotope.

Removal of target species

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

Evidence

Spongia officinalis (a Mediterranean species) has been targeted as a commercial species for use as bath sponges, although this species does not occur in the British Isles and no record of commercial exploitation of sponges in the British Isles could be found.  Hiscock (2003) stated that the greatest loss of Axinella dissimilis at Lundy might have been due to collecting during scientific studies in the 1970s. No indication of recovery was evident.  Axinella damicornis was harvested in Lough Hyne during the 1980s (for molecular investigations) and the populations were reduced to very low densities, which subsequently recovered very slowly, although they are now considered to be back to their original densities (Bell, 2007).  No evidence of targeted removal of the characterizing species could be found.  Despite historic harvesting of the hydroid Sertularia cupressinain in the Wadden Sea (Wagler et al., 2009), no evidence for the harvesting of the characterizing hydroids could be found and targeted extraction is highly unlikely.  Despite novel proposals to farm Ciona intestinalis as a potential feedstock for aquaculture in Sweden (Laupsa, 2015), it is very unlikely that solitary ascidians, would be targeted for extraction. 

Sensitivity assessment. None of the characterizing species are harvested and targeted removal is, therefore ‘Not relevant’ to this biotope.

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

The characteristic species probably compete for space within the biotope, so that loss of one species would probably have little if any effect on the other members of the community. However, removal of the characteristic epifauna due to by-catch is likely to remove a proportion of the biotope and change the biological character of the biotope. 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.  Whilst a large proportion of the sponge community is likely to be affected by abrasion events, there is some debate as it the level of effects depending on the size of the sponge and the type of abrasion effect (see Coleman et al., 2013).  The majority of the literature agrees that damage would fall within the ‘Low’ bracket of 25-75% reduction.  Ascidians are also likely to be significantly affected, although given their high resilience, they are likely to recover quickly (Bradshaw et al., 2000). 

Sensitivity assessment. Based on the broad agreement of trawl impacts on sponge communities and the likely disturbance to the sessile epifaunal ascidians and hydroids, resistance is recorded as ‘Low’, resilience is recorded as ‘Medium’ and Sensitivity is ‘Medium’.