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

Growth rates were reported to increase with temperature in several bryozoans species, however, zooid size decreased, which may be due to increased metabolic costs at higher temperature (Menon, 1972; Ryland, 1976; Hunter & Hughes, 1994). Temperature is also a critical factor stimulating or inhibiting reproduction in hydroids, most of which have an optimum temperature range for reproduction (Gili & Hughes, 1995). Most of the hydroid and bryozoan species within the biotope are recorded to the north or south of the British Isles and are unlikely to be adversely affected by long-term increases in temperature at the benchmark level, e.g. Hartlaubella gelatinosa has been recorded from southeast Sweden to the Mediterranean (Cornelius, 1995b). Several members of the community may also survive acute temperature change, e.g. in Conopeum reticulum and Electra pilosa upper lethal temperatures of 30-32°C and 25-29°C respectively were reported (Menon, 1972, 1974). Therefore, an increase in temperature at the benchmark level is likely to affect growth and reproduction in the encrusting bryozoan and hydroid species but otherwise have few adverse effects.  Therefore, a resistance of 'High' has been recorded. Resilience is likely to be 'High', and, therefore, the biotope 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

Conopeum reticulum and Electra pilosa were reported to survive below freezing temperatures (Menon, 1972) although colonies are probably more tolerant of low temperatures in winter than summer (see sprecies review for details). Low temperatures may trigger regression or dormancy in hydroids (e.g. Cordylophora caspia). Brault & Bourget (1985) noted that recruitment was delayed until spring on settlement plates deployed in winter. However, all the dominant species within the biotope are boreal or recorded from north of the British Isles. Therefore, although growth and reproduction may be reduced, they are unlikely to be adversely affected by reductions in temperature in British waters.  Therefore, resistance and resilience are likely to be 'High' and the biotope 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

The hydroids and bryozoans present in the biotope are characteristic of brackish waters. The biotope itself is only recorded from 'low' or 'reduced' salinity environments. The dominance of hydroid and bryozoans species in this biotope is probably due to the exclusion of predators and competitors (con-specifics and ascidians) by reduced and variable salinity. An increase in salinity at the benchmark level (i.e from 'reduced' to 'variable' to 'full') is likely to allow more marine species to colonize the biotope and potentially out-compete the hydroid and bryozoan members of the community. A year long increase in salinity will probably result in loss of the community at the marine limit of its range. It may be able to colonize new space at the upper estuarine limit of its range providing suitable hard substrata are present. Therefore, a resistance of 'Low'has been recorded to represent a loss of the extent of the biotope and/or the change in abundance of characteristic species. Resilience is probably 'High'. Therefore, an overall sensitivity of 'Low' is recorded.  

Salinity decrease (local)

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

Evidence

The hydroids and bryozoans present in the biotope are characteristic of brackish waters. Lower limits of tolerance include, for example, 6.2 psu for Hartlaubella gelatinosa, 1 psu for Obelia geniculata, 12 psu for Obelia dichotoma, 21.5 psu for Conopeum reticulum and 13.7 psu in  Einhornia crustulenta (as Electra crustulenta) (Ryland, 1970; Cornelius, 1995b; Hayward & Ryland, 1998). [Please note:Conopeum seurati is considered a truly brackish water species surviving down to 1 psu, and often confused with Conopeum reticulum (Ryland, 1970; Hayward & Ryland, 1998)]. Therefore, they are tolerant of the low and variable salinities experienced within the habitat and probably not sensitive to short-term changes in the salinity for a week. However, a long-term change in salinity from e.g. low to <5psu for a year is likely to adversely affect the community. In the Tamar estuary, Devon, this biotope (IR.LIR.IFaVS.HarCon) is replaced by the IR.LIR.IFaVS.CcasEin at the riverine/estuarine transition where the salinity is always below 20 psu and can drop to 0 psu (Hiscock & Moore, 1986). Therefore, a long-term decrease in salinity is likely to result in loss of the biotope in the upper most parts of the estuary although it may extend its range into reduced salinity waters further down the estuary. Therefore, a resistance of 'None' is suggested. However, resilience is probably 'High', so that sensitivity is '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

Water movement is essential for hydroids and other suspension feeders such as encrusting bryozoans and barnacles, to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Most hydroids have a narrow range of water flow rates for effective feeding, and feeding efficiency decreasing a high water flow rates (Gili & Hughes, 1995). Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). Okamura (1985) noted that encrusting bryozoans live and feed in the boundary layer, so that the water flow rates they experience are much lower than that of the surrounding environment.  In the laboratory, feeding efficiency of Conopeum reticulum colonies was higher at slow water flow (0.01-0.02 m/s) that fast (0.1-0.2 m/s), depending on colony size, with large colonies experiencing less effect (Okamura, 1985). However, Conopeum reticulum has been reported in strong to weak tidal streams (JNCC, 1999). Therefore, it is probably tolerant of a wide range of water flow rates. Similarly, Hartlaubella gelatinosa has been recorded in very strong, moderately strong and weak tidal streams (JNCC, 1999).  Therefore, the characteristic species in the biotope are probably tolerant of changes in the water flow. This biotope is recorded from moderately strong tidal flow (0.5-1.5 m/s), so a change of 0.1-0.2 m/s is probably not significant. Therefore, a resistance of 'High' is recorded to represent potential changes in feeding efficiency, while resilience is probably 'High', and the biotope is probably '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

This biotope is subtidal occurring from 0-5 m. However, its upper extent may be exposed during periods of low water level. Intertidal populations of hydroids, Conopeum reticulum, and Electra sp. are restricted to damp habitats such as underboulders, overhangs or the interstices between macroalgae. The branched growth form of hydroids is likely to retain water on emersion (e.g. see Cordylophora caspia).  The biotope is found on a mixture of hard substrata, included cobbles and small boulder, so that damp areas probably exist to protect parts of the population form dessication.  A decrease in emergence may allow the biotope to extent its range if suitable habitat exists. 

Nevertheless, hydroids and bryozoans are likely to be adversely affected by desiccation as a result of increase in emergence at the benchmark level and the upper limit of the population may be removed.  Therefore a resistance of 'Low'  has been recorded to represent the potential loss of a proportion of the biotope. Resilience is probably 'High' so that the biotope has a 'Low' sensitivity to this pressure. 

Wave exposure changes (local)

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

Evidence

The biotope occurs in very wave sheltered situations, although storms may create significant oscillatory water movement in shallow depths. The oscillatory water flow caused by wave action is potentially more damaging to delicate marine organisms than unidirectional flow. The encrusting bryozoans are tolerant of a wide range of wave exposures (e.g. see Electra pilosa review). Wave exposed conditions tend to favour small, less branched species of hydroid than are found in this biotope (Boero, 1984; Gili & Hughes, 1995). Hartlaubella gelatinosa has only been recorded from wave sheltered conditions. The most likely adverse effect of wave action is the displacement of hard substrata (e.g. small rocks, cobbles or pebbles) and attached organisms. The resultant movement of the substratum and sediment scour may remove attached hydrorhizae and even resting stages of hydroids but many are likely to survive. Therefore, it is likely than an small increase in wave exposure at the benchmark level  is likely to result in loss or damage of the hydroid and bryozoan colonies. Therefore, a resistance of 'Medium' is suggested, so that with a resilience of 'High', sensitivity to this pressure is probably 'Low'. 

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

Various heavy metals have been show to have sublethal effects on growth in the few hydroids studied experimentally (Stebbing, 1981; Bryan, 1984; Ringelband, 2001). Bryozoans are common members of the fouling community and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints, and bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1977; Holt et al., 1995). 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. Rees et al. (2001) reported that the abundance of epifauna had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that several species of epifauna may be at least inhibited by the presence of TBT. However, Hartlaubella gelatinosa has been recorded in the Crouch estuary where TBT were very high, and was recorded under boat moorings at Cargreen in the river Tamar, and may be relatively tolerant of TBT (Keith Hiscock pers. comm.). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoans with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100 ng/l TBT.

Howethis pressure is Not assessed.

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.

Although subtidal, this biotope is relatively shallow and may be exposed to oils and hydrocarbons adsorbed onto particulates and ingested or through the water soluble fractions of oils and hydrocarbons. Species of the encrusting bryozoan Membranipora and the erect bryozoan Bugula were reported to be lost or excluded from areas subject to oil spills (Mohammad, 1974; Soule & Soule, 1979). Houghton et al. (1996) reported a reduction in the abundance of intertidal encrusting bryozoans (no species given) at oiled sites after the Exxon Valdez oil spill. Encrusting bryozoans are also probably intolerant of the smothering effects of oil pollution, resulting in suffocation of colonies. The water soluble fractions of Monterey crude oil and drilling muds were reported to cause polyp shedding and other sublethal effects in the athecate Tubularia crocea in laboratory tests (Michel & Case, 1984; Michel et al., 1986; Holt et al., 1995). However, 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). Calder (1976) suggested that hydroids found in the low salinity areas of south Carolina, such as Cordylophora caspia, were also present in relatively polluted waters, such as Charleston Harbour. Loss of the dominant, abundant species Conopeum reticulum and Electra spp. from the biotope would result in significant change in the community and perhaps loss of the biotope as described.

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.

Chemical contaminants are likely to affect different species inthe biotope to varying degrees, depending on the nature of the contaminant and its concentration.

• Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth.
• 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.
• Rees et al. (2001) reported that the abundance of epifauna had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that several species of epifauna may be at least inhibited by the presence of TBT.
• However, Hartlaubella gelatinosa has been recorded in the Crouch estuary where TBT were very high, and was recorded under boat moorings at Cargreen in the river Tamar, and may be relatively tolerant of TBT (Keith Hiscock pers. comm.).
• Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1977; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoans with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100 ng/l TBT.
• Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey.

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). Bryozoans are probably intolerant of chemical pollution.

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

Sagasti et al. (2000) reported that epifauna communities, including dominant species such as the bryozoans Conopeum tenuissimum and Membranipora tenuis, and the hydroid Obelia bicuspidata were unaffected by periods of moderate hypoxia (ca 0.35 -1.4 ml/l) and short periods of hypoxia (<0.35 ml/l) in the York River, Chesapeake Bay. Although the exact species examined differ, their study suggests that estuarine epifaunal communities are relatively tolerant of hypoxia.  Therefore, a resistance of 'High' is suggested.  As resilience is 'High' the biotope is recorded as 'Not sensitive' at the benchmark level. 

Nutrient enrichment

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

Evidence

Estuarine habitats are generally higher in nutrient levels than coastal waters. A moderate increase in nutrients may increase food availability for suspension feeders, in the form of organic particulates. Eutrophication may result in local hypoxic conditions (see below) and /or blooms of ephemeral algae. However, in this turbid environment, ephemeral algae are likely to be limited to the very shallow water near the top of the shore, and unlikely to adversely affect the biotope. Therefore, 'Not sensitive' has been recorded based on expert judgement. 

Organic enrichment

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

Evidence

Little direct evidence was found.  Hartlaubella gelatinosa was recorded from both reference and disposal sites examined in the Weser estuary (Witt et al., 2004).  The disposal sample sites had consistently higher silt and organic content compared to reference sites (dominated by find sand) yet there was no significant difference in the abundance of Hartlaubella gelatinosa between reference and disposal sites, while Obelia sp. was more abundant at disposal sites (Witt et al., 2004).  This evidence suggests that Hartlaubella gelatinosa is either resistant of both silt deposition and a high organic content or can recover quickly from disturbance by regrowth or rapid colonization.  Nevertheless, Conopeum reticulum was not recorded.  Therefore, it suggests that a proportion of the community in the biotope is resistant of organic enrichment and siltation, while other members of the community are probalby adversely affected. Therefore, a resistance of 'Medium' is suggested. However, as resilience is likely to be 'High', sensitivity is, therefore, 'Low'.  

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

All members of this biotope require hard substratum for attachment, ranging from bedrock and cobbles, to shell, artificial substrata and plants. Therefore a permanent change from a hard rock (or similar substratum) to soft (sedimentary) substratum would result in loss of the biotope.  Members of the biotope may survive on scattered shell and stones on a sedimentary substratum but the biotope would not remain.  Therefore, a resistance of 'None' is recorded. As the change in permanent, the resilience is 'Very low' (by definition) and sensitivity is 'High'. 

Physical change (to another sediment type)

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

Evidence

All members of this biotope require hard substratum for attachment, ranging from bedrock and cobbles, to shell, artificial substrata and plants. Where the biotope occurs on mixed substrata, the characterizing species occur on stones, shell and debris. Therefore, a change in one Folk class from mixed to sand or gravel dominated sediment that resulted in removal of stones or other suitable hard substrata, would result in loss of the biotope.  A change to fine sediment may also increase localised scour resutling in loss of the part of the biotope.  Therefore, a resistance of 'Low' is recorded. As the change in permanent, the resilience is 'Very low' (by definition) and sensitivity is 'High'. 

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

Where the biotope occured on bedrock or other hard substrata, extraction is unlikely, and the pressure is not relevant. However, where the biotopes occurs on stones, shell and other hard substrata on mixed sediment then extraction (to 30 cm) would probably remove the biotope completely within the affected area. The biotope would be lost and resistance is 'None'.  Resilience is probably 'High' so that sensitivity is 'Medium'. 

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

Abrasion by an anchor or fishing gear is likely to remove relatively delicate uprights of hydroids, and damage bryozoan colonies, and erect epifauna are considered particularly sensitive to physical abrasion. 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 is incremental with damage increasing with 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. Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Erect epifauna can be directly removed and brought to the surface in trawl hauls. De Groot (1984), for example, found that beam trawls with or without tickler chains removed the hydrozoan Tubularia spp. (mostly Tubularia indivisa). He suggested that nearly all individuals in the path of a beam trawl would be destroyed. This study was based on observations of species caught as by-catch and did not assess in-situ damage rates. 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. Other population level effects have also been recorded. The scallop fishery has been implicated for altering genetic diversity within Sertularia cupressina populations on commercial scallop grounds in Atlantic Canada where increased damage rates have increased clonality from injury-induced fragmentation (Henry & Kenchington 2004). This means that genetic diversity in fished areas is lower than unfished areas.

No specific information was available to assess the resistance of the characteristic species within this biotope. But erect epifauna exposed to abrasion could displace, damage and remove individuals (De Groot 1984; Veale et al., 2001; Boulcott & Howell, 2011). Colonies of hydroids and bryozoans attached to mobile substrata may suffer damage during displacement of stones and small boulders but are likely to survive. However, removal of a bryozoan of hydroid colony from its substratum would probably be fatal, and encrusting bryozoans are not known to be able to reattach. Fragmentation is thought to be a possible mode of asexual reproduction in hydroids (Gili & Hughes, 1995) and it is possible that a proportion of displaced hydroid fragments may attach to new substrata, enhancing recovery. However, the encrusting bryozoans, hydroids and barnacles are likely to be lost and an intolerance of high has been recorded.  Although, trawling activities may be unlikely where this biotope occurs, anchoring and dredging activities may have simialr effects.  Overall, the biotope is likely to have a 'Low' resistance to this pressure. However, resilience is likely to be 'High', so that sensitivity is probably 'Low. 

Penetration or disturbance of the substratum subsurface

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

Evidence

Subsurface penetrative activities are not relevant where this biotope occurs on bedrock and other hard substratum.  Where the biotope occurs on small boulders, cobbles and stones on mixed substrata, then sub-surface penetrative activities may physically displace or remove the biotope (see abrasion above). However, where suitable substrata remain, the biotope is highly resilient. Therefore, a resistance of 'Low' is suggested, with 'High' resilience and hence 'Low' sensitivity. 

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

The biotope occurs in high turbidity estuarine waters.  Estuarine waters can exhibit high turbidities, measured in grammes per litre rather than milligrammes per litre and probably exceed the lower limit of 'very turbid' (>300 mg/l) on the UKTAG scale (see benchmark).  Therefore, an increase in turbidity is unlikely. If a decrease is turbidity was to occur, then conditions may favour macroalgae, which would compete for space with the dominant epifauna and potentially grow over the epifauna.  However, records of the biotope suggest that it contains bare areas and the hydroid and bryozoan epifauna can probably tolerate being grown over, so that the effects are limited. If conditions return to prior norm aftera year, then recovery will be rapid. Therefore, a resistance of 'Medium' is suggested to represent changes in the abundance epifauna, but with a resilience of 'High', sensitivity is 'Low'. 

Smothering and siltation rate changes (light)

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

Evidence

No direct evidence was found to assess the impact of this pressure at the pressure benchmark. The characteristic species are attached to the substratum and are usually shorter than 30 cm (e.g. Obelia dicotoma can form large, erect and loosely fan-shaped or elongate up to 35 cm in height, or short and either bushy or unbranched colonies up to 5 cm in height in British waters (see review), and Hartlaubella gelatinosa can grow up to 20 cm in height (see review). Encrusting bryozoans are easily buried.  Siltation by fine sediments would also prevent larval settlement by the characteristic species which require hard substratum (Berghahn & Offermann,  1999). The intensity and duration of siltation will be mediated by site-specific hydrodynamic conditions, such as water-flow and wave action that determine the dispersal of deposits.

In general it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea has 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. However, Hartlaubella gelatinosa was recorded from both reference and disposal sites examined in the Weser estuary (Witt et al., 2004).  The disposal sample sites had consistently higher silt and organic content compared to reference sites (dominated by find sand) yet there was no significant difference in the abundance of Hartlaubella gelatinosa between reference and disposal sites, while Obelia sp. was more abundant at disposal sites (Witt et al., 2004).  This evidence suggests that Hartlaubella gelatinosa is either resistant of both silt deposition and a high organic content or can recover quickly from disturbance by regrowth or rapid colonization.  

Smothering by 5 cm of sediment (see benchmark) is likely to prevent feeding and hence reduce growth and reproduction in encrusting bryozoans. The hydroid hydranths are relatively large and some parts of the colony are likely to protrude above 5 cm of sediment. However reduced feeding, together with local hypoxic conditions under the sediment layer will probably reduce growth and reproduction rates. In addition, associated sediment abrasion may remove the bryozoan colonies.  Therefore, a biotope resistance of ' Medium' has been recorded, as a proportion of the community may be adversely affected. The resilience is probably 'High'  so that the sensitivity is likely to be 'Low'. 

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

No direct evidence was found to assess the impact of this pressure at the pressure benchmark. The characteristic species are attached to the substratum and are usually shorter than 30 cm (e.g. Obelia dicotoma can form large, erect and loosely fan-shaped or elongate up to 35 cm in height, or short and either bushy or unbranched colonies up to 5 cm in height in British waters (see review), and Hartlaubella gelatinosa can grow up to 20 cm in height (see review). Encrusting bryozoans are easily buried.  Siltation by fine sediments would also prevent larval settlement by the characteristic species which require hard substratum (Berghahn & Offermann,  1999). The intensity and duration of siltation will be mediated by site-specific hydrodynamic conditions, such as water-flow and wave action that determine the dispersal of deposits.

In general it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea has 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. However, Hartlaubella gelatinosa was recorded from both reference and disposal sites examined in the Weser estuary (Witt et al., 2004).  The disposal sample sites had consistently higher silt and organic content compared to reference sites (dominated by find sand) yet there was no significant difference in the abundance of Hartlaubella gelatinosa between reference and disposal sites, while Obelia sp. was more abundant at disposal sites (Witt et al., 2004).  This evidence suggests that Hartlaubella gelatinosa is either resistant of both silt deposition and a high organic content or can recover quickly from disturbance by regrowth or rapid colonization.  

Smothering by 30 cm of sediment (see benchmark) is likely smother encrusting bryozoans. The hydroid hydranths are relatively large and a limited number may protrude above even 30 cm of sediment. However reduced feeding, together with local hypoxic conditions under the sediment layer will probably reduce growth and reproduction rates. In the moderately strong tidal  flow the sediment may remain for some time, so that the hydroid uprights may die back. In addition, associated sediment abrasion may remove the bryozoan colonies. Therefore, while Hartlaubella gelatinosa and Obelia sp.may be tolerant, other members of the community are likely to be removed, so a resistance of 'Low' is suggested. As resilience is probably 'High, sensitivity is likely to be 'Low'. 

Litter

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

Evidence

Not assessed

Electromagnetic changes

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

Evidence

No evidence

Underwater noise changes

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

Evidence

Not relevant to the charaterizing species of this biotope. 

Introduction of light or shading

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

Evidence

This is a epifaunal dominated biotope with any macroalgae present restricted to shallow water due to the high turbidity conditions in which the biotope occurs.  Artificial light sources are unlikely to affect light availability for algae as the turbidity is the may reason for poor light penetration. Shading may restrict macroalgal growth further, but is unlikely to adversely affect the biotope.  Therefore a resistance of 'High' is recorded, with a resilience of 'High', so that the biotope is probably '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

Barrier to water flow may restrict larval recruitment for this biotope.   The hydroid or colonies are probably not dependent on external recruitment for it continued survival, except if severely damaged. In addition, members of this biotope are typical of fouling communities and noted for high rates of recovery and recruitment. Therefore, only complete cessation of water flow and water transport is likely to adversely affect recruitment of the dominant characteristic species. Therefore, this pressure is 'Not relevant' to this biotope. 

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

None of the charateristic species within this biotope have any know visual acuity, and while they may react to shading of the polyps themselves, this presure is '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

None of the characteristic species are known to be 'translocated', subject to breeding programmes, or liable to hybridize with other species. Therefore this pressure is 'Not relevant' to this biotope. 

Introduction or spread of invasive non-indigenous species

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

Evidence

This epifaunal dominated biotope could be threatened by other more aggresive epifauna,e.g. Didemnum vexillum or Perophora japonica.  However, the latter species have not been reported from the upper and mid Tamar estuary where the biotope is found. No evidence of impact was found, although the evidence may need to be revisited in the future. 

Introduction of microbial pathogens

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

Evidence

No evidence found

Removal of target species

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

Evidence

It is extremely unlikely that any of the species indicative of sensitivity would be targeted and we have no evidence for the indirect effects of removal of other species on this biotope. Therefore, this pressure is not relevant. 

Removal of non-target species

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

Evidence

It is extremely unlikely that any of the species indicative of sensitivity would be targeted and we have no evidence for the indirect effects of removal of other species on this biotope. Therefore, this pressure is not relevant.