Biodiversity & Conservation

CR.C.FaV.Bug

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum will result in removal of all the sessile species, together with most of the slow mobile species (crustaceans, sea urchins and starfish) and an intolerance of high has been recorded.
Recoverability will depend on recruitment from neighbouring communities and subsequent recovery of the original abundance of species, which may take many years, especially in slow growing sponges and Anthozoa. Therefore, a recoverability of high has been recorded (see additional information below).
Smothering
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Although, overhangs and vertical surfaces are unlikely to suffer from smothering, this biotope occurs on steep slopes and the sides of boulders, which may collect sediment. Smothering by 5cm of sediment will prevent feeding and reduce growth and reproduction, interfere with respiration and potentially cause localised anoxia, and interfere with larval settlement. Tall erect species, e.g. Nemertesia antennina or large Alcyonium digitatum may escape the smothering due to their size, while some hydroids may survive as dormant stages. However, the dominant Bugula species, encrusting sponge species and ascidians are likely to be damaged or killed by smothering. Therefore, an intolerance of high has been recorded. A recoverability of high has been suggested (see additional information below).
Increase in suspended sediment
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Suspension feeding organisms may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus. Bryozoan turfs form preferentially on steep surfaces and under overhangs and larvae preferentially settle under overhangs, presumably to avoid smothering and siltation (Ryland, 1977; Hartnoll, 1983). Wendt (1998) noted that Bugula neritina grew faster on downward facing surfaces than upward facing surfaces, presumably due to siltation and reduced feeding efficiency on upward facing surfaces. But where water flow is sufficient to prevent siltation, Bugula turbinata may colonize upward facing surfaces (Hiscock & Mitchell, 1980). Large massive sponges tend to favour fast flowing waters that are free of silt while encrusting species can tolerate more turbid conditions, (e.g. Halichondria panicea), although the response to suspended sediment loads varies with species (Morton & Miller, 1968; Moore, 1977). The tolerance of ascidians to suspended sediment varies with species, e.g. Clavelina lepadiformis and Morchellium argus are probably relatively tolerant (see species reviews) whereas Aplidium pallidum and Botrylloides leachi may be more intolerant.

This biotope occurs in moderately wave exposed sites with moderately strong to weak tidal streams. Where water flow is adequate to prevent excessive siltation, an increase in suspended sediment at the benchmark level is likely to reduce feeding efficiency and hence growth and reproduction, which may be important for species with several generations per year (e.g. Bugula spp.). The biotope also occurs extensively in areas subject to high levels of suspended sediment, e.g. North Devon (Keith Hiscock pers comm.). However, in areas of weak water flow and increased depth (reduced effects of wave action), an increase in suspended sediment increase siltation to the detriment of several members of the community, especially the bryozoans. Therefore an intolerance of low has been recorded for most examples of the biotope. Recovery is likely to be rapid (see additional information below).

Decrease in suspended sediment
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A decrease in suspended sediment may decrease food availability for the duration of the benchmark (one month) but otherwise not adversely affect the biotope. Therefore, an intolerance of low has been recorded.
Desiccation
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Bryozoans, sponges, soft corals, and hydroids are probably highly intolerant of desiccation. However, this biotope is circalittoral, occurring below 5-10m depth and possibly to great depths and unlikely to be exposed to the air and desiccation.
Increase in emergence regime
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An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action and tidal streams may be increased (see water flow rate below).
Decrease in emergence regime
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An increase or decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action and tidal streams may be increased (see water flow rate below).
Increase in water flow rate
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The abundance of bryozoans is positively correlated with supply of stable hard substrata and hence with current strength (Eggleston, 1972b; Ryland, 1976). The community stability and diversity also requires stable substrata (Osman, 1977; Dyrynda, 1994). Water movement is essential for suspension feeders such as hydroids, bryozoans, sponges, amphipods and ascidians to supply adequate food, remove metabolic waste products, prevent accumulation of sediment (siltation) and disperse larvae or medusae. Most hydroids utilize a narrow range of water flow rates for effective feeding, and feeding efficiency decreases at high water flow rates (Gili & Hughes, 1995). Similarly, water flow rates affect filter feeding efficiency in bryozoans, the preferred ranges depending on species, e.g. Okamura (1984) reported that an increase in water flow from slow flow (1-2cm/s) to fast flow (10-12cm/s) reduced feeding efficiency in small colonies but not in large colonies of Bugula stolonifera.
This biotope occurs in moderately strong to weak tidal streams and moderate wave exposure. The oscillatory water movement caused by wave action is probably of greater importance in sites with only weak tidal streams. However, an increase in water flow from moderately strong to very strong (see benchmark) is likely to adversely affect some members of the community due to drag. For example, species tolerant of strong water flow, e.g. Tubularia indivisa, Halichondria panicea, Actinothoe sphyrodeta, Alcyonium digitatum and Flustra foliacea may increase in abundance, while species that are less tolerant of strong water flow, e.g. Nemertesia spp., Caryophyllia smithii and Ascidia mentula may be excluded (see Hiscock, 1983). A proportion of the Bugula colonies may also be damaged or removed by very strong currents. In addition, very strong water flow may interfere with larval settlement and recruitment. Therefore, an intolerance of intermediate has been recorded. Loss of several intolerant species is likely to reduce species richness. Recovery is likely to be rapid (see additional information below).
Decrease in water flow rate
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Water movement is essential for suspension feeders such as hydroids, bryozoans, sponges, amphipods and ascidians to supply adequate food, remove metabolic waste products, prevent accumulation of sediment (siltation) and disperse larvae or medusae. A decrease in water flow rates in the proximity of sediment is likely to result in greater siltation (see above). Most hydroids utilize a narrow range of water flow rates for effective feeding, and feeding efficiency decreasing a high water flow rates (Gili & Hughes, 1995). Similarly, water flow rates affect filter feeding efficiency in bryozoans, the exact preferred ranges depending on species, e.g. Okamura (1984) reported that an increase in water flow from slow flow (1-2cm/s) to fast flow (10-12cm/s) reduced feeding efficiency in small colonies but not in large colonies of Bugula stolonifera.
A decrease in water flow, e.g. from moderately strong to very weak or negligible will probably result in impaired growth and reproduction of suspension feeders due to a reduction in food availability, an increased risk of siltation (see above) and encourage colonization by other species of hydroids, ascidians, sponges and anemones, resulting in significant changes in the community and possibly the loss of the dominant hydroid/ bryozoans turf. For example, species of Bugula, Tubularia indivisa, Actinothoe sphyrodeta may be lost, while the remaining species may increase in abundance. Therefore, an intolerance of high has been recorded as the biotope is likely to change. Recoverability is likely to be rapid as many of the characteristic species would remain in reduced abundance and could re-colonize rapidly (see additional information below).
Increase in temperature
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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. For example, Bugula turbinata is a predominantly southern species in British water (Lewis, 1964; Hayward & Ryland, 1998) but has been recorded as far north as Shetland. A long term increase in temperature may increase its abundance in northern British waters and allow the species to extend its range. Similarly, the sponges Clathrina coriacea occurs from the Arctic to South Africa, Pachymatisma johnstonia occurs south to Spain, while Haliclona oculata is widespread. Asterias rubens and Echinus esculentus are probably intolerance of short term increases in temperature at the benchmark level.

Growth rates were reported to increase with temperature in several bryozoans species but zooid size decreased, possibly 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). Therefore, an intolerance of low has been recorded to represent the effects of temperature on growth and reproduction in many species.

Decrease in temperature
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Temperature is 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. However, Bugula turbinata and Bugula plumosa are predominantly southern species extending in range to the Mediterranean (Lewis, 1964; Hayward & Ryland, 1998). A long term decrease in temperature may reduce their extent in British waters, probably by interfering with growth and reproduction. They will probably be replaced by more northern species (perhaps Bugula flabellata or Bugula purpurotincta) and therefore not change the biotope. Therefore, an intolerance of low has been recorded to represent the effects of temperature on growth and reproduction in many species and changes in species composition.
Increase in turbidity
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An increase in turbidity is likely to result in a decrease in phytoplankton and macroalgal primary production, which may reduce food available to the suspension feeders within the community. As a result, growth rates and reproduction may be decreased, and some species may not be able to keep up with predation (e.g. see Gaulin et al., 1986). Similarly, rapid growing bryozoans with multiple generation per year (e.g. Bugula species and Bicellaria ciliata) probably have relatively high food demands and their growth and reproduction may be impaired, as would their ability to cope with predation. Therefore, increased turbidity may result in loss of condition and reduced growth rates but no mortality, so an intolerance of low has been recorded. Recovery is likely to be rapid (see additional information below).
Decrease in turbidity
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An decrease in turbidity may increase phytoplankton and hence zooplankton productivity and potentially increase food availability. Increased light penetration may allow macroalgae to colonize deeper water. Macroalgae effectively compete for space and grow over and may smother fauna. Hydroid and bryozoan communities in the infralittoral tend to occupy steep or vertical surfaces, while macroalgae dominate horizontal, flat surfaces (Hartnoll, 1983, 1998). Therefore, decreased turbidity may allow macroalgae to colonize the more shallow examples of this biotope, resulting in loss of a proportion of the biotope, although some members of the community are likely to survive even in the presence of macroalgae. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).
Increase in wave exposure
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This biotope occurs in moderately wave exposed habitats. The oscillatory flow generated by wave action is potentially more damaging than unidirectional flow but is attenuated with depth (Hiscock, 1983). Many of the species in the biotope are likely to tolerate an increase in wave exposure from moderately exposed to very exposed, for example, Alcyonium digitatum, Bugula species, the sponges Halichondria panicea and Esperiopsis fucorum, and the hydroids Tubularia indivisa Sertularia argentea. However, less flexible or weaker hydroids and bryozoans may be removed, e.g. Nemertesia antennina and Nemertesia ramosa.
Increased wave action at the benchmark level is likely to decrease sea urchin and starfish predation, allowing larger, massive species (e.g. sponges, Alcyonium digitatum, anemones and ascidians) increase in dominance, becoming a different successional community (see Sebens, 1985). For example, the similar biotope £IR.AlcByH£ occurs in wave exposed conditions in the infralittoral (less deep) and is dominated by Alcyonium digitatum.
Therefore, it is likely that some species within the biotope, especially delicate hydroids may be lost, and the community structure change in favour of more massive species, and hence fewer bryozoans. Therefore, a proportion of the biotope is likely to be lost or changed and an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).
Decrease in wave exposure
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The moderately strong currents and tidal streams in this biotope are probably more important for water movement than wave induced oscillatory flow. Where the tidal streams are weak, the biotope is likely to be more intolerant. A decrease in wave action from e.g. moderately exposed to very sheltered may allow more delicate species, such as Nemertesia ramosa, ascidians and sponges to increase in abundance. However, species adapted to strong water movement may be reduced in abundance or lost e.g. Tubularia indivisa, Flustra foliacea, Pachymatisma johnstonia and Actinothoe sphyrodeta. However, species more tolerant of sheltered conditions may increase in abundance e.g. Caryophyllia smithii (see Hiscock, 1983). Reduced wave action may also result in an increase in sea urchin predation and hence increased patchiness and species richness (Sebens, 1985; Hartnoll, 1998).
Overall, the dominant bryozoans in this biotope will probably not be adversely affected by a decrease in wave exposure in moderately strong currents but examples of the biotope in weak currents are likely to be more intolerant due to the overall decrease in water movement (see water flow). In addition, the decreased wave action is likely to result in an increase in starfish and sea urchin predation. Therefore, an intolerance of high has been recorded. Recovery of the full community may take many years (see additional information below).
Noise
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Hydroids, bryozoans, sponges and ascidians are unlikely to be sensitive to noise or vibration at the benchmark level. Mobile fish species may be temporarily scared away from the areas but few if any adverse effects on the biotope are likely to result.
Visual Presence
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Hydroid and bryozoan polyps or barnacle cirri may retract when shaded by potential predators, however the community is unlikely to be affected by visual presence. Mobile fish species may be temporarily scared away from the areas but few if any adverse effects on the biotope are likely to result.
Abrasion & physical disturbance
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Erect epifaunal species are particularly vulnerable to physical disturbance. Hydroids and bryozoans are likely to be removed or damaged by bottom trawling or dredging (Holt et al., 1995). Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Hydroid and bryozoan matrices were reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998 and references therein). The removal of rocks or boulders to which species are attached by the passage of mobile fishing gears (Bullimore, 1985; Jennings & Kaiser, 1998) results in substratum loss (see above). Magorrian & Service (1998) reported that queen scallop trawling removed emergent epifauna from horse mussel beds in Strangford Lough. They suggested that the emergent epifauna such as Alcyonium digitatum were more sensitive than the horse mussels themselves and reflected early signs of damage. However, Alcyonium digitatum is more abundant on high fishing effort grounds suggests that this seemingly fragile species is more resistant to abrasive disturbance than might be assumed (Bradshaw et al., 2000), presumably owing to good recovery due to its ability to replace senescent cells, regenerate of damaged tissue and early larval colonization of available substrata. Species with fragile tests that occur in the biotope such as Echinus esculentus and the brittlestar Ophiocomina nigra and edible crabs Cancer pagurus were reported to suffer badly from the impact of a passing scallop dredge (Bradshaw et al., 2000). Scavengers such as Asterias rubens and Buccinum undatum were reported to be fairly robust to encounters with trawls (Kaiser & Spencer, 1995) may benefit in the short term, feeding on species damaged or killed by passing dredges. However, Veale et al. (2000) did not detect any net benefit at the population level.

Overall, physical disturbance by mobile fishing gear is likely to remove a proportion of all groups within the community and attract scavengers to the community in the short term. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high due to repair and regrowth of hydroids and bryozoans and recruitment within the community from surviving colonies and individuals (see additional information below). Severe physical disturbance will be similar in effect to substratum loss (see above).

Displacement
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Most sessile species, such as bryozoans (e.g. Bugula species), sponges (e.g. Halichondria panicea), ascidians (e.g. Clavelina lepadiformis) and hydroids (e.g. Nemertesia species) can not reattach to the substratum if removed, and may be damaged or destroyed in the process. Hydroids and sponges may be able to grow and reattach from fragments, aiding recovery. Mobile species, such as amphipods, gastropods, small crustaceans, crabs and fish are likely to survive displacement. Anemones (e.g. Actinothoe sphyrodeta) are strongly but not permanently attached and will probably reattach to suitable substrata. However, the dominant bryozoans and hydroids are likely to be lost and an intolerance of high has been recorded. Recovery of the full community is likely to take many years and a recoverability of high has been recorded (see additional information below).

Chemical Factors

Synthetic compound contamination
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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, 1979; Holt et al., 1995). But Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Crisia spp. and Cellaria sp. did not occur less than 600m from the effluent source and noted that Bugula flabellata did not occur within the bay. Moran & Grant (1993) reported that settlement of marine fouling species, including Bugula neritina was significantly reduced in Port Kembla Harbour, Australia, exposed to high levels of cyanide, ammonia and phenolics.

The species richness of hydroid communities decreases with increasing pollution (Boero, 1984; Gili & Hughes, 1995). However, Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth.

Alcyonium digitatum at a depth of 16m in the locality of Sennen Cove (Pedu-men-du, Cornwall) died resulting from the offshore spread and toxic effect of detergents e.g. BP 1002 sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill (Smith, 1968). Possible sub-lethal effects of exposure to synthetic chemicals, may result in a change in morphology, growth rate or disruption of reproductive cycle. Smith (1968) also noted that large numbers of dead Echinus esculentus were found between 5.5 and 14.5 m in the vicinity of Sennen, presumably due to a combination of wave exposure and heavy spraying of dispersants in that area (Smith, 1968). Smith (1968) also demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus.

Tri-butyl tin (TBT) has a marked effect on numerous marine organisms (Bryan & Gibbs, 1991). The encrusting bryozoan Schizoporella errata suffered 50% mortality when exposed for 63 days to 100ng/l TBT. 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. Copepod and mysid crustaceans were particularly intolerant of TBT while crabs were more resistant (Bryan & Gibbs, 1991), although recent evidence suggests some endocrine disruption in crabs. The adverse effect of TBT on reproduction in Nucella lapillus and other neogastropods is well known (see review), and similar effects on reproduction may occur in other gastropod molluscs, including nudibranchs. 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. Rees et al. (2001) suggested that TBT inhibited settlement in ascidian larvae. This report suggests that epifaunal species (including, bryozoan, hydroids and ascidians) may be at least inhibited by the presence of TBT.

Therefore, hydroids crustaceans, gastropods, and ascidians are probably intolerant of TBT contamination while bryozoans are probably intolerant of chemical pollution and an intolerance of high has been recorded, albeit at low confidence. A recoverability of high has been recorded (see additional information below).
Heavy metal contamination
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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. Bryozoans were also shown to bioaccumulate heavy metals to a certain extent (Soule & Soule, 1979; Holt et al., 1995). However, Bugula neritina was reported to survive but not grow exposed to ionic Cu concentrations of 0.2-0.3 ppm (larvae died above 0.3ppm) but die where the surface leaching rate of Cu exceeded 10µg Cu/cm²/day (Ryland, 1967; Soule & Soule, 1979). Ryland (1967) also noted that Bugula neritina was less intolerant of Hg than Cu. Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton and their tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez 1999). Waters containing 25 µg / l Cu caused developmental disturbances in Echinus esculentus (Kinne, 1984) and heavy metals caused reproductive anomalies in the starfish Asterias rubens (Besten, et al., 1989, 1991). Sea urchin larvae have been used in toxicity testing and as a sensitive assay for water quality (reviewed by Dinnel et al. 1988), so that echinoderms are probably intolerant of a heavy metal contamination. Gastropod molluscs have been reported to relatively tolerant of heavy metals while a wide range of sublethal and lethal effects have been observed in larval and adult crustaceans (Bryan, 1984).
Overall, the dominant bryozoans may be tolerant and hydroids manifest only sublethal effects. The sea urchin Echinus esculentus is probably highly intolerant of heavy metal contamination. Heavy metals contamination may, therefore, reduce reproduction and recruitment in starfish and sea urchins, potentially reducing predation pressure in the biotope. Therefore, an intolerance of low has been recorded to represent the sublethal effects on dominant bryozoans and hydroids. Loss of predatory sea urchins, may result in an increased dominance by some species and a slight decrease in species richness.
Hydrocarbon contamination
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Circalittoral communities are likely to be protected from the direct effects of oil spills by their depth. However, the biotope may be exposed to emulsified oil treated with dispersants, especially in areas of turbulence, or may be exposed to water soluble fractions of oils, PAHs or oil adsorbed onto particulates. For example:
  • 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). Soule & Soule (1979) reported that Bugula neritina was lost from breakwater rocks in the vicinity of the December 1976 Bunker C oil spill in Los Angeles Harbour, and had not recovered within a year. However, Bugula neritina had returned to a nearby area within 5 months (May 1977) even though the area was still affected by sheens of oil. Houghton et al. (1996) also reported a reduction in the abundance of intertidal encrusting bryozoans (no species given) at oiled sites after the Exxon Valdez oil spill.
  • The water soluble fractions of Monterey crude oil and drilling muds were reported to cause polyp shedding and other sublethal effects in the athecate hydroid Tubularia crocea in laboratory tests (Michel & Case, 1984; Michel et al., 1986; Holt et al., 1995).
  • Laboratory studies of the effects of oil and dispersants on several red algal species concluded that they were all sensitive to oil/dispersant mixtures, with little difference between adults, sporelings, diploid or haploid life stages (O'Brien & Dixon, 1976; Grandy, 1984, cited in Holt et al., 1995).
  • Suchanek (1993) reported that the anemones Anthopleura spp. and Actinia spp. survived in waters exposed to spills and chronic inputs of oils. Similarly, one month after the Torrey Canyon oil spill, the dahlia anemone, Urticina felina, was found to be one of the most resistant animals on the shore, being commonly found alive in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968).
  • Amphipods, especially ampeliscid amphipods, are regarded as especially sensitive to oil (Suchanek, 1993).
  • Smith (1968) reported dead colonies of Alcyonium digitatum at depth in the locality of Sennen Cove (Pedu-men-du, Cornwall) resulting from the combination of wave exposure and heavy spraying of dispersants sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill (see synthetic chemicals).
  • Crude oil from the Torrey Canyon and the detergent used to disperse it caused mass mortalities of echinoderms; Asterias rubens, Echinocardium cordatum, Psammechinus miliaris, Echinus esculentus, Marthasterias glacialis and Acrocnida brachiata (Smith, 1968).
  • Halichondria panicea survived in areas affected by the Torrey Canyon oil spill, although few observations were made (Smith 1968).
If the physiology within different animals groups can be assumed to be similar, then bryozoans, amphipods, echinoderms and soft corals may be highly intolerant of hydrocarbon contamination, while hydroids may demonstrate sublethal effects and anemones and some species of sponge are relatively tolerant. Therefore, the dominant bryozoans and several members of the community may be lost or damaged as a result of acute hydrocarbon contamination, and an intolerant of high has been suggested, albeit at very low confidence. Recoverability is likely to be high (see additional information below).
Radionuclide contamination
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Insufficient information found.
Changes in nutrient levels
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An increase in nutrient levels from e.g. sewage sludge, sewage effluent or riverine flooding, may result in an increase in inorganic and organic suspended particulates (see above), increased turbidity (see above) and increased phytoplankton productivity. An increase in nutrient levels may stimulate macroalgal growth, competing with the faunal turf especially in its shallower examples, although increased turbidity due to phytoplankton abundance may offset the effect of nutrient enrichment (Hartnoll, 1998). Moderate nutrient enrichment may increase the food available to the community in the form of phytoplankton, zooplankton or organic particulates. However, eutrophication may result in deoxygenation (see below) or algal blooms. While the biotope is unlikely to be directly affected by algal blooms, the biotope may be adversely affected by toxins from toxic algae that accumulate in zooplankton, or smothered by dead 'bloom' algae and deoxygenation resulting from their subsequent decay (see below).

Death of a bloom of the phytoplankton Gyrodinium aureolum in Mounts Bay, Penzance in 1978 produced a layer of brown slime on the sea bottom. This resulted in the death of invertebrates, including Echinus esculentus, Marthasterias glacialis, while sessile bryozoans, sponges and Alcyonium spp. appeared moribund, presumably due to anoxia caused by the decay of the dead dinoflagellates (Griffiths et al. 1979). A bloom of the toxic flagellate Chrysochromulina polypedis in the Skagerrak resulted in death or damage of numerous benthic animals, depending on depth. The red algae Delesseria sanguinea lost pigmentation, and ascidians exhibited high mortalities even at 17m depth, while in shallow water all dominant species (including Ciona intestinalis, Halichondria panicea and Asterias rubens) were killed. The toxic effects of the algal bloom resulted in a marked change in the community structure (Lundälv, 1990).

This biotope occurs in areas subject to moderately strong tidal streams, so that prolonged deoxygenation is unlikely to occur. However, the potential toxic effects of the algal blooms and the siltation caused by death of an algal bloom may result in loss of several members of the community, especially ascidians, and an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).
Increase in salinity
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This biotope occurs in full salinity and is unlikely to encounter variation in salinity.
Decrease in salinity
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Although circalittoral, shallow examples of the biotope may be affected by variable or reduced salinity, resulting, for example, from hyposaline effluents. Several of the species identified as indicative of intolerance may be of 'intermediate' or 'high' intolerance to a reduction in salinity. Ryland (1970) stated that, with a few exceptions, the Gymnolaemata were fairly stenohaline and restricted to full salinity (ca 35 psu) and noted that reduced salinities result in an impoverished bryozoan fauna.

The majority of hydroids are subtidal and, although some brackish water species exist (Gili & Hughes, 1995), they are probably intolerant of prolonged decreases in salinity. Similarly, most sponges are subtidal and probably intolerant of reduced salinity. Halichondria panicea occurs in damp areas of the intertidal but probably only experiences short periods of reduced salinity due to rainfall.

Echinoderms are generally unable to tolerate low salinity (i.e. they are stenohaline) and possess no osmoregulatory organ (Boolootian, 1966). At low salinity e.g. sea urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal. Although, local adaptation to reduced salinity may occur (see Stickle & Diehl, 1987), the inability of echinoderms to osmoregulate makes them intolerant of short term acute or chronic long term reductions in salinity, e.g., a sudden inflow of river water into an inshore coastal area caused mass mortality of the Asterias vulgaris at Prince Edward Island, Canada (Smith, 1940, in Lawrence, 1995).

Overall, the majority of the species in the community are likely to be intolerant of a reduction in salinity, resulting in loss off many species and an impoverished community. The biotope would probably be replaced by reduced salinity epifaunal communities (e.g. the estuarine biotope £SIR.HarCon£) and therefore lost. Hence, an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).
Changes in oxygenation
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Little information on the effects of oxygenation on bryozoans was found. Sagasti et al. (2000) reported that epifauna communities, including dominant species such as the bryozoans 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 bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation.
Diaz & Rosenberg (1995) reported that the abundance of crustaceans and echinoderms decreased in hypoxic conditions, and suggested that the amphipods Gammarus tigrinus and Ampelisca agassizi were intolerant of hypoxia. Echinoderms are probably intolerant of hypoxia (see reviews). Mobile fauna are also likely to begin to leave the habitat once the oxygen fall below ca 2.8 mg/l (Diaz & Rosenberg, 1995).
This biotope occurs in moderate water movement (see water flow and wave exposure) and is unlikely to experience low oxygen levels, except due to algal blooms (see nutrients), smothering or decrease water movement (see water flow). Although evidence is limited, the dependence of this community (especially bryozoans and hydroids) on water movement suggests a high dependence on a turnover of nutrients and oxygen. Therefore, hypoxia at the benchmark level will probably result in the loss of a proportion of both sessile and mobile species, and a decrease in species richness, and an intolerance of intermediate has been recorded, albeit with very low confidence. Recoverability is likely to be rapid (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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Epizooics were shown to reduce growth rates in Flustra foliacea (Stebbing, 1971a) and may have similar effects on other bryozoans. The sea urchin Echinus esculentus has been reported to suffer mass mortalities due to 'bald urchin disease' (see review), although not in the British Isles. However, sea urchin populations have been reported to undergo large fluctuations in numbers, with increased numbers forming swarms that denude areas, followed by mass mortalities due to disease. Diseases in sea urchins may be an important natural controlling factor in sea urchin population dynamics. Periodic fluctuations in sea urchin populations will probably affect succession, and the dominant bryozoans faunal in circalittoral faunal turfs (see Sebens, 1985; Hartnoll, 1998). A decrease in sea urchin grazing due to disease induced mass mortality may allow epifaunal succession to proceed to more massive growth forms, e.g. ascidians, sponges and anemones, resulting in a loss of at least a proportion of the biotope as described. Therefore, an intolerance of intermediate has been recorded, albeit at very low confidence. Once predation and grazing pressure return to prior levels recovery will probably be rapid.
Introduction of non-native species
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No information found.
Extraction
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Although, not subject to extraction at present, many bryozoans have been recently found to contain pharmacologically active substances (Hayward & Ryland, 1998). Therefore, species of Bugula and other bryozoans may be subject to harvesting in the future. Few species present within this biotope are known to be subject to extraction or harvesting. However, the use of mobile fishing gear in the vicinity of the biotope, such as scallop dredges and beam trawls result in physical disturbance to the sediment surface, and an increase in suspended sediment (Hartnoll, 1998). Furthermore, potting and fixed netting (their placement and collection) for crabs, crayfish and lobster would probably result in abrasion and physical disturbance although this has been dealt with in 'Physical Disturbance' above. Echinus esculentus has been collected by diving in the past (Nichols, 1984). The loss of functionally important predators such as sea urchins, and to a lesser extent crabs and lobster may affect community structure (see microbial pathogens, Sebens, 1985; Hartnoll, 1998). However, other important functional species such as Asterias rubens are likely to remain and a low intolerance has been suggested.

Additional information icon Additional information

Recoverability
The majority of the species within this biotope have short lived pelagic larvae, with limited powers of dispersal, resulting in good local recruitment but poor long distance dispersal (see recruitment). Exceptions include, mobile crustaceans and echinoderms with long-lived planktonic larvae, and Nemertesia antennina and Alcyonium digitatum which can probably disperse up to 50m or over 100km respectively (Hughes, 1977; Hartnoll, 1998).

Bugula and other bryozoan and hydroid species exhibit multiple generations per year (see recruitment), that involve good local recruitment, rapid growth and reproduction. Bryozoans and hydroids are often opportunistic, fouling species, that colonize and occur space rapidly. For example, hydroids would probably colonize with 1-3 months and return to their original cover rapidly, while Bugula species have been reported to colonize new habitats within 6 -12 months (see recruitment). However, Bugula has been noted to be absent form available habitat even when large populations are nearby (Castric-Frey, 1974; Keough & Chernoff, 1987), suggesting that recruitment may be more sporadic.

Where the population is reduced in extent or abundance but individuals remain, local recruitment, augmented by dormant resistant stages and asexual reproduction, is likely to result in rapid recovery of the dominant bryozoan species, hydroids, probably within 12 months. Colonial ascidians would probably recover their original cover with 2 years, while sponges and anemones may take longer to recover but would probably regain original cover within less than 5 years (see 'time for community to reach maturity').

Where the community was destroyed and recovery is dependant on recruitment from other areas, bryozoans, hydroids and ascidians would probably recruit rapidly from other neighbouring areas (see Jensen et al., 1994; Hatcher, 1998). However, sponges and especially Anthozoa may take many years to recruit and develop. In addition, recruitment of rare and scarce species, where they occur, such as Leptopsammia pruvoti, Hoplangia durotrix, and Alcyonium hibernicum, is likely to take a very long time, e.g. up to 25-30 years or not occur at all in Leptopsammia pruvoti (see MarLIN review).

Therefore, studies of settlement and the effects of disturbance suggest rapid colonization, so that the biotope would be restored and recognisable within 5 years. But the species richness and a full community may take 5-10 years to recover, depending on local conditions. Communities isolated by distance from reproductive populations by geography or hydrography may take longer to develop. The recovery of any rare and scarce species with infrequent recruitment, e.g. Leptopsammia pruvoti and Hoplangia durotrix is likely to be very low (Keith Hiscock pers comm.).


This review can be cited as follows:

Tyler-Walters, H. 2002. Bugula spp. and other bryozoans on vertical moderately exposed circalittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 20/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=105&code=1997>