Biodiversity & Conservation

CR.MCR.EcCr.FaAlCr.Pom

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum would result in removal of the community and its associated species, therefore an intolerance of high has been recorded. Most of the species in the biotope, including the sea urchins would probably recover within less than 5 years (see additional information below).
Smothering
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Smothering by 5 cm of sediment will prevent feeding and reduce growth and reproduction, interfere with respiration and potentially cause localized anoxia, and interfere with larval settlement. Tall erect species, e.g. Nemertesia antennina, may survive due to their size, while some hydroids may survive as dormant stages. But encrusting sponge species and ascidians are likely to be damaged or killed by smothering, while vertical surfaces and overhangs will provide refuges from the effects of the factor.

Echinus esculentus is slow moving and unlikely to escape smothering by 5cm of sediment but even if immobilized is unlikely to starve within a month. But juveniles and small individuals may be adversely affected. Large specimens of Alcyonium digitatum may be up to 20cm tall and probably unaffected but the smaller colonies more typical of this biotope may be smothered, impairing respiration, and hence adversely affected (see MarLIN reviews). Encrusting corallines and encrusting bryozoans are unlikely to be affected since they were reported to survive being overgrown by other species and hence smothering (Gordon, 1972; Sebens, 1985; Todd & Turner, 1988)

Overall, smothering may result in loss of small or juvenile urchins, potentially resulting in a reduction in the grazing intensity. Therefore, an intolerance of intermediate has been recorded. Recoverability is probably high (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. Animal dominated communities develop preferentially on steep surfaces and under overhangs, e.g. bryozoan 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 sensitive.

Moore (1977) suggested that Echinus esculentus was unaffected by turbid conditions. Similarly, Comely & Ansell (1988) recorded this species in the presence of suspended material up to 5-6 mg/l. Echinoderm pedicellariae keep the test clear of settling larvae, spores and presumably sediment particles. Similarly Alcyonium digitatum has been shown to be tolerant of high levels of suspended sediment. Hill et al. (1997) demonstrated that Alcyonium digitatum sloughed off settled particles with a large amount of mucus, although mucus production incurs an energetic cost.

This biotope occurs in moderately wave exposed to wave exposed sites with weak to very 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 in suspension feeders, which may be important for species with several generations per year (e.g. Bugula spp. or Nemertesia spp. In areas of weak water flow and increased depth (reduced effects of wave action), an increase in suspended sediment will increase siltation to the detriment of several members of the community, especially the hydroids and bryozoans. Therefore, an overall intolerance of low has been recorded to represent the additional energetic costs associated with increased suspended sediment (e.g. mucus production), although Echinus esculentus, Alcyonium digitatum and encrusting corallines are unlikely to be adversely affected. 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 to suspension feeders within the biotope for the duration of the benchmark (one month) but otherwise not adversely affect the biotope. Therefore, not sensitive has been recorded.
Desiccation
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Encrusting algae, sea urchins, bryozoans, sponges, soft corals, and hydroids are probably highly intolerant of desiccation. However, this biotope is circalittoral, occurring below 5-10m depth (most records between 10-30m depth) and unlikely to be exposed to the air and desiccation.
Increase in emergence regime
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An increase in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action may be increased (see water flow rate below).
Decrease in emergence regime
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A decrease in tidal emergence is unlikely to affect circalittoral habitats, except that the influence of wave action may be decreased (see water flow rate below).
Increase in water flow rate
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This biotope was recorded from weak to very weak tidal streams (Connor et al., 1997a; JNCC, 1999). 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. An increase in water flow from weak to 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, 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, Ophiothrix fragilis and Ascidia mentula may be excluded (see Hiscock, 1983). In addition, very strong water flow may interfere with larval settlement and recruitment.

Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 1 knot. Outside the beds specimens were occasionally seen being rolled by the current (Comely & Ansell 1988), which may have been up to 2.6 knots. Urchins are removed from the stipe of kelps by wave and current action and are also displaced by storm action. However, Echinus esculentus also occurs in strong tidal streams (e.g. £MIR.Lhyp.TPk£) and even in shallow areas where wave action is extremely strong such as Rockall (Laffoley & Hiscock, 1988). But in the above examples the sea urchins occurred in developed macroalgal communities. Jones & Kain (1967) demonstrated that monthly removal of Echinus esculentus allowed a macroalgal community to develop within two to three years. Experimental removal significantly reduced the density of the sea urchin in the experimental area, and Jones & Kain (1967) concluded that the lower limit of the kelp beds on Port Erin breakwater were controlled by sea urchin grazing intensity. Therefore, any factor that reduces the density of the sea urchin population will reduce grazing intensity. An increase in water flow from weak to strong (see benchmark) is unlikely to result in removal of the Echinus esculentus population. But an increase in water flow may reduce the abundance of Echinus esculentus and reduce grazing intensity. Sebens (1985) demonstrated that the experimental addition of sea urchins to vertical rock faces removed the epifaunal community back to bare rock and encrusting corallines but on their subsequent removal the epifaunal community had partly recovered within a year.

Overall, a reduction in grazing intensity, together with an increase food supply due to increased water flow, is likely to result an increase in the abundance of suspension feeders and other epifauna and major changes in the community leading to loss of the biotope as described. Therefore, an intolerance of high has been recorded. Loss of several intolerant species is likely to reduce species richness in the short term, while subsequent development of a faunal turf communities will increase species richness in the long term. The biotope may become replaced by epifaunal turf communities characteristic of strong water flow e.g. £CR.Bug£ or £MCR.Flu£. Recovery is likely to be high (see additional information below).

Decrease in water flow rate
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This biotope occurs in areas of weak to very weak tidal streams. While wave induced water movement is probably of more importance in theses sites, a further reduction in water flow is unlikely.
Increase in temperature
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Most 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, the hydroid Abietinaria abietina occur from the Arctic south to Madeira, the sponge Pachymatisma johnstonia occurs south to Spain, and Alcyonium digitatum occur from Norway to Portugal. 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). The bottle-brush hydroid Thuiaria thuja may occur in northern records of this biotope, and is thought to be susceptible to climate change, probably retreating further north in response to long term increases in temperature.

Echinus esculentus is recorded from the north and south of the British Isles, experiencing for example temperatures between 0 -18 °C in the Limfjord, Denmark (Ursin 1960), and is unlikely to be affected by long term changes in temperature at the benchmark level. But Bishop (1985) noted that prolonged exposure to 19 ° C disturbed gametogenesis and suggested that Echinus esculentus could not tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Therefore, Echinus esculentus may be intolerant of short term acute increases in temperature.

Overall, an intolerance of intermediate has been recorded to represent the potential effects of short term temperature shock on Echinus esculentus. The biotope has only been recorded from northern areas suggesting that a long term increase in temperature may cause a retreat northwards. Recoverability is likely to be high (see additional information below).
Decrease 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. Temperature is a critical factor stimulating or inhibiting reproduction in hydroids, most of which have an optimum temperature range for reproduction (Gili & Hughes, 1995). Sebens (1986) reported that growth rates of most species were higher in the warmer months, except in Alcyonium spp. and Spirorbis spp. which showed little seasonal differences.

Echinus esculentus is recorded from the north and south of the British Isles, experiencing for example temperatures between 0 -18 °C in the Limfjord, Denmark (Ursin 1960), and is unlikely that this species will be adversely affected by a long term temperature change in British waters. Similarly, Alcyonium digitatum occurs from Iceland in the North, to Portugal and was also reported to be apparently unaffected by the severe winter of 1962-1963 (Crisp, 1964).

Therefore, a decrease in temperature at the benchmark level is unlikely to adversely affect the biotope but an intolerance of low has been recorded to represent the effects on growth rates.
Increase in turbidity
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The only species likely to be affected by a reduction in light intensity due to increased turbidity are the encrusting algae. But encrusting coralline algae are amongst the deepest water species of macroalgae occurring in the circalittoral, at great depths, and a light levels as low as 0.05 -0.001% of surface incident light (Lüning, 1990). A reduction in light intensity may reduce their growth rates, especially in the deepest examples of the biotope. However, their extremely slow growth rates mean that the corallines will probably not be adversely affected for the duration of the benchmark. Therefore, not sensitive has been recorded.
Decrease in turbidity
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A decrease in turbidity, and hence increased light, will increase the growth rates of macroalgae within the biotope (e.g. Delesseria sanguinea) especially in shallower examples of the biotope. This may allow macroalgae to increase in abundance, depending on grazing intensity. However, this biotope is characterized by grazing, so that although periodic escapes of macroalgae may be favoured , the biotope will probably not be significantly changed.
Increase in wave exposure
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This biotope was recorded from wave exposed to moderately exposed habitats, with a few examples from very wave exposed habitats (JNCC, 1999). Many of the epifaunal 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 the hydroid Kirchenpaueria pinnata. 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 in the shallow extent of the biotope may decrease sea urchin and starfish predation, allowing larger, massive species (e.g. sponges, Alcyonium digitatum, anemones and ascidians) to 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.

Echinus esculentus may be removed form kelp stipes by the action of currents and wave action or the effects of storms. Increase wave action is likely to depress the upper extent of shallow sublittoral populations. But Echinus esculentus occurred in significant numbers as shallow as 15m below low water level in the most wave exposed location in the British Isles at Rockall (Keith Hiscock pers comm.).

Hence, 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. Therefore, a proportion of the biotope is likely to be lost or changed (especially in shallow examples) 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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This biotope is found in moderately wave exposed to wave exposed habitats with a few records from sheltered or very wave sheltered habitats. However, species adapted to strong water movement may be reduced in abundance or lost e.g. Flustra foliacea and Pachymatisma johnstonia, while species more tolerant of sheltered conditions may increase in abundance e.g. Ascidia mentula (see Hiscock, 1983) and hydroids. But wave sheltered conditions are likely to encourage sea urchin grazing, which may reduce the abundance of epifaunal species including hydroids and bryozoans yet further, to leave only bare rock or encrusting coralline. However, given the grazed nature of the biotope, little significant difference in the biotope is likely to occur, so that not sensitive has been recorded.
Noise
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Hydroids, bryozoans, sponges, sea urchins and ascidians are unlikely to be intolerant of noise or vibration at the benchmark level. Mobile fish or decapod 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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Many of the species within the biotope probably respond to light levels, detecting shade and shadow to avoid predators, and day length in their behavioural or reproduction. However, their visual acuity is probably very limited and they are unlikely to be intolerant of visual disturbance at the benchmark level.
Abrasion & physical disturbance
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Erect epifaunal species, e.g. soft corals, bryozoans and hydroids are thought to be especially intolerant of physical disturbance, and epifaunal assemblages show reduced biomass and abundance in areas impacted by fishing (e.g. trawls and dredges). For detail see MarLIN reviews e.g. £CR.Bug£, £MCR.Flu£ or £MCR.ModT£. But, in this biotope, the epifaunal and algal crusts probably experiences greater levels of physical disturbance in the form of grazing, and are either tolerant (e.g. encrusting corallines) or rapidly recolonize available space (e.g. hydroids and tubeworms). However, urchins themselves may be sensitive.

Abrasion by lobster pots, anchor chains, or mobile fishing gear will probably result in displacement, loss of spines and some damage to tests of adult Echinus esculentus. Species with fragile tests, such as Echinus esculentus and Echinocardium cordatum were reported to suffer badly as a result of impact with a passing scallop dredges (Bradshaw et al., 2000; Hall-Spencer & Moore, 2000a). Adults can repair non-lethal damage to the test and spines can be re-grown but most dredge impact is likely to be lethal. Therefore, physical abrasion due to a passing anchor or dredge is likely to kill or remove a proportion of the sea urchin population. Any activity that significantly reduces the sea urchin population and hence grazing intensity it likely to result in major changes in the community. Therefore, an intolerance of high has been recorded. Recoverability is likely to be very high as sea urchins are likely to migrate into the area before any significant growth of epifauna can occur (see additional information below).

Displacement
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Echinus esculentus is probably regularly displaced to deeper water by storms. Displaced specimens are able to move up the shore after displacement (Lewis & Nichols 1979b). Large erect epifaunal species, e.g. Alcyonium digitatum and hydroids can not reattach if removed and would be lost. Encrusting species such as encrusting corallines would probably be killed if removed although removal is unlikely. Mobile scavengers and epifauna, e.g. crabs and brittlestars will probably survive displacement and return to suitable substrata. Overall, any activity that significantly reduces the sea urchin population and hence grazing intensity it likely to result in major changes in the community. Therefore, an intolerance of high has been recorded. But displacement of the sea urchins is likely to be temporary as they are likely to return to the biotope quickly, so that little long term effect on the biotope is likely and a recoverability of very high has been recorded.

Chemical Factors

Synthetic compound contamination
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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 suggested that epifaunal species (including bryozoans, hydroids and ascidians) may be at least inhibited by the presence of TBT.

Smith (1968) reported large numbers of dead Echinus esculentus at between 5.5 and 14.5 m depth in the vicinity of Sennen, presumably due to a combination of wave exposure and heavy spraying of dispersants in that area after Torrey Canyon oil spill (Smith, 1968). Smith (1968) also demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in echinopluteus larvae of Echinus esculentus. Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez 1999).

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.

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, 1986). Possible sub-lethal effects of exposure to synthetic chemicals, may result in a change in morphology, growth rate or disruption of reproductive cycle.

Therefore, hydroids crustaceans, gastropods, and ascidians are probably intolerant of TBT contamination while sea urchins and bryozoans are probably intolerant of chemical pollution. Therefore 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). Heavy metals caused reproductive anomalies in the starfish Asterias rubens (Besten, et al., 1989, 1991). 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).

Little is known about the effects of heavy metals on echinoderms. Bryan (1984) reported that early work had shown that echinoderm larvae were intolerant of heavy metals. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu). Sea-urchins, especially the eggs and larvae, are used for toxicity testing and environmental monitoring (reviewed by Dinnel et al. 1988). Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) and showed developmental abnormalities in the skeleton (Gomez & Miguez-Rodriguez 1999). Therefore, sea urchins may be intolerant of heavy metal contamination.

Bryozoans and hydroids may only manifest sublethal effects due to heavy metal contamination. But Echinus esculentus and its larvae are probably highly intolerant. Loss of sea urchin grazing is likely to result in major changes in the community and a biotope intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).
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. Invertebrates groups vary in their sensitivities, e.g.:
  • 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).
  • 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).
  • Smith (1968) reported dead colonies of Alcyonium digitatum in the locality of Sennen Cove (Pedu-men-du, Cornwall) resulting from the combination of wave exposure and heavy spraying of dispersants 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).
  • Crump et al. (1999) described "dramatic and extensive bleaching" of 'lithothamnia' following the Sea Empress oil spill. Observations following the Dona Marika oil spill (K. Hiscock, pers. comm.) were of rock pools with completely bleached coralline algae. However, Chamberlain (1996) observed that although Lithophyllum incrustans was rapidly affected by oil during the Sea Empress spill, the oil only destroyed about one third of the thallus surface thickness and regeneration occurred from thallus filaments below the damaged area so that recovery occurred within about a year.
If the physiology within different animals groups can be assumed to be similar, then bryozoans, 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. While the direct affects of oil spills will be ameliorated by depth in this circalittoral habitat, the abundance of sea urchins is likely to reduced, resulting in community change and possibly, change to a different biotope. Several other members of the community may be lost or damaged as a result of acute hydrocarbon contamination, and an intolerance of intermediate has been suggested, albeit at very low confidence. Recoverability is likely to be high (see additional information below).
Radionuclide contamination
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No 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 crusts especially in its shallower examples, although increased turbidity due to phytoplankton abundance may offset the effect of nutrient enrichment (Hartnoll, 1998) while increased macroalgal growth may benefit sea urchins. Moderate nutrient enrichment may increase the food available to the community in the form of phytoplankton, zooplankton or organic particulates. Sea urchins are unlikely to be directly affected since Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age. Comely & Ansell (1988) also suggested that Echinus esculentus could absorb dissolved organic material. However, eutrophication may result in indirect effects such as deoxygenation (see below) or algal blooms.

Whilst 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). For example, 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 and 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).

The potential toxic effects of the algal blooms and the siltation and anoxia caused by death of an algal bloom may result in loss of several members of the community, especially sea urchins, and an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).
Increase in salinity
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This is a circalittoral biotope, occurring in fully saline conditions and unlikely to experience increased salinity.
Decrease in salinity
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The majority of species within the biotope are only found in full salinity.
Echinoderms are generally unable to tolerate low salinity (stenohaline) and possess no osmoregulatory organ (Boolootian, 1966). At low salinity urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal. Echinoderm larvae have a narrow range of salinity tolerance and develop abnormally and die if exposed to reduced or increased salinity. While Stickle & Diehl (1987) noted that local adaptation can occur, echinoderms are generally considered to be intolerant to reduced salinity. For example, Lawrence (1996) reported that sudden river discharge in 1977, killed Strongylocentrotus droebachiensis in Newfoundland and Labrador fjords and at one locality 60-75% of the population was killed to a depth of 12-15m.

Ryland (1970) stated that, with a few exceptions, the Gymnolaemata Bryozoa were fairly stenohaline and restricted to full salinity (ca 35 psu) and noted that reduced salinities result in an impoverished bryozoan fauna. Similarly, 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.

Gessner & Schramm (1971) summarized the effects of salinity changes on marine algae and noted that most sublittoral red algae cannot withstand salinities below 15 psu. The maerl forming coralline algae, Phymatolithon calcareum and Lithothamnion corallioides are found only in fully saline waters (between 30-40 psu) and the growth of some maerl species is impaired below 24 psu. However, other species such as Lithophyllum incrustans are intertidal and tolerate fluctuating salinities.

Overall, the majority of the species in the community are likely to be intolerant of a reduction in salinity, resulting in loss of many species within the community. Loss of or marked reduction in the sea urchin population will probably result in major changes in the community. Therefore, an intolerance of high has been recorded. Recoverability is likely to be high (see additional information below).

Changes in oxygenation
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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. Echinoderms are probably intolerant of hypoxia (see reviews). For example, 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 and 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). 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 wave action. 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. Loss of or marked reduction in the sea urchin population will have a significant effect on the community and an intolerance of high has been recorded. Recoverability is likely to be rapid (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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Alcyonium digitatum acts as the host for the endoparasitic species Enalcyonium forbesi and Enalcyonium rubicundum (Stock, 1988). Parasitization may reduce the viability of a colony but not to the extent of killing them but no further evidence was found to substantiate this suggestion. Sebens (1985, 1986) noted that Lithothamnion species were susceptible to boring polychaetes, which increased their susceptibility to grazing damage. But the result susceptibility was probably one and important factor in the competitive equilibrium between Lithothamnion and Phymatolithon species. 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. Populations of the sea urchin Strongylocentrotus droebachiensis have also 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 epifaunal (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 epifaunal turfs and massive growth forms, e.g. ascidians, sponges and anemones. Therefore, although no mass mortalities of sea urchins in British waters have been observed or attributed to disease, disease could potentially result in major changes in the community and an intolerance of high has been recorded, albeit with very low confidence. Recoverability is likely to be high (see additional information below).
Introduction of non-native species
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No information found.
Extraction
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Echinus esculentus is the key functional species within this biotope. Collecting of Echinus esculentus for the curio trade was studied by Nichols (1984). He concluded that the majority of divers collected only large specimens that are seen quickly and often missed individuals covered by seaweed or under rocks, especially if small. As a result, a significant proportion of the population remains. He suggested that exploited populations should not be allowed to fall below 0.2 individuals per square metre. But in this heavily grazed biotope, any reduction in grazing pressure may significantly affect the community, probably allowing increased escapes of erect epifauna, and possibly macroalgae in shallow examples of the biotope. Therefore, an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below).

Additional information icon Additional information

Recoverability The development of this community is probably dependant on the abundance and density of the sea urchin population. The majority of the epifaunal and algal crust species were shown to re-colonize cleared areas quickly. For example red crustose algae, encrusting bryozoans, tubeworms, tubicolous amphipods and worms, erect hydroids and bryozoans were reported to cover cleared areas within 1-4 months in spring, summer and autumn (Sebens, 1986). Colonial ascidians re-appeared and achieved significant cover within a year. Sponges (e.g. Halichondria panicea) recovered cover within >2 years while only a few individuals of Alcyonium and Balanus balanus established after 4 years, probably requiring longer to achieve prior cover (Sebens, 1985; 1986). Encrusting coralline algae (e.g. Lithothamnion and Phytomatolithon took 1-2 years to recolonize cleared areas (Sebens, 1985; 1986) and with their slow growth rates probably take many years to recover their original abundance.

In the absence of sea urchin grazing the community will probably develop into a faunal turf community, following a similar successional sequence to that demonstrated by Sebens (1985). But Sebens (1986) demonstrated that addition of sea urchins removed the majority of species, leaving only bare rock and encrusting algae within 2-3months. Therefore, biotope recovery is likely to be rapid once sea urchin abundance returns to prior levels after disturbance.

Echinus esculentus produces long-lived planktonic larvae with high dispersal potential. Settlement is thought to occur in autumn and winter (Comely & Ansell, 1988). But recruitment is sporadic or variable depending on locality, e.g. Millport populations showed annual recruitment, whereas few recruits were found in Plymouth populations during Nichols studies between 1980-1981 (Nichols, 1984). Lewis & Nichols (1979) found that adults were able to colonize an artificial reef in small numbers within 3 months and the population steadily grew over the following year. Bishop & Earll (1984) suggested that the population of Echinus esculentus at St Abbs had a high density and recruited regularly whereas the Skomer population was sparse, ageing and had probably not successfully recruited larvae in the previous 6 years. But Echinus esculentus is a widespread, mobile species so that recovery is probably improved by migration from neighbouring areas.

Overall, most of the species within the biotope could probably recolonize bare rock, and regain significant cover within 5 years, with the exception of sponges and anemones. Loss of sea urchins would allow faunal turf communities to develop, however, on their return the biotope would probably be recognizable within 2-3 months. Sea urchin recruitment is sporadic and dependant on location but populations would probably recover within 5 years, except in locations isolated by geography or hydrography.


This review can be cited as follows:

Tyler-Walters, H. 2002. Faunal and algal crusts, Echinus esculentus, sparse Alcyonium digitatum and grazing-tolerant fauna on 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 23/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=337&code=2004>