Biodiversity & Conservation

IR.SIR.EstFa.MytT

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum will include the removal of all the species within the biotope. Therefore, an intolerance of high has been recorded. For recoverability, see additional information below.
Smothering
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Although smothering by sediment is unlikely in the strong tidal currents to which this biotope is exposed, mortality of mussels might occur as a result on smothering by large scale movements of sand (Daly & Mathieson, 1977; Holt et al., 1998). Similarly, biodeposition within a mussel bed results in suffocation or starvation of individuals that cannot re-surface. Young mussels have been shown to move up through a bed, avoiding smothering, while many others were suffocated (Dare, 1976; Holt et al., 1998). This suggests that a proportion of the population may be able to avoid smothering in subtidal conditions, and, therefore, an intolerance of intermediate has been recorded. Associated species will most likely also survive some smothering. Although a single good recruitment event for mussels may recolonize the substratum within a year, recovery in relation to size of mussels and presence of associated species may take up to 5 years, and is some circumstances significantly longer (see additional information below). Therefore, a recoverability of high has been recorded.
Increase in suspended sediment
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Mytilus edulis has been reported to be relatively tolerant of suspended sediment and siltation and survived over 25 days at 440 mg/l and on average 13 days at 1200 mg/l (Purchon, 1937; Moore, 1977). Similarly, Asterias rubens flourishes in naturally turbid conditions and is capable of cleansing itself of adherent mud particles (Moore, 1977). However, mussels and the species living on mussels probably suffer a metabolic cost resulting from the cleansing mechanisms, mucus production and interrupted or impaired feeding. Therefore, a biotope intolerance of low, at the benchmark level, has been recorded. The majority of the organisms within the biotope probably have mechanisms to deal with siltation and suspended sediment, so that recoverability of immediate has been recorded.
Decrease in suspended sediment
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Decrease in the amount of suspended sediment is likely to reduce available food for the suspension feeders that dominate this biotope. However, the result would be a slowing of growth and not decline.
Desiccation
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The dominant species in the biotope, Mytilus edulis, is capable of closing valves and surviving significant exposure to air. Although Balanus crenatus has more permeable shell plates than other littoral barnacles and therefore loses water quicker and dies sooner when exposed to air, it is still capable of surviving for long periods in air (see the Balanus crenatus review). Halichondria panicea is likely to be less resistant to desiccation. However, as this is a sublittoral biotope that would only be subject to desiccation for short periods if at all, intolerance is assessed as low and recoverability as very high.
Increase in emergence regime
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The dominant species in the biotope, Mytilus edulis, is capable of closing valves and surviving significant exposure to air. Although Balanus crenatus has more permeable shell plates than other littoral barnacles and therefore loses water quicker and dies sooner when exposed to air, it is still capable of surviving for long periods in air (see the Balanus crenatus review). Halichondria panicea is likely to be less resistant to increased emergence and therefore desiccation. However, as this is a sublittoral biotope that would only be subject to emergence for short periods if at all, intolerance is assessed as low and recoverability as very high.
Decrease in emergence regime
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This is a sublittoral biotope and decrease in emergence is not therefore relevant.
Increase in water flow rate
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The dominant species in the biotope, Mytilus edulis, together with other sessile species in the biotope lives in very strong tidal currents and in extremely wave exposed situations. It might be expected that an increase in water flow may dislodge a proportion of the Mytilus edulis. However, Young (1985) reported that Mytilus edulis increased byssus thread production in response to increased agitation and water flow rates. Therefore it is not expected that increased water flow rate would remove the dominant species and therefore the species attached to it. Mobile species such as Asterias rubens may be swept away but this seems unlikely based on the occurrence of the species in very strong flows.
Decrease in water flow rate
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Decreases in water flow rates are likely to increase siltation and allow increased predation pressure from crabs, lobsters and starfish such as Asterias rubens. The biotope is likely to suffer from competition from species adapted to more sheltered conditions. Also, the supply of food will be reduced, mussels will have to rely more on active suspension feeding and growth will be lessened. At the benchmark level of a decrease by two categories of tidal stream exposure for one year, it is likely that the biotope will be substantially altered and may no longer be SIR.EstFa.MytT. Species richness may increase as Mytilus edulis will no longer dominate the rock surfaces. Therefore, an intolerance of high has been recorded. On return to previous conditions, recoverability is likely to be high (see Additional information below).
Increase in temperature
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An upper, sustained temperature tolerance limit of about 29°C has been reported for Mytilus edulis in the United Kingdom (Read & Cumming, 1967; Almada-Villela et al., 1982). Therefore, Mytilus edulis is considered to be of low intolerance to temperature change. Temperatures that adversely affect Balanus crenatus and Asterias rubens are higher than the benchmark level and, even though outflow from enclosed areas might be of water that has significantly heated in sunny weather, it is unlikely to reach the levels suggested as harmful (for instance Naylor, 1965 for Balanus crenatus; Schäfer, 1972 for Asterias rubens). Overall, the biotope has been assessed as of low intolerance to increased temperatures. Recovery is likely to be rapid (see additional information below).
Decrease in temperature
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Mytilus edulis tolerates decreases in temperature and even freezing for short periods. Similarly, Balanus crenatus and Asterias rubens were unaffected by the severe winter of 1962/63 (Crisp, 1964) when average temperatures were 5 to 6 °C below normal. However, Halichondria panicea was recorded as being killed by frost in North Wales. It appears, therefore, that most of the characterizing species within the biotope are tolerant of an acute short term temperature decrease and a biotope intolerance of low has been recorded.
Increase in turbidity
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Filamentous brown algae are present in this biotope (Hiscock, 1984). However, the biotope is primarily an animal dominated community, dependant on secondary production and not dependant on light. Therefore, the biotope is probably only intolerant of changes in turbidity and light attenuation in that some algae might be reduced in abundance or lost.
Decrease in turbidity
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Decrease in turbidity will increase light penetration and therefore the possibility that abundance of algae might increase. Seaweed growth may adversely affect the survival of mussels: Suchanek (1985) found that urchin grazing removed seaweeds and increased survival of mussel beds. However, the overwhelming dominance of mussels in the conditions of tidal stream exposure make it unlikely that addition of algae will change the biotope present. Thus an intolerance of low is suggested.
Increase in wave exposure
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Oscillatory water movement is potentially far more destructive than tidal streams due to the 'to and fro' motion that is more likely to loosen mussels. Although, mussels occur in extremely wave exposed situations, the large and loosely attached mussels likely to occur in this biotope will be easily dislodged. However, Young (1985) reported that Mytilus edulis increased byssus thread production in response to increased agitation and water flow rates. Any loss of mussels will result in loss of associated species and therefore at least a reduction in extent of the biotope. Therefore, an intolerance of intermediate has been recorded. For recoverability see 'Additional Information below'.
Decrease in wave exposure
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This biotope already occurs in extremely sheltered conditions so that further decrease in wave exposure is probably not relevant.
Noise
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The species in this biotope are insensitive to noise disturbance at the levels of the benchmark. However, wildfowl are a major predator and several species are highly intolerant of noise. Therefore, noise at the level of the benchmark may disturb predatory wildfowl, so that the mussel populations and the species that live attached to them may benefit indirectly.
Visual Presence
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The species living in the biotope are unlikely to be sensitive to visual disturbance. However, wildfowl are highly intolerant of visual presence and are likely to be scared away by increased human activity, therefore reducing the predation pressure on the mussels and the species attached to them. Therefore, visual disturbance may be of indirect benefit to the biotope.
Abrasion & physical disturbance
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It is likely that abrasion or impact at the level of the benchmark would damage or remove patches of the population of mussels and the species attached to the mussels. Several long term studies have shown that gaps took a long time to heal in intertidal populations, and in some cases enlarged (presumably due to wave action and predation), with little recovery within 3-5 years, leading to estimated recovery times of 8-34 years (Pain & Levin, 1981) or several hundreds of years (Seed & Suchanek, 1992). However, enlargement of gaps due to wave action will not occur in the sheltered situations where this biotope occurs and settlement rates of mussels in tide-swept sublittoral situations is rapid. Taking account of time for mussels to grow to a significant size and to be colonized by the associated species (see further information below), a recoverability of 'high' has been reported.
Displacement
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The dominant species in the biotope, Mytilus edulis, is capable of re-attaching itself to suitable substrata once displaced. Overall, however, displacement will result in loss of mussels and associated species from this biotope. Displaced starfish are unlikely to be adversely affected and could probably return. Overall, a proportion of the mussel bed would probably survive displacement and an intolerance of intermediate has been recorded. However, other members of the community are probably more intolerant, resulting in a decline in species richness until they are able to recolonize. Recovery is dependant on recruitment of Mytilus edulis from outside the biotope and a recoverability of high has been reported (see additional information below).

Chemical Factors

Synthetic compound contamination
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The effects of contaminants on Mytilus edulis were extensively reviewed by Widdows & Donkin (1992) and Livingstone & Pipe (1992). Overall, Mytilus edulis is probably relatively tolerant of contaminants, although mortalities have been recorded (see species review for details). For example,
  • Widdows et al. (1995) noted that polar organics, and organo-chlorines reduced scope for growth in Mytilus edulis
  • Mytilus edulis has been shown to accumulate PCBs and ivermecten (Hummel et al., 1989; Holt et al., 1995; Cole et al., 1999);
  • the presence of poly-aromatic hydrocarbons, cis-chlordane pesticides and cadmium gas been associated with an increase in tumours in Mytilus edulis (Hillman, 1993; Holt et al., 1998), and
  • mussels may be absent from areas of high boating activity, presumably due to TBT (Holt et al., 1998).
Barnacles, such as Balanus crenatus were considered to be highly sensitive to chemical contaminants (Holt et al., 1995). PCB exposure resulted in defective larvae in Asterias rubens (Besten et al., 1989). Although only filamentous brown algae are named in the biotope description, kelp plants may occur and possibly some other algal species. Cole et al. 1999 report the following as very toxic to macrophytes: atrazine; simazine; diuron; and linuron (herbicides). It is likely therefore that Laminariales such as Laminaria hyperborea have an intermediate intolerance to atrazine and some other herbicides. No information was found concerning the effect of contaminants on other named species in the biotope. Therefore, chemical contamination may cause mortalities and sub-lethal effects in the Mytilus edulis bed but affect other members of the community to varying degrees, and an overall intolerance of intermediate has been recorded. Since likely effects at the benchmark level are generally sublethal, a recoverability of very high is recorded.
Heavy metal contamination
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Lethal threshold concentrations for several heavy metals have been determined in Mytilus edulis (see species review; Widdows & Donkin (1992) and Livingstone & Pipe (1992) for reviews). Mussels were also reported to be missing from a wider area of the Cumbrian coast than other organisms in the vicinity of a phosphate rich effluent contaminated by heavy metals (Holt et al., 1998). Widdows & Donkin (1992) noted that lethal responses give a false impression of high tolerance. However, Mytilus edulis is probably relatively tolerant of heavy metal contamination. Besten et al. (1989) suggested that cadmium (Cd) pollution posed a significant threat to populations of Asterias rubens since it affected reproduction. Little information concerning heavy metal toxicity was found for other animal species in the biotope. Although only filamentous brown algae are named in the biotope description, kelp plants may occur and possibly some other algal species. Hopkin & Kain (1978) examined the effect of Cu, Zn, Hg and Cd on Laminaria hyperborea gametophytes and sporophytes. Sublethal effects on sporophyte development, growth and respiration were shown at concentrations higher than the short term benchmark for Hg, Zn and Cd. Hg was found to be lethal at 0.05 mg/l. However, Cu affected sporophyte development at 0.01 mg/l but was lethal at 0.1 mg/l. Given the evidence of sub-lethal and lethal effects of heavy metals in Mytilus edulis especially, a biotope intolerance of intermediate has been reported. Effects are likely to be sublethal and recovery rapid.
Hydrocarbon contamination
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The effects of contaminants including hydrocarbons on Mytilus edulis were extensively reviewed by Widdows & Donkin (1992) and Livingstone & Pipe (1992). Overall, Mytilus edulis is probably relatively tolerant of contaminants, although mortalities have been recorded (see species review for details). Hydrocarbon contamination in infralittoral populations is limited to exposure to lighter oil fractions and PAHs in solution or as droplets as a result of turbulent flow or adsorbed onto particulates.
  • Toxic hydrocarbons and PAHs contribute to a decline on the scope for growth in Mytilus edulis (Widows & Donkin, 1992; Widdows et al., 1995).
  • The presence of poly-aromatic hydrocarbons, cis-chlordane pesticides and cadmium gas been associated with an increase in tumours in Mytilus edulis (Hillman, 1993; Holt et al., 1998).
  • Mesocosm experiments have shown high mortalities of Mytilus edulis exposed to the water accommodated fraction of diesel (Widdows et al., 1987; Bokn et al., 1993).
  • Ingestion of droplets of sunflower oil, from a tanker spill off the Anglesey coast resulted in mortalities after spawning in Mytilus edulis (Mudge et al., 1993; Holt et al., 1998).
  • Asterias rubens suffered mass mortalities after the Torrey Canyon oil spill and was reported to be lost from mesocosms treated with the water accommodated fraction of diesel (Smith, 1968; Bokn et al., 1993).
  • Whilst it is unclear what the full species complement of this biotope is, it seems likely that some likely species including crustaceans and echinoderms may be adversely affected by oil which might cause narcotization and possible mortality.
  • Mytilus edulis dominated jetty piles immediately adjacent to an oil refinery effluent in Milford Haven, suggesting a high tolerance of hydrocarbon contamination (K. Hiscock, pers. comm.).
Overall, Mytilus edulis and many of the associated species in this biotope are appear to be relatively tolerant of hydrocarbon pollution. However, due to the incidence of mortality after exposure to diesel and oils Mytilus edulis was regarded as of intermediate intolerance to hydrocarbon contamination.
Although, Asterias rubens has been assessed as highly intolerant, the mussel bed may benefit from a reduction in starfish predation. An overall biotope intolerance of intermediate has been recorded. Since dominant species and most other species at least are likely to survive hydrocarbon contamination, a recoverability of very high is recorded.
Radionuclide contamination
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No information has been found.
Changes in nutrient levels
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Moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material, is likely to increase food availability for all the suspension feeders within the biotope. Therefore a 'tolerant*' has been recorded. However, long term or high levels of organic enrichment may result in deoxygenation and algal blooms. Mytilus edulis has been reported to suffer mortalities due to algal blooms of Gyrodinium aureolum and Phaeocystis poucheri (Holt et al., 1998). In this biotope, there is unlikely to be any danger of de-oxygenation as a result of the death of algal blooms caused by nutrient enrichment because of the strong tidal flow and water mixing to which the biotope is exposed.
Increase in salinity
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The dominant species, Mytilus edulis, thrives in strong tidal currents whether or not subject to low salinity. However, Mytilus is able to tolerate low salinity conditions much more than some other species likely to thrive in full salinity. Those other species, for instance Halichondria panicea, other encrusting sponges and Tubularia indivisa, may out-compete and dominate in the place of Mytilus edulis and therefore the biotope will no longer be SIR.EstFa.MytT.
Decrease in salinity
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Mytilus edulis thrives in brackish lagoons and estuaries, including the reduced salinity that this biotope is exposed to. Overall, Mytilus edulis can acclimate to a wide range of salinities and a change of salinity at the benchmark level is unlikely to adversely affect this species. However, some of the associated species in this biotope do not penetrate far into estuarine or reduced salinity situations and are most likely intolerant of a fall in salinity below that to which the biotope is usually exposed. Therefore an intolerance of intermediate is indicated. Once conditions return to normal, most of the associated species will recolonize from planktonic stages. A recoverability of high is given.
Changes in oxygenation
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Mytilus edulis was regarded to be tolerant of a wide range of oxygen concentrations including zero (Zwaan de & Mathieu, 1992; Diaz & Rosenberg, 1995; see species review). However, echinoderms such as Asterias rubens are highly intolerant of anoxic conditions. Similarly, Balanus crenatus was considered to be highly intolerant to anoxia. Although Mytilus edulis is likely to tolerate hypoxic conditions, an intolerance of intermediate has been recorded due to the likely intolerance of the other members of the community. It should be noted that in the presence of strong to moderate tidal streams, anoxic conditions are unlikely to occur unless combined with reduced water flow rates. Recoverability of the associated species is likely to be rapid (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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The diseases and parasites of Mytilus edulis were reviewed by Bower (1992) and Bower & McGladdery (1996) (see the species review).
Asterias rubens may be parasitised by the ciliate Orchitophyra stellarum (Vevers, 1951; Bouland & Clareboudt, 1994) resulting in castration of males, and subsequent reduction in population size (Vevers, 1951). However, little information on diseases of other members of the community has been found.
Therefore, an intolerance of intermediate has been recorded. Recovery of the mussel beds will be dependant on recruitment from other populations and a recoverability of high has been recorded (see additional information below).
Introduction of non-native species
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Mytilus edulis is an effective space occupier and few other species are able to out-compete it. The South American mytilid Aulocomya ater, which has been reported recently in the Moray Firth (Eno et al., 2000), is not expected to be a threat to this reduced salinity populations.
Extraction
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Mytilus edulis is the only species indicative of sensitivity within this biotope that is subject to commercial extraction. Subtidal mussel beds may be exploited by dredging. Holt et al. (1998) suggest that in particular embayments over-exploitation may reduce subsequent recruitment leading to long term reduction in the population or stock. Extraction of Mytilus edulis is likely to remove much of the epifaunal and infaunal community, resulting in a decline in species richness. Overall, an intolerance of intermediate has been recorded at the benchmark level of extraction. However, recovery is likely to occur within 5 years and a recoverability of high has been recorded (see additional information below).

Additional information icon Additional information

Recoverability
Recovery will depend mainly on recruitment of the dominant species, Mytilus edulis. Although a single good recruitment event for mussels may recolonize the substratum within a year, recovery in relation to size of mussels and presence of some associated species may take up to 5 years, and is some circumstances significantly longer. In the case of Balanus crenatus, the species is an important early colonizer of sublittoral rock surfaces (Kitching, 1937) and it densely colonized a site that had been dredged for gravel within 7 months (Kenny & Rees, 1994). Halichondria panicea also appears to be a rapid colonizer, suggested by its presence dominating kelp stipes that would not be more than a few years old. Therefore, recovery is predicted to be high.

This review can be cited as follows:

Hiscock, K. 2001. Mytilus edulis beds on reduced salinity tide-swept infralittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 24/04/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=259&code=1997>