Biodiversity & Conservation

LR.MLR.MusF.MytFves

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. Although a single good recruitment event may recolonize the substratum within a year, recovery may take up to 5 years, and is some circumstances significantly longer (see additional information below). Therefore, a recoverability of moderate has been recorded.
Smothering
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Intertidal Mytilus edulis beds have been reported to suffer mortalities as a result on smothering by large scale movements of sand or sand scour (Holt et al., 1998; Daly & Mathieson, 1977). 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 Mytilus edulis population may be able to avoid smothering. Gastropods (e.g. Littorina littorea) may be suffocated by the sediment. Smothering may also adversely affect interstitial fauna and epifauna, resulting in a decrease in species richness and an increase of infaunal species (Tsuchiya & Nishihira, 1985, 1986). However, on moderately wave exposed to exposed coasts sediment is unlikely to remain in place resulting in scour which may remove a proportion of the mussels and probably adversely affect Fucus vesiculosus and other macroalgae. After one month (see benchmark) although fronds may have been removed or died back, a proportion of holdfasts and hence plants would probably survive to grow back. Therefore, an overall intolerance of intermediate has been recorded. Smothering by impermeable or immobile materials, e.g. oil, may result in a higher intolerance (see hydrocarbons). Recoverability has been recorded as moderate (see additional information below).
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 1200mg/l (Purchon, 1937; Moore, 1977). Mytilus edulis also has efficient pseudofaeces discharge mechanisms (Moore, 1977; de Vooys, 1987), although increased suspended sediment may reduce feeding efficiency (Widdows et al., 1998).
The gastropods and amphipods within the biotope occur in more sheltered habitats and are probably tolerant of a range of suspended sediment levels. Increased siltation will probably interfere with larval recruitment in some species, e.g. macroalgae. Fucus vesiculosus may suffer as a result of increased scour (see above) and the associated turbidity will reduce photosynthesis (see below) , but occurs in more sheltered environments and estuaries and is probably tolerant of siltation. Increased siltation may fill the mussel matrix, resulting in increase infauna but loss of more mobile species and species richness (Tsuchiya & Nishihira, 1985, 1986). Overall, the biotope will be little affected but species richness will probably decline and an intolerance of low has been recorded.
Decrease in suspended sediment
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A decrease in suspended sediment, especially organic particulates could potentially reduce the food available to Mytilus edulis and the other suspension feeders within the biotope. However, little other effects are likely. Therefore, an intolerance of low has been recorded.
Desiccation
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The upper limit of Mytilus edulis population is primarily controlled by the synergistic effects of temperature and desiccation (Suchanek, 1978; Seed & Suchanek, 1992; Holt et al., 1998). For example, on extremely hot days in the summers of 1974 -1976 on Strawberry Island, Washington State, Suchanek (1978) reported mass mortality of mussels at the upper edge of the mussel bed. Mortality decreased down the shore. The upper limit of mussels fluctuated, increasing up the shore in winter and decreasing again in summer (Suchanek, 1978). Therefore, a increase in desiccation at the benchmark level is likely to result in mortality of mussels at the upper limit of the bed, and loss of their associated organisms, with patches of mussels restricted to depressions and rocks pools at their upper limit.
Similarly the upper limit of most intertidal species are partly determined by desiccation. The upper limit of Fucus vesiculosus and red algae is likely to be reduced, so that the entire biotope is likely to become 'squeezed' between a reduced upper limit and its lower limit caused primarily by predation. Similarly, the upper limit of Nucella lapillus will be reduced, decreasing dog whelk predation within the bed. The biotope is likely to be more vulnerable to desiccation in moderately wave exposed conditions than in wave exposed conditions, since the later tend to exhibit a greater humidity due to wash, spray and wave crash.
Overall, the extent of the biotope is likely to be reduced and an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information below).
Increase in emergence regime
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Mytilus edulis can only feed when immersed, therefore, changes in emergence regime will affect individuals ability to feed and their energy metabolism. Growth rates decrease with increasing shore height and tidal exposure, due to reduced time available for feeding and reduced food availability, although longevity increases (Seed & Suchanek, 1992; Holt et al., 1998). Increased emergence will expose mussel populations to increased risk of desiccation (see above) and increased vulnerability to extreme temperatures, potentially reducing their upper limit on the shore, and reducing their extent in the intertidal. Therefore, the upper limit of the biotope and its associated community will probably decrease, being replaced by barnacles, and an intolerance of intermediate has been recorded. Recoverability will probably be high (see additional information below).
Decrease in emergence regime
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A decrease in emergence will reduce exposure to desiccation and extremes of temperature and allow the resident Mytilus edulis to feed for longer periods and hence grow faster. Therefore, the biotope will probably be able to colonize further up the shore into depressions or gaps in the barnacle cover.
However, the lower limit of the biotope may become susceptible to greater predation pressure from crabs and/or dog whelks, resulting in greater turnover of individuals and a reduced number of size classes, and reduced age of mussels. In addition, the Fucus vesiculosus may be lost at its lower limit, replaced by patchy Fucus serratus and an increased abundance of red algae.
Therefore, in the short term, a decrease in emergence is likely to change the population structure of the mussel bed at its lower limit, probably reducing the species richness of the mussel matrix, and the replacement of the lower limit of the biotope by another mussel dominated biotope e.g. £MLR.MytFR£. Although the mussel beds will effectively survive the lower limit of the biotope as described will be lost and an intolerance of intermediate has been recorded. This biotope (MLR.MytFves) will probably colonize further up the shore and recovery will be rapid (see additional information below).
Increase in water flow rate
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The biotope is found in wave exposed conditions where water movement from wave action will greatly exceed the strength of any possible tidal flow. The biotope is therefore considered to be not sensitive.
Decrease in water flow rate
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The biotope is characteristic of wave exposed conditions where water movement from wave action will greatly exceed the effects of any reduction of tidal flow. If the biotope occurred in areas where water flow was more important to provide an adequate supply of food and prevent siltation some adverse effects on feeding and reproduction may occur. Therefore an intolerance of low has been recorded.
Increase in temperature
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Most species within the biotope a widely distributed to the north or south of the British and Ireland and unlikely to be adversely affected by long term changes in temperature at the benchmark level. Low to mid shore turf forming red algae (e.g. Mastocarpus stellatus, Palmaria palmata and Osmundea pinnatifida) were damaged or died at their upper limit during the exceptionally hot summer of 1983 (Hawkins & Hartnoll, 1985). Therefore, their abundance or upper limits may be reduced by short term increases in temperature at the benchmark level. Similarly, an acute temperature change (e.g. 5 °C) will probably interfere with feeding activity in Nucella lapillus and in summer may result in direct mortality or indirect mortality due to heat coma and desiccation (see MarLIN review). Bousfield (1973) reported that amphipod tolerance to extremes of temperature was low. However, they probably derive protection within the macroalgal fronds or mussel matrix. Ephemeral algae become more abundant in summer months and may be stimulated by increases in temperature

In the British Isles an upper, sustained thermal tolerance limit of about 29 °C was reported in Mytilus edulis (Read & Cumming, 1967; Almada-Villla et al., 1982). However, Seed & Suchanek (1992) noted that European populations were unlikely to experience temperatures greater than about 25 °C. Mytilus edulis is generally considered to be eurythermal.

Fucus vesiculosus can also withstand a wide range of temperatures and has been found to tolerate temperatures as high as 30 °C (Lüning, 1990). The species is well within its temperature range in the British Isles so would not be affected by a change of 5 °C. The species showed no sign of damage during the extremely hot summer of 1983, when the average temperature was 8 °C hotter than normal (Hawkins & Hartnoll, 1985).

Overall, the dominant characterizing species will probably survive an increase in temperature at the benchmark level, while some red algae may be reduced in abundance and species richness suffer a minor decline. An increase in temperature is likely to decrease the threat of dog whelk predation. Therefore an intolerance of low has been recorded.

Decrease in temperature
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The dominant characterizing species are widely distributed to the north or south of Britain and Ireland. Mytilus edulis can withstand extreme cold and freezing, surviving when its tissue temperature drops to -10 °C (Williams, 1970; Seed & Suchanek, 1992) or exposed to -30 °C for as long as six hours twice a day (Loomis, 1995). Bourget (1983) also reported that cyclic exposure to otherwise sublethal temperatures, e.g. -8 °C every 12.4 hrs resulted in significant damage and death after 3-4 cycles. This suggests that Mytilus edulis can survive occasional, sharp frost events, but may succumb to consistent very low temperatures over a few days. Mytilus edulis was relatively little affected by the severe winter of 1962/63, with 30% mortality reported from south-east coasts of England (Whitstable area) and ca. 2% from Rhosilli in south Wales (Crisp,1964) mainly due to predation on individuals weakened or moribund due to the low temperatures rather than the temperature itself. Overall, Mytilus edulis is considered to be eurythermal.

Fucus vesiculosus, and Littorina littorea can withstand a wide range of temperatures. For example, Fucus vesiculosus was reported to tolerate -30 °C in Maine (Lüning, 1990). Nucella lapillus can probably survive temperatures as low as 3 °C and possibly 0 °C, although evidence for duration is lacking, the effects of low temperatures being sub-vital (see MarLIN reviews). Bousfield (1973), reported that amphipod tolerance to extremes of temperature is low but they probably derive protection within the macroalgal fronds or mussel matrix.

Overall, the dominant characterizing species will probably survive short term acute or long term chronic decreases in temperature at the benchmark level, while some mobile species may be lost by migration, reducing species richness. Therefore, an intolerance of low has been recorded to represent sublethal effects on growth and reproduction.
Increase in turbidity
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Increased turbidity may reduce phytoplankton primary productivity, therefore reducing the food available to Mytilus edulis and other suspension feeders. However, mussels use a variety of food sources and the effects are likely to be minimal, and this species is probably not sensitive to changes in turbidity. Increased turbidity will decrease photosynthesis and primary productivity in seaweeds when immersed but they will probably be able to compensate when emersed. For example, Fucus vesiculosus occurs in the intertidal in turbid estuaries and red algae are regarded as shade tolerant. Therefore an intolerance of low has been recorded.
Decrease in turbidity
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Decreased turbidity may increase phytoplankton primary productivity, therefore potentially increasing the food available to Mytilus edulis and other suspension feeders. Macroalgae may benefit from decreased turbidity resulting in rapid growth, especially of ephemeral green algae. Increased algal growth may destabilize the bed by increasing drag and smothering the mussels, although, grazers will probably compensate for the increased growth. Therefore, an intolerance of low has been recorded.
Increase in wave exposure
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This biotope occurs in moderately wave exposed and exposed shores. Mussels are tolerant of wave action, replacing fucoids and barnacles with increasing wave exposure and increase their byssus thread production (and hence attachment) with increased by water agitation (Young, 1985). However, Young (1985) suggested concluded that mussels would be susceptible to sudden squalls and surges. Fouling organisms, e.g. barnacles and seaweeds, may also increase mussel mortality by increasing weight and drag, resulting in an increased risk of removal by wave action and tidal scour (Suchanek, 1985; Seed & Suchanek, 1992). Winter storms and increased wave exposure are likely to result in removal of patches of mussels, especially where hummocks form, creating gaps in the bed.

However, with increasing wave exposure, the fucoids are likely to be lost, replaced by exposure tolerant algae such as Porphyra. The mussel bed is likely to become more patchy and dynamic with cycles of losses of mussels and recovery perhaps resembling £ELR.MytB£. Once formed gaps may be enlarged by wave action. In Mytilus californianus gaps were enlarged during winter, while recolonization and recovery rates increased in summer (Seed & Suchanek, 1992). A reduction in macroalgae will result in loss of associated mesoherbivores. Similarly, mobile gastropods such as top shells and littorinids are likely to be lost.

Overall, an increase in wave exposure is likely to result in a more patchy mussel beds interspersed by barnacles, few fucoids and red algae at the lower limit of the biotope, similar to £ELR.MytB£. Although the mussel bed will probably survive, the biotope as described will be lost, and probably replaced by a mussel and barnacle biotope characteristic of more wave exposed shores. Therefore, an intolerance of high has been recorded. Recoverability will probably be moderate (see additional information below).
Decrease in wave exposure
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A decrease in wave exposure from e.g. moderately exposed to very sheltered will have marked effects on the biotope. While many of the species present are tolerant of sheltered conditions, including Mytilus edulis, this biotope is likely to become replaced by fucoid dominated communities. Therefore, an intolerance of high has been recorded. Once conditions return to their prior state recoverability is likely to be moderate (see additional information below).
Noise
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Most of the invertebrates within the community are probably not sensitive to noise at the benchmark level. Mytilus edulis can probably detect slight vibrations in its immediate vicinity, however, it probably detects predators by touch (on the shell) or by scent. Therefore, it is probably insensitive to noise disturbance at the levels of the benchmark. Birds are major predators and several species are highly intolerant of noise. Therefore, noise at the level of the benchmark may disturb predatory birds, so that the mussel populations may benefit indirectly.
Visual Presence
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Mytilus edulis can probably detect changes in light commensurate with shading by predators. But its visual acuity is probably very limited and it is unlikely to be sensitive to visual disturbance. Birds 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. Therefore, visual disturbance may be of indirect benefit to mussel populations.
Abrasion & physical disturbance
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Daly & Mathieson (1977) reported that the lower limit of Mytilus edulis populations at Bound Rock, USA, was determined by burial or abrasion by shifting sands. Wave driven logs have been reported to influence Mytilus edulis populations, causing the removal of patches from extensive beds that subsequently open the beds to further damage by wave action. It is likely that abrasion or impact at the level of the benchmark would also damage or remove patches of the population.

The effects of trampling on Mytilus californianus beds in Australia were studied by Brosnan & Cumrine (1994). They concluded that mussel beds were intolerant of trampling, depending on bed thickness, and noted that in heavily tramped site mussels were uncommon and restricted to crevices. Trampling also inhibited subsequent recovery. Trampling pressure was most intense in spring and summer, so that gaps and patches created by storms in winter were not repaired but exacerbated. Fucoid cover has also been reported to be reduced by trampling (Holt et al., 1997). Brosnan & Cumrine (1994) also observed that barnacles were crushed and removed by trampling in California but recovery took place within one year following the cessation of trampling.

Therefore, it is likely that abrasion and physical disturbance at the benchmark level will result in loss of a proportion of the mussel patches, fucoids and their associated species and an intolerance of intermediate has been recorded. Recoverability is likely to be high (see additional information below). Large scale abrasion e.g. due to a vessel grounding, is likely to be similar to substratum loss in effect.
Displacement
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The dominant characterizing species are sessile. Barnacles and fucoids can not re-attach one removed from the substratum. Dare (1976) reported that individual mussels swept or displaced from mussel beds rarely survived, since they either became buried in sand or mud, or were scattered and eaten by oystercatchers. However, mussels can attached to a wide range of substrata and should a mussel be displaced to a suitable substratum it is likely to be able to attach itself using byssus threads quickly. If displaced, limpets may not be able to self-right themselves before succumbing to predation, especially from birds. Mobile gastropods may be washed to deeper water, only to return, while mobile crustaceans are unlikely to be adversely affected. However, displacement would result in removal of the mussel patches, fucoid and barnacle cover, and loss of their associated community, and hence the biotope. Intolerance has, therefore, been recorded as high. Recoverability is likely to be moderate (see additional information below).

Chemical Factors

Synthetic compound contamination
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The effects of contaminants on mussels, barnacles, limpets and fucoids have been particularly well studied. Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarised in the MarLIN review and Holt et al. (1998). A variety of chemical contaminants have been shown to produce sublethal effects and reduce scope for growth (e.g. PCBs, and organo-chlorides) (Widdows et al., 1995), while others (e.g. the detergent BP1002, the herbicide trifluralin and TBT) cause mortalities.

Barnacles (e.g. Semibalanus balanoides) have a low resilience to chemicals such as dispersants, dependant on the concentration and type of chemical involved (Holt et al., 1995). Limpets are extremely intolerant of aromatic solvent based dispersants used in oil spill clean-up (Smith, 1968; see MarLIN review of Patella vulgata for details). In addition, populations of dog whelk Nucella lapillus have been significantly reduced in areas subject to TBT population (see Bryan & Gibbs, 1991 and MarLIN review for discussion). Similarly, most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods, isopods, mysids, shrimp and crabs) and fish (Cole et al., 1999). The pesticide ivermectin is very toxic to crustaceans, and has been found to be toxic towards some benthic infauna such as Arenicola marina (Cole et al., 1999).

Fucoids are generally quite robust in terms of chemical pollution but Fucus vesiculosus is extraordinarily highly intolerant of chlorate, as found in pulp mill effluents (Holt et al., 1997). Laboratory studies of the effects of oil and dispersants on several red algae species (Grandy 1984 cited in Holt et al. 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination.

Overall, a number of chemical contaminants are likely to result in reduced growth and condition and loss of a proportion of the mussel population and hence the bed. Loss of intolerant dog whelks may be advantageous, especially at the lower limit of the mussel bed. Loss of intolerant epifaunal and epifloral grazers such as gastropods, isopods and amphipods may result in an increase in fouling of the mussels themselves by fucoids in particular resulting in increased loss due to wave action. Therefore a proportion of the mussel bed will be lost, while the species richness may show a marked decline, an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information below).
Heavy metal contamination
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Heavy metal contamination affects different taxonomic groups and species to varying degrees.
  • The effects of contaminants on Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarised in the MarLIN review. Heavy metals were reported to cause sublethal effects and occasionally mortalities in mixed effluents.
  • Bryan (1984) suggested that adult gastropod molluscs (e.g. Littorina littorea and Nucella lapillus) were relatively tolerant of heavy metal pollution.
  • Crustaceans are generally regarded to be intolerant of cadmium (McLusky et al., 1986). In laboratory investigations Hong & Reish (1987) observed 96 hour LC50 of between 0.19 and 1.83 mg/l in the water column for several species of amphipod.
  • Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes. However, it is generally accepted that adult fucoid are relatively tolerant of heavy metal pollution (Holt et al., 1997).
Overall, a proportion of the mussel bed and some intolerant species such as amphipods may be lost. An increase in fucoid abundance due to loss of mesoherbivores may also result in a increased vulnerability to wave related damage (see wave exposure above). Therefore, an intolerance of intermediate has been recorded. Recoverability will probably be high (see additional information below).
Hydrocarbon contamination
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Hydrocarbon contamination, e.g. from spills of fresh crude oil or petroleum products, may cause significant loss of component species in the biotope, through impacts on individual species viability or mortality, and resultant effects on the structure of the community.
    The effects of contaminants on Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarised in the MarLIN review and Holt et al. (1998). Overall, hydrocarbon tissue burden results in decreased scope for growth and in some circumstances may result in mortalities, reduced abundance or extent of Mytilus edulis (see review).
  • Fucus vesiculosus shows limited intolerance to oil. After the Amoco Cadiz oil spill Fucus vesiculosus suffered very little (Floc'h & Diouris, 1980). Indeed, Fucus vesiculosus, may increase significantly in abundance on a shore where grazing gastropods have been killed by oil, although very heavy fouling could reduce light available for photosynthesis and in Norway a heavy oil spill reduced fucoid cover.
  • Littoral barnacles (e.g. Semibalanus balanoides) have a high resistance to oil (Holt et al., 1995) but may suffer some mortality due to the smothering effects of thick oil (Smith, 1968).
  • Gastropods (e.g. Littorina littorea and Patella vulgata) and especially amphipods have been shown to be particularly intolerant of hydrocarbon and oil contamination (see Suchanek, 1993).
  • The abundance of littorinids decreased after the Esso Bernica oil spill in Sullom Voe in December 1978 (Moore et al., 1995). The abundance of Patella sp., Littorina saxatilis, Littorina littorea and Littorina neglecta and Littorina obtusata were reduced but had returned to pre-spill levels by May 1979. In heavily impacted sites, subjected to clean-up, where communities were destroyed in the process, Littorina saxatilis recovered an abundance similar to pre-spill levels within ca 1 year, while Littorina littorea took ca 7 years to recover prior abundance (Moore et al., 1995).
  • Widdows et al. (1981) found Littorina littorea surviving in a rockpool, exposed to chronic hydrocarbon contamination due to the presence of oil from the Esso Bernica oil spill.
  • Laboratory studies of the effects of oil and dispersants on several red algae species (Grandy 1984 cited in Holt et al. 1995) concluded that they were all intolerant of oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination.
Loss of grazing gastropods and mesoherbivores after oil spills results in marked increases in the abundance of ephemeral green algae (e.g. Ulva spp.) and fucoids (Southward & Southward, 1978; Hawkins & Southward, 1992; Raffaelli & Hawkins, 1999). As a result, surviving mussels may be smothered by macroalgae and subsequently lost due to wave action. The mussels may succumb directly to smothering by oil which is likely to be retained within the mussel matrix resulting is additional mortality to interstitial and infauna species. Although a proportion of the mussel population may survive hydrocarbon contamination, the additional effects on the community and potential for smothering suggest that the biotope will be lost. Therefore, an intolerance of high has been recorded.

On wave exposed rocky coasts oil will be removed relatively quickly. Recovery of rocky shore populations was intensively studied after the Torrey Canyon oil spill in March 1967. Loss of grazers results in an initial flush of ephemeral green then fucoid algae, followed by recruitment by grazers including limpet, which free space for barnacle colonization (see recoverability for details). On shores that were not subject to clean up procedures, the community recovered within ca 3 years, however, in shores treated with dispersants recovery took 5-8 years but was estimated to take up to 15 years on the worst affected shores (Southward & Southward, 1978; Hawkins & Southward, 1992; Raffaelli & Hawkins, 1999). Recovery of the patches of mussels would probably depend on a reduction in macroalgal cover and recovery of the barnacle cover. Therefore, a recoverability of moderate has been recorded (see additional information below).

Radionuclide contamination
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Insufficient information
Changes in nutrient levels
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Nutrient enrichment may lead to an increase in algal growth but also leads to eutrophication and associated increases in turbidity and suspended sediments (see above), deoxygenation (see below) and the risk of algal blooms.

Increased nutrient may increase growth in fast growing species (e.g. Ulva spp. and Ulva lactuca) to the detriment of slower growing species of macroalgae. However, Fucus vesiculosus was observed to grow in the vicinity of a sewage outfall (Holt et al., 1997) and is probably not sensitive. An increase in ephemeral algae may be detrimental to the mussel bed due to smothering of the mussels.

Mytilus edulis may benefit from moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic material. The resultant increased food availability may increase growth rates, reproductive potential and decrease vulnerability to predators.

Mussels are suspension feeders and accumulate toxins from toxic algae resulting in closure of shellfish beds (Shumway, 1992). The toxic algal blooms themselves (or deoxygenation resulting from their death) have been shown to cause tumours, sublethal effects, reproductive failure and to be highly toxic to Mytilus edulis, and result in mass mortalities in the dog whelk Nucella lapillus (Pieters et al., 1980; Shumway, 1990; Landsberg, 1996; Holt et al., 1998; Gibbs et al., 1999).

Therefore, algal blooms may result in loss of a proportion of the biotope and its associated species and an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information).
Increase in salinity
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This biotope occurs in full salinity and is unlikely to experience an increase in salinity, save due to short term evaporation of interstitial water.
Decrease in salinity
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Mytilus edulis and Fucus vesiculosus are considered to be tolerant of a wide range of salinity (see MarLIN reviews for details). Most of the characterizing species (e.g. Littorina littorea, Semibalanus balanoides, and Patella vulgata) are tolerant of variable salinity, although Patella is not tolerant of reduced salinity. The intertidal interstitial invertebrates and epifauna probably experience short term fluctuating salinities, with increased salinity due to evaporation or reduced salinities due to rainfall and freshwater runoff when emersed.

Prolonged reduction in salinity, e.g. from full to reduced is likely to adversely affect species richness of the biotope. While the dominant species will probably survive, the species richness of the biotope will be reduced due to loss of less tolerant red algae and some intolerant invertebrates. Areas of freshwater runoff in the intertidal promote the growth of ephemeral greens, probably due to their tolerance of low salinities and inhibition of grazing invertebrates. Therefore, an intolerance of intermediate has been recorded, together with a decline in species richness. Recoverability is likely to be high (see additional information below).

Changes in oxygenation
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In moderately wave exposed to exposed habitats the resultant water movement and turbulence probably provides adequate oxygenation so that deoxygenation at the benchmark is unlikely to occur except under extreme circumstances.

Biological Factors

Introduction of microbial pathogens/parasites
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Information of the effects of diseases or parasites in all characterizing species in the community was not available.
  • Mytilus spp. hosts a wide variety of disease organisms. parasites and commensally from many animal and plant groups including bacteria, blue green algae, protozoa, boring sponges, boring polychaetes, boring lichen, the intermediary life stages of several trematodes, the copepod Mytilicola intestinalis (red worm disease) and decapods e.g. the pea crab Pinnotheres pisum (Bower, 1992; Bower & McGladdery, 1996). Bower (1992) noted that mortality from parasitic infestation in Mytilus sp. was lower than in other shellfish in which the same parasites or diseases occurred. Mortality may result from the shell boring species such as the polychaete Polydora ciliata or sponge Cliona celata, which weaken the shell increasing the mussels vulnerability to predation (see MarLIN review for details).
  • Barnacles are parasitised by a variety of organisms and, in particular, the cryptoniscid isopod Hemioniscus balani , in which heavy infestation can cause castration of the barnacle.
  • Intertidal gastropods often act a secondary hosts for trematode parasites of sea birds. For example, Nucella lapillus may be infected by cercaria larvae of the trematode Parorchis acanthus. Infestation causes castration and continued growth (Feare, 1970b; Kinne, 1980; Crothers, 1985).
Overall, the occurrence of diseases and parasites are probably highly variable but significant infestations may result in loss of the proportion of the mussel bed and important members of the community, either through mortality or reproductive failure. Therefore, an intolerance of intermediate has been recorded. Recovery is likely to be high (see additional information below).
Introduction of non-native species
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The most significant non-native species currently likely to occur in this biotope is the barnacle Elminius modestus, which may replace Semibalanus balanoides in estuaries but is less competitive on exposed coasts (Raffaelli & Hawkins, 1996). The South American mytilid Aulocomya ater was reported recently in the Moray Firth, Scotland in 1994 and again in 1997 (McKay, 1994; Holt et al., 1998; Eno et al., 1997). Aulocomya ater is thought to have a stronger byssal attachment than Mytilus edulis and may replace Mytilus edulis in more exposed areas if it reproduces successfully (Holt et al., 1998). However, there is no evidence of competition at present. Overall, there is little evidence of this biotope being adversely affected by non-native species.
Extraction
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The only regularly harvested species to occur in this biotope are Mytilus edulis and Littorina littorea. Holt et al., (1998) suggest that when collected by hand at moderate levels using traditional skills mussel beds will probably retain most of their biodiversity. They also cite incidences of over-exploitation of easily accessible small beds by anglers for bait. 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. The edible winkle Littorina littorea is harvested by hand, without regulation, for human consumption. In some areas, notably Ireland, collectors have noted a reduction in the number of large snails available (see MarLIN review). Fucoids may be harvested by hand locally, but the abundance in this biotope is low and unlikely to attract commercial harvesting. Overall, removal of 50% of the key or important characterizing species (see benchmark) is likely to result in a reduction of the extent of the mussel bed and its associated species, and an intolerance of intermediate has been recorded. Prolonged un-regulated collection may result in loss of the bed e.g. a small bed close to a road on Anglesey was almost eliminated by anglers and bait diggers over a period of years (Holt et al., 1998). Recoverability is likely to be high.

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Recoverability
Macroalgae such as ephemeral greens (e.g. Ulva spp.) with recruit rapidly and Fucus vesiculosus recruits readily to cleared areas of the shore and full recovery takes 1-3 years (Holt et al., 1997).

Mytilus edulis is highly fecund but larval mortality is high. Larval development occurs within the plankton over ca 1 month (or more), with high dispersal potential. Recruitment within the population is possible when larvae may be entrained within enclosed coasts but it is likely that larval produced in open coast examples of the biotope are swept away from the biotope to settle elsewhere. Larval supply and settlement could potentially occur annually, however, settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992). Once settled, Mytilus edulis can reproduce within its first year if growth conditions allow.

On rocky shores, gaps in beds of mussels are often colonized by barnacles and fucoids, barnacles enhancing subsequent recruitment of mussels. The presence of macroalgae in disturbance gaps in Mytilus califorianus populations, where grazers were excluded, inhibited recovery by the mussels. In New England, U.S.A, prior barnacle cover was found to enhance recovery by Mytilus edulis (Seed & Suchanek, 1992). Cycles of loss and recruitment leads to a patchy distribution of mussels on rocky shores. High intertidal and less exposed sites recovered slower than low shore, more exposed sites. Several long term studies showed that gaps in mussel beds took a long time to heal, but 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). Recruitment in mobile species may be rapid once the mussel matrix develops.

Development of the community from bare or denuded rock is likely to follow a similar succession to that occurring after an oil spill. The loss of grazing species results in an initial proliferation of ephemeral green then fucoid algae, which then attracts mobile grazers, and encourages settlement of other grazers. Limpet grazing reduces the abundance of fucoids allowing barnacles to colonize the shore. Recovery of rocky shore populations was intensively studied after the Torrey Canyon oil spill in March 1967. Areas affected by oil alone recovered rapidly, within 3 years. But other sites suffered substantial damage due to the spilled oil and the application of aromatic hydrocarbon based dispersants. In the latter sites, populations of fucoids were abnormal for the first 11 years, and limpet Patella vulgata populations were abnormal for at least 10-13 years. Recovery rates were dependant on local variation in recruitment and mortality so that sites varied in recovery rates, for example maximum cover of fucoids occurred within 1-3 years, barnacle abundance increased in 1-7 years, limpet number were still reduced after 6-8 years and species richness was regained in 2 to >10 years. Overall, recovery took 5-8 years on many shores but was estimated to take about 15 years on the worst affected shores (Southward & Southward, 1978; Hawkins & Southward, 1992; Raffaelli & Hawkins, 1999).

This biotope is characterized by the presence of dense Mytilus edulis. Mytilus spp. populations were considered to have a strong ability to recover from environmental disturbance (Holt et al., 1998; Seed & Suchanek, 1992). While good annual recruitment is possible, recovery of gaps in the mussel population may take up to 5 years. However, where the biotope is significantly damaged recovery of the mussel population may be delayed by 1-7 years for the initial macroalgal cover to reduce and barnacle cover to increase. Therefore, a recognisable biotope may take between 5 -10 years to recover depending on local conditions. However, it should be noted that in certain circumstances and under some environmental conditions recovery may take significantly longer.

This review can be cited as follows:

Tyler-Walters, H. 2002. Mytilus edulis and Fucus vesiculosus on moderately exposed mid eulittoral 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=46&code=2004>