Biodiversity & Conservation

SS.SMu.CFiMu.SpnMeg

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Most species are infaunal or epifaunal and will be lost if the substratum is removed so the overall intolerance of the biotope is high. Although some of the mobile species in the biotope may be able to escape, most, such as the harbour swimming crab Liocarcinus depurator and the starfish Asterias rubens are not very fast moving and so are also likely to be removed. Nothing is known about the life cycle and population dynamics of British sea pens, but data from other species suggest that they are likely to be long-lived and slow growing with patchy and intermittent recruitment. The burrowing megafauna in the biotope vary in their longevity and reproductive strategies and some species do not reach sexual maturity for several years. Calocaris macandreae, for example, does not reproduce until five years old. Therefore, it seems likely that a community of sea pens and burrowing megafauna may take longer than five years to recover and so a recoverability rank of moderate is reported (see additional information).
Smothering
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The biotope will have low intolerance to smothering by 5 cm of sediment because most species are burrowing and live within the sediment anyway. The burrowing thalassindean crustaceans, the echiuran worm Maxmuelleria lankesteri, infaunal polychaetes, brittlestars and bivalves are not likely to be affected by smothering by 5 cm of sediment. There may be an energetic cost expended to either re-establish burrow openings or to move up through the sediment though this is not likely to be significant. The sea pens Virgularia mirabilis and Pennatula phosphorea are able to withdraw rapidly into the sediment and appear to be able to recover from smothering. Although the sea pen Funiculina quadrangularis is not able to withdraw into the sediment its height, up to 2m, means that it is unlikely to be affected by smothering of 5cm of sediment. Most animals will be able to reburrow or move up through the sediment within hours or days so recovery is set at immediate (see additional information). Intolerance to smothering by other factors such as oil may be higher.
Increase in suspended sediment
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Most species in the biotope are burrowing infauna so will not be affected by an increase in suspended sediment. There may be possible clogging of the feeding organs of the suspension feeding sea pens although since these animals are able to self-clean this is not likely to be very energetically costly, particularly at the level of the benchmark. Some species may benefit from increased food supply if suspended sediment has a high organic content. However, since most species in the biotope have low intolerance to an increase in suspended sediment at the benchmark level an overall rank of low is also reported for the biotope. Overall species composition and richness is not expected to be affected. On return to normal, suspended sediment levels recovery will be immediate as affected species will be able to self-clean within a few days.
Decrease in suspended sediment
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A decrease in suspended sediment and siltation will reduce the flux of particulate material to the seabed. Since this includes organic matter the supply of food to the biotope would probably also be reduced. However, the benchmark is a reduction in suspended sediment of 100 mg/l for a month which is unlikely to have a significant effect on the biotope and would not alter species composition. Intolerance is therefore, assessed as low. On return to normal conditions, recovery will be rapid and a rank of very high is recorded.
Desiccation
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The biotope only occurs in the circalittoral zone (below 15 m) and is not subject to desiccation.
Increase in emergence regime
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The biotope only occurs in the circalittoral zone (below 15 m) and is not subject to emergence.
Decrease in emergence regime
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The biotope only occurs in the circalittoral zone (below 15 m) and is not subject to emergence.
Increase in water flow rate
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The biotope is found in areas of weak or very weak tidal streams and so is likely to be intolerant of increases in water flow. Strong tidal currents keep most of the organic particles in the sediment in suspension which can support suspension feeders even in low organic content sediments. The horizontal supply of small and light nutritious particles by resuspension and advective transport has been shown to influence the growth rate of suspension-feeding benthos (Dauwe, 1998). However, some suspension feeders in the biotope will be unable to feed if the water flow rate increases by two categories in the water flow scale (see benchmarks). The sea pen Virgularia mirabilis for example, will retract into the sediment at water currents speeds greater than 0.5m/s (i.e. 1 knot). If water speeds remain at this level or above, the sea-pen will be unable to extend above the sediment, unable to feed and will die. Increases in flow rate will change the surface layer of the sediment structure, removing the fine mud element to leave the coarser particles behind. A long term increase (i.e. the benchmark level of one year) will change the nature of the top layers of sediment, becoming coarser and possibly unsuitable for some shallow burrowing species such as the brittle stars Amphiura. Deeper burrowing species such as the thalassinidean crustaceans Callianassa subterranea and Nephrops norvegicus are not likely to be affected by sediment changes at the surface. The overall impact of an increase in water flow rate on the biotope may be the loss of some key species, such as sea pens, which changes the biotope, and some other species such as brittle stars and so intolerance is assessed as high. In slightly more energetic conditions and coarser sediment the biotope CMS.AfilEcor which includes Callianassa subterranea and sparse Virgularia mirabilis is more likely to be present. Recovery has been assessed as high (see additional information).
Decrease in water flow rate
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The biotope exists in habitats where tidal streams are already very weak so a decrease in flow rate would result in almost non-moving water. Tidal currents keep most of the organic particles in the sediment in suspension which can support suspension feeders even in low organic content sediments. Therefore, if water movement becomes negligible suspended organic particles available to filter feeders such as the sea pens will decline. Growth and fecundity will be affected and over a period of a year may result in the death of sea pens. In enclosed or semi-enclosed water bodies, such as sea lochs, negligible water flow may result in some deoxygenation of the overlying water and the loss of some intolerant species. The sea pen Virgularia mirabilis for example, has high intolerance to deoxygenation and may die. However, other species such as Callianassa subterranea and many other thalassinidean crustaceans are tolerant of reduced oxygenation and are not likely to die. The overall impact on the biotope is likely to be the loss of a few key species such as sea pens and so intolerance is assessed as high. Recovery has been assessed as high (see additional information).
Increase in temperature
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In shallow sea lochs, sedimentary biotopes typically experience seasonal changes in temperature of about 10 °C and so CMU.SpMeg may be tolerant of long term increases although growth and fecundity of some species may be affected. No information was found on the upper limit of sea pens tolerance to temperature increases. However, the distribution of the sea pens typically found in the biotope, Virgularia mirabilis, Pennatula phosphorea and Funiculina quadrangularis, extends south into the warmer waters of the Mediterranean suggesting they may be able to tolerate a long term increase in temperature of 2 °C. However, sea pens are subtidal animals where wide and rapid variations in temperature, such as experienced in the intertidal, are not so common and so may be more intolerant of a short term increase of 5 °C. The reported intolerance to changes in temperature for Virgularia mirabilis is intermediate. Since the loss of sea pens changes the biotope the intolerance of the biotope to increased temperature is also recorded as intermediate. For most deep burrowing species temperature changes in the water column are likely to be buffered to some extent by the sediment and so many individuals will not be affected. See additional information for details of recovery.
Decrease in temperature
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In shallow sea lochs, sedimentary biotopes typically experience seasonal changes in temperature of about 10 °C and so CMU.SpMeg may be tolerant of long term decreases although growth and fecundity of some species may be affected. No information was found on the lower limit of sea pens tolerance to temperature decreases. However, the distribution of the sea pens typically found in the biotope, Virgularia mirabilis, Pennatula phosphorea and Funiculina quadrangularis, extends into the northern North Atlantic where waters are colder than in the UK suggesting they may be able to tolerate a long term decrease in temperature of 2°C. However, sea pens and other species in the biotope are subtidal where wide and rapid variations in temperature, such as experienced in the intertidal, are not so common and so may be more intolerant of a short term decrease in temperature of 5°C. For most deep burrowing species temperature changes in the water column are likely to be buffered to some extent by the sediment and so many individuals will not be affected. During the very cold winter of 1962-63 a few dead Nephrops norvegicus were caught in the North Sea although the majority were caught alive (Crisp, 1964) therefore it seems likely that burrowing species will probably be not sensitive to the factor. Since one of the key faunal groups, the sea pens may be intolerant of a short term decrease and the viability of populations may be threatened the intolerance of the biotope to decreased temperature is recorded as intermediate. See additional information for details of recovery.
Increase in turbidity
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An increase in turbidity, reducing light availability may reduce primary production by phytoplankton in the water column. However, productivity in the CMU.SpMeg biotope is secondary (detritus) and is not likely to be significantly affected by changes in turbidity and so intolerance is assessed as low. In estuaries and surf zones on the lower shore turbidity can be measured in g/l so the benchmark level is low in comparison. Nevertheless, primary production by pelagic phytoplankton and microphytobenthos do contribute to benthic communities and so long term increases in turbidity may reduce the overall organic content of the detritus. Reduced food supply may affect growth rates and fecundity of some species in the biotope so intolerance is assessed as low. On return to normal turbidity levels recovery will be high as food availability returns to normal.
Decrease in turbidity
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A decrease in turbidity, increasing light availability may increase primary production by phytoplankton in the water column. However, productivity in the CMU.SpMeg biotope is secondary (detritus) and is not likely to be significantly affected by changes in turbidity and so intolerance is assessed as low. In estuaries and surf zones on the lower shore turbidity can be measured in g/l so the benchmark level is low in comparison. Nevertheless, primary production by pelagic phytoplankton and microphytobenthos do contribute to benthic communities and long term decreases in turbidity may increase the overall organic input to the detritus. Increased food supply may increase growth rates and fecundity of some species in the biotope. Nephrops norvegicus avoid bright light and exposure to high intensities causes blindness (Loew, 1976) and so a decrease in light attenuation resulting from decreased turbidity may affect the depth at which the species is present or more likely that Nephrops will only feed at night. See additional information for details of recovery.
Increase in wave exposure
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The biotope exists in areas with physically-sheltered conditions of low wave exposure and weak tidal currents. An increase in wave exposure is likely to change the composition of species present in the biotope because it is likely to disrupt feeding and burrowing and may also have an impact on reproduction and recruitment. An increase in the factor can also change the sediment characteristics which may result in a change in the proportion of suspension to deposit feeders within it. Sea pens, for example, may be unable to feed and may be damaged or broken by increased wave exposure. Virgularia mirabilis is able to withdraw into the sediment to avoid the factor but will be unable to feed if wave exposure increases are long term and will be likely to die. Coarser material is more difficult to burrow through, and organisms need to be robust the survive and so a major decline in the number of species able to inhabit the biotope is likely to result. Even very deep burrowing species like Callianassa subterranea are likely to be affected because increased wave exposure will probably disturb burrow openings and water flow through the burrows making feeding difficult. With the loss of key species, in particular the sea pens, the biotope will change so intolerance is assessed as high. See additional information for details of recovery.
Decrease in wave exposure
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The biotope occurs in areas of very low or no wave exposure so a decrease is not relevant.
Noise
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Some of the important characterizing species associated with this biotope, in particular the sea pens, may respond to sound vibrations and can withdraw into the sediment. Feeding will resume once the disturbing factor has passed. However, most of the species are infaunal and likely to be not sensitive to noise disturbance at the benchmark level. It is possible that predator avoidance behaviour in Liocarcinus depurator and other species may be triggered by noise vibrations although this has not been recorded. Therefore, unless predation pressure is reduced increased noise disturbance is not likely to have an impact on the nature and function of the biotope and a rank of not sensitive is recorded.
Visual Presence
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Most species within the biotope are burrowing and have no or poor visual perception and are unlikely to be affected by visual disturbance such as shading. Epifauna such as crabs have well developed visual acuity and are likely to respond to movement in order to avoid predators. However, it is unlikely that the species will be affected by visual disturbance at the benchmark level. The biotope is therefore, not sensitive to the factor.
Abrasion & physical disturbance
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The biotope is subject to physical disturbance because it supports a major fishery for one of its characteristic species, Nephrops norvegicus. Information on the effects of trawling on the other fauna in the biotope is limited but it is likely that the deep burrowing species such as the crustaceans Callianassa subterranea and Jaxea nocturna and the echiuran worm Maxmuelleria lankesteri and some burrowing fish will be little affected by this type of disturbance. Individual burrowing crustaceans may occasionally be displaced from burrow openings by towed gear (Atkinson, 1989). However, the animals will be able to re-establish burrow openings if these become blocked so recovery would be immediate.

Of the three sea pen species Funiculina quadrangularis is likely to be the most sensitive to abrasion and disturbance because it has a long brittle stalk and is unable to retract into the sediment. However, experimental studies have shown that all three species of seapen can re-anchor themselves in the sediment if dislodged by fishing gear (Eno et al., 1996). Eno et al. (1996) found that even if damaged Funiculina quadrangularis appeared to remain functional and this could also be true of the other sea pens. However, the apparent absence of Funiculina from open-coast Nephrops grounds may be a consequence of its susceptibility to trawl damage (D.W. Connor, pers. comm. in Hughes, 1998b).

In long term experimental trawling Tuck et al. (1998) found no effect on Virgularia mirabilis populations and Kinnear et al. (1996) found that sea pens were quite resilient to being smothered, dragged or uprooted by creels. The investigation by Tuck et al. (1998) examined the effects of extensive and repeated experimental trawl disturbance on whole benthic communities over an 18 month period in a Scottish loch that had previously been un-fished for 25 years. The subsequent patterns of recovery over a further 18 month period were also investigated. Trawling disturbance resulted in reduced species diversity and a disproportionate increase in the abundance of a few dominant species, in particular the opportunistic polychaetes Chaetozone setosa and Caulleriella zetlandica. Other species, also found in this biotope, that were observed to be sensitive include the bivalves Nucula nitidosa and Corbula gibba and the polychaetes Nephtys sp. and Terebellides stroemi. For epifaunal species, no long-term effects on the total number of species or individuals were detected, but individual species did show effects, notably an increase in the density of Ophiura sp. and a decrease in numbers of the fish Hippoglossoides platessoides and the whelk Buccinum undatum.

Other authors have also suggested that increases in echinoderm populations in the North Sea are associated with fishing disturbance (Aronson, 1990; Lindley et al., 1995).

Scavenging species such as Liocarcinus depurator, Pagurus bernhardus and Asterias rubens might be expected to benefit from fishing disturbance, through increased food availability. Kaiser & Spencer (1994) found that benthic disturbance by fishing gear caused an increase in the density of epifaunal scavengers, in response to an increase in food availability in the form of damaged and disturbed organisms. The long term effects on infauna were still noticeable after 18 months and short term effects on epifauna recovered 6 months after fishing ceased. During long term monitoring of fishing disturbance on the Northumberland coast Frid et al. (1999) observed a decrease in numbers of sedentary polychaetes, echinoid echinoderms and large (>5 cm) brittlestars. Observations of the effects of Nephrops trawl fishing in the Irish Sea led Ball et al. (2000a) to suggest that the bivalves Corbula gibba and Thyasira flexusa were sensitive to fishing disturbance.

Thus, it appears that abrasion and physical disturbance, such as that caused by fish trawling or scallop dredging, is likely to affect the species composition of the biotope and so intolerance is assessed as intermediate. Recovery is expected to be high (see additional information).

Displacement
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Most species in the biotope are burrowing and have low intolerance to displacement, such as that caused by a passing trawl that does not kill species but throws them into suspension, because animals can reburrow into suitable substrata. Displaced individuals of Virgularia mirabilis, which are not damaged (see Abrasion above for damage), will re-burrow (Jones et al., 2000) and recover completely within 72 hours, provided the basal peduncle remains in contact with the sediment surface. Eno et al. (1996) found that even when damaged Funiculina quadrangularis appeared to remain functional. Recovery will be immediate because burrowing species can rapidly reburrow into the sediment, thereby avoiding predation. Full burrow construction for some species may take longer however.

Chemical Factors

Synthetic compound contamination
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There was no information found on the effect of chemical pollutants on the biotope, probably because burrowing megafauna are generally too difficult to sample to be included in standard pollution monitoring studies. However, effects on some of the individual species in the biotope have been reported. Dahllöf et al. (1999) studied the long term effects of tri-n-butyl-tin (TBT) on the function of a marine sediment system. TBT spiked sediment was added to a sediment that already had a TBT background level of approximately 27ng g-1 (83 pmol TBT g-1) and contained the following fauna: Amphiura spp., Brissopsis lyrifera, the bivalve Abra alba and several species of polychaete. Within two days of treatment with a TBT concentration above 13.7 µmol / m² all species except the polychaetes had crept up to the surface and after six weeks these fauna had started to decay. Thus, increased contamination from TBT is likely to result in the death of some intolerant species such as brittle stars and heart urchins. Bryan & Gibbs (1991) report that crabs appear to be relatively resistant to TBT although some deformity of regenerated limbs has been observed. However, arthropods are very intolerant of the insecticide carbaryl (1-napthol n-methyl carbamate; sold under the trade name Sevin®) which has been used to control burrowing shrimp in oyster farms (Feldman et al., 2000). There is no information available on the possible consequences of chemicals to British sea pens. Different species will be affected by different chemicals but a general trend in areas of increasing pollution is a reduction in species diversity with habitats becoming dominated by pollution tolerant polychaete worms. However, Ivermectin, an anti-louse treatment coming into use in the salmon fish farming industry, has been shown to be highly toxic to sediment dwelling polychaetes (Hughes, 1998b).

Overall the biotope has been recorded as highly intolerant to synthetic chemicals. Recovery is likely to take longer than five years and has been recorded as moderate (see additional information).

Heavy metal contamination
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In Norwegian fjords Rygg (1985) found a relationship between species diversity in benthic fauna communities and sediment concentrations of heavy metals Cu, Pb and Zn. Cu in particular showed a strong negative correlation and the author suggested a cause-effect relationship. Those species not present at sites where Cu concentrations were greater than ten times higher than the background level, such as Calocaris macandreae, Amphiura filiformis and several bivalves including Nucula sulcata and Thyasira equalis, were assessed as non-tolerant species. The tolerant species were all polychaete worms. Therefore, increased heavy metal contamination in sediments may change the faunal composition of the community and decrease overall species diversity and intolerance has been assessed intermediate. Some burrowing crustaceans, brittle stars and bivalves may disappear from the biotope and lead to an increasing dominance of polychaetes. There was no information found on the effect of heavy metals on sea pens. Recovery is likely to be high.
Hydrocarbon contamination
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There was no information found on the effect of hydrocarbon pollution on the biotope. The best documented oil spill for protected habitats with soft mud/sand substrates is the West Falmouth, Florida spill of 1969. Immediately after the spill virtually the entire benthic fauna was eradicated immediately following the incident and populations of the opportunistic polychaete Capitella capitata increased to abundances of over 200,000/m² (Sanders, 1978). Persistent toxicity of Amoco Cadiz oil in sediment prevented the start of the recovery period (Clark, 1997). Callinanassa subterranea appears to be highly intolerant of sediment contaminated by oil-based drilling muds (Daan et al., 1992). Oil from spills would have to be dispersed deep into the water column to affect the biotope and since the biotope occurs in very sheltered conditions this is unlikely to occur. However, should the sediment become contaminated with oil there is likely to be the loss of many species and so intolerance is assessed as high. Nothing is known about the life cycle and population dynamics of British sea pens, but data from other species suggest that they are likely to be long-lived and slow growing with patchy and intermittent recruitment. The burrowing megafauna in the biotope vary in their longevity and reproductive strategies and some species do not reach sexual maturity for several years. Calocaris macandreae, for example, does not reproduce until five years old. Therefore, it seems likely that a community of sea pens and burrowing megafauna may take longer than five years to recover and so a recoverability rank of moderate is reported.
Radionuclide contamination
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In an investigation of bioturbation in the north-eastern Irish Sea Hughes & Atkinson (1997) surveyed several sites close to the Sellafield nuclear reprocessing plant. At a station immediately offshore from the Sellafield outfall pipeline a community similar to the CMU.SpMeg biotope was present. Burrow openings and shafts indicated the presence of the burrowing crustaceans Callianassa subterranea, Goneplax rhomboides and Upogobia deltaura. Epifauna were abundant, particularly Ophiura ophiura and Asterias rubens. The sea pen Virgularia mirabilis occurred at high density. Dragonets and small gobies were also common. Other species in the biotope such as Nephrops norvegicus and the echiuran worm Maxmuelleria lankesteri were also present at sites close to the outfall pipeline. Thus, the biotope occurs in bottom sediments that contain particles of long half-life radionuclides derived from the liquid effluent released from the reprocessing plant at Sellafield and so intolerance is assessed as low. However, species diversity may be slightly reduced compared to unpolluted sites. Recovery is likely to take less than five years and has been recorded as high (see additional information).
Changes in nutrient levels
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Most species in the biotope appear to tolerate sediments relatively high in organic content. In Loch Sween in Scotland, for example, where the organic content is about 5% and as high as 9% in some areas burrowing species such as the crustaceans Callianassa subterranea, Calocaris macandreae and Nephrops norvegicus and the echiuran worm Maxmuelleria lankesteri are present in high densities. Although absent from the most enriched areas the sea pen Virgularia mirabilis was present at organic contents of 4.5% (Atkinson, 1989). Very large increases in organic content can result in significant changes in community composition of sedimentary habitats and sometimes defaunation. Typically an increasing gradient of organic enrichment results in a decline in the suspension feeding fauna and an increase in the number of deposit feeders, in particular polychaete worms (Pearson & Rosenberg, 1978). For example, in areas under fish farm cages gross organic pollution has been observed to result in the loss of megafaunal burrowers. However, these changes generally refer to gross nutrient enrichment. At the level of the benchmark, a 50% increase in nutrients is likely to impact only the most intolerant species and may result in a reduction in the number of sea pens so intolerance is assessed as intermediate. A high recovery is expected (see additional information).
Increase in salinity
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The biotope is found in fully marine conditions so is likely to be intolerant of increases in salinity. The overall effect on the biotope of a chronic decrease in salinity for a period of a year is likely to be the loss of most species and so intolerance is reported as high. Recovery is likely to take longer than five years and has been recorded as moderate (see additional information).
Decrease in salinity
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The biotope is found in fully marine conditions and does not extend into estuaries so is likely to be intolerant of decreases in salinity. The key species are highly intolerant of salinity changes although Jones et al. (2000) suggest that Virgularia mirabilis appears to be somewhat tolerant of occasional lowering of salinity. However, the species is found only in fully marine environments and so is likely to be intolerant of a long term, chronic decrease; e.g., a change of one category from the MNCR salinity scale for one year. The overall effect on the biotope of a chronic decrease in salinity for a period of a year is likely to be the loss of most species and so intolerance is reported as high. Recovery is likely to take longer than five years and has been recorded as moderate (see additional information).
Changes in oxygenation
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Large active animals with high respiratory demands will be most affected by oxygen depletion. In moderately hypoxic conditions (1mg l-1) Nephrops norvegicus compensates by increasing production of haemocyanin (Baden et al., 1990). In the laboratory this compensation lasted one week so at the level of the benchmark the species will not be killed. However, at levels of about 0.6 mg l-1 the species died within 4 days. Catches of Nephrops norvegicus have been observed to be high when oxygenation in the water is low, probably because animals are forced out their burrows. Thalassinidean mud-shrimps are very resistant to oxygen depletion and enriched sulphide levels. Callianassa subterranea, for example, often lives in hypoxic or even anoxic conditions. Virgularia mirabilis is often found in sea lochs so may be able to tolerate some reduction in oxygenation. However, Jones et al., (2000) found sea pen communities to be absent from areas which are deoxygenated and characterized by a distinctive bacterial community and Hoare & Wilson (1977) reported Virgularia mirabilis absent from sewage related anoxic areas of Holyhead harbour. Therefore, the benchmark level of 2 mg/l of oxygenation for one week will result in the death of only the most intolerant species and maybe some individual sea pens. The total loss of populations of the key is not likely to occur at the benchmark level and since the faunal composition of the overall biotope is unlikely to change to any great extent intolerance is assessed as low. On return to normal oxygenation recovery will be immediate as respiratory rates return to normal. However, recruitment of intolerant species that are killed will be required to return the biotope to pre-impact species diversity.

Biological Factors

Introduction of microbial pathogens/parasites
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The only major disease causing organism found in the biotope is the dinoflagellate parasite, Hematodinium sp. found in Nephrops populations from the west of Scotland, Irish Sea and North Sea (Hughes, 1998b). The parasite occurs in the blood and connective tissue spaces and appears to cause death by blocking the delivery of oxygen to the host's tissues (Taylor et al., 1996). Infection is at its highest in the spring and early summer when a dense concentration of parasite cells in the blood give Nephrops an abnormal bright orange body and milky white ventral abdomen. Heavily infected animals become moribund, spend more time out of their burrows than healthy individuals making them more vulnerable to predation and fishing gear. Heavy infestation is fatal. The ecological consequences of Hematodinium infection and host mortality in Nephrops populations are unknown, but there are potential economic implications, since the disease adversely affects meat quality. Since the parasite can cause mortality of a species within the biotope intolerance is assessed as intermediate. However, so far the Nephrops fishery has not suffered any serious decline. The infection appears to be cyclical. In the Clyde Sea infection peaked in 1991-92 at 70% and had declined to 10 - 20% by 1996-7 so recovery appears to be possible within five years and so a rank of high is reported.
Introduction of non-native species
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There are no records of any non-native species invading the biotope and so is assessed as not sensitive. However, as several species have become established in British waters there is always the potential for new introduced non-native species to have an effect on the biotope.
Extraction
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Nephrops norvegicus is a characterizing species and Nephrops fisheries are of major economic importance. The species is fished throughout most of the geographic range of the biotopes in which it occurs including CMU.SpMeg. In trawled areas it is likely that the density of Nephrops norvegicus has been reduced but Hughes (1998b) reports that most stocks have the potential to recover even after heavy fishing pressure. Atkinson (1989) concluded that trawling for Nephrops was unlikely to affect other megafaunal burrowers to any great extent. The upper section of burrows will be disrupted by trawling but observations in Loch Sween have shown that surface openings are soon re-established (Hughes, 1998b). Some sea pens are likely to be uprooted by trawling activities although in observations of the impact of creeling activities all three British species proved able to re-anchor themselves provided the basal peduncle remained in contact with the sediment surface. Crabs such as Liocarcinus depurator are often extracted as a by-catch species in benthic trawling. A reduction in the density of predators may affect species abundance but is not likely to have a significant effect on overall species diversity. Removal of Nephrops norvegicus would probably not change the nature of the biotope because there are likely to be other megafaunal burrowers present. None of the key or important species in the biotope are targeted for collection or harvesting. An intolerance of intermediate has been suggested to reflect likely loss of Nephrops norvegicus. Recovery is likely to be high.

Additional information icon Additional information

Recoverability
There is very little known about community development for this biotope. Almost nothing is known about the life cycle and population dynamics of British sea pens, but data from other species suggest that they are likely to be long-lived and slow growing with patchy and intermittent recruitment. The burrowing decapods that characterise the biotope vary in their reproductive strategies and longevity. In the North Sea the life span of Callianassa subterranea appears to be 2-3 years (Rowden & Jones, 1994) and individuals become sexually mature in their first year. Time to sexual maturity is longer in Nephrops norvegicus, about 2.5 - 3 years, and for the very long-lived Calocaris macandreae individuals off the coast of Northumberland did not become sexually mature until five years of age, and produced only two or three batches of eggs in their lifetime. Although little is known of the life cycle of the echiuran worm Maxmuelleria lankesteri long term observations of populations in situ have provided no evidence of any major fluctuations in population size, and it has been suggested that the species is long-lived with stable populations and low recruitment rates. Many of the other species in the biotope, such as polychaetes and bivalves, are likely to reproduce annually, be shorter lived and reach maturity much more rapidly. Since most key species reproduce regularly but take a while to grow, recruitment will be rapid but it will take several years to reach maturity and so it will probably take at least five years for the overall community to reach maturity. Therefore, recovery will probably be moderate from factors to which the biotope is highly intolerant.

This review can be cited as follows:

Hill, J.M. 2004. Sea pens and burrowing megafauna in circalittoral soft mud. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 01/11/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=131&code=2004>