Biodiversity & Conservation


Explanation of sensitivity and recoverability

Physical Factors

Substratum Loss
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The biotope is characterized predominantly by benthic species that would be lost as a part of substratum removal and the intolerance is, therefore, high. The mobile species that might be left (especially those living in the water column: mysid shrimps and Gasterosteus aculeatus) would not constitute the biotope. For recoverability see additional information below.
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Smothering by 5 cm of sediment would most likely isolate algal growths from the small amount of water movement that exists in this habitat and de-oxygenated conditions with consequent death of algae and animals is likely to occur amongst the algal mats. However, mobile species such as Asterias rubens, Hydrobia ulvae and Akera bullata would most likely dig themselves out of smothering sediment and survive whilst Arenicola marina is unlikely to be perturbed by smothering by 5cm of sediment where it occurs on open sediments. Some attached animals are likely to be intolerant. For instance, Dare (1976) reported that mussel beds accumulated ca. 0.4-0.75m of 'mussel mud' (a mixture of silt, faeces, and pseudo-faeces) between May and September 1968 and 1971 in Morecambe Bay. Young mussels moved upwards becoming lightly attached to each other, but many were suffocated. Therefore, it appears that mussels are able to move upwards through accumulated sediment, but that a proportion will succumb. Smothering is not relevant to species in the water column except that food sources may be covered and nests of three-spined sticklebacks affected. There are sufficient species that would be likely to survive for the biotope to be identified as IMX.FiG and so intolerance is recorded as intermediate although recovery may take more than six months. For all species except those in the water column, smothering by impermeable materials would lead to high intolerance.
Increase in suspended sediment
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Increased levels of suspended sediment will increase turbidity and therefore light penetration (see below) with possibly adverse effects on algal growth. Benthic animal species in the biotope are most likely tolerant of high levels of suspended sediment. For instance, Moore (1977) reported that Mytilus edulis was relatively tolerant of turbidity and siltation, thriving in areas that would be harmful to other suspension feeders. Arenicola marina is unlikely to be perturbed by increased concentrations of suspended sediment since it lives in sediment and is probably adapted to re-suspension of sediment by wave action or during storms. Animal species in the water column (particularly mysid shrimps and Gasterosteus aculeatus) may be adversely affected by high levels of suspended sediment because of its effects on vision (see turbidity below). However, the impact is likely to be short-lived and loss of condition as a result of reduced ability to feed the consequence. Therefore a rank of low intolerance has been reported.
Decrease in suspended sediment
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Decreases in suspended sediment levels may result in reduced food supply for suspension feeding species but will improve light penetration (see turbidity below) and therefore algal growth and will improve the ability of hunting species (mysid shrimps and Gasterosteus aculeatus) to catch prey. Overall, it is expected that decreased suspended sediment levels will improve prospects for many species in the biotope.
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The biotope occurs in shallow waters and may be subject to drying out in exceptional conditions. Algae would be expected to dry and be bleached causing death. Arenicola marina is protected from desiccation because it lives in a deep, water filled burrow. Mytilus edulis, where present, can close its valves to prevent water loss. Because significant species in the biotope would be likely to be killed in the fringing parts of the biotope, intolerance is described as intermediate. Recovery is likely to be very high as re-growth of algae will occur and mobile animal species recolonize.
Increase in emergence regime
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The biotope occurs in shallow waters and may be subject to drying out in increased emergence. Algae would be expected to dry and be bleached causing death although the mat of algae would protect species under it including algal filaments. Many animal species in the biotope are mobile and would escape. Arenicola marina is protected from desiccation because it lives in a deep, water filled burrow. Mytilus edulis, where present, can close its valves and survive for significant periods out of water. Because significant species in the biotope would be likely to be killed in the fringing parts of the biotope, intolerance is described as intermediate. Recovery is likely to be very high as re-growth of algae will occur and mobile animal species recolonize.
Decrease in emergence regime
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The biotope is subtidal and decrease in emergence is not relevant.
Increase in water flow rate
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This biotope occurs in very still conditions and increase in tidal flow rate is likely to adversely affect some species so that the biotope may change to a different one. In particular, algal mats may offer significant resistance to flowing water and, as they are poorly attached or not attached, be swept away together with associated species such as Akera bullata. Many of the animal species in the biotope occur in conditions of at least moderate flow. Some mobile species such as Neomysis integer (see Lawrie et al., 1999) avoid strong flow but occur in estuaries so that they would be expected to persist. Burrowing species would be protected providing that the sediment was not swept away. For instance, increases in water flow rate are unlikely to affect Arenicola marina directly since it lives in a deep burrow. Mytilus edulis, which is abundant in high tidal currents at the exits of variable salinity habitats, may increase in abundance or occur for the first time in the biotope if increase in water flow rates is long-term. Overall, some of the key characterizing species are likely to be lost and recruitment of species that are favoured by flowing water (for instance, brown seaweeds sponges, mussels, hydroids, bryozoans and barnacles) will occur possibly switching the biotope (perhaps, where sediment predominates, to £IMX.LsacX£ - Laminaria saccharina, Chorda filum and filamentous red seaweeds on sheltered infralittoral sediment or £IMX.MytV£ - Mytilus edulis beds on variable salinity infralittoral mixed sediment. Once conditions return to low tidal flow, demise of species thriving in strong flow will occur and re-growth or recolonization is likely to be rapid.
Decrease in water flow rate
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The biotope occurs in extremely shelter situations with no or little water flow so that this factor is considered not relevant.
Increase in temperature
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The sort of location in which this biotope occurs may be subject to significant increased temperatures as a result of its isolation. Increased temperature may stimulate increased bacterial activity, increased oxygen consumption and therefore depletion of oxygen from the interstitial waters resulting in reduced oxygen levels (hypoxia) or absence of oxygen (anoxia) (see deoxygenation) in the sediment (Hayward, 1994). The lack of water circulation resulting from isolation of the habitat will be exacerbated especially in summer by the blanket of filamentous seaweed reducing water flow. Since hypoxia already occurs, increased temperature might significantly increase de-oxygenation at the seabed especially where blanketed by algae and mortality of at least some benthic species. An intolerance of intermediate has therefore been indicated. Regrowth should be rapid from remaining species and many mobile species would return rapidly.
Decrease in temperature
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The sort of location in which this biotope occurs may be subject to significant decreased temperatures as a result of its isolation. However, in winter, the characterizing algae species will be in low abundance and species such as mysids and Gasterosteus aculeatus can migrate to deeper waters. Decreased temperature is, however, likely to result in mortalities amongst some characterizing species. For instance, in Akera bullata in the Fleet, high mortality coincided with cold winds with low water and rain although it recolonized from deeper populations (Thompson & Seaward, 1989) . During the cold winter of 1962-63, Carcinus maenas adults were found moribund or dead all around the coast of Britain but smaller individuals were less affected and dominated the surviving population in March-April (Crisp, 1964). Some other benthic species may be highly tolerant. Mytilus edulis, where it occurs, can withstand extreme cold and freezing, surviving when its tissue temperature drops to -10 °C (Williams, 1970; Seed & Suchanek, 1992) . Therefore, some species might be expected to be lost or their population numbers decline as a result of decrease in temperature and an intolerance of intermediate is indicated. Since recoverability of intolerant species is likely to be by migration, recoverability is indicated as very high.
Increase in turbidity
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Increased turbidity will reduce light penetration with possibly adverse effects on growth of the dense mats of algae characteristic of this biotope. Benthic diatom productivity is also likely to be reduced possibly reducing this source of food for Arenicola marina. However, Arenicola marina also feeds on meiofauna, bacteria and organic particulates in the sediment is unlikely to be affected significantly. Similarly, Akera bullata switches to feeding on the muddy bottom when algal growth is sparse in winter (Thompson & Seaward, 1989) and may be adversely affected by lack of diatom production. The extent of the biotope with depth may be reduced and therefore an intolerance of intermediate is indicated. The algal species likely to be affected will colonize and grow very rapidly once turbidity declines and so a very high recoverability is indicated.
Decrease in turbidity
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Decreased turbidity will improve light penetration and therefore algal growth and will improve the ability of hunting species (mysid shrimps and Gasterosteus aculeatus) to catch prey. Overall, it is expected that decreased turbidity will improve prospects for many species in the biotope.
Increase in wave exposure
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Increased wave exposure may cause erosion of fine sediments decreasing the extent of the available habitat for some species. The larval nursery areas of Arenicola marina may be particularly intolerant since the larvae inhabit the top few centimetres of the substratum. Increased wave exposure may also dislodge mats of filamentous algae and species such as Akera bullata displacing them to unfavourable habitats. Some other species may not be affected. Adult Arenicola marina living below the sediment surface and anyway known from moderately exposed situations is likely to survive. Mobile species such as Carcinus maenas will find shelter or move to deeper water. Mysid shrimps are also likely to find shelter or to abandon the area. Increased wave exposure may improve oxygenation and prevent hypoxia. However, overall, since the filamentous algae are likely to be adversely affected the biotope may become difficult to recognise and switch to another more characteristic of wave exposed conditions and an intolerance of high is suggested.
Decrease in wave exposure
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The biotope occurs in ultra sheltered situations and so decrease in wave exposure is considered not relevant.
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The 'fright' response of some mobile species in the biotope, especially fish and mysids, will most likely be elicited by sudden noise. Acclimation is likely to occur in relation to continuous noise. However, most species in the biotope are unlikely to be sensitive to noise per se. For instance, Arenicola marina may respond to vibrations from predators or bait diggers by retracting to the bottom of their burrow. Strong vibrations may therefore interfere with feeding in some animals in the biotope.
Visual Presence
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Fish and mysids in the biotope rely on vision for feeding and are susceptible to possible predators - most likely diving into algal mats when apparently threatened. Visual presence may therefore interfere with normal feeding activity and result in very minor loss of condition.
Abrasion & physical disturbance
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Both the epibiota and the infauna in the biotope are likely to be intolerance of abrasion, such as dredging or dragging an anchor. Mats of algae will be displaced with any associated species but, in the still conditions that prevail in this biotope, are likely to survive displacement. Soft bodied epifauna, such as ascidians, are most vulnerable, and are likely to suffer mortality. Crabs may be crushed. Fish and mysids in the water column are highly mobile and unlikely to be unaffected directly. The infaunal annelids are predominantly soft bodied, live within a few centimetres of the sediment surface and may expose feeding or respiration structures where they could easily be damaged by a physical disturbance such as a dragging anchor. The species with robust exoskeletons, such as bivalves and crustaceans, are likely to be the most resistant. The overall intolerance of the biotope is recorded as intermediate. The recoverability is assessed to be very high (see additional information below).
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Algal mats with their associated species will most likely continue to survive even if unattached unless displaced to deeper water or onto the shore. Mytilus edulis should re-attach if moved to a suitable substratum. Some algae with holdfasts and sessile species, for instance ascidians, may suffer mortality, probably after surviving for some time free-living, as they will be unable to reattach. Mobile species such as fish, mysids, starfish and crabs will migrate back. Displacement from the sediment is likely to expose species such as Arenicola marina to an increased risk of predation. However, once on the substratum surface worms are capable of burrowing back into the sediment. Some loss of individuals is likely and so an intolerance of intermediate is indicated. Regrowth and migration is likely to result in rapid recolonization.

Chemical Factors

Synthetic compound contamination
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The impact of pesticides on two of the characterizing algal species in the biotope was studied by Ramachandran et al. (1984). At a concentration of 50 ppb, DDT reduced photosynthesis in Chaetomorpha linum by 63% and in Ulva intestinalis(as Enteromorpha intestinalis) by 74%. Lindane at 50 ppb reduced photosynthesis to 50% in Ulva intestinalis. Significant work has also been undertaken on effects of different chemicals on three of the characterizing animal species in the biotope. The three-spined stickleback Gasterosteus aculeatus, the opossum shrimp Neomysis integer and the blow lug Arenicola marina. For Gasterosteus aculeatus:
  • Oestrogenic endocrine disrupting chemicals have been found (Bell, 2001) to exert "subtle yet important effects on behaviour".
  • Exposure to the organotin compound bis(tri-butyl tin) oxide caused significant changes in the spatial position of the fish in the aquarium.
For Neomysis integer:
  • Roast et al. (1999, 1999b, 1999c) suggested a relatively low 96 hour LC50 for pesticides and trace metals and that 'scope for growth' was significantly affected by chlorpyrifos exposure.
  • Davies et al., (1997) investigated the acute toxicity of Ivermectin (22, 23-dihydroavermectin B1), a chemotherapeutant for the treatment of farmed salmon infested with sea lice, to Neomysis integer. The 95 hour LC50 of Ivermectin to Neomysis integer was 70 ng/l , with 95% confidence limits of 44 ng/l and 96 ng /l.
For Arenicola marina:
  • Ivermectin was found to produce a 10 day LC50 of 18µg ivermectin /kg of wet sediment. Sub-lethal effects were apparent between 5 - 105 µg/kg. Cole et al. (1999) suggested that this indicated a high sensitivity.
  • Naphthalene (a poly-aromatic hydrocarbon, PAH) was found to accumulate from the water column rather than sediment, however it was nearly completely lost from Arenicola marina within 24 hrs (Cole et al. 1999).
  • Bryan & Gibbs (1991) reviewed the reported effects on tributyl tin (TBT). They concluded that Arenicola cristata larvae were unaffected by 168 hr exposure to 2000 ng TBT/ l seawater and was probably relatively tolerant.
  • Besten, et al. (1989) reported that exposure of Asterias rubens to polychlorinated biphenyls (PCBs) resulted in production of defective offspring and an intolerance of intermediate is recorded as the viability of the species was affected.

Whilst recovery of the biotope may occur rapidly following many events, contaminated sediments would probably take longer to recover, so that a recovery of high has been reported.
Heavy metal contamination
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The toxicity of copper to Gasterosteus aculeatus was tested by Svecevicius & Vosyliene (1996) and found that, in comparison with perch, roach, dace and rainbow trout, three-spined sticklebacks had a high tolerance.
Roast et al. (2001) and Wildgust & Jones (1998) studied the effects of cadmium exposure on Neomysis integer (see review of Neomysis integer for details) and found variable effects on behaviour and mortality that was also linked to salinity levels. For Neomysis integer, exposure to cadmium caused significant disruption of hyperbaric behaviour and more individuals were in the water column than in control experiments.
Arenicola marina is presently used routinely as a standard bioassay organism for assessing the toxicity of marine sediments.
  • At high concentrations of Cu, Cd or Zn the blow lug left the sediment (Bat & Raffaelli, 1998).
  • Bryan (1984) suggested that polychaetes are fairly resistant to heavy metals, based on the species studied. Short term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb whereas Cd, Ni, Co and Se the least toxic.
  • Exposure to 10 ppm Cd in seawater halted feeding in Arenicola marina although they continued at 1 ppm (Rasmussen et al., 1998). Rasmussen et al., (1998) pointed out that bioturbation by the blow lug increases the rate of uptake of Cd from the water to the sediment, however, where sediments were already contaminated, bioturbation ensured that some fraction of the contaminant would be mobilised to the surface sediment and the environment.
  • Arenicola marina was found to accumulate As, Cd, Sb, Cu, and Cr when exposed to pulverised fuel ash (PFA) in sediments (Jenner & Bowmer, 1990).
  • Heavy metals have also been reported to effect gametogenesis and early larval development in the starfish Asterias rubens likely to pose a considerable threat to populations of Asterias rubens in terms of recruitment success (Besten et al.,1991).
The intolerance of the biotope has been assessed to be low as effects seem limited to sub-lethal effects (for instance, disruption of swimming, disruption of reproduction) on some members of the biotope.
Hydrocarbon contamination
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The locations in which IMX.FiG occurs are mainly very enclosed and shallow so that oil is unlikely to be dispersed and is likely to penetrate to the seabed. Oil may therefore be particularly damaging to this biotope.
  • Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected.
  • Hailey (1995) cited substantial kills of Nereis, Cerastoderma, Macoma, Arenicola and Hydrobia as a result of the Sivand oil spill in the Humber estuary in 1983.
  • Levell (1976) examined the effects of experimental spills of crude oil and oil: dispersant (BP1100X) mixtures on Arenicola marina. Single spills caused 25-50% reduction in abundance and additional reduction in feeding activity. Up to 4 repeated spillages (over a 10 month period) resulted in complete eradication of the affected population either due to death or migration out of the sediment.
  • Asterias rubens is intolerant of hydrocarbon pollution. For instance Asterias rubens disappeared entirely from upper sublittoral samples in a mesocosm receiving a high dose of WAF diesel oil (Bokn et al.,1993) and experienced high mortality during the Torrey Canyon oil spill (Smith, 1968).
Because of the sheltered nature of the habitat, several component members of the biotope are likely to be killed and an intolerance of high has been reported. Recovery will depend on whether residual oil is in sediments but, if removed or of low toxicity, would be expected to be high (see additional information).
Radionuclide contamination
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No information has been found on effects of radionuclide accumulation on species in the biotope. However, there is information on accumulation of radionuclides.
  • Following the Chernobyl nuclear incident, the highest accumulation of caesium137 was found in Chaetomopha linum of the green algae (Gueven et al., 1990).
  • Kennedy et al. (1988) reported levels of 137Cs in Arenicola sp. of 220-440 Bq/kg from the Solway Firth.
  • Guary et al. (1982) measured the body burden of 239Pu 240Pu and 238Pu and found 94.5% and 95.6% respectively in the body wall of Asterias rubens. It became apparent that in the environment the water pathway predominates in the uptake of radionuclides by asteroids as the body wall is in constant contact with the transferring medium.
Changes in nutrient levels
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The development of algal growth in likely to be nutrient limited (see, for instance Pedersen & Borum, 1996). Increased nutrient levels may, therefore, increase algal growth including of phytoplankton to the benefit of suspension feeders such as Mytilus edulis and solitary ascidians. The abundance and biomass of Arenicola marina increases with increased organic content in their favoured sediment (Longbottom, 1970; Hayward, 1994). Therefore, moderate nutrient enrichment may be beneficial. However, increasing nutrient enrichment may result in a well studied succession from the typical sediment community, to a community dominated by opportunist species (e.g. capitellids) with increased abundance but reduced species richness and eventually to abiotic anoxic sediments (Pearson & Rosenberg, 1978). Indirect effects may include algal blooms and the growth of algal mats (e.g. of Ulva sp.) on the surface of the intertidal flats. Algal mats smother the sediment, reducing water and oxygen exchange and resulting in localised hypoxia and anoxia when they die. Algal blooms have been implicated in mass mortalities of lugworms, e.g. in South Wales where up to 99% mortality was reported (Holt et al. 1995; Olive & Cadman, 1990; Boalch, 1979). Increased nutrients are likely to cause algal blooms and subsequent de-oxygenated conditions that may kill a significant part, but not all, of the biota (see oxygenation below). Some species might thrive in increased nutrients.
Increase in salinity
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The biotope occurs in reduced salinity conditions and some of the component species are only or mainly found in such conditions. For instance Gasterosteus aculeatus and mysid shrimps may thrive in low salinity conditions and may be lost if salinity changes in the long term (for instance from reduced to variable salinity for a year). Carcinus maenas is also found in low to variable salinity conditions but would most likely survive at the benchmark change level. The filamentous algae that dominate this biotope appear to thrive in low salinity conditions. Some of the species in the biotope have a wide tolerance to salinity. For instance, Akera bullata was found to occur in salinities from full to 6 psu in the Fleet (Thompson & Seaward, 1989) whilst Henry et al. (1999) suggests that Carcinus maenas can survive in salinities from 8 to 40 psu.
Most importantly, it seems likely that species previously unable to colonize the biotope because of its low salinity may settle and grow changing the biotope to a different one perhaps dominated by ascidians and sponges (for instance £SCR.SubSoAs£). The biotope would therefore be lost and intolerance is high. For recoverability, see additional information below.
Decrease in salinity
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The biotope occurs in reduced salinity conditions where some of the component species are at the limits of their distribution along the salinity gradient. For instance, echinoderms are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate. Therefore Asterias rubens would be expected to be an early casualty of reduced salinity. Some species can survive to very low salinity levels. For instance Akera bullata survives down to 6 psu but high mortality coincided with cold winds with low water and rain (Thompson & Seaward 1989), while Carcinus maenas survives down to 8 psu (Henry et al., 1999). Arenicola marina is unable to tolerate salinities below 24 psu and is excluded from areas influenced by freshwater runoff or input (e.g. the head end of estuaries) where it is replaced by Hediste diversicolor (Hayward, 1994). Arenicola marina in the Baltic are more tolerant of reduced salinity. For example, Barnes (1994) reports that Arenicola marina occurs at salinities down to 18 psu in Britain, but survives at salinities as low as 8 psu in the Baltic, whereas Shumway & Davenport (1977) reported that Arenicola marina cannot survive less than 10 psu in the Baltic. The reported salinity tolerance in the Baltic is probably a local adaptation. However, species such as Arenicola marina may disappear in response to decrease in salinity. Some species are likely to thrive in lower salinity, for instance Gasterosteus aculeatus and mysid shrimps. Overall, it is expected that a reduction in salinity from reduced to low for one year would result in the loss of a minority of species in the biotope and the biotope would remain IMX.FiG. Intolerance is therefore indicated as intermediate. Species in the biotope are rapid settlers or will migrate from other areas so recoverability is indicated as very high.
Changes in oxygenation
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The presence of Beggiatoa sp. in the biotope suggests that de-oxygenated pockets occur and that hypoxia may be a feature that component species need to be tolerant of. For instance, Arenicola marina has been found to be unaffected by short periods of anoxia and to survive for 9 days without oxygen (Borden, 1931 and Hecht, 1932 cited in Dales, 1958; Hayward, 1994). Therefore, Arenicola marina is likely to have a low intolerance if exposed to oxygen concentration as down to 2mg/l (the benchmark). Many others benthic species can move away but algae may be affected by severe de-oxygenation. At the level of the benchmark, some mortality of fixed species might occur but, because of tolerance of many species and the ability to move away of others, the biotope should persist during the one week the benchmark level of 2 mg/l persists. In conditions of more severe hypoxia or anoxia, the biotope might become £CMU.Beg£ (Beggiatoa spp. on anoxic sublittoral mud).

Biological Factors

Introduction of microbial pathogens/parasites
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There are microbial parasites that might affect several of the characteristic or commonly occurring species and only examples are given here.
  • The cestode parasite Schistocephalus solidus inhibits the female three-spined stickleback from producing clutches of eggs.
  • Carcinus maenas may be affected by the parasitic barnacle Sacculina carcini.
  • A range of diseases and other potential biological control measures are identified by Goggin (1997) for Carcinus maenas.
  • Male Asterias rubens are liable to gonad parasitisation by the ciliate parasite Orchitophrya stellarum (Vevers, 1951; Bouland & Claereboudt, 1994) that may cause population decline.
There are no doubt other pathogens that affect species in the biotope but overall, the biotope is likely to persist as dominant species seem unaffected. The intolerance assessment of low reflects the possibility that viability and condition of several species in the biotope may be affected.
Introduction of non-native species
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There are no non-native species known from this biotope or likely to invade it.
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Arenicola marina might be subject to some extraction by bait diggers especially at the shallow margins of the habitat. McLusky et al. (1983) examined the effects of bait digging on blow lug populations in the Forth estuary. Dug and infilled areas and unfilled basins left after digging re-populated within 1 month. However, bait digging may also disturb the filamentous algal cover although recolonization would be expected to be rapid (see displacement).

Where present, Mytilus edulis may also be extracted and accordingly, intolerance has been assessed as intermediate. Recovery is expected to be high (see additional information).

Additional information icon Additional information

The dominant characteristic species during summer at least are filamentous green algae. These algae colonize readily and grow rapidly so that, if recovery was occurring in spring or summer, it would be very high. Recolonization by mobile species including Carcinus maenas, and Asterias rubens would also be expected to be rapid by migration. In the Fleet, Thompson & Seaward (1989) observed the destruction of an entire population of Akera bullata in February 1986 with recolonization taking several months. For Arenicola marina, McLusky et al. (1983) examined the effects of bait digging on populations in the Forth estuary. Dug and infilled areas and unfilled basins left after digging re-populated within 1 month, whereas mounds of dug sediment took longer and showed a reduced population. Overall recovery potential is regarded as very high or high.

This review can be cited as follows:

Hiscock, K. 2002. Filamentous green seaweeds on low salinity infralittoral mixed sediment or rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25/11/2015]. Available from: <>