Biodiversity & Conservation

LS.LGS.S.Lan

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Characterizing species within this biotope are infaunal and would therefore be removed along with the substratum. Intolerance has been assessed to be high because the species which characterize the biotope would be lost. Recoverability has been assessed to be high. See additional information below.
Smothering
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Although all of the species in the biotope are able to move within the substratum to some extent, some species live at specific depths and/or have to maintain contact with the surface. For instance, Ziegelmeier (1952) showed that Lanice conchilega increased the height of its tube top with increasing sedimentation so that it could continue feeding and respire. The bivalve Cerastoderma edule has short siphons and needs to keep in contact with the surface of the sediment. It will quickly burrow to the surface if covered by as little as 2 cm of sediment (Richardson et al., 1993b) but Jackson & James (1979) reported that cockles buried under 10 cm of sediment were unable to burrow back to the surface and over a period of six days 83% mortality was recorded. In the same experiment, most cockles buried to a depth of 5 cm were able to regain contact with the surface. In muddy substrata all cockles died between three and six days. Nephtys species are highly mobile within the sediment. Vader (1964) observed that Nephtys hombergii relocated throughout the tidal cycle and is unlikely to be affected by smothering with a sediment consistent with that of the habitat. Intolerance has been assessed to be intermediate as mortality of some cockles (especially smaller individuals) and probably other species may occur. At the benchmark level the composition of the community would probably not alter to the extent that the biotope would not be recognised. In years of good cockle recruitment recovery of the population may occur within a year, however, recruitment tends to be sporadic (see Cerastoderma edule, reproduction) and may take longer in 'bad' years.
Increase in suspended sediment
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Suspension feeding species within the biotope may benefit from an increase in suspended sediment, e.g. Lanice conchilega and Cerastoderma edule, especially if the suspended material includes a significant proportion of organic matter and food was previously limiting. Lanice conchilega uses its 'feeding crown' of tentacles to capture particles and unless the 'feeding crown' becomes clogged and requires excessive cleaning at energetic cost the species is unlikely to be especially affected. Navarro & Widdows (1997) considered that Cerastoderma edule was well adapted to living in environments with high concentrations of suspended sediment. The cockles compensate by increased pseudofaeces production but with concomitant loss of energy and carbon as mucus. The intolerance of the community has been assessed to be low as species may suffer loss of condition over the period of one month as a consequence of excessive clearance.
Decrease in suspended sediment
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A decrease in suspended sediment would reduce the amount of food available for suspension feeders such as Lanice conchilega and Cerastoderma edule. Deposit feeding polychaetes such as Arenicola marina are unlikely to be significantly affected, as over a period of one month deposits of organic matter are unlikely to become limiting as a consequence of reduced supply. Navarro & Widdows (1997) suggested that Cerastoderma edule was able to compensate for a decrease in particulate quality (i.e. proportion of organic to inorganic seston) between 1.6 to 300 mg/l, accomplished by excessive preingestive selection of organic particles, together with increasing filtration and rejection rates. Over a period of one month loss of condition may occur but mortality is considered to be unlikely and intolerance has been assessed to be low. On return to prior conditions optimal feeding is likely to resume and recoverability has been assessed to be very high.
Desiccation
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In the eulittoral the biotope experiences regular periods of emersion and hence the biotope is subjected to desiccation. The majority of the species in the biotope live infaunally and so are likely to be protected from desiccation stress. For instance, Lanice conchilega can retract into its tube, which can be up to 65cm long and bivalves, such as Cerastoderma edule are able to respond to desiccation stress by valve adduction during periods of emersion, so it is likely that bivalves would be able to retain enough water within their shells to avoid mortality. Mobile species such as polychaetes and amphipods are able to migrate and avoid the factor. During especially hot summers mortality of sessile species might be expected in the mid eulittoral owing to effects of desiccation and an intolerance assessment of intermediate is reported. Recoverability has been assessed to be high (see additional information below).
Increase in emergence regime
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The majority of the species in the biotope, including the important characterizing species, live infaunally and are therefore protected from the short term stresses of an increase in emergence regime. However, over time the increased emergence would be likely to result in an energetic cost due to reduced feeding opportunities. Many of the species, e.g. mobile polychaetes and amphipods, would be able to relocate to their preferred position on the shore. Lanice conchilega is not a mobile species and mortalities might be expected as a result of increased predation by birds, increased exposure to desiccation and changes in temperature. Over a period of one year a decrease in the density of Lanice conchilega may occur and intolerance has been assessed to be intermediate. The biotope would probably still be recognizable but would be impoverished.
Decrease in emergence regime
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Lanice conchilega also thrives in the shallow subtidal zone (See £IGS.Lcon£ - Dense Lanice conchilega and other polychaetes in tide swept infralittoral sand) therefore would not be intolerant of a decreased emergence regime. It is possible that decreased emergence would allow Lanice conchilega and other species in the biotope to colonize further up the shore. Hence, not sensitive* has been recorded.
Increase in water flow rate
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The nature of the substratum is, in part, determined by the hydrodynamic regime including water flow rate. Changes in the water flow rate will change the sediment structure and have concomitant effects on the community, as many sediment dwelling species have defined substratum preferences (e.g. amphipods such as Bathyporeia pelagica). Moderate to high velocities of water flow have been reported to enhance settlement of Lanice conchilega larvae (Harvey & Bourget, 1995), but the benchmark increase in water flow from moderately strong to very strong, would probably winnow away smaller particulates, increasing average particle size in favour of gravels and pebbles. Therefore, the density of the Lanice conchilega population would decline, in part due to lack of suitable substrata with which to build its tubes, and partly from interference with its feeding. In the absence of Lanice conchilega the biotope would not be recognized and intolerance has been assessed to be high. The community would probably become dominated by water flow tolerant species with a preference for/or tolerant of a coarser more mobile substratum, e.g. crustaceans (burrowing haustoriid and oedecerotid amphipods) and errant polychaetes (Elliott et al., 1998). Intolerance has been assessed to be high as the dense Lanice biotope would not be recognized. On return to prior conditions recovery of the Lanice conchilega dominated community is probable, although recovery may take several years as recolonization would be dependent on larval recruitment, which is more successful in the presence of conspecific adults.
Decrease in water flow rate
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The nature of the substratum is, in part, determined by the hydrodynamic regime including water flow rate. Changes in the water flow rate will change the sediment structure and have concomitant effects on the community.
Reduced water flow is a factor that has been identified as affecting the density of Lanice conchilega. Recruitment to the benthos is reduced under low flow as a result of reduced turbulence (Harvey & Bourget, 1995). Furthermore, at the benchmark level, decreased water flow rate would probably increase deposition of finer sediments, and increase siltation. The sediment would probably begin to favour deposit feeders and detritivores, to the detriment of the suspension feeders. The average grain size of the sediment would be reduced, and the community may start to be replaced over a period of one year by communities more characteristic of muddy sands, e.g. predominantly sessile tube-dwelling polychaetes and bivalves that primarily deposit feed, e.g. Macoma balthica. Intolerance has been assessed to be high as the Lanice conchilega biotope would not be recognized. Recovery is likely on return to prior conditions, but a transitional community may persist for several years.
Increase in temperature
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The distribution of polychaete and bivalve species characteristic of the biotope extend to the south of the British Isles, so are likely to be tolerant of a long-term chronic temperature increase of 2 °C , e.g. Cerastoderma edule (Wilson, 1993). In the intertidal, acute temperature increases at the surface can be avoided to some extent by burrowing, however, in very hot weather mortalities can occur, but it is rarely clear whether such mortalities are a direct result of high temperatures or an indirect consequence of oxygen deficiency resulting from increased bacterial activity and oxygen consumption. Lethal temperatures (LT50) have been reported for some species, e.g. Ansell et al. (1981) reported an upper median lethal temperature of 35 °C after 24 hrs (29 °C after 96 hrs), whilst Wilson (1981) reported an upper LT50 of 42.5 °C for Cerastoderma edule. However, temperatures in excess of 20 °C within the substratum are likely to be uncommon in the British Isles. An intolerance assessment of intermediate has been made as some individuals may die as a consequence of acute increases in temperature. Recoverability has been assessed to be high (see additional information below).
Decrease in temperature
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Although recorded to the north of the British Isles, the dominant and key structural polychaete Lanice conchilega is intolerant of acute decreases in temperature (Beukema, 1990). An intertidal population of Lanice conchilega, in the northern Wadden Sea, was wiped out during the severe ice winter of 1995/96 (Strasser & Pielouth, 2001), and Crisp (1964) described mortality of Lanice conchilega between the tidemarks but not at lower levels during the severe winter of 1962/63.
Other characterizing species in the biotope are recorded to the north of the British Isles and therefore are likely to be tolerant of a long-term chronic decrease in temperature, but also may be vulnerable to acute temperature decreases. For example, high mortalities of cockle populations attributed to severe winters have been reported by many authors (Kristensen, 1958; Hancock & Urquhart, 1964; Beukema, 1990; Ducrotoy et al., 1991) and high shore populations are likely to be more vulnerable to extremes of temperature owing to their longer emergence time. Other infaunal species may be protected to some extent by the ability to burrow deeper within the sediment. Intolerance has been assessed to be high as evidence suggests that intertidal populations of Lanice conchilega and Cerastoderma edule are vulnerable to acute decreases in temperature. Recoverability has been assessed to be high. Following the severe winter in the Dutch Wadden Sea, a population of Lanice conchilega took three years to fully recover, as there was low recruitment for the first two years (Strasser & Pielouth, 2001). Cerastoderma edule may recover within a year, however, given the sporadic nature of recruitment in the species, recovery may be more protracted and take several years.
Increase in turbidity
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Production within the biotope is predominantly secondary, derived from detritus and to some extent phytoplanktonic production. Characteristic infauna do not require light and therefore the effects of increased turbidity on light attenuation are not directly relevant. However an increase in turbidity may affect primary production by phytoplankton and the microphytobenthos thereby affecting food supply. However as organic material would be transported in to the biotope from other areas on the flood tide the effect of increased turbidity may be mitigated to some extent. At the benchmark level increased turbidity persists for a year, so reduced food availability may affect the condition of species and an intolerance of low has been recorded. As soon as light levels return to normal, phytoplanktonic and microphytobenthic primary production would increase, the species would resume optimal feeding, so recoverability has been assessed to be very high.
Decrease in turbidity
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It is possible that decreased turbidity would increase primary production in the water column by phytoplankton and by the microphytobenthos of the interstices of the sand. Increased food availability may enhance growth and reproduction of both suspension and deposit feeding species but only if food was previously limiting. An intolerance assessment of not sensitive* has been made.
Increase in wave exposure
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The predominant factor controlling the intertidal community is wave exposure (Eleftheriou & McIntyre, 1976), and a sedentary fauna of sedentary tube dwelling polychaetes, such as Lanice conchilega, and long-lived bivalves dominate in sheltered areas (Elliott et al., 1998). Such communities are likely to be severely affected by increased wave exposure for the period of one year and a large reduction in species richness and abundance is likely, owing to wash-out, disrupted feeding and burrowing activity in addition to inhibited recruitment by larvae. A transitional community of opportunistic species is likely to result, consisting of agile haustoriid (Bathyporeia spp.) and oedecerotid (Pontocrates spp.) amphipods and errant polychaetes in which diversity increases towards the low shore area (Eleftheriou & McIntyre, 1976). The biotope would change to another, e.g. £LGS.AP£ (Burrowing amphipods and polychaetes in clean sand shores) and thus intolerance has been assessed to be high. Recoverability of the Lanice conchilega community would be determined by the degree of change. Over a period of one year, a change in shore topography (gradient and shore width) and grain size is likely following intense wave action, with concomitant changes in the physical and biological integrity of the habitat, perhaps leading to a persisting change in community composition despite a return to prior conditions. However, a succession in the type of species that dominate the habitat is likely as species that were lost as a result of increased exposure gradually return. A recoverability of moderate has been suggested.
Decrease in wave exposure
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The biotope occurs in 'moderately' to 'very sheltered' locations (Connor et al., 1997a). A further decrease in wave exposure may result in increased siltation and a consequent change in sediment characteristics (Hiscock, 1983). A substratum with a higher proportion of fine sediment would probably result in the increased abundance of the deposit feeders within the biotope, particularly species which favour finer sediments, such as the polychaete Aphelochaeta marioni and the echinoid Echinocardium cordatum. However, in the absence of wave action, tidal flow is likely to be a more significant factor structuring the community, replenishing oxygen, supplying planktonic recruits and would maintain a supply of suspended organic matter in suspension for suspension feeders. Therefore the biotope has been assessed not to be sensitive.
Noise
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Species within the biotope can probably detect vibrations caused by noise and in response may retreat in to the sediment for protection. However, at the benchmark level the community is unlikely to be sensitive to noise.
Visual Presence
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Important characterizing species within the biotope may possess the visual acuity to detect changes in light or predators if active at the surface, but are likely to be not sensitive to the visual presence of boats etc. in their vicinity and an assessment of not sensitive has been suggested.
Abrasion & physical disturbance
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Lanice conchilega inhabits a permanent tube and is likely to be damaged by any object that penetrates or drags through the sediment, as are all other infaunal polychaetes. Ferns et al. (2000) recorded significant losses of common infaunal polychaetes from areas of muddy sand worked with a tractor-towed cockle harvester. For instance, 31% of the polychaete Scoloplos armiger (initial density of 120 per m²) and 83% of Pygospio elegans (initial density 1850 per m²) were removed. The population of Pygospio elegans remained depleted for more than 100 days after harvesting, whilst those of Nephtys hombergii, Scoloplos armiger and Bathyporeia spp. were depleted for over 50 days. The tubes of Lanice conchilega were damaged but this damage was seen to be repaired.

In locations of cleaner sand with lower densities of Cerastoderma edule and dense aggregations of Lanice conchilega, recovery occurred more rapidly. Cockles are often damaged during mechanical harvesting, e.g. 5-15% were damaged by tractor dredging (Cotter et al, 1997) and ca 20% were too damaged to be processed after hydraulic dredging (Pickett, 1973). Therefore, an overall biotope intolerance of intermediate has been recorded. Abrasion due to mooring, or anchoring is likely to result in less damage to the population. Recoverability has been assessed to be high (see additional information below).

Displacement
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Yonow (1989) observed Lanice conchilega re-establishing tubes immediately after removal from the sediment when replaced on a suitable substratum. Other polychaetes and bivalves are all active burrowers and are capable of reburying themselves if displaced. However, while at the sediment surface the infauna are vulnerable to predation by mobile epifauna, e.g. crabs, and bottom feeding fish, so some mortality may occur and intolerance has been assessed to be intermediate. However, it is likely that the majority of displaced specimens would obtain protection within the substratum relatively quickly, so recoverability has been assessed to be immediate.

Chemical Factors

Synthetic compound contamination
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No evidence concerning specific effects of synthetic chemical contaminants on Lanice conchilega, Nephtys cirrosa or Nephtys hombergii was found. Boon et al. (1985) reported that Nephtys species in the North Sea accumulated organochlorines but, based on total sediment analyses, organochlorine concentrations in Nephtys species were not correlated with the concentrations in the sediment which they inhabited. Specific deleterious effects of synthetic chemicals (Ivermecten, tri-butyl-tin (TBT)) have been reported for other polychaetes, e.g. Hediste diversicolor, Arenicola marina, Scoloplos armiger and Cirratulus cirratus (see Collier & Pinn, 1998; Beaumont et al., 1989; Bryan & Gibbs, 1991). Beaumont et al. (1989) concluded that bivalves are particularly sensitive to TBT. For example, when exposed to 1-3 µg TBT/l, Cerastoderma edule suffered 100% mortality after two weeks. Bryan & Gibbs (1991) presented evidence that TBT caused recruitment failure in bivalves, either due to reproductive failure or larval mortality. Waldock et al. (1999) examined recovery of benthic infauna of the Crouch estuary after a ban on the use of TBT on small boats. They observed marked increase in species diversity, especially of Ampeliscid amphipods and polychaetes (Tubificoides species and Aphelochaeta marioni) which mirrored the decline in sediment TBT concentration. Whilst a causal link could not be shown, the study by Waldock et al. (1999) suggested that crustacean and polychaete diversity may be inhibited by TBT contamination.
Polychaetes vary greatly in their tolerance of chemical contamination. However, evidence suggests that those within this biotope are potentially highly intolerant of chemical contamination and that the abundance and reproduction of bivalves and crustaceans may also be adversely affected. An intolerance of high has been recorded, albeit at low confidence. Species richness is likely to decline markedly and a few species dominate. On return to prior conditions and assuming deterioration of the contaminants recoverability is likely to be high (see additional information below).
Heavy metal contamination
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Bryan (1984) reported that short term toxicity in polychaetes was highest to Hg, Cu and Ag, declined with Al, Cr, Zn and Pb with Cd, Ni, Co and Se being the least toxic. However, polychaetes vary in their tolerance to heavy metals.
Nephtys hombergii and Hediste diversicolor are recorded in Restronguet Creek, Cornwall, a branch of the Fal estuary system which is heavily contaminated with metals. Concentrations of dissolved Zn typically range from 100-2000 µg/l, Cu from 10-100 µg/l and Cd from 0.25-5.0 µg/l. The sediments of Restronguet Creek are also highly contaminated, the levels of Cu, Zn, As and Sn being in the order of 1500-3500 µg/g (Bryan & Gibbs, 1983). Analyses of organisms from Restronguet Creek revealed that Nephtys hombergii from the middle and lower reaches of the creek contained appreciably higher concentrations of Cu (2227 µg/g dry wt), Fe and Zn than comparable specimens of Hediste diversicolor (as Nereis diversicolor). There was evidence that some metals were regulated and suggestion that some species (Hediste diversicolor and Nephtys hombergii) had developed metal resistant populations.
Bryan (1984) stated that Hg is the most toxic metal to bivalve molluscs while Cu, Cd and Zn seem to be most problematic in the field. Studies of Cerastoderma edule populations from polluted and un-contaminated sites in Southampton Water showed that tissue heavy metal concentrations were lower in summer than winter/spring, tissue heavy metal concentrations decreased with size of the cockle, and that cockles in sediments contaminated with metals and hydrocarbons had lower life expectancies, growth rates and body condition index (Savari et al., 1991(a), (b)). Transplantation of Cerastoderma edule into Restronguet Creek resulted in 10-15%mortality within 63 days but 100% within ca. 4 months (Bryan & Gibbs, 1983). An intolerance assessment of intermediate has been suggested as it is probable that in the short term heavy metal exposure may be deleterious to populations not previously exposed. On return to prior conditions, and assuming deterioration of the contaminants, recoverability has been assessed to be high (see additional information below).
Hydrocarbon contamination
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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. A 20 year study investigating community effects after the Amoco Cadiz oil spill of 1978 (Dauvin, 2000) found that a population of Lanice conchilega re-established between 1978-84 but disappeared after 1985. Nephtys hombergii, cirratulids and capitellids were largely unaffected by the Amoco Cadiz oil spill (Conan, 1982). The Amoco Cadiz oil spill resulted in reductions in abundance, biomass and production of the community through the disappearance of the dominant populations of the amphipods Ampelisca sp. which were very sensitive to oil contamination (Dauvin, 1998) The sediment rapidly de-polluted and, in 1981, benthic recruitment occurred under normal conditions (Dauvin, 1998). However, the recovery of Ampelisca populations took up to 15 years, which was probably due to the amphipods' low fecundity, lack of pelagic larvae and the absence of local unperturbed source populations (Poggiale & Dauvin, 2001).
Hailey (1995) cited substantial kills of Hediste diversicolor, Cerastoderma edule, Macoma balthica, Arenicola marina and Hydrobia ulvae as a result of the Sivand oil spill in the Humber estuary in 1983. Similarly large numbers of moribund and dead infaunal animals were washed ashore after the Sea Empress oil spill (SEEEC, 1998). Savari et al. (1991a) stated that there was a concentration related reduction in scope for growth (SFG) of Cerastoderma edule with increasing concentrations of hydrocarbons in the water column. McLusky (1982) examined the fauna of the intertidal mudflats at Kinneil in the Forth estuary that received petroleum, chemical and domestic effluents. Spat fall of Cerastoderma edule occurred in 1976 but the abundance declined steadily between 1976 and 1980. Cerastoderma edule, Nephtys hombergii and many other species were excluded from the substratum within 1.5 km of effluent discharges.
Evidence suggests that soft sediment communities are highly intolerant of hydrocarbon contamination. In the littoral zone especially, oil spills resulting from tanker accidents are likely to be deposited directly on the biotope, preventing oxygen transport to the substratum and oil pushed in to the substratum by tidal-pulsing will destabilize the sediment (Elliott et al., 1998). Therefore, an intolerance of intermediate has been recorded. Recovery of some species, such as amphipods and Cerastoderma edule, to the biotope may be quite slow depending upon the availability of recruits and the severity of disturbance. However, the dominant and key structural species, Lanice conchilega was shown to be relatively opportunistic after the Amoco Cadiz oil spill, colonizing shortly after the spill (Dauvin, 2000), so that the biotope may be identifiable within several years after disturbance (in terms of species present). Recoverability has been reported to be high.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Nutrient enrichment can lead to significant shifts in community composition in sedimentary habitats. Typically the community moves towards one dominated by deposit feeders and detritivores, such as polychaete worms (see review by Pearson & Rosenberg, 1978). The LGS.Lan biotope includes some species tolerant of nutrient enrichment, such as the polychaete Capitella capitata (Pearson & Rosenberg, 1978). It is likely that such species would increase in abundance following nutrient enrichment (Elliott, 1994), with an associated decline in suspension feeding species and organisms adapted to low nutrient levels, such as Magelona mirabilis (Niermann, 1996). Algal blooms are symptomatic of eutrophication, the decomposition and mineralization of which may result in localised deoxygenation. Overall, the structure of the community is likely to change in favour of deposit feeders, with an increase in the abundance of opportunistic species and a decrease in species richness. The dense Lanice conchilega bed is likely to become impoverished and an intolerance of high has been recorded. On return to prior conditions recoverability is likely to be high (see additional information below).
Increase in salinity
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LGS.Lan occurs in locations of full to variable salinity (Connor et al., 1997a). No information was found concerning the intolerance of the community to hypersaline conditions.
Decrease in salinity
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Intertidal sandflats are intolerant of increased rainfall and thus increased freshwater input which, may cause scouring of intertidal areas, changes in intertidal creeks and possibly a reduction in salinity in localized areas (Elliott et al., 1998). McLusky (1989) described the physiological effects of salinity change. In the intertidal, species are well adapted to tolerate alterations in salinity by osmoregulation, or if mobile by moving seawards or deeper in to the sediment. For instance, Kristensen (1958) reported death of young cockle spat (1-2 mm) in the Dutch Wadden Sea due to heavy rain, whereas adults were able to dig deeper and avoid the freshwater. Although Lanice conchilega may be found around the mouth of estuaries in reduced salinities, it occurs at lower abundance, but in the short term it is likely that the polychaete would withdraw in to its burrow to avoid temporary episodes of reduced surface salinity. Nephtys species penetrate in to the mouths of estuaries and estuarine lagoons until the salinity falls to around 20 psu (Barnes, 1994). Nephtys hombergii seems able to tolerate salinities as low as 16 psu (Clark & Haderlie, 1960). Studies of the salinity tolerance of Cerastoderma edule suggest a wide tolerance range for both adults and larvae (e.g. Rygg, 1970; Russell & Peterson, 1973; Kingston, 1974; Wilson, 1984), but that its preferred range is between 15 and 35 psu (Boyden & Russell, 1972). An intolerance of low has been suggested as whilst reduced salinity may cause a reduction in the abundance of some important characterizing species, it is likely that in open coastal locations that the biotope would still be recognizable. On return to prior conditions recoverability is suggested to be very high.
Changes in oxygenation
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Relatively coarse sands, such as those of the LGS.Lan biotope, tend to have a relatively high oxygen concentration and a deep reducing layer. Brafield (1964) concluded that the most significant factor influencing oxygenation is the drainage of the beach which, in turn, is determined by the slope and particle size. Oxygen depletion becomes a severe problem at all states of the tide on only the finest grained beaches. Dense aggregations of Lanice conchilega serve to increase the oxygenation of the sediment. The species periodically withdraws from the surface into the sediment for a few seconds and in doing so exchanges the tube water with overlying water (Forster & Graf, 1995). So in normal circumstances oxygen is unlikely to become limiting. However, as a consequence of organic enrichment, oxygen concentration in the sediment may become depleted. Over a longer period concomitant changes in the infauna would occur and tend to show a consistent sequence of response, such as that of the Pearson-Rosenberg model (Pearson & Rosenberg, 1978), and the biotope would change. However, at the benchmark level assessment is made over the period of one week. Some important characterizing species seem tolerant of hypoxia whilst others are less so. For example, Nephtys hombergii was found to be particularly tolerant of severe hypoxia and hydrogen sulphide (Alheit, 1978; Arndt & Schiedek, 1997). Rosenberg et al. (1991) observed that Cerastoderma edule migrated to the sediment surface in response to reduced oxygen concentrations and reported 100% mortality of Cerastoderma edule exposed to 0.5-1.0 ml/l oxygen for 43 days. Theede et al. (1969) reported 50% mortality after 4.25 days at 1.5 ml/l oxygen. Theede et al. (1969) also noted that Cerastoderma edule only survived 4 days exposure of < 6.1 cm²/l of hydrogen sulphide, which is associated with anoxic conditions. Cerastoderma edule may therefore survive several days of anoxia but fatalities may occur over the duration of a week.
intolerance has been assessed to be intermediate as some populations of species may be adversely affected. Recoverability has been suggested to be high (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information was found concerning microbial pathogens and parasites of polychaete species. However, more than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996). Cerastoderma edule may be infected by numerous larval digenean trematodes, and the parasitic copepod Paranthessius rostatus but no evidence of mass mortalities of cockles in the British Isles attributable to parasites was found. Boyden (1972) reported castration by parasites of a population of cockles from the River Couch. Parasitic infection is likely to directly or indirectly result in a reduced condition or abundance of affected populations, so intolerance has been assessed to be low.
Introduction of non-native species
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No evidence was found to suggest that important characterizing species of the biotope are threatened by alien species.
Extraction
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The common cockle, Cerastoderma edule and the polychaetes Nephtys are characterizing species targeted for extraction within the biotope. Nephtys species are used by anglers as bait and the biotope may be subjected to bait digging. Jackson & James (1979) observed that bait digging disturbs sediment to a depth of 30-40 cm and probably buries many cockles below 10 cm, so that they are smothered (see smothering).
Cerastoderma edule of marketable size can be harvested in both the intertidal and subtidal more rapidly and efficiently using mechanical methods, such as tractor-powered harvesters and hydraulic suction dredgers than by traditional methods. Hydraulic suction dredgers operate by fluidising the sand using water jets and then lifting the sediment and infauna into a revolving drum for sorting. The tractor-towed dredge utilises a blade between 70 -100 cm wide that penetrates to a depth of between 20-40 cm. Sediment is sorted through a rotating drum cage (Hall & Harding, 1997). Such machinery adversely impacts on non-target infaunal species as they are sucked or displaced from the sediment and sustain damage as 'by-catch' (see abrasion & physical disturbance). Cotter et al. (1997) noted that tractor dredging reduced the Cerastoderma edule stock by 31-49% depending on initial density, while Pickett (1973) reported that hydraulic dredging removed about one third of the cockle fishery. Franklin & Pickett (1978) noted that subsequent spat survival was markedly reduced and Pickett (1973) noted reduced survivability of 1-2 year old cockles after hydraulic dredging. Furthermore, tractor dredging leaves visible tracks in the sediment, which can act as lines for erosion accelerating erosion of the sediment (Moore, 1991; Gubbay & Knapman, 1999). However, most studies concluded that the impact of mechanised dredging on cockle populations and other macrofauna in the long term was low (Pickett, 1973; Franklin & Pickett, 1978; Cook, 1990; Moore, 1991; Cotter et al., 1997; Hall & Harding, 1997; Ferns et al., 2000). Time of year of exploitation will influence recovery and avoiding seasonal spawning or larval settlement periods is likely to reduce the time taken for recovery (Gubbay & Knapman, 1999). Cockle beds have been subjected to mechanical fishing for decades but several beds are closed form time to time depending on settlement and recruitment to the population, which is sporadic. Recovery may take less than a year in years of good recruitment but would take longer in years of poor recruitment. A recoverability of high has been suggested.

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Recoverability
In many instances recoverability of the biotope has been assessed to be high. The time required for the community to recover will be in part determined by the proximity of other source populations and the season during which a disturbance occurs. Recolonization by some groups is likely to be more rapid than others. For instance, diatoms may be transported by resuspension in the water column and by lateral sediment transport. The rapid colonization (within days) by diatoms establishes food resources for other species, usually nematodes, that subsequently colonize. Dittmann et al. (1999) observed that the number of nematode species returned to pre-impact levels within seven days following a month long disturbance. Polychaetes tend to rapid colonizers, and species recorded by Dittmann et al. (1999) within two weeks of disturbance included the polychaetes Pygospio elegans, Polydora sp., Nephtys hombergii, Capitella capitata, Heteromastus filiformis, Eteone longa, Hediste diversicolor (as Nereis diversicolor) and Scoloplos armiger, and the molluscs Macoma balthica and Mytilus edulis. Next to polychaetes, amphipods e.g. Urothoe poseidonis, are also rapid colonizers owing to their mobility. However, species that did not recolonize within the period of subsequent monitoring (14 months) included Arenicola marina, Lanice conchilega and its commensal Malmgreniella lunulata. Although it is likely that these species would recolonize suitable substrata, settlement of Lanice conchilega, for instance, has been reported to be more successful in areas with existent adults than areas without (see full MarLIN review; Heuers & Jaklin, 1999). Strasser & Pielouth (2001) reported that establishment of a mature population took three years in the absence of an established population. Thus the time taken for the community to reach maturity and be considered recovered, is likely to be in the order of several years.

This review can be cited as follows:

Budd, G.C. 2006. Dense Lanice conchilega in tide-swept lower shore sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=195&code=1997>