The species composition in shallow inshore areas may be similar to that of intertidal sand flats (Willems et al. 1982; Atkins 1983). The presence of any species in an area is dependent on its tolerance to those environmental variables such that considerable spatial and temporal variation occurs within estuarine and coastal sediment areas.

The physical environment of infralittoral sand with strong currents is often too harsh for vegetation to become established. However, more sheltered sand may support the sugar kelp Laminaria saccharina attached to stones and shallow conditions with adequate light will maintain a microphytobenthic (diatoms) community. Mobile sandbanks are colonised by infaunal/epifaunal small crustaceans, polychaetes and molluscs which are adapted to the changing hydrography and substratum; they are able to reburrow rapidly following being washed-out of the sediment during storms (Vanosmael et al. 1982). For example, the body form and mobility of magelonid polychaetes and species such as Nephtys cirrosa and Micropthalmus similis are well suited to burrowing in mobile sands. These features indicate that the communities are clearly shaped by physical rather than biological factors. The sediment in a mobile sandbank system may range from fine to coarse clean sands, and the density of individuals and species richness is often highest in the coarsest grade, mainly due to large numbers of interstitial polychaetes (Vanosmael et al. 1982). The mean macrobenthic diversity and species richness of clean mobile sandbanks is generally lower than the surrounding sea bed (reflecting the greater stresses inherent in these environments) although the fauna is essentially comparable with that of the open sea. Due to the continual sediment disturbance, the community may have a large opportunistic component, including species such as Chaetozone setosa, and may be prevented from reaching a climax community. The heart urchin Echinocardium cordatum may also be common but is replaced by another heart urchin Brissopsis lyrifera in more silty areas. Sand-eels e.g. Ammodytes tobianus and A. marinus are widespread. The meiofauna also form an important component of the sandbank fauna. Interstitial organisms occur in sediments with a median grain size above 200 å m and polychaetes are found abundantly in sediments with a particle size above 300 å m (Willems et al. 1982). The meiofauna may be characterised by low densities of nematodes and high densities of copepods, annelids and halacarid mites.

The population dynamics of the fauna in exposed habitats may be based on long-term breeding success, e.g. 6-7 years for tellinids with a cohort produced which may then dominate the population (Pearson & Barnett 1987). The opportunistic pollution-tolerant polychaete Capitella capitata has both benthic and planktonic larvae and breeds thoughout the year; this means it is able to colonise impacted or stressed areas very quickly. Subtidal mobile sandbanks are usually dependent on an input of colonising organisms and have few species with benthic reproduction, thus any disruption to the delivering currents will cause changes.

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Infralittoral sandbanks provide a source of prey for demersal fish, especially those mobile small crustaceans which migrate from the sediment and thus become available for predation (Costa & Elliott 1991). The habitat is often important as fish nursery areas e.g. for plaice Pleuronectus platessa (Gibson 1973), and may be characterized by low organic enrichment though there may be localised pockets of organic matter or areas which receive anthropogenic waste. The sandbanks are also important areas for crab populations and other epifauna, particularly echinoderms. The epifaunal component may represent a large proportion of the biomass of the sand bank fauna with large numbers of Asterias rubens and brittlestars such as Ophiura albida. Predatory fauna such as hermit crabs e.g. Pagurus bernhardus, the swimming crab Liocarcinus depurator and the edible crab Cancer pagurus may also be present. Birds such as the gullimot, razorbill, puffin and terns will feed on the fish such as sand-eels Ammodytes spp. which are found in mobile sands (Batten et al. 1990).

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