Intertidal Sand and Mudflats

Subtidal Mobile Sandbanks

In general, and because of the extensive nature of these biotope complexes, the soft-sedimentary marine habitats are unlikely to have rare or unusual species or communities. Because of this, these elements of community structure are of limited value in assessing and defining change, and so it is considered necessary to place emphasis on functioning and thus rate-processes. Sustainable functioning, within limits, may be regarded as an indication of the maintenance of integrity of the site, for example whether intertidal mudflats continue to maintain their support for over-wintering waterfowl, or whether subtidal mobile sandbanks support sandeels and juvenile flatfishes. The change of physical or biological attributes is of importance and the rate of that change should be used in decision making; however, it is unlikely that sufficient data exist for this to be determined.

It is considered necessary to put emphasis on assessing any decline in well-being rather than rarity. It is further considered necessary to separate a process rather than a structural attribute for monitoring; this is particularly the case where the components of a system may change but not result in a net loss of ecological function. For example, in estuarine mudflats, a variation in river flow, and thus salinity, will alter the community structure and components (e.g. change from Nereis to Nephtys as the dominant polychaete) but its functioning and carrying capacity will remain largely unaffected.

Rare species are unlikely to occur given the nature of the areas in general. However, rare species can occur, for example, the anemone, Nematostella vectensis and polychaete, Armandia cirrhosa - both are scheduled species and occur in intertidal areas within the Fleet, Portland Harbour and Poole Harbour (Copeley, pers. comm.). The intertidal sand and mud within Portland Harbour supports a diverse assemblage of polychaete worms and molluscs due in part to the extreme conditions imposed by the double low tide. On the lower shore, species occur which are normally considered subtidal and these include a number of rare species or species on the edge of their range.

Similarly, the Subtidal Mobile Sandbanks are unlikely to contain rare species given the dispersal mechanisms of the species and the dynamic nature of the areas. However, in some cases the SAC will be designated because of the presence of rare species. Those species which are rare due the extent of their geographical distribution may be difficult to protect, whereas those which are rare due to a paucity of habitat can be protected more easily. Given the extensive nature of this biotope complex, the latter is unlikely.

Despite this, it is emphasised that the maintenance of the physical habitats of the subtidal sandbanks should ensure the protection of even the rare species. However, it is considered of greater relevance that the functioning of the populations of the dominant and/or characterising species is protected as these will support the predators, e.g. the sandeel Ammodytes in the case of seabirds, and infaunal amphipods and polychaetes in terms of benthic and demersal fishes. In addition, these dominant species may be required to stabilise the sandbank and thus protect its integrity, e.g. Lanice beds and Zostera meadows. Any reduction in these will influence the physical stability of the sandbank and perhaps increase levels of erosion.

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