Candidate and possible SACs for estuaries: Solway Firth, Drigg Coast, Llyn Peninsula & Sarnau, Pembrokeshire Island, Bury Inlet, Severn Estuary, Plymouth Sound & Estuaries, Solent & Isle of Wight Maritime, Essex Estuaries, Dornoch Firth.

Candidate and possible SACs for mudflats and sandflats not covered by seawater at low tide : Solway Firth, Morecambe Bay, Severn Estuary, Isles of Scilly Complex, Fal and Helford, Essex Estuaries, the Wash and North Norfolk Coast, Berwickshire & North Northumberland Coast.

Estuaries are one of a number of types of inlet found along the UK coastline. The Nature Conservancy Council's 'Estuaries Review' defined nine different categories (on the basis of geomorphology and topography), and identified 155 estuaries around the British coastline (Davidson et al., 1991). As well as being physiographic features in their own right, estuaries are habitat complexes. Tidal flats, saltmarshes, areas of shingle, rocky shores, lagoons, sand dunes and coastal grassland may be elements of coastal and intertidal areas, and muddy and sandy seabed, gravels and rocky areas may be found in the subtidal zone.

There is a rich source of invertebrates within the sediments of many estuaries, making them extremely productive areas as well as important feeding and overwintering grounds for waders and wildfowl. The UK has the largest single national area of estuaries in Europe, making up around one quarter of the total estuarine habitat of North Sea shores and the Atlantic seaboard of western Europe (Davidson et al., 1991).

Mudflats and sandflats which are uncovered at low tide are one of the habitat types found within estuaries and embayments. They can cover large areas and are often the most extensive habitat in many estuaries. The characteristics of the flats will depend on a combination of factors, the most important being degree of exposure to wave action, particle size, position on the shore and salinity regime. There may be a gradation of sediment types with fine muds on the sheltered upper shore, and coarser grained sediments on the lower shore. Species diversity may be low but these flats often support very dense populations of invertebrates so the overall biomass of the area can be extremely high. Sand flats appear to be more common in northern and western parts of the country and finer grained flats more common in southern and eastern areas.

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