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Special Areas of Conservation

1140 Mudflats and sandflats not covered by seawater at low tide

Marine, coastal and halophytic habitats

Description and ecological characteristics

Intertidal mudflats and sandflats are submerged at high tide and exposed at low tide. They form a major component of 1130 Estuaries and 1160 Large shallow inlets and bays in the UK but also occur extensively along the open coast and in lagoonal inlets. The physical structure of the intertidal flats ranges from mobile, coarse-sand beaches on wave-exposed coasts to stable, fine-sediment mudflats in estuaries and other marine inlets. This habitat type can be divided into three broad categories (clean sands, muddy sands and muds), although in practice there is a continuous gradation between them. Within this range the plant and animal communities present vary according to the type of sediment, its stability and the salinity of the water.

  1. Clean sands. These occur particularly on open coast beaches and in bays around the UK where wave action or strong tidal currents prevent the deposition of finer silt. Owing to the mobility of the sediment and consequent abrasion, species that inhabit clean sands tend to be robust and include amphipod crustaceans, such as sandhoppers Bathyporeia spp., some polychaete worms and certain bivalve molluscs.
  2. Muddy sands. These occur particularly on more sheltered shores of the open coast and at the mouths of estuaries or behind barrier islands, where sediment conditions are relatively stable. A wide range of species, such as lugworm Arenicola marina, and other polychaete worms and bivalve molluscs, can colonise these sediments. Substantial beds of mussels Mytilus edulis may develop on the lower shore. Intertidal beds of eelgrass Zostera spp. may also occur. In estuaries, reduced salinity conditions may give rise to a variety of other communities.
  3. Mudflats. These form in the most sheltered areas of the coast, usually where large quantities of silt derived from rivers are deposited in estuaries. The sediment is stable and communities are typically dominated by polychaete worms and bivalve molluscs and may support very high densities of the mud-snail Hydrobia ulvae. The high biomass of invertebrates in such sediments often provides an important food source for waders and wildfowl, such as common shelduck Tadorna tadorna, knot Calidris canuta and dunlin Calidris alpina.

Distribution of SACs/SCIs/cSACs with habitat 1140 Mudflats and sandflats not covered by seawater at low tide. Click image for enlarged map.

European status and distribution

Mudflats and sandflats not covered by sea water at low tide are a widespread habitat type on coasts of Atlantic Europe, particularly around the North Sea.

UK status and distribution

Mudflats and sandflats not covered by sea water at low tide occur widely throughout the UK.

Click here view UK distribution of this species

Site accounts

  • Berwickshire and North Northumberland Coast Eastern Scotland, Extra-Regio, Northumberland and Tyne and Wear
    This is an extensive and diverse stretch of coastline in north-east England and south-east Scotland. There is variation in the distribution of features of interest along the coast. Stretches of the coast in England support a very extensive range of intertidal mudflats and sandflats, ranging from wave-exposed beaches to sheltered muddy flats with rich infaunal communities. These have been selected as biologically diverse and extensive examples of clean sandflats on the east coast. Those in the Lindisfarne and Budle Bay area and on the adjacent open coast flats north of Holy Island are the most extensive in north-east England, with the largest intertidal beds of narrow-leaved eelgrass Zostera angustifolia and dwarf eelgrass Z. noltei on the east coast of England, a diverse infauna, and some large beds of mussels Mytilus edulis. Some of the bays along the open coast have mobile sediments, with populations of sand-eels Ammodytes sp., small crustaceans and polychaete worms. More sheltered sediments have very stable lower shore communities of burrowing heart-urchins Echinocardium cordatum and bivalve molluscs.
  • Carmarthen Bay and Estuaries/ Bae Caerfyrddin ac Aberoedd East Wales, Extra-Regio, West Wales and The Valleys
    Carmarthen Bay and Estuaries on the south coast of Wales includes extensive areas of intertidal mudflats and sandflats. Large areas of these intertidal flats are dominated by bivalves. In areas of fine sand cockles Cerastoderma edule are abundant, along with other bivalves, amphipods and worms. In muddier sediments the sand-gaper Mya arenaria, peppery furrow-shell Scrobicularia plana and mud-snail Hydrobia ulvae are also found in large numbers. The lower Loughor Estuary is one of the few places in the UK where the worm Ophelia bicornis has been found. There are also beds of the nationally scarce dwarf eelgrass Zostera noltei.
  • Dee Estuary/ Aber Dyfrdwy Cheshire, East Wales, Extra-Regio, Merseyside, West Wales and The Valleys
    Habitat occurrence description not yet available.
  • Dornoch Firth and Morrich More Extra-Regio, Highlands and Islands
    The Dornoch Firth is the most northerly complex estuary in the UK. Situated on the Scottish east coast, the estuary contains extensive areas of mudflats and sandflats. The flats extend in a wide belt along the northern and southern shores and are characteristic of a range of environmental conditions. There is a continuous gradient in the physical structure of the flats, from medium-sand beaches on the open coast to stable, fine-sediment mudflats and muddy sands further inland. This results in a high diversity of animal and plant communities supporting polychaetes, oligochaetes, amphipods, gastropods and bivalves. The sheltered bays provide a habitat for communities of algae, eelgrass Zostera spp. and the pioneer saltmarsh plant glasswort Salicornia spp.
  • Essex Estuaries Essex, Extra-Regio
    Essex Estuaries represents the range of variation of this habitat type found in south-east England and includes the extensive intertidal mudflats and sandflats of the Colne, Blackwater, Roach and Crouch estuaries, Dengie Flats and Maplin Sands. The area includes a wide range of sediment flat communities, from estuarine muds, sands and muddy sands to fully saline, sandy mudflats with extensive growths of eelgrass Zostera spp. on the open coast. The open coast areas of Maplin Sands and Dengie Flats have very extensive mudflats and an unusually undisturbed nature. Maplin Sands is particularly important for its large, nationally-important beds of dwarf eelgrass Zostera noltei and associated animal communities.
  • Fal and Helford Cornwall and Isles of Scilly, Extra-Regio
    This area supports examples of sheltered intertidal mudflats and sandflats representative of south-west England, and is particularly recognised for the importance of the species living in the sediments, including amphipods, polychaete worms, the sea cucumber Leptopentacta elongata and bivalve molluscs. Most of the shores of the Fal and Helford rias, and their upper reaches, are fringed by sandflats and mudflats. Owing to the sheltered nature of the site, the sediments are stable as well as being diverse, and include muds, muddy sand and clean sand. These support particularly rich and nationally important sediment communities in the Fal/Ruan estuary, Percuil River and in Passage Cove, including beds of dwarf eelgrass Zostera noltei and diverse invertebrate communities.
  • Humber Estuary East Yorkshire and Northern Lincolnshire, Extra-Regio, Lincolnshire
    the Humber Estuary includes extensive intertidal mudflats and sandflats not covered by seawater at low tide. Upstream from the Humber Bridge, extensive mud and sand bars in places form semi-permanent islands.
  • Isles of Scilly Complex Cornwall and Isles of Scilly
    The Isles of Scilly archipelago supports extensive areas of undisturbed intertidal sandflats in the extreme south-west of the UK. The islands are particularly important for exceptionally rich communities occurring in coarse sediments, including clean sand, a substrate that is usually poor in species. Although sheltered, the sediments include little mud because the surrounding seas have a low suspended sediment concentration, resulting from the islands’ isolation and the presence of oceanic water. The sandflats exposed at low tide between the northern islands are of international marine nature conservation importance, owing to their extent and diversity and the presence of species rarely found elsewhere in the intertidal. The lower shore sandflats are particularly notable, for they include the fringes of the most extensive and diverse beds of eelgrass Zostera marina known in southern England (Hocking & Tompsett 2001), with an unusually species-rich associated biota, including various seaweeds and fish and rich sediment communities of anemones, polychaete worms, bivalve molluscs and burrowing echinoderms. These include many species restricted to the sublittoral elsewhere in the UK. Many southern species are present, often in large numbers, including some, such as the hermit crab Cestopagurus timidus and the spiny cockle Acanthocardia aculeata, that are recorded only rarely in the UK.
  • Morecambe Bay Cumbria, Extra-Regio, Lancashire
    Morecambe Bay in north-west England is the confluence of four principal estuaries, the Leven, Kent, Lune and Wyre (the latter lies just outside the site boundary), together with other smaller examples such as the Keer. Collectively these form the largest single area of continuous intertidal mudflats and sandflats in the UK and the best example of muddy sandflats on the west coast. At low water, large areas of sandflats are exposed, and these range from the mobile fine sands of the outer Bay to more sheltered sands in the inner areas. With increasing shelter in the Bay’s adjoining estuaries, finer sediments settle out and form extensive mudflats, supporting a particularly rich and diverse range of infaunal species.
  • Severn Estuary/ Môr Hafren Dorset and Somerset, East Wales, Extra-Regio, Gloucestershire, Wiltshire and Bristol/Bath area
    Habitat occurrence description not yet available.
  • Solway Firth Cumbria, Extra-Regio, South Western Scotland
    The Solway Firth is representative of highly mobile, predominantly sandy intertidal flats on the west coast. It contains the third-largest area of continuous littoral mudflats and sandflats in the UK. These occur within a natural estuary system substantially unaffected by human activities, such as industrial development and dredging. The Solway is an unusually dynamic estuarine system, with mobile channels and banks. Fine sandy sediments occur in the inner estuary, and more stable and diverse conditions in the outer reaches. Salinity ranges from fully marine to estuarine in character, and these gradients in physical conditions add to the ecological diversity within the site. The presence of intertidal sediment flats of fine sands, rather than muds, in conditions of estuarine salinity is a notable feature.
  • Strangford Lough Northern Ireland
    The intertidal mudflats and sandflats in the north of Strangford Lough represent the largest single continuous area of such habitat in Northern Ireland. There are very extensive areas of muddy sand from Newtownards to Ardmillan Bay in the west and to Greyabbey in the east. The habitat also occurs in the south-west reaches of the Lough along the northern shore of Lecale. The northern flats support luxuriant beds of the eelgrasses Zostera noltei and Z. angustifolia. Common eelgrass Z. marina and tasselled pondweed Ruppia maritima are also present, the latter being widespread but quite local in its distribution. Such extensive beds are rare in the British Isles. The green algae Enteromorpha spp. and Ulva lactuca tend to occur where there is seepage of nutrient-enriched freshwater. Many of the invertebrate species present in muds also occur in muddy sand. However, lugworm Arenicola marina and nereid worms are generally dominant, along with bivalve molluscs such as Angulus tenuis, Mya arenaria and Cerastoderma edule.
  • The Wash and North Norfolk Coast East Anglia, Lincolnshire
    The Wash, on the east coast of England, is the second-largest area of intertidal flats in the UK. The sandflats in the embayment of the Wash include extensive fine sands and drying banks of coarse sand, and this diversity of substrates, coupled with variety in degree of exposure, means that there is a high diversity relative to other east coast sites. Sandy intertidal flats predominate, with some soft mudflats in the areas sheltered by barrier beaches and islands along the north Norfolk coast. The biota includes large numbers of polychaetes, bivalves and crustaceans. Salinity ranges from that of the open coast in most of the area (supporting rich invertebrate communities) to estuarine close to the rivers. Smaller, sheltered and diverse areas of intertidal sediment, with a rich variety of communities, including some eelgrass Zostera spp. beds and large shallow pools, are protected by the north Norfolk barrier islands and sand spits.
  • Tweed Estuary Northumberland and Tyne and Wear
    The Tweed is a long narrow estuary with a wide variety of intertidal mudflat and sandflat communities. Sandstell Point, at the mouth of the estuary, is a wide spit of clean mobile sand. This sand is subject both to wave action and, in places, the scouring action of the outflowing river, and is characterised by a mobile infauna (mainly crustaceans such as Eurydice pulchra and Bathyporeia spp. and a few polychaetes) which reflect these conditions. On the more sheltered west-facing shore of this spit, and on Calot Shad on the opposite bank, reduced mobility of the sand allows robust polychaetes such as Scolelepis squamata and Paraonis fulgens to occur with the crustaceans. Both biotopes are highly representative of the north-east of England. Further upstream at Yarrow Slake, more sheltered areas of muddy sand are characterised by polychaetes, amphipods, oligochaetes and enchytraeids that are characteristic species tolerant of brackish conditions.
  • Y Fenai a Bae Conwy/ Menai Strait and Conwy Bay Extra-Regio, West Wales and The Valleys
    The intertidal mudflats and sandflats of the Menai Strait and Conwy Bay on the north Wales coast include Traeth Lafan, the shores of the Menai Strait, and the Foryd estuary. Traeth Lafan is an example of an almost fully marine extensive mud and sandflat that experiences a broad range of wave exposure, providing a range of sediment types with typical associated communities. For example, the shrimps Haustorius arenarius and Bathyporeia sarsi are found in mobile clean sand, whilst bivalves such as the cockle Cerastoderma edule, the gaper Mya arenaria and Baltic tellin Macoma balthica are common in more sheltered fine and muddy sand. The sand-mason worm Lanice conchilega is found in more tide-swept areas. The mixed sediment shores between Beaumaris and Lleiniog are highly productive shores that are rich in animal and plant species. These shores include a nationally important biotope that is rare in the UK. The nationally scarce dwarf eelgrass Zostera noltei is also found at this site.

SACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection

Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.

Please note that the map shows sites where the presence of a feature is classed as ‘grade d’, but these sites are not listed. This is because ‘grade d’ indicates a non-significant presence.