Intertidal mudflats and sandflats form a major component of both estuaries and embayments, but also occur on the open coast. These intertidal flats can range from the mobile, coarse-sand beaches of wave-exposed coasts to the stable, fine sediment mudflats of estuaries and embayments. They are often restricted to the mid and lower shore by rocky features, and therefore can exist as small ‘pockets’ of sediment through to extensive mudflats fringing inlets and estuaries.
These mud and sand flats shelter large diverse communities of invertebrates and algae and so are particularly important as feeding grounds for wildfowl and waders.
The flats of the Gann near Dale are the most biologically diverse intertidal sediment site within the SAC, despite being used heavily for bait digging. This area of very mixed substrate supports a mosaic of distinct communities including sandy Echinocardium cordatum and muddy sand Macoma balthica communities. The muddy gravel Venerupis senegalensis community at the Gann is considered to be the richest in south-west Wales. A Code of Conduct for bait digging at the Gann is in place following much research and engagement with stakeholders. Please see here.
Zostera noltii, intertidal eelgrass, is a nationally scarce marine flowering plant which can be found in the intertidal flats of the SAC. Such areas are also important as nursery areas for fish such as bass and mullet.
For ease of description intertidal flats can be divided into three broad categories, although in practice they merge into each other. The three categories are:
Clean sands. Found on clean, sandy beaches on the open coast and in bays where wave action or strong tidal streams prevent fine silt being deposited or where the supply of suspended silt is low.
Muddy sands. These occur where there is enough shelter from wave action to allow the deposition of fine sediments, but some water movement or lack of supply of silt leads to a more sandy substrate. A wide range of organisms can colonise these sediments, such as lugworms Arenicola marina, mussels Mytilus edulis, and eelgrasses Zostera spp.
Mudflats. These form in the most sheltered areas of the coast, usually in estuaries which have large supplies of silt from the rivers draining into them. Due to the sheltered nature of these areas the sediment is stable and dominated by polychaete worms and bivalve molluscs often with very high numbers of the snail Hydrobia ulvae.