Intertidal sandflats and mudflats

Intertidal mudflats and sandflats are areas of sediment that are submerged at high tide and exposed at low tide.

The intertidal mudflats and sandflats of the SAC are present within:

  • The three main estuaries, the Glaslyn/Dwyryd, Mawddach and the Dyfi;
  • In fully marine open coast situations on moderately exposed and exposed sandy shores at Porth Dinllaen on the north Llŷn coast, along the south Llŷn coast between Pen-ychain and Criccieth and between Criccieth and Afon Glaslyn and along the Meirionnydd coast at Harlech and the mouth of the Mawddach and Dyfi estuaries.

The structure of intertidal mudflats and sandflats varies depending on the physical conditions and forces acting on them (in particular the degree of exposure to wave action and tidal currents) as well as the nature of the sediments occurring in any one location. The sediments vary from mobile coarse sand in more wave exposed areas to stable, fine sediment expanses of mudflat in estuaries and other marine inlets.  

Intertidal mudflats and sandflats support a variety of different wildlife communities. These are predominantly infaunal communities with a variety of different animals species such as worms, molluscs and crustaceans living within the sediment habitat. The type of sediment, its stability and the salinity of the water have a large influence on the wildlife species present.

In areas of clean, more mobile sand, communities of robust species such as sandhoppers, some polychaete worms and certain bivalve molluscs occur. In muddy sands, more species-diverse communities develop, including species such as the lugworm, and other polychaete worms and bivalve molluscs.  Substantial beds of mussels Mytilus edulis may develop on the lower shore of muddy sand areas, and intertidal beds of eelgrass Zostera spp. may be present. Mudflats are the most stable and can support communities that are typically dominated by large numbers of polychaete worms and bivalve molluscs. In addition to these infaunal communities, the intertidal mudflats and sandflats can support epifaunal and mobile species assemblages and often provide an important food source for other species, such as waders and wildfowl in estuarine and coastal locations.


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