Marine Atlantic littoral mud with associated communities

Intertidal flats along the open coast and near tidal inlets consist mainly of medium to coarse sand. In sheltered areas and near tidal watersheds the sediment is finer and may entirely consist of fine mud. This habitat type can occur in patches or grade into intertidal flats dominated by other soft sediments. Similarly whilst intertidal mudflats in large bays may be considered fully marine, there can still be a river influence and therefore reduced salinities across some of the habitat depending on location and outflow levels. Mudflats in fully marine waters, periodically falling dry at low tide, subject to great amplitudes of temperature, light and salinity, and may be subject to high eutrophication and input of organic substances from rivers and the open sea. Free of vegetation, of higher plants and of macroalgae but mostly covered by thin layers of diatoms and bluegreen algae. In some situations they may be colonised by seagrass. Sediments consist mainly of fine particles, mostly in the silt and clay fraction (particle size less than 0.063 mm in diameter), though sandy mud may contain up to 80% sand (mostly very fine and fine sand), often with a high organic content. Little oxygen penetrates these cohesive sediments, and an anoxic layer is often present within millimetres of the sediment surface. Intertidal mudflats in fully marine open sea (coast) only develop under macrotidal conditions such as those found in the German Bight and Mont Saint Michel, France. Also they form part of a habitat complex on a landscape scale within bays, barrier systems and estuaries. The intertidal mudflats support communities characterised by polychaetes, bivalves, snails and oligochaetes. The species composition of the macrobenthic communities are likely to show zoning from high to lower intertidal levels. In the Dutch, German and Danish Waddens Sea, for example, the high coastal tidal flats are generally characterised by a low number of species with numerous small individuals that are deposit feeders. Typical examples are Corophium volutator and Hydrobia ulvae. Large-sized species become numerous below mean tide levels and below this the biomass is dominated by the deposit feeding Arenicola marina. At mudflats well below mean low water, large filter-feeding bivalves such as Cerastoderma edule, Mya arenaria, Mytilus edulis and Ensis directus make up a significant proportion of the total biomass.

Indicators of quality:

Both biotic and abiotic indicators have been used to describe marine habitat quality. These include: the presence of characteristic species as well as those which are sensitive to the pressures the habitat may face; water quality parameters; levels of exposure to particular pressure, and more integrated indices
which describe habitat structure and function, such as trophic index, or successional stages of development in habitats that have a natural cycle of change over time.

There are no commonly agreed indicators of quality for this habitat, although particular parameters may have been set in certain situations e.g. protected features within Natura 2000 sites, where reference values have been determined and applied on a location-specific basis.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats