MBDP-NI: Priority habitat

Sheltered Muddy Gravels (Mixed Sediments)

Sheltered Muddy Gravels (Mixed Sediments)

Correspondence with existing habitats

Intertidal mixed sediments:

Habitats Directive Annex I: Mudflats and sandflats not covered by seawater at low tide

Habitats Directive Annex I: Estuaries

Habitats Directive Annex I: Large shallow inlets and bays

Subtidal mixed sediments: Not covered

Description: Sheltered muddy gravel habitats occur principally in areas protected from wave action and strong tidal streams. In fully marine conditions on the lower shore this habitat can be extremely species-rich, however, good quality examples of this habitat are very scarce. One such area can be found at Ballyhenry Island, Strangford Lough where the rare burrowing sea anemone Edwardsia timida and the burrowing sea cucumber Leptosynapta inhaerens can be found. Polychaetes and bivalve molluscs are normally dominant and the most varied, but representatives of most marine phyla can be present. The fauna is often characterised by a large range in body size.

At lower salinities there is a marked reduction in species richness. Lower salinity muddy gravels are characterised by the carpet shell mollusc Venerupis corrugata, the blunt gaper Mya truncata, the polychaetes Neanthes virens and Cirriformia tentaculata, the cockle Cerastoderma edule and the native oyster Ostrea edulis. Oligochaetes and the ragworm Hediste diversicolor usually dominate upper estuarine low salinity muddy gravels.

Full salinity community can include the tube-dwelling polychaetes Sabella pavonina, Myxicola infundibulum and Amphitrite edwardsi, the sipunculan worm Golfingia sp., the anemones Cylista troglodytes and Cereus pedunculatus, and the holothurian Labidoplax digitata. The presence of coarse gravel and stones at the sediment surface often provides a substratum for the attachment of a variety of fauna and epiflora, for example fucoids, ephemeral green algae with associated littorinids and filamentous red algae.

Shallow subtidal muddy gravel can contain communities of burrowing anemones such as Mesacmaea mitchelli, Capnea sanguinea, Cereus pedunculatus and Synarachnactis lloydii.

Associated biotopes:

Intertidal mixed sediment

Priority species: Capnea sanguinea, Thyonidium drummondi, Panningia hyndmani, Cestopagurus timidus, Edwardsia timida, Ostrea edulis

Threats:

Physical disturbance: Coastal developments including the construction of marinas and slipways, sediment extraction, the widening and dredging of channels and sea defences such as barrages. Such activity may alter tidal flow patterns, affecting the sedimentary conditions across the gravel beds.
Bait digging: Threat is especially prevalent where king rag Neanthes virens is common.
Fisheries: Intertidal mollusc beds, including Venerupis corrugata, have been the subject of small fisheries in the past. The current fishery is small, but has the potential for resurgence, whereas Mercenaria mercenaria dredging in Southampton Water has severely disrupted this habitat.
Organic enrichment, especially sewage pollution stress: Severe pollution can lead to anoxic conditions and a decrease in macrobenthic populations and species diversity.
Persistent bio-accumulating chemicals (e.g. polychlorinated biphenyls and tri-butyl tin), waste discharges containing heavy metals and chemicals.
Introduction of non-native species: Crepidula fornicata can dominate the fauna resulting in the smothering of the sediment surface leading to anoxia in the sediment. They are also considered a pest of oyster beds. Non-native bivalves such as the American jack-knife razor clam can replace native species.

References:

This habitat description has been adapted from the 1994 UK BAP Action Plan for Sheltered Muddy Gravels.

Marine Biodiversity Data Portal - NI

Sheltered Muddy Gravels (Mixed Sediments)