Marine Biodiversity Data Portal - NI

Correspondence with existing habitats

OSPAR habitat: Intertidal mudflats

Habitats Directive Annex I: Large shallow inlet and bays

Description: Mudflats are sedimentary intertidal habitats that occur in low energy coastal environments, particularly estuaries, inlets and other sheltered areas. Their sediment consists mostly of silts and clays with a high organic content. In estuaries and inlets they commonly form the largest part of the intertidal area. In Belfast Lough much of the mudflats have been much reduced by land claim. Mudflats, like other intertidal areas, dissipate wave energy, thus reducing the risk of eroding saltmarshes, damaging coastal defences and flooding low-lying land. In areas receiving pollution, organic sediments sequester contaminants and may contain high concentrations of heavy metals. Mudflats support important populations of marine worms, molluscs and other invertebrates and are important source of food for many shore birds. The mudflat biota is influenced by the prevailing physical conditions such as salinity and exposure to wave action.

Areas of lowered salinity, are characterised by the common cockle Cerastoderma edule, sand-hopper Corophium volutator, laver spire shell Peringia ulvae, ragworm Hediste diversicolor and, when salinity is very low, large numbers of oligochaete annelids (principally Tubificoides spp).

With a slight increase in the proportion of sand, the polychaetes Nephtys hombergi and lugworm Arenicola marina occur. In slightly coarser areas, seagrass (Zostera spp.) beds may develop. Where stones and shells provide an initial attachment for byssus threads, beds of the common mussel Mytilus edulis occur and accrete material through faecal deposition. Occasional stones or shells may also provide suitable attachment for stands of fucoid macroalgae such as Fucus vesiculosus or F. spiralis. The surface of the sediment is often apparently devoid of vegetation, although mats of benthic microalgae (diatoms and euglenoids) are common. These produce mucilage (mucopolysaccharides) that binds the sediment. Under nutrient-rich conditions, there may be mats of the macroalgae Enteromorpha spp. or Ulva spp.

Mudflats are highly productive areas which, together with other intertidal habitats, support large numbers of predatory birds and fish. They provide feeding and resting areas for internationally important populations of migrant and wintering waterfowl, and are also important nursery areas for flatfish.

In Northern Ireland, intertidal mudflats can be found in Strangford Lough, Belfast Lough, Killough Harbour and Inner Dundrum Bay.

Associated biotopes:

• LS.LSa.MuSa – Polychaete/bivalve dominated muddy sand shores
• LS.LMu – Littoral mud

• EUNIS code: A2.24 and A2.3

Priority species: Ostrea edulis

Threats:

• Sea level rise: Sea defences prevent a landward migration of the high water mark with the result that intertidal flats are squeezed out.
• Land claim: Urban and transport infrastructure and for industry
• Barrage schemes: Water storage, amenity, tidal power and flood defence continue to pose a threat to mudflats.
• Diffuse and point source discharges from agriculture, industry and urban areas: Including polluted storm-water run-off, can create abiotic areas or produce algal mats which may affect invertebrate communities. They can also remove embedded fauna and destabilising sediments thus making them liable to erode.
• Oil and gas extraction and related activities.
• Dredging for navigation can have an important effect on sediment biota and on sediment supply and transport.
• Fishing and bait digging can have an adverse impact on community structure and substratum. For example, suction dredging for shellfish or juvenile flatfish bycatch from the shrimp fisheries may have a significant effect on important predator populations.
• Human disturbance: Affects bird populations’ roosting and feeding areas.
• Introduction of new or non-native species: For example the spread of cord-grass Spartina anglica which has vegetated some upper-shore mudflat areas with important ecological consequences in some areas.
• Estuarine dynamics: Within estuaries, mudflats deposited in the past may erode due to changed estuarine dynamics and remobilised sediment may be re-deposited elsewhere in the same littoral sediment cell.
• Higher sea level and increased storm frequency: Resulting from climate change, may further affect the sedimentation patterns of mudflats and estuaries.

References:

This habitat description has been adapted from the 1994 UK BAP Action Plan for Mudflats.