Habitat Action Plan

Mudflats are sedimentary intertidal habitats created by deposition in low energy coastal environments, particularly estuaries and other sheltered areas. Their sediment consists mostly of silts and clays with a high organic content. Towards the mouths of estuaries where salinity and wave energy are higher the proportion of sand increases. Mudflats are intimately linked by physical processes to, and may be dependent on, other coastal habitats such as soft cliffs and saltmarshes. They commonly appear in the natural sequence of habitats between subtidal channels and vegetated saltmarshes. In large estuaries they may be several kilometres wide and commonly form the largest part of the intertidal area of estuaries. However, in many places they 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. The mud surface also plays an important role in nutrient chemistry. In areas receiving pollution, organic sediments sequester contaminants and may contain high concentrations of heavy metals.

Mudflats are characterised by high biological productivity and abundance of organisms, but low diversity with few rare species. The mudflat biota reflects the prevailing physical conditions. The JNCC Marine Nature Conservation Review (MNCR) biotope codes for mudflats are LMU.SMu (Sandy mud shores), LMU.Mu (Soft mud shores) and LMS.MS (Muddy sand shores). In areas of lowered salinity, the macroinvertebrate fauna is predominantly of the Petersen Macoma community, characteristic species being: common cockle Cerastoderma edule, sand-hopper Corophium volutator, laver spire shell Hydrobia ulvae, ragworm Hediste diversicolor and, when salinity is low, large numbers of oligochaete annelids (principally Tubificoides spp). With a slight increase in the proportion of sand, the polychaetes catworm 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.

The total UK estuarine resource has been estimated as c588,000 ha of which 55% is intertidal area, mostly mud and sandflats with a lesser amount of saltmarsh. Intertidal flats cover about 270,000 ha. The UK has approximately 15% of the north-west European estuarine habitat.

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. They are widespread in the UK with significant examples in the Wash, the Solway Firth, Mersey Estuary, Bridgwater Bay and Strangford Lough.

Reference should be made to the saltmarsh and seagrass beds habitat action plans.

It has been estimated that sea level rise will result in a loss of 8000 to 10,000 ha of intertidal flats in England between 1993 and 2013. Much of this loss is expected in southern and south-east England although research suggests that the major firths in Scotland will also be affected. The rise results from sinking of the land following the end of the last ice age, plus the effects of global warming. Low water moves landward, but sea defences prevent a compensating landward migration of high water mark with the result that intertidal flats are squeezed out.

Land claim, for urban and transport infrastructure and for industry, has removed about 25% of Great Britain estuarine intertidal flats and up to 80% in some estuaries. Loss of mudflats reduces estuary productivity and may influence other estuary habitats such as saltmarsh. Although land claim has slowed considerably in recent years, it has not stopped.

Barrage schemes for water storage, amenity, tidal power and flood defence continue to pose a threat to the integrity and ecological value of mudflats in estuaries and enclosed bays.

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, and dredging for navigation, have an important effect on sediment biota and on sediment supply and transport. Many coastal areas, including estuaries, are now either licensed or available for exploration and development.

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.

The introduction of new or non-native species, for example the spread of cord-grass Spartina anglica whichhas vegetated some upper-shore mudflat areas with important ecological consequences in some areas.

Within estuaries, mudflats deposited in the past may erode due to changed estuarine dynamics and remobilised sediment may be redeposited 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.

Protection for mudflats is provided by various international and EU agreements and is implemented by the relevant UK enabling legislation. In addition the UK has its own domestic measures which can protect mudflats. Some of this legislation provides direct protection for the habitat +whilst other measures provide indirect protection by controlling water quality.

International designations of major significance to mudflats are the Ramsar Convention protecting wetlands of international importance, the Bonn Convention to protect migratory species of wild animals, and the Bern Convention to conserve European wildlife and habitats.

Sites designated under EU law form part of the Natura 2000 series of protected habitats, ie Special Protection Areas (SPA) under the 1979 EC Birds Directive or Special Areas of Conservation (SAC) under the 1992 EC Habitats Directive. SACs may be designated for the Annex I habitat 'Mudflats and sandflats not covered by seawater at low tide'. Mudflats are also included within several other designated Annex I Habitats: 'Estuaries', 'Lagoons' and 'Large shallow inlets and bays'.

Under the Wildlife and Countryside Act 1981, over 300 SSSIs which include mudflats have been designated on estuaries. In addition there are 22 (November 1998) coastal ASSIs in Northern Ireland, 10 of which contain significant areas of mudflats.

The water quality on mudflats will be improved by a number of EC Directives including the Dangerous Substances, Shellfish (Waters), Integrated Pollution Control, Urban Waste Waters and Bathing Waters Directives. The forthcoming Water Framework Directive will also be relevant. The Oslo and Paris Convention (OSPAR) and the North Sea Conference declarations are also important. These commitments provide for the regulation of discharges to the sea and have set targets and quality standards covering many metals and pesticides, and other toxic persistent and bioaccumulative substances.

Government departments (MAFF, SE, CEC, DTI and DETR) are responsible for the assessment of the potential impacts of oil and gas exploration and production aggregate extraction, marine construction work, land reclamation and dumping of dredged material prior to licensing. The conditions attached to these licences can stipulate that measures are adopted to minimise environmental impacts. Licenses may be refused on environmental grounds.

The statutory nature conservation agencies have included the management of mudflats within several wider schemes. The Environment Agency (EA), and local authorities in England and Wales, with guidance from the Ministry of Agriculture Fisheries and Food (MAFF) and the National Assembly for Wales (NAW), develop Shoreline Management Plans (SMPs). The main remit of SMPs is for flood and coast protection, and includes recognition of the important role played by mudflats in protecting low lying coastal features. In Scotland, SNH is leading the Firths Initiative and SMPs are also being developed. The UK Marine SAC project is funded by the EU LIFE programme to develop management schemes to be implemented under the EC Habitats Directive. Mudflats are included in the project's study sites. Many intertidal flats are also covered by Estuary Management Plans (EMPs). The EA takes similar considerations into account in its Local Environment Agency Plans (LEAPs). Many Local Nature Reserves, designated by local authorities, but often managed by Wildlife Trusts, are in upper intertidal areas and can benefit both saltmarsh and mudflats. Wildlife Trusts and the RSPB also own and/or manage mudflats within estuarine and coastal reserves.

The Natural Environment Research Council Special Research Topic Land Ocean Interaction Study (1994-8), included estuarine and coastal processes, with a component on littoral sediment processes (LISP). Similarly, the EU Marine, Science and Technology (MAST) programme ECOFLAT studied mudflat physical and biological processes and interactions.

English Nature has studied the loss of intertidal areas in the Essex estuaries and is collaborating with EA and MAFF on two different projects studying managed set-back as a flood defence option in the Blackwater estuary: at Orplands and Tollesbury. At Orplands research with the EA on estuary hydrodynamics covers sediment erosion and accretion. At Tollesbury work has been done with MAFF on the re-creation of intertidal habitats for nature conservation and flood defence. These schemes have made a small contribution to the creation of new mudflats within the study areas. In Northern Ireland, a study of mudflats at the north end of Strangford Lough, on behalf of the Department of Agriculture Northern Ireland Rivers Agency, has been carried out prior to a major upgrade of the seawall.

Local Government planning guidelines (eg PPG20, NPPG7 and NPPG13) identify the consultation required on wetland habitat development. The publications Strategy for Flood and Coastal Defence (MAFF/WO, 1993) and Towards Best Practice for Coastal Zone Management (Department of the Environment, 1996) also consider mudflat habitats in the light of proposed developments.

The successful implementation of this habitat action plan will have resource implications for both the public and private sectors. The data in the table opposite provide an estimate of the current expenditure on the habitat and the likely additional resource costs. These additional costs are based on the annual average over 5 and 10 years. The total expenditure for these time periods is also given. Almost all the costs will relate to the public sector, although some costs (eg for research) will be met by the private sector/non-governmental organisations).

Anon. 1996. Birds of Conservation Concern. Royal Society for the Protection of Birds, Sandy.

Cambridge Environmental Research Consultants Ltd. 1993. Targets for coastal habitat recreation. English Nature Science No.13, English Nature, Peterborough.

Cambridge Coastal Research Unit. 1996. Geomorphological Trends in the Blackwater Estuary, 1978 - 1994. Report to National Rivers Authority and English Nature. CCRU, University of Cambridge.

Cranswick, P.A., Waters, R.J., Musgrove, A.J. & Pollitt, M.S. 1997. The Wetland Bird Survey 1995-1996: Wildfowl and Wader Counts. British Trust for Ornithology, The Wildfowl and Wetlands Trust, Royal Society for the Protection of Birds and Joint Nature Conservation Committee.

Davidson, N.C., D.A. Laffoley, D., Doody, J.P., Way, L.S., Gordon, J., Key, R., Drake, C.M., Pienkowski, M.W., Mitchell, R. & Duff, K.L. 1991. Nature conservation and estuaries in Great Britain. Nature Conservancy Council, Peterborough.

Department of the Environment. 1996. Coastal Zone Management - towards best practice. HMSO, London.

English Nature. 1992. Campaign for a Living Coast. English Nature, Peterborough

McLusky D.S., Bryant, D.M. & Elliott, M. 1992. The impact of land-claim on macrobenthos, fish and shorebirds on the Forth estuary, eastern Scotland. Aquatic Conservation: Marine and Freshwater Ecosystems, 2 , 211-222.

Ministry of Agriculture, Fisheries and Food. 1993. A strategy for flood and coastal defence in England and Wales. MAFF PB 1471.

Ministry of Agriculture, Fisheries and Food. 1995. Shoreline Management Plans: A guide for coastal defence authorities. MAFF PB 2197.

Prater, A.J. 1981. Estuary birds of Britain and Ireland. T & AD Poyser, Carlton.

Raffaelli, D.G., Raven, J. & Pool, L. 1998. Ecological impact of green macro-algal blooms. In: Oceanography and Marine Biology: an Annual Review 36 , 97-126.

Duncan Huggett, Environment Agency Tel:

The following LBAPs are working on Mudflats:

Originally published in: UK Biodiversity Group Tranche 2 Action Plans - Volume V: Maritime species and habitats (October 1999, Tranche 2, Vol V, p179)