Habitat Action Plan

Seagrass beds develop in intertidal and shallow subtidal areas on sands and muds. They may be found in marine inlets and bays but also in other areas, such as lagoons and channels, which are sheltered from significant wave action.

Three species of Zostera occur in the UK, and all are considered to be scarce (present in 16-100 ten km squares). Dwarf eelgrass Zostera noltii is found highest on the shore, often adjacent to lower saltmarsh communities, narrow-leaved eelgrass Zostera angustifolia on the mid to lower shore and eelgrass Zostera marina predominantly in the sublittoral. The plants stabilise the substratum, are an important source of organic matter, and provide shelter and a surface for attachment by other species. Eelgrass is an important source of food for wildfowl, particularly brent goose and widgeon which feed on intertidal beds. Where this habitat is well developed the leaves of eelgrass plants may be colonised by diatoms and algae such as Enteromorpha spp, Cladophora rectangularis, Rhodophysema georgii, Ceramium rubrum, stalked jellyfish and anemones. The soft sediment infauna may include amphipods, polychaete worms, bivalves and echinoderms. The shelter provided by seagrass beds makes them important nursery areas for flatfish and, in some areas, for cephalopods. Adult fish frequently seen in Zostera beds include pollack, two-spotted goby and various wrasse. Two species of pipefish, Entelurus aequoraeus and Syngnathus typhie are almost totally restricted to seagrass beds while the red algae Polysiphonia harveyi which has only recently been recorded from the British Isles is often associated with eelgrass beds.

Five different community types have been identified for seagrass beds from the southern North Sea and the Channel and 16 microhabitats including the seagrass itself, sessile epifauna, infauna and free swimming animals not confined to a special part of the community. The diversity of species will depend on environmental factors such as salinity and tidal exposure and the density of microhabitats, but it is potentially highest in the perennial fully marine subtidal communities and may be lowest in intertidal, estuarine, annual beds.

The Cromarty Firth supports what is most probably the largest total area of dwarf eelgrass and narrow leaved eelgrass in Britain (approximately 1200 ha) while the Maplin Sands is estimated to be the largest surviving continuous population of dwarf eelgrass in Europe (covering around 325 ha). The Fleet has the most extensive population of all three Zostera species in Britain. Other important sites are the Exe Estuary, Maplin Sands, the Solents marshes and the Isles of Scilly, Morfa Nefyn, Milford Haven, the Moray Firth, Carlingford Lough, Dundrum Bay, Strangford Lough and Lough Foyle.

Reference should be made to the habitat action plans for saline lagoons, saltmarsh and mudflats.

Disease. A wasting disease was responsible for die-back of large areas of seagrass in the UK in the 1930s. The fungus and slime mould which colonised the weakened seagrass have recently reappeared in seagrass beds around the Isles of Scilly.

Natural cycles . The extent of seagrass beds may change as a result of natural factors such as severe storms, exposure to air, and freshwater pulses. Grazing by wildfowl can have a dramatic seasonal effect with more than 60% reduction in leaf cover reported from some sites. Warm sea temperatures coupled with low level of sunlight may cause significant stress and die back of seagrass.

Physical disturbance, for example by trampling, dredging, and use of mobile bottom fishing gear, land claim and adjacent coastal development through the construction of sea defences and potential for changes in the hydrological regime.

Introduction of, and competition from, alien species such as Spartina anglica and Sargassum muticum

Increased turbidity reducing photosynthesis.

Nutrient enrichment , at low levels, may increase production in Zostera while high nitrate concentrations have been implicated in the decline of mature Z. marina Phytoplankton blooms, resulting from nutrient enrichment, have been shown to reduce biomass and depth penetration of eelgrass. Eutrophication can also result in a shift to phytoplankton epiphyte or macroalgal dominance.

Marine pollution. Eelgrass is known to accumulate Tributyl, tin and possibly other metals and organic pollutants. Several heavy metals and organic substances have been shown to reduce nitrogen fixation which may affect the viability of the plant, particularly in nutrient poor conditions. Accumulated pollutants may become concentrated through food chains.

Areas of seagrass are included in some coastal ASSIs/SSSIs, Ramsar sites, SPAs (under the EC Birds Directive) and voluntary marine protected areas. Two out of the three UK Marine Nature Reserves have seagrass beds and the habitat occurs in a number of areas proposed as SACs under the EC Habitats Directive.

Information on the distribution of seagrass beds is being collected as part of the JNCC Marine Nature Conservation Review.

Seagrass beds around the Isles of Scilly were monitored for several years in the late 1980s by the Nature Conservancy Council and have been re-surveyed by volunteers. This work is on-going.

In Milford Haven, re-mapping of the location, extent and density of narrow-leaved eelgrass was completed by the Pembrokeshire National Park, as part of a rolling programme of research and monitoring administered by the Milford Haven Waterway Environment Monitoring Steering Group. Repeat surveys of eelgrass in Milford Haven are likely to be the next focus for attention. Eelgrass in North Haven, Skomer, is monitored on a regular basis as part of the Marine Nature Reserve work programme. This work has been given an extra focus by the events surrounding the Sea Empress oil spill in 1996.

In Northern Ireland research, part funded by the Department of Agriculture (NI), has examined the utilisation of seagrass by wildfowl in Strangford Lough. There have also been investigations by the Department of the Environment (NI) into methods of controlling Spartina, which in some situations is encroaching onto seagrass beds. Spartina control in Strangford Lough using the herbicide Dalapon was resumed in 1997.

A report on the status of eelgrass in Scotland was published in 1993 covering latest information on taxonomy and systematics, distribution, threats and suggestions for further work.

A major review of the key conservation, management and monitoring requirements of the genus Zostera in the UK was completed in 1997 on behalf of EHS.

A two year research project at the Royal Botanic Gardens, Kew, to improve understanding of seagrass seed biology and conserve eelgrass in the seed bank, has also been completed.

A baseline study of the mudflats (including detailed Zostera work) at the north end of Strangford Lough has been completed ahead of a major up-grade of the sea wall in the area.

Limited data on habitat restoration and management of seagrass beds does not permit a full costing to be undertaken for this action plan. However, an estimate of potential costs is provided on the basis of several recent US studies. One project, in Tampa Bay, Florida, will require approximately £6,000 per hectare for full restoration of a 263 hectare site. It should be noted that this project incorporates other objectives as well as seagrass restoration. In the UK, a 1974 study concluded that transplanting of seagrass was feasible at a cost of approximately £4,200 (1994/95 prices) per hectare.

Until surveys to ascertain the extent of the seagrass resource are completed it is not feasible to provide a specific target for restoration. However, the data in Table 1 below provide indicative costs on the basis of assumptions that at least 1,000 hectares will require restoration during the programme and that this will be at an average cost of £5,000 per hectare.

Habitat Type: Seagrass beds (£000 per annum)

Renny Mckeown, Environment and Heritage Service Tel:

The following LBAPs are working on Seagrass beds:

Originally published in: Biodiversity: The UK Steering Group Report - Volume II: Action Plans (December 1995, Tranche 1, Vol 2, p262)