Coastal grazing marsh
This article provides an introduction to a habitat created by the enclosure of saltmarsh and its subsequent use for grazing by domestic stock. Conversion to intensive agriculture by drainage and infilling creeks does not normally take place. The habitat therefore retains many of the original features of saltmarsh in a modified form. Coastal grazing marsh lies towards the most maritime end of the spectrum of coastal wet grasslands. This article distinguishes between the general term ‘lowland wet grassland’, which can be applied to virtually all river valley grasslands and coastal marshes [1] (Ward 1994), 'coastal wet grassland' and 'coastal grazing marsh'.
Contents
Introduction
Two principal features are important in defining the ‘coastal’ nature of the lowland wet grassland: its origins and the degree of maritime influence. A general term of ‘coastal wet grassland’ or coastal and floodplain grazing marsh [2] as defined by the UK Habitat Action Plan, is used to describe those grasslands which lie on the margins of tidal influence. These are subject to some, often limited maritime influence. More specifically ‘coastal grazing marsh’ is applied to sites where the coastal wet grassland is derived from marine sediments (mostly those from which a salt marsh develops).
Lowland wet grassland
Wet grassland can occur in any areas of low-lying land where the soil is subjected to varying water levels. Impeded drainage, flooding as a result of river banks over-topping or excessive precipitation are among the factors helping to create permanent or seasonally wet grasslands. Those which are unploughed and traditionally managed as grazing pasture or used for hay, with little use of artificial fertiliser, tend to have a rich flora and fauna.
Coastal wet grassland
What distinguishes coastal wet grassland from lowland wet grassland which happens to lie near the sea, is the nature of the soil and/or the influence of saline water. Their origins are usually the result of drainage of swamps and other low-lying wetlands derived from marine sediments around the margins of tidal embayments. These enclosures may date back to ‘Roman Times’ as in the case of the Wash (the Fenland Basin, East Anglia) and Romney Marsh (c200AD) in Kent. Precise definitions are difficult because the marine sediments may be overlain by river alluvium as relative sea level movements alter the landward limits of tidal influence. Although now largely intensive agriculture, the fenland basin around the Wash, for example, is derived from the enclosure of freshwater and brackish swamps and salt marshes. The Ouse Washes in Cambridgeshire lie adjacent to the canalised river Ouse which stretches many miles inland and is influenced by tidal water along much of its length. The extent of saline water intrusion today is limited, and only occurs in the lowest tidal reaches of the river. Pevensey Levels on the south coast in East Sussex was formed on the site of a former inlet (Steers 1969) [2] but today has no saline influence. Other coastal wet grasslands may receive limited saline water intrusion from seepage zones through coastal barriers or as a result of overtopping by the sea. There will be gradations from brackish to freshwater, especially in the drainage ditches.
Coastal grazing marsh
Coastal grazing marsh is a specific term applied to land derived from the enclosure of salt marsh. Coastal grazing marsh is recognised as a distinct habitat type in Great Britain (Section 11.2.17 of the SSSI Guidelines define it as “Enclosed, unimproved or semi-improved salt marsh.”) [3]. These areas represent some of the most recent (within the last 200-300 years) enclosures. The habitat is something of an anomaly in European terms and not recognised in the classification used to identify Special Areas of Conservation (European Commission 1999 [4]). It is defined by the presence of permanent and semi-permanent grassland, drainage ditches and enclosing earth dykes. Features of the original marsh are present including old creek lines. Saline waters derive from seepage zones through the sea walls and intrusion of sea water along channels which may remain partially open to the tide. As with other wet grasslands the wildlife interest has developed alongside the traditional use of the land for agricultural, notably grazing or hay-making.
From the above it might be inferred that these definitions are mutually exclusive and easy to identify. However this is far from the case and historically the drainage and enclosure of tidal lands, such as those around the Wash inevitably include transitions between salt marsh, brackish water swamps and fresh water fens. The Norfolk Broads are also derived from fen peat deposits though in this case many of the wetlands are the result of excavations.
Coastal grazing marsh is best considered as a sub-set of coastal wet grassland. The principal components which help to define these systems include:
- Sea walls;
- Saline seepage areas;
- Vehicle rutted ground;
- Grassland (alongside the sea wall);
- Counter ditches (alongside sea wall);
- Grazing marsh;
- Cattle poached areas;
- Dredged spoil heaps;
- Fleet (former tidal channel);
- Reed beds and scrub (after Gray, 1977 p. 257[5]).
This description relates to land derived from salt marsh enclosure. Although this falls within a general term “reclaimed land” used by Gray (1977) he makes a distinction between two types of 'reclaimed land' (perhaps more accurately referred to as land claim), based on the method of reclamation. The former involves the enclosure of an intertidal surface, usually salt marsh covered, the land has unique character and is described as “permanent” retaining a high wildlife interest (the ‘coastal grazing marsh’ as defined above). This is distinguished from “temporary” land derived from newly enclosed bare surfaces, subsequently developed for more intensive uses. The latter involves either pumping dry the existing tidal mud, or infilling the void behind a sea wall with imported material. This may include dredged sediments from the seabed or other material such as refuse.
"Coastal grazing marsh" is thus one of the habitat sub-categories within the section dealing with biodiversity of coastal and marine habitats and ecosystems.
Animals and plants
The unimproved permanent pasture when used for low intensity grazing, developes a vegetation structure attractive to nesting birds. These include redshank, lapwing, snipe, curlew and oystercatcher which also breed on salt marsh. In the winter surface flooding of parts of these same areas attract wintering wildfowl such as teal, wigeon and waders. The presence of short grassland also provides grazing for waterfowl, including the brent goose.
Rare species of plants are often found in association with the pasture and the brackish water drainage ditches. The brackish water ditches are particularly important for a number of rare invertebrates. Some of the more important sites include, for example a high proportion of the total number of dragonflies which occur in Britain. 12 out of the 44 species were found on the Gwent Level (Severn Estuary) and 14 in the Essex grazing marshes, respectively. These sites and the Suffolk and North Kent marshes include more than 10% of all the species which have been identified as being nationally rare or scarce in samples of both aquatic and "terrestrial" species.
References
- ↑ Ward, D., 1994. Management of lowland wet grassland for breeding waders. British Wildlife, 6/2, 89-98.
- ↑ Steers, J.A., 1969. The Coastline of England and Wales. 2nd edition. Cambridge University Press, Cambridge.
- ↑ Nature Conservancy Council 1989. Guidelines for the Selection of Biological SSSIs. Nature Conservancy Council, Peterborough
- ↑ European Commission, 2003. Interpretation Manual of European Habitats. Natura 2000. European Commission, DG Environment, Natrue and Biodiversity, Brussels. Source: [1]
- ↑ Gray, A.J., 1977. Reclaimed land. In: The Coastline, ed., R.SK. Barnes, John Wiley & Sons, Chichester, 253-270.
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