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Grazing marsh is aBritish Isles term for flat,marshygrassland inpolders. It consists of large grass fields separated by fresh orbrackish ditches, and is often important for its wildlife.
Grazing marshes were created frommedieval times by buildingsea walls (earth banks) across tidalmudflats andsalt marsh to makepolders (though the term "polder" is little used in Britain). Polders in Britain are mostly drained by gravity, rather than active pumping. The original tidal drainage channels were augmented by new ditches, andflap valves in the sea walls let water drain out at lowtide and prevent the sea ortidal river from entering at high tide. Constructing polders in this way is calledinning orreclaiming from the sea.
Grazing marshes have been made in most lowlandestuaries in Britain, often leaving only theriver channel and the lowest part of theestuary tidal. In a few cases (such asNewtown Harbour on theIsle of Wight, andPagham Harbour inWest Sussex) thesea walls have been breached, and the estuaries have returned to a tidal state. Grazing marshes have also been made on low-lying open coasts.
Many grazing marshes were inned in stages, and the old sea walls (calledcounter walls) may be found marooned far from the current sea wall. Land levels on either side of a counter wall often differ by several metres. Paradoxically, the lower side is the land inned earlier, because sediment continued to build up on the side that remained tidal.
Winteringwildfowl are characteristic of grazing marshes, often including large flocks ofEurasian wigeon,brent goose,white-fronted goose andBewick's swan. Many of these birds are hunted by predators such asperegrine andmarsh harrier.
In spring,waders such ascommon redshank,Eurasian curlew,snipe, andnorthern lapwing breed.[1]
The ditches often have a range ofsalinity, depending on how close to the sea wall they are. The more saline ditches hostspecialist brackish-water plants and animals. These include, for example, the rare brackishamphipodGammarus insensibilis andsea club-rush (Bolboschoenus maritimus). Fresher ditches may support rare animals, such as thegreat silver water beetle (Hydrophilus piceus) and thegreat raft spider (Dolomedes plantarius), and a wide range of pondweeds (Potamogeton and relatives).
The grassland vegetation usually has a fairly small number of species, but those present are often scarce elsewhere, such assea arrowgrass (Triglochin maritimum),divided sedge (Carex divisa) and strawberry clover(Trifolium fragiferum).
Many grazing marshes have been converted intoarable land, often using pumped drainage to lower the water levels enough to grow crops, though most are used for grazing cattle.[2] The low ditch levels andagricultural runoff combine to remove much of the aquatic wildlife, although the arable fields may still be used by some winteringwildfowl.
Some areas of grazing marsh and other polder land have been used to recreate tidal habitats by a process ofmanaged retreat.
Many of the larger areas of grazing marsh bear nature conservation designations, includingSite of Special Scientific Interest,Special Protection Area,Special Area of Conservation andRamsar Site.
