Securing Biodiversity in Breckland - European Commission

influenced by seasonal water tables and damp grassland, while undrained intact peat can supportfen communities.In the headwaters and tributaries of the Little Ouse, Lark, Wissey and Thet spring-fed valley-headfens have developed on chalky nutrient poor water that percolates upward through thin peat layers(Rothera 1989). Fen exists in a number of sites including at Great Cressingham Fen, ThompsonCommon and Gooderstone Fen in Norfolk and Market Weston Fen in Suffolk which also has sawsedge Cladium mariscus, as does Hopton Fen nearby. Fen with saw sedge Cladium mariscus alsoexists at Talent’s Fen on Foulden Common and at Swangey Fen on the edge of the NationalCharacter Area.Organic peat occurs over most of the river valley floodplains, reaching 2-3 metres depth in thecentre of larger valleys, but is generally drained and humified to a eutrophic state (Corbett, 1973).Little of the valley fen resource remains, though areas do exist along the River Little Ouse betweenThetford, Brandon and Weeting and in fragments elsewhere.Peats have also formed locally in meres (Corbett 1973) – fluctuating ground-water fed water bodieson the plateaus, all restricted to the Norfolk Breckland. These unusual hydro-geological features arefed by base-rich groundwater with no in-flow or out-flow streams and exhibit concentric zones ofdifferent vegetation due to fluctuation in the water table. Importantly, they are thought to benaturally self sustaining, not silting up or filling in, as vegetation is killed off by drying and reflooding(Rothera 1989). There are five principal meres which are recognised as true fluctuatingground-water fed meres, these are: Langmere, and Ringmere (both at East Wretham Heath),Fowlmere, Home Mere (in STANTA at Thorpe Great Heath) and the Devil’s Punchbowl. There are 3other meres within STANTA which demonstrate some characteristics of fluctuating meres -Smoker's Hole, West Mere and West Tofts Mere. Of these, Smoker's Hole is the most like theprincipal five, the other two are less typical, West Mere may have with underlying impermeablestrata, whilst West Tofts is spring-fed), but which nevertheless exhibit a response to high watertables in the surrounding chalk/sands. The identification of remaining meres is less certain butessentially comprises a series of hollows around the East Wretham/Roudham area, which clearlyhave the same form as the principal meres but that hold water only occasionally when the watertable is particularly high. There are at least seven of these on East Wretham Heath and a further 8through the Roudham Forest block to the east of East Wretham Heath (B. Nichols pers. comm.).Some water bodies near Larling have also been suggested to exhibit fluctuating mere characteristics(Watson, 1974). A number of other waterbodies exist that are rain-fed and lack the characteristicspecies of fluctuating meres.Pingo systems, created by ice lenses during tundra conditions c20, 000 years ago, comprisecomplexes of pools. Within one system, different pools may vary in the extent to which they areground water fed, their pH, whether water levels fluctuate and in their vegetation. This providescomplex range of ecological conditions. Surviving systems occur at Thompson Common, FouldenCommon, East Harling Common, Great Hockham (Hills and Holes), Breckles Heath and FrostsCommon and within STANTA (Rothera 1989; Walmsley 2008).The floristic diversity and interest of the pingo systems arises from variations in substrate,hydrology, water pH and the resulting habitat mosaic (Walmsley 2008). Where water levels withinthe hollows fluctuate, vegetation may be unable to establish for long periods and bare mud on thedraw-down zones can provide opportunities for species that need low levels of competition. High30