Ecology of eutrophic waterbodies in a coastal grazing marsh

The ecology of a mainly interconnected, complex system of smallwater-bodies on a coastal grazing marsh in the Greater Thames estuary,north-east of London, England, is described. Sixteen sites were sampled on aregular basis over a two-year period while 37 sites were sampled once in Mayto develop a site classification. Most sites were brackish and alkaline but,following re-wetting after drying out, the pH in some water-bodies decreasedsharply (minimum pH 2.7). All sites had high concentrations of phosphorusbut there were large differences; some, with good growths of submergedaquatic macrophytes, having lower phosphorus than other sites, which weredevoid of macrophytes. Internal loading with phosphorus was important.Nitrate levels were low. Waters were mostly turbid, Secchi depths sometimes falling as low as 2cm, while chlorophyll a was very high, indicating extreme hypertrophy.Diatoms and euglenoids were characteristic members of the phytoplankton;cyanobacterial blooms did not occur. The zooplankton was dominated bycopepods, while Cladocera were much less numerous, largely due to predationby shrimps in waterbodies without macrophytes. The classification of siteswas influenced primarily by water depth, some waterbodies drying out, whilephosphorus and turbidity were also important in differentiating sites. Animal guano is the likely cause of the hypertrophic state but livestockgrazing is an essential management tool for grazing marshes. The dredging ofwaterbodies (with removal of sediments from the site) and the instigation ofa flushing regime, requiring a new system of sluices, should reduce nutrientloading and assist the establishment of submerged macrophytes, providingrefuges for cladocerans from predation.