ECOLOGY OF EUTROPHIC WATERBODIES IN A COASTAL GRAZING MARSH

The ecology of a mainly interconnected, complex system of small waterb odies on a coastal grazing marsh in the Greater Thames estuary, north- east of London, England, is described. Sixteen sites were sampled on a regular basis over a two-year period while 37 sites were sampled once in May to develop a site classification. Most sites were brackish and alkaline but, following re-wetting after drying out, the pH in some w aterbodies decreased sharply (minimum pH 2.7). All sites had high conc entrations of phosphorus but there were large differences; some, with good growths of submerged aquatic macrophytes, having lower phosphorus than other sites, which were devoid of macrophytes. Internal loading with phosphorus was important. Nitrate levels were low. Waters were mo stly turbid, Secchi depths sometimes falling as low as 2 cm, while chl orophyll a was very high, indicating extreme hypertrophy. Diatoms and euglenoids were characteristic members of the phytoplankton; cyanobact erial blooms did not occur. The zooplankton was dominated by copepods, while Cladocera were much less numerous, largely due to predation by shrimps in waterbodies without macrophytes. The classification of site s was influenced primarily by water depth, some waterbodies drying out , while phosphorus and turbidity were also important in differentiatin g sites. Animal guano is the likely cause of the hypertrophic state bu t livestock grazing is an essential management tool for grazing marshe s. The dredging of waterbodies (with removal of sediments from the sit e) and the instigation of a flushing regime, requiring a new system of sluices, should reduce nutrient loading and assist the establishment of submerged macrophytes, providing refuges for cladocerans from preda tion.