EXISTENCE OF A MACROPHYTE-DOMINATED CLEAR WATER STATE OVER A VERY WIDE-RANGE OF NUTRIENT CONCENTRATIONS IN A SMALL SHALLOW LAKE

Little Mere, a small shallow lake, has been monitored for four years, since its main source of nutrients (sewage effluent) was diverted. The lake has provided strong evidence for the persistence of a dear water state over a wide range of nutrient concentrations. It had clear wate r at extremely high nutrient concentrations prior to effluent diversio n, associated with high densities of the large body-sized grazer, Daph nia magna, associated with low fish densities and fish predation. Foll owing sewage effluent diversion in 1991, the nutrient concentrations s ignificantly declined, the oxygen concentrations rose, and fish predat ion increased. The dominance of large body-sized grazers shifted to on e of relatively smaller body-sized animals but the clear water state h as been maintained. This is probably due to provision of refuges for g razers by large nymphaeid stands (also found prior to diversion). Ther e has been a continued decrease in nutrient concentrations and expansi on of the total macrophyte coverage, largely by submerged plants, foll owing effluent diversion. The grazer community of Little Mere has also responded to this latter change with a decline in daphnids and increa se in densities of weed-associated grazers. The presence of large dens ities of such open water grazers was the apparent main buffer mechanis ms of the clear water state until 1994. The lake has, so far, maintain ed its clear water in the absence of such grazers. Thus, new buffer me chanisms appear to operate to stabilize the ecosystem. Little Mere app ears to have shifted from previous top-down controlled clear water sta te to a bottom-up controlled clear water state.