LAKE VOLKERAK-ZOOM - A LAKE SHIFTING FROM THE CLEAR TO THE TURBID STATE

Lake Volkerak-Zoom (area 6000 ha, mean depth 5 m) was created in 1987 by isolating a part of the Eastern Scheldt estuary. The new freshwater lake was clear despite a high phosphorus loading a few years after en closure. However, from 1990 onwards the transparency dropped. This pap er describes the transition from the clear to the turbid state in Lake Volkerak-Zoom. The processes responsible for the final lowering of th e transparency in the period 1988-1994 are discussed and also the meas ures necessary to meet the target situation: clear water with a transp arency of 2 m. From 1990 on transparency decreased due to an increasin g algal biomass, which was probably caused by reduced grazing of the z ooplankton. It is possible that deterioration of the food quality (by increased cyanobacterial blooms) reduced the zooplankton grazing. Howe ver, there are indications that increased mortality, caused by fish pr edation, played the most important role in the reduction of the zoopla nkton grazing. In 1992 the recruitment of fish, especially of roach (R utilus rutilus), was high (c. 40 kg ha(-1)). This caused a high predat ion pressure on the zooplankton and is shown by a decrease of the mean length and vertical migration of Daphnia in 1992. In the same year th e largest zooplankton species, Daphnia pulex, disappeared from the lak e. Smaller zooplankton species with a lower grazing capacity remained. Because of the decreased grazing, the algal biomass increased and the transparency dropped. We expect that in the future the lake will beco me more turbid. In addition to P-reduction, removal of fish is suggest ed to attain the target situation: a lasting clear water state.