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 enclosure. However, from 1990 onwards the transparency dropped. This paper describes the transition from the clear to the turbid state in Lake Volkerak-Zoom. The processes responsible for the final lowering of the transparency in the period 1988-1994 are discussed and also the measures necessary to meet the target situation: clear water with a transparency of 2 m. From 1990 on transparency decreased due to an increasing algal biomass, which was probably caused by reduced grazing of the zooplankton. It is possible that deterioration of the food quality (by increased cyanobacterial blooms) reduced the zooplankton grazing. However, there are indications that increased mortality, caused by fish predation, played the most important role in the reduction of the zooplankton grazing. In 1992 the recruitment of fish, especially of roach (Rutilus rutilus), was high (c. 40 kg ha(-1)). This caused a high predation 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 the largest zooplankton species, Daphnia pulex, disappeared from the lake. 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 become more turbid. In addition to P-reduction, removal of fish is suggested to attain the target situation: a lasting clear water state.