Modelling the eutrophication process of lake Ijssel

Eutrophication can be considered as the most urgent problem in water quality management in The Netherlands. The main freshwater source for The Netherlands is the border crossing river Rhine, which is highly polluted by more or less treated industrial and domestic waste water, and therefore contains high concentration of nutrients. A branch of the river Rhine flows into the largest freshwater lake in The Netherlands, the Lake IJssel. Since this lake has to fulfil many functions, inhibited by severe (blue green) phytoplankton blooms the Dutch government initiated in 1979 a project in which a eutrophication model for the delta region of the river Rhine basin (Lake IJssel e.g. the estuary closed in 1928), consisting of two models e.g. BLOOM II, a phytoplankton model and CHARON, a model which describes the chemistry of the lake was applied. The models have been calibrated, verified and applied to the system of the river Rhine and the Lake IJssel in the framework of a eutrophication study. The models involved in this study are shortly explained. Special attention is paid to the calibration and verification step. The models are used to analyse the eutrophication process in the lake and to quantify the influence of policy and management scenarios including waste water treatment and measure with respect to nutrient reduction. The results of application of the models are discussed and evaluated against the background of the decision making process of the water manager.