A dynamic, deterministic model is presented to simulate the phosphorus cycle and plankton growth in the shallow, hypertrophic Loosdrecht Lakes (The Netherlands) before and after restoration measures. Both the water and the upper sediment layer are modelled. The model comprises three algal groups, zooplankton, fish detritus, zoobenthos and upper sediment (all modelled both in carbon and in phosphorus) besides inorganic phosphorus (SRP) in both the surface water and the interstitial water. Within the model system, the phosphorus cycle is completely closed. Carbon and phosphorus are described independently, so that dynamics of P/C ratios can be modelled. Sediment processes are described in a simplified form. Simulated values are largely within the range of observed ones. De detrital fraction of the seston (= phytoplankton + detritus) varies from 50-60 % in summer to about 90% in winter. SRP in the surface water is very low during most of the year. The sensitivity for external phosphorus input is larger for algal and detrital P than for algal and detritial C and chlorophyll-a. So the P/C ratio of the seston descreases following restorations measures, as is observed in the lakes, while the P/C ratios of zooplankton and fish remain constant and much higher. Phosphorus mobilisation from the sediment decreases with decreasing external input. Adaption of the model system to the reduced loading takes place in about two years. This means that without additional measures no further changes in the system are to be expected. Further reduction of phosphorus loading will lead to an improvement of water quality. Sources of uncertainty in the model include the limited knowledge on selective grazing as well as on mortality and mineralisation processes.<br>