APPLICATION OF SWITCH, A MODEL FOR SEDIMENT WATER EXCHANGE OF NUTRIENTS, TO LAKE VELUWE IN THE NETHERLANDS

The formulations of SWITCH, a model for prediction of nutrient fluxes across the sediment-water interface, are presented. Results of the application to data on the sediment of Lake Veluwe are presented and discussed. SWITCH calculates the thicknesses of the aerobic and denitrifying layers on the basis of a step-wise steady state approach. The concentrations of detritus, ammonium, nitrate and phosphate in the sediments and the pore water are simulated dynamically using mass balance equations. Analysis of the data for Lake Veluwe show large spatial heterogeneity. This presents a major draw-back for the calibration of SWITCH, which focused on the silty part of the lake. The results show that the model simulates realistically and consistently layer thicknesses, concentrations and mass fluxes connected with the transport and conversion processes. The model appears to have potential for describing both seasonal patterns and developments on the long term. SWITCH calculates strongly increased phosphate return fluxes, following total reduction of the top sediments. An important hypothesis in the model is that phosphate precipitated in reduced sediment layers is transferred to the oxidized layer and dissolves instantaneously. This results in a decrease of the phosphorus content of the sediment, but also maintains high release rates of phosphorus after the reduction of the external phosphorus loading of Lake Veluwe. Model results and mass balance studies for the overlying water indicate that the removal of phosphorus to deeper sediment layers is underestimated or that dilution of the sediments occurs as the result of sedimentation.