Mathematical modelling as a tool for management in eutrophication control of shallow lakes

The eutrophication model Delwaq-Bloom-Switch is developed to be a functional tool for water management. Therefore it includes nutrients, algal biomass and composition as well as water transparency. A module describing the interaction between water and bottom gives the model the flexibility to deal with measures, such as a decrease of the external phosphorus loading and flushing with water differing in composition from the lake water. This paper focuses on the functional aspects of the model, the results of an application on Lake Veluwe, The Netherlands, and the implications for water management.With one set of coefficients DBS reproduces the most important characteristics of Lake Veluwe for a period of two years before measures (reduction of the external loading and flushing during the winter months) and eight years after the measures. The phosphorus concentration decreased and became growth limiting for algae instead of nitrogen and light. Both in measurements and modelling results, the algal composition changed from blue-green algae dominance to green algae and diatom dominance. Lake Veluwe had a relatively short transient phase after reduction of external loading, because high nitrate concentrations in the flushing water inhibited a long period with high phosphorus releases from the bottom.Model calculations were carried out to investigate the effects of fish stock management and optimization of flushing. Both measures are promising.