Modelling the effects of macrophytes on algal blooming in eutrophic shallow lakes

A dynamic model for eutrophication incorporating phytoplankton and nutrients in the overlying water, submerged macrophytes Potamogeton pectinatus L., and nutrient dynamics in sediments was developed as a functional tool to understand the effects of macrophytes on algal blooming in shallow lakes. The submodule for the overlying water treats three types of phytoplankton (diatoms, green and blue green algae), the macro-nutrients, as well as dissolved oxygen in the water, while the macrophyte submodule consists of equations for the time-dependence of the macrophyte components: shoots, secondary shoots, roots, tubers, and new tubers. The sediment submodule describes the concentrations of detritus, nitrates, ammonium, dissolved oxygen, and phosphates. These modules are processed simultaneously. The model was applied to the Lake Veluwe (Netherlands) experiment, successfully reproducing the seasonal changes in the lake, such as the transition of the dominant species of phytoplankton from diatoms during spring, green algae in summer, to blue green algae in late autumn depending on the nutrient and light condition, nutrient concentration in the water, and the yearly life-cycle of macrophytes. The model also describes the long term transition in the lake: after artificial flushing with water rich in nitrates and calcium, the return flux from the sediment decreased, and together with the reduction in the external load, the phosphate concentration in the overlying water decreased. This lessened the phytoplankton biomass in the water, which resulted in the remarkable development of macrophytes under the improved light condition. Sensitivity analyses of important coefficients indicated that the maximum photosynthesis and respiration rate are the most sensitive parameters for macrophyte and algal development. The parameters have sufficient accuracy to be successfully applied to other experiments.