Modelling macrophyte-nutrient-phytoplankton interactions in shallow eutrophic lakes and the evaluation of environmental impacts

A numerical model, incorporating phytoplankton, submerged macrophytes, Pota mogeton pectinatus L., their decomposition process and resulting nutrient d ynamics in the overlying water, was developed. The model was first applied to Swartvlei Lake, South Africa, to test the possible general application o f the model, and second, to a simplified hypothetical lake to evaluate the effects of external nutrient loading, water temperature, depth and the rete ntion time of water on the macrophyte growth and phytoplankton development. Total macrophyte biomass, standing stock of phosphorus, as well as phospho rus content in shoots, secondary shoots and roots of macrophytes in Swartvl ei Lake were reproduced well, providing insights into phosphorus stock capa city of P. pectinatus in the lake. Numerical experiments on a hypothetical lake provided information about the interaction between macrophyte growth a nd phytoplankton development. Except for extremely high inflow nutrient loa ding, the repeated depletion and accumulation of nutrients causes frequent blooms of phytoplankton standing stock, which then creates the associated s hading in the water. As a slow responder, however, macrophytes monotonicall y decrease with increasing nutrient inflow loading and shading effects due to phytoplankton biomass. Increasing water temperature shifts the phytoplan kton blooms earlier than macrophyte development, which reduces the shading of phytoplankton for the subsequent macrophyte growth. Together with lower phytoplankton biomass due to macrophyte biomass, macrophytes develop faster with rising temperature. In a shallower lake, although the phytoplankton c oncentration is high, macrophytes, which have already grown sufficiently du ring the decline phase of phytoplankton blooms, continue to grow due to the favorable light conditions near the water surface. As retention time is re duced to a critical level, if not too short, the phytoplankton standing sto ck increases with higher nutrient concentrations, suppressing the macrophyt e development. (C) 2001 Elsevier Science B.V. All rights reserved.