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

Abstract

A numerical model, incorporating phytoplankton, submerged macrophytes, Potamogeton pectinatus L ., their decomposition process and resulting nutrient dynamics in the overlying water, was developed. The model was first applied to Swartvlei Lake, South Africa, to test the possible general application of the model, and second, to a simplified hypothetical lake to evaluate the effects of external nutrient loading, water temperature, depth and the retention time of water on the macrophyte growth and phytoplankton development. Total macrophyte biomass, standing stock of phosphorus, as well as phosphorus content in shoots, secondary shoots and roots of macrophytes in Swartvlei Lake were reproduced well, providing insights into phosphorus stock capacity of P. pectinatus in the lake. Numerical experiments on a hypothetical lake provided information about the interaction between macrophyte growth and phytoplankton development. Except for extremely high inflow nutrient loading, the repeated depletion and accumulation of nutrients causes frequent blooms of phytoplankton standing stock, which then creates the associated shading in the water. As a slow responder, however, macrophytes monotonically decrease with increasing nutrient inflow loading and shading effects due to phytoplankton biomass. Increasing water temperature shifts the phytoplankton 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 concentration is high, macrophytes, which have already grown sufficiently during the decline phase of phytoplankton blooms, continue to grow due to the favorable light conditions near the water surface. As retention time is reduced to a critical level, if not too short, the phytoplankton standing stock increases with higher nutrient concentrations, suppressing the macrophyte development.