Modeling the effects of macrophyte growth and decomposition on the nutrient budget in Shallow Lakes

A numerical model was developed to illustrate the annual life cycle and the decomposition process of Potamogeton pectinatus L. Growth, decay, and their interactions were considered simultaneously for five fractions: main and secondary shoots, roots, tubers, and new tubers. The model was applied to field experiments carried out in Swartvlei Lake (South Africa). The predictions of the model for biomass of each fraction, standing stock of phosphorus and phosphorus content in shoots, secondary shoots and roots well agree with the observed values. The model successfully simulated the decomposition process, the remaining dry mass, and the phosphorus and nitrogen content of the remaining biomass. A series of numerical experiments were subsequently carried out to investigate the long-term effects of macrophyte decomposition on the nutrient budget of a hypothetical lake. Results show that phosphorus released in the decomposition process of macrophytes in the lake can be reduced significantly, at least by 75%, if the above-ground macrophyte biomass is harvested at the end of the growing season. Moreover, phosphate leached to the overlying water can be reduced to less than half within 10 years, if the above-ground biomass (AGB) is removed.