Modelling the effects of macrophyte decomposition on the nutrient budget in shallow lakes

A model of the annual life cycle of submerged macrophytes, Potamogeton pectinatus L. , and their decomposition process was developed. Their growth and decay were considered for five fractions simultaneously: main and secondary shoots, roots, tubers and new tubers of P. pectinatus in the growth submodel. In the decomposition submodel, different decomposition rates were incorporated for the easily and hardly degradable fractions of the above ground biomass. The model was applied to the field experiment in Swartvlei Lake (South Africa) rich in Potamogeton pectinatus L.. The biomass of each fraction, standing stock of phosphorus and the phosphorus content in shoots, secondary shoots and roots were reproduced well. The model was also able to simulate successfully the decomposition process, the remaining dry mass, and the phosphorus and nitrogen content of the remaining biomass. A series of numerical experiments were carried out for an imaginary lake to investigate the long term effects of macrophyte decomposition on the lake ecosystem.