Since the sixties eutrophication has been recognized to affect the quality of surface waters. The prolonged loading with nutrients has led to high algal concentrations in the water and to concommitant environmental problems such as the depletion of oxygen, the production of toxins and the development of tedious animal populations e.g. bream. A series of management measures have been carried out to combat eutrophication in the Netherlands. However, despite all efforts, the majority of the dutch surface waters is still considered to be eutrophic. The disappointing results are often attributed to processes in the sediment that can delay the improvement of the water quality. This thesis deals with the phosphorus dynamics in the sediment of an eutrophic lake. In contrast to most studies which are chemically orientated, attention here is mainly given to the role of microbial processes in the phosphorus cycle. The research was performed in eutrophic Lake Loosdrecht where the external phosphorus loading was recently decreased from 35-40 to 10-15 mmol.m -2.y -1.The importance of microbial processes for the release of phosphate by the sediment was investigated by comparing sterilized and non-sterilized columns (chapter 2). Columns were sterilized by γ-irradiation (25 kGy). The combination of temperature and γ-irradiation experiments made it possible to distinguish between microbial and physico-chemical processes. The release of dissolved phosphate from the sediment is controlled by microbial processes on a short-term and a long-term basis. Microbial mediated release responds directly on an increase in temperature. This is probably due to the induction of changes in the chemical environment such as a decrease in oxygen content of the surface layer. Mineralization of organic matter results in a mobilization of phosphate and is prerequisite to sustain the phosphate release on a long-term basis.Phosphate fluxes over the sediment-water interface were calculated using measured concentration gradients in the pore water and were ...