This study concentrates on eutrophication effects in shallow lakes and ponds on the one hand and in ditches (small water channels in agricultural areas) on the other. In shallow lakes (up to ca 4 m of depth), ihe clear-water community characterized by macrophytes is generally replaced by a dominance of phytoplankton and turbid water, while a diverse fish community including piscivores is transferred into a species-poor community dominated by bream. In ditches, eutrophication causes the typical, richly structured community of submerged macrophyles to be replaced by a monotonous layer of small floating plants, duckweeds. This leads, among olher things, to an anaerobic environment and deterioration of aquatic life. As these biotic effects are considered as undesirable, it is important to be able to predict, as far as possible in a quantitative way, at what degree of eutrophication these changes will occur, and whether they are reversible or not. Mathematical models are a useful tool to address such questions and support management decisions. This thesis describes two such mathematical model, a model for lakes and a model for ditches