Abstract A short description is given of a dynamic ecological model (SALMO) of the pelagic zone of reservoirs and lakes. The model involves only three state-variables (two groups of phytoplankton, zooplankton, orthophosphate), but nevertheless a great number of internal control mechanisms are considered. The applicability of this model to quite different water bodies is checked by comparing simulation results with observations. Four water bodies are used for these comparisons: Lake Stechlin (deep, oligotrophic), the Saidenbach reservoir (deep, mesotrophic), the Bautzen reservoir (shallow, hypereutrophic), and the Bleiloch reservoir (deep, hypereutrophic, high light extinction). Despite numerous deviations between the simulations and observations, the general result of this validation justifies the use of SALMO in water-quality management. A relative comparison of simulated scenarios is regarded to be the most adequate method in such an application because it minimizes the influence of inaccuracies in the model. An example of a scenario analysis for decision-making in the management of a multipuroose reservoir is given in detail.