The study presents results of decadal simulations of the dynamics of the ecosystem of the Baltic Sea with a three-dimensional coupled physical chemical biological model for the 1980s. The model consists of a circulation model, which is an implementation of the Modular Ocean Model (MOM 2) for the Baltic Sea and an embedded ecosystem model based on a nine-component model (ERGOM <Neumann, 2000>). The model was driven with realistic atmospheric forcing, river loads, and atmospheric deposition of nutrients. A simulation with the realistic forcing was carried out as a control run in order to study in a further experimental simulation the response of the model ecosystem to a 50% reduction of riverine nutrient loads. It was found that the model food web reacts on the load reduction in a complex manner. The changes in dissolved nutrients and in phytoplankton display a spatial variability and different species-specific reactions. While the total biomass and nutrient inventories are reduced, there are significant regional differences. In particular, the role of cyanobacteria, which can fix atmospheric nitrogen, is important, and the biomass of cyanobacteria increases in response to the reduced loads. Although the model system needs further improvements, the present study is a step toward an understanding of the quantitative effects of a reduction of nutrient river loads to the Baltic Sea ecosystem based on a full three-dimensional coupled model.