Аннотация:In a case study using the Slnava Reservoir on the river Vah (Czechoslovakia) and the catchment agricultural area (4000ha of cropland), a comparison was made of data obtained by laboratory experiments, field measurements and mathematical simulation. In the reservoir body were found the negative consequences of strongly increased input rates of ammonia-N and nitrate-N from the applied fertilizers and the inhibitory effect of pesticides. Due to continual increase of agrochemicals input to water reservoir an important part of the biogeochemical cycle of N is damaged. Results show a strong suppression of nitrification process in water environment and accumulation of toxic nitrite in the system. Serious symptoms of unstability were found in the water ecosystem.