In many European water bodies with a high dilution rate (flowing waters, reservoirs, mountain lakes), the supply of nitrate from fertilized agricultural areas is so high that it may exceed the demand of the photoautotrophic organisms and also of the bacteria on the sediment/water interface by more than one order of magnitude. In oligotrophic drinking water reservoirs in Saxony, the atomic N/P ratio may exceed a magnitude of 5000:1. This nitrate excess is an indication of a lack of electron donors. Our data confirm the thesis that a feedback exists in these cases between phosphate deficiency and nitrate excess, i.e. nitrate may operate as a factor of oligotrophication. It is, however, unavoidable to control nitrate if the critical threshold for drinking water supply (CTDW) might be exceeded. This control has to be focused upon agricultural practices. In waste treatment plants, a final nitrification stage (instead of nitrification/denitrification) may be sufficient in situations where the mass flow of nitrate is governed by the non-point sources and where the CTDW is still not exceeded. On the other hand, control of phosphate is imperative in all cases where control of eutrophication is the primary management goal.

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