Eutrophication and self-purification: Counteractions forced by large-scalecycles and hydrodynamic processes

During the 1990s nitrogen reserves have shown rising trends in the northern Baltic Sea. Changes in denitrification explain some of the fluctuations ob served in nitrogen reserves. Although denitrification is an anaerobic proce ss, the most efficient removal of nitrogen by denitrification occurs where the sediment is moderately well oxidized. A dramatic decrease in the ratio of SiO4 to DIN (DIN = NO3 + NH4) in the northern Baltic proper during the p eriod 1973-1999 was recorded. If dissolved silica limits phytoplankton spri ng bloom, diatom blooms fade and become replaced by flagellates leading to changes in summer phosphate reserves and sedimentation. Seven years have el apsed from the previous strong saltwater inflow and anoxia has spread over large areas. Deep bottom phosphorus levels have started to increase and den itrification capacity is weakening. Thus, all efforts to minimize N- and P- loading will, in the long run, help the Baltic Sea to recover from the unac ceptable status of eutrophication.