Eutrophication and self-purification: counteractions forced by large-scale cycles 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 observed in nitrogen reserves. Although denitrification is an anaerobic process, 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 period 1973-1999 was recorded. If dissolved silica limits phytoplankton spring bloom, diatom blooms fade and become replaced by flagellates leading to changes in summer phosphate reserves and sedimentation. Seven years have elapsed from the previous strong saltwater inflow and anoxia has spread over large areas. Deep bottom phosphorus levels have started to increase and denitrification 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 unacceptable status of eutrophication.