Recent nutrient trends in the western and central Baltic Sea, 1958-1989

Abstract

Trend studies on concentrations of phosphates and nitrates, covering the period of 1958-1989 show that a period of a strong increase in phosphates and nitrates accumulated in the winter surface layer of the Baltic proper was followed by a period of generally small changes, beginning in 1978. Since 1977, the positive trend coefficient of phosphates has been also decreasing in the oxic intermediate layer below the permanent halocline in the central Baltic Sea, whereas nitrate accumulation was continuing nearly unchanged in the whole period under investigation. In connection with the absence of major inflows of highly saline water masses into the Baltic proper, the salinity and oxygen content have been decreasing in the stagnant deep layer of its central part since 1977, whereas phosphate contents increased mainly due to the release from sediments in the presence of hydrogen sulphide. Very high peaks of nutrient concentrations were observed in shallow coastal waters (Pomeranian Bay) not only in the period of low biological activity, but also in late spring and summer. Consequences of the recent nutrient conditions are discussed with respect to eutrophication of the Baltic Sea.