Current Trends in Hydrographic and Chemical Parameters and Eutrophication in the Baltic Sea

Analysis of phosphate and nitrate data recorded in the winter surface layer of the Baltic Sea from 1958 to 1989 indicate positive overall trends, although the rate of accumulation has decreased since 1978. The long term accumulation of these algal nutrients in the oxic deep water below the halocline has also decreased since 1977. Under anoxic conditions, however, the trend shown by the phosphate concentration is distinctly positive in recent times owing to phosphate remobilization from the sediment. The salinity has decreased in the surface water and even more rapidly below the halocline in the central Baltic basins during the period marked by the absence of major salt water inflows since 1977. The oxygen conditions have also deteriorated in the near-bottom water layer during this time as shown by the considerable rise in the hydrogen sulphide concentrations. These changes, including the recent phosphate accumulation under anoxic conditions, can thus be considered natural rather than anthropogenic. The impact of long-term variations in nutrients and hydrographic parameters is discussed with respect to the development of Baltic fish stocks.