The hydrographic-hydrochemical state of the Baltic Sea in 1996

The severe winter 1995/96 in the western and central Baltic Sea which is among the strongest since 1946 (4th rank) caused negative temperature anomalies in winter surface and summer intermediate waters. The surface temperatures were below the long-term mean up to July but reached positive anomalies between 2 and 4 K in August due to sunny high pressure weather conditions and weak winds.

Compared with 1995, the mean inflow of saline water into the Baltic decreased. This supported the continuation of stagnation in the deep water of the central Baltic in 1996. The very intensive inflow in the beginning of November, which just missed the criteria for major Baltic inflows, transported about 80 km³ water with salinities up to 26 and 20 PSU across the Drodgen and Darss Sills into the Baltic.

The oxygen conditions in central Baltic deep waters deteriorated in 1996, due to the continuation of the present stagnation period beginning in 1995. For the first time since 1994, hydrogen sulphide was observed in the Gotland and Fårö Deeps. Advective processes linked with small inflow events in March and June 1996 supplied the deep water of the Bornholm and Gdańsk Basins with oxygen.

Whereas the winter concentrations of nitrate were nearly unchanged high in the surface layer in 1996, the decreasing tendency of the phosphate concentrations continued in all Baltic areas under investigation. The changes of the redox potential depending on the oxygen conditions decided the distribution of the inorganic phosphorus and nitrogen compounds in central Baltic deep waters in 1996.

In 1996, the abiotic environmental conditions were unfavourable for the development of good year classes of cod, sprat and herring due to the deteriorating oxygen conditions and the severe winter.

Dr. Wolfgang Matthäus