The hydrographic-hydrochemical state of the Baltic Sea in 1997

The winter of 1996/97 can be characterized as moderate in the western and central Baltic Sea but was only weak in the Gulfs of Bothnia, Finland and Riga. The temperatures in the winter surface and summer intermediate layers met the long-term mean. The summer of 1997 was the warmest since 1890. The water temperatures reached positive anomalies up to 6 K in the 10 m upper layer due to sunny weather conditions and weak winds during August.

The meteorological conditions in 1997 did not give rise to major Baltic inflows. The Darss Sill area was characterized by a distinct estuarine circulation only interrupted by two long lasting outflow events in May and November. Between the end of May and the beginning of November, persistend inflow was recorded in the near-bottom layers which culminated in two storms in September and at the beginning of October transporting larger volumes of water across the sills. Thereby exceptional warm and saline water entered the Baltic Sea which caused temperatures from 11.5 to 13 °C and salinities up to 18 PSU in the Bornholm Basin deep water at the end of October. Such positive temperature anomalies of 3 to 4 K were rarely measured in the deep water during the present century. The Bornholm Basin deeper than 60 m was filled by water with salinities > 15 PSU at the end of October. Saline (14 to 15 PSU), warm (> 10 °C) and oxygen-rich water (up to 2cm³/dm³) crossed the Stolpe Sill into the central Baltic Sea.

Due to the stagnation beginning in 1995 the deep water in the Gotland and Fårö Deeps became anoxic below 175 m depth until the end of 1996. Small inflows shortly interrupted this development between February and May 1997. From August onward the below 150 m in the eastern Gotland Basin again became anoxic. The deep water of the western Gotland Basin continues to be oxic.

The decreasing tendency of phosphate concentrations measured in the mixed winter surface layer since several years continued in all Baltic areas under investigation. The winter concentrations of nitrate are still high but in 1997 first indications for a decrease could be observed.

Significant impacts of the exceptional Oder flood in summer 1997 could only be observed locally in the Pomeranian Bight. The measured concentrations of most inorganic nutrients ranged in the same order of magnitude as observed earlier during times of increased Oder discharges in spring. Significantly increased concentrations were, however, measured for silicate and organic carbon and nitrogen compounds. In August, hydrogen sulfide occured in the bottom layer in front off Usedom Island. Long-term effects of the nutrient load on the ecosystem in the Pomeranian Bight and in the whole Baltic Sea are not expected.

Dr. Wolfgang Matthäus