The hydrographic-hydrochemical state of the Baltic Sea in 2002

The winter of 2001/2002 was the fifth mild winter in succession. Moreover, it belongs to a period of relative mild winters which started 1987/1988. In the southern Baltic Sea anomalies up to +4.8 ^oC were observed in February. Indeed, positive deviations from the long-term mean air temperature continued until September, with very warm air masses from southern Europe causing unusually stable "subtropical" conditions in August and the first part of September. All in all, the summer of 2002 was the second warmest since 1890, at least for the southern Baltic Sea area. Only that of 1997 was warmer.

However, not only was the first half of 2002 unusually warm. During all months precipitation in the western Baltic drainage area was abnormally high. For example, precipitation in north-east Germany in February was 273% of the average. With a spatial average of 1018 mm the year 2002 was the richest in percipitation since the beginning of German wide calculations in 1901. Consequently, riverine flows and with them nitrogen inputs into the Baltic Sea were exceptionally high.

High nutrient inputs enhanced algal blooms from spring onwards. With the development of a strong thermohaline stratification in the western Baltic Sea in June, oxygen concentrations in the near bottom waters began to decrease through processes of mineralization of sedimenting organic matter. A long period of sunny, warm, and calm weather conditions in late summer then caused the oxygen situation in the near bottom waters to deteriorate rapidly. Oxygen levels fell to the limit of detection and there was production of hydrogen sulphide. The result was a wide-range depletion of bottom fauna in Danish and German waters and reports of fish kills over wide areas of the western Baltic Sea. The affected areas were larger than ever observed before, covering the area between Kattegat and Mecklenburg Bight. Stronger winds in early autumn, however, led to a slight improvement in the situation. In the further course of the year, oxygen conditions returned to normality.

In 2002, the western Baltic experienced an exceptional frequency and duration of easterly and southerly winds, which hardly favoured major gale-forced inflow events from the North Sea, but partly led to unusual ways of water exchange between the Kattegat and the Baltic. In August/September, the Darss Sill mast dominantly recorded inflowing waters in a thick bottom layer with high salinity and oxygen content between 4 and 1 ml/l, which carried exceptionally warm water into the adjacent basins. In October/November 2002 an additional small inflow occurred, driven by westerly winds, which added even more remarkably warm water and apparently caused a ventilation of the Gdansk Basin by November 2002. Eye-catching warm waters later propagated further towards the eastern Gotland Basin and improved along their way the stagnation conditions below the halocline.

The intermittent water renewal in January, caused by arriving North Sea waters which entered the Baltic in autumn 2001, could already no longer be observed in summer 2002.

Dr. Günther Nausch