The hydrographic-hydrochemical state of the Baltic Sea in 1999

The winter of 1998/1999 was mild in the entire Baltic Sea area. The summer was among the five warmest since 1946, all of it occured in the 1990s. The September 1999 was the warmest since 1946.

The mild winter led to sea surface temperatures in the Baltic Sea that were about 1 K above the long-term mean during winter and early spring. This resulted in relatively warm water of 3-4 °C being trapped in the summer intermediate water layer of the central Baltic Sea. The surface temperatures in the summer reached positive anomalies up to 4 K in the 20 m upper layer due to the warm summer. The deep water temperatures of the central Baltic Sea were anomalously high due to the inflow of warm water in 1997, 0.2-1.7 K above the mean for period 1950-1998.

The weather conditions in winter 1998/1999 gave rise to a low inflow activity across the sills into the Baltic with only small inflows Inflows in October and late December 1998 led only to the aeration of the Bornholm Basin up to the end of November when other small inflows led again to oxic conditions. A major inflow event did not ocur.

In the eastern Gotland Basin, the anoxic water covered the layer between bottom and about 130 m depth all year round. In the deep water of the western Gotland Basin, oxygen depletion has continued since 1993. The decrease in oxygen concentration led in August 1999 to the formation of hydrogen sulphide in the near-bottom layer for the first time since the mid-1980s. In 1999, the area of the whole central Baltic deep water which was affected by oxygen deficiency and anoxic conditions was the largest in the past 15 years.

The concentration of the inorganic nutrients phosphate and nitrate in the mixed winter surface layer did not show any clear tendencies since 1995. But, comparing the first and second half of the 1990s, disctinct decrease in the phosphate concentrations can be observed, especially in the near-coastal areas of the western Baltic, but also in the central Baltic Sea. Measures undertaken to reduce point sources seem to be effective. A new equilibrium is already established and, therefore, a further decrease in phosphate concentrations is at least questionable. In contrast, nitrate concentrations do not show any significant decrease between both periods of the 1990s. This may be attributed to the predominance of diffuse sources and a huge contribution by atmospheric input.

Concentrations of organic nutrients observed since 1994 show a pronounced seasonal signal in the central Baltic surface water for both particulate and dissolved organic carbon. In near-coastal areas, which are strongly influenced by riverine inputs, and in the western Baltic, which is affected by exchange processes with the North Sea, no clear seasonal signals could be identified.

Dr. Wolfgang Matthäus