The hydrographic-hydrochemical State of the Baltic Sea 2000

The winter of 1999/2000 was mild in the whole Baltic Sea area and even very mild in the southern Baltic. While the air temperatures in spring were higher than average the summer temperatures only fluctuated by the long-term average. By contrast, the autumn was unusual mild.

The mild winter led to sea surface temperatures in the Baltic Sea that were about 1 - 1.5 K above the long-term mean during winter and early spring. This resulted in relatively warm water of 3.5 - 4 ^oC being trapped in the summer intermediate water layer of the central Baltic Sea. Due to the average summer, the surface layer only reached temperatures in the range of the long-term mean. The delay in temperature decrease between October and December due to the unusual mild autumn caused positive anomalies of 2 - 3 K in the surface water of the western and central Baltic.

Caused by an inflow of warm water in autumn 1999 the temperature of the Bornholm Basin deep water had, on average, a positive anomaly of 2.6 K. The deep water temperatures of the central Baltic Sea were still anomalously high due to the inflow of warm water in 1997, 0.7 - 0.8 K (eastern Gotland Basin) and 0.3 - 0.4 K (western Gotland Basin) above the mean for the period 1950 - 1998.

The weather conditions in the winter of 1999/2000 gave rise to a very low inflow activity across the sills into the Baltic. Weak inflows in December 1999 and February 2000 led to the renewal of the Bornholm Basin deep water in spring 2000. From August onwards hydrogen sulphide was measured in that basin up to mid-November when effects of small inflows in autumn led again to oxic conditions. A major inflow event did not occur.

In the eastern Gotland Basin, the anoxic water covered the layer between bottom and 120 - 125 m depth all year round, in August even up to about 100 m. In the deep water of the western Gotland Basin, oxygen depletion has continued since 1993. The decrease in oxygen concentration led since August 1999 to the permanent occurrence of hydrogen sulphide in the deep water of the Landsort Deep. In 1999/2000, the area of the whole central Baltic deep water which was affected by oxygen deficiency and anoxic conditions was the largest during the past 16 years (Fig. 1).

The stagnation period in the deep basins is also documented by nutrient conditions. Phosphate as well as ammonium concentrations have further increased in the Gotland and Fårö Deeps further and reached highest values within the last five years. Starting in August 1999 the permanent anoxic conditions in the deep water of the Landsort Deep also resulted in 2000 in increased ammonium and phosphate concentrations whereas nitrate was not detectable. Only in the Bornholm Basin oxic conditions prevailed, mainly in the first half of the year, accompanied by high nitrate and low phosphate and ammonium concentrations.

Nitrate concentrations in the mixed winter surface layer have a level comparable to that of the previous years. Since the beginning of the 1990s no significant changes were observed. Phosphate concentrations, however, were high in winter 1999/2000. A new equilibrium is already established and winter concentrations in the open sea are mainly influenced by the intensity of vertical mixing.

In the near surface water layer of the open Baltic Sea, concentrations of particulate organic material are, on average, approximately 25 µmol/dm³ for carbon and close to 4 µmol/dm³ for nitrogen. These concentrations are almost the same in the areas examined. However, deviations from mean values are high, caused by the seasonal effects and interannual variability. The C/N-ratio of the particulate material is nearly constant and is close to the Redfield ratio. In contrast to the particulate material, dissolved organic matter shows a distinct gradient in the open Baltic Sea with higher concentrations in the eastern parts and lower concentrations in the transition area of the Kattegat region. Compared to the particulate matter, the C/N-ratio in the dissolved material is more than twice as high. Areas directly influenced by riverine inputs often show significantly higher concentrations of organic material. Caused by seasonal or interannual variability of river runoff, amounts of organic matter in these regions can vary over a broad range.

Detailed results will be published in "Meereswissenschaftliche Berichte" (Marine Science Reports) of the Institut für Ostseeforschung Warnemünde. Please contact: Institut für Ostseeforschung Warnemünde - Library. Seestrasse 15, D-18119 Warnemünde, Germany (E-mail: olivnullia.diehr@io-warnemuende.de).

Dr. Wolfgang Matthäus, Dr. Günther Nausch