The hydrographic-hydrochemical state of the Baltic Sea in 2005

The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2005. Based on the meteorological situation, the horizontal and vertical distribution of temperature, salinity, oxygen/hydrogen sulphide, inorganic and organic nutrients are described on a seasonal scale.

In 2005, no important inflow events occurred. Thus, the stagnation period in the deeper basins which started in 2004 continued. At the end of August, with 3.12 ml/l the highest hydrogen sulphide concentration ever observed in the Bornholm Basin was detected near to the bottom. In the eastern Gotland Basin, the layer free of oxygen increased continuously. At the end of 2005 the whole column between 150 m depth and the bottom was anoxic. Near to the bottom a negative oxygen concentration of 3.75 ml/l was measured.

The winter phosphate concentrations in the surface layer were high in all areas of the central Baltic Sea. They are comparable with values from 1993. Extremely high concentrations were measured in the Bornholm Basin, but also in the Karlso Deep area. Thus, in February a maximum of 1.14 umol/l was detected. Such high concentrations were found in the mid 1980s for the last time. The winter nitrate concentrations were similar to the years before.

Whereas in the whole eastern, northern and western Gotland Basin, the Gulf of Finland and the southern Bothnian Sea very intense cyanobacteria blooms were observed in 2005, no development of cyanobacteria worth to note occurred in the Bornholm Basin. This is especially remarkable because during the whole summer a considerable amount of phosphate with 0.3 umol/l was available. The reasons for this development have to be clarified still.

Dr. Günther Nausch