The hydrographic-hydrochemical state of the Baltic Sea in 2006

The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2006. 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 2006, the lack of important barotropic inflow events lead to a continuation of the stagnation period in the deep basins of the Baltic Sea. Oxygen conditions deteriorated further, hydrogen sulphide concentrations as well as their spatial extension increased. Thus, the annual mean at 200 m water depth in the eastern Gotland Basin decreased from 0.88 ml/l (2004) over -0.23 ml/l (2005) to -1.58 ml/l in 2006. Two baroclinic inflow events from September to December 2005 and from June to August 2006 could improve the oxygen condition in the Bornholm and eastern Gotland Basin only shortly. The increasing anoxic layer caused an enrichment of phosphate. The H2S zone was free of nitrate, and ammonium did accumulate. The annual mean of phosphate at 200 m water depth in the eastern Gotland Basin increased from 3.12 µmol/l (2005) to 4.20 µmol/l (2006), ammonium increased from 1.7 µmol/l (2005) to 9.2 µmol/l in 2006. The extreme values from the end of the last stagnation period were not reached yet.

Phosphate winter concentrations were lower than in the year before, nitrate was comparable to that of the last years. The resulting N/P ratios were favourable for the development of cyanobacteria in summer. Additionally, the high water temperatures caused an early start of blooming in the Bornholm and Arkona Basin and in the western Baltic which continued until the end of August/beginning of September. The Gotland Basin was clearly less affected.

Dr. Günther Nausch