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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.
Günther Nausch
The
hydrographic-hydrochemical state of the Baltic Sea in 2006
