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The article summarizes the hydrographic-hydrochemical conditions in the western
and central Baltic Sea in 2007. 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 2007, three barotropic inflow events carrying more than 200 km³ occurred
in January, March and November, two additional events with less than 150 km³
took place in June/July and August/September. Additionally, effects of baroclinic
inflows from summer 2006 could be seen. Especially the near-bottom layer region
from the Bornholm Basin to the Gdansk Deep was ventilated repeatedly. The different
inflow events could only shortly improve the oxygen situation in the eastern
Gotland Basin. At the end of 2007 the water column of the Gotland Deep between
137 m and the bottom (239 m) was completely anoxic. The annual mean for phosphate
at 200 m water depth of 4.03 µmol/l was in the same range as in 2006,
nitrate was not existent under anoxic conditions and ammonium concentrations
increased further from 1.7 µmol/l (2005) over 9.2 µmol/l (2006)
to 11.1 µmol/l in 2007. In the western Gotland Basin the stagnation period
prevailed throughout the whole year.
Noteworthy are also the high phosphate winter concentrations in all sea areas.
Together with relatively low winter nitrate concentrations a shift took place
in the N:P-ratio. In general, N:P-ratios in the mixed winter surface layer are
far away from the classical Redfield ratio of 16:1. Over a longer period of
time, a stable N:P- ratio of 7:1 - 9:1 had been established. In the last five
years, a remarkable decrease was observed. In 2007, the N:P ratio ranged between
4.3:1 in the Bornholm Basin and 6.6:1 in the Landsort Deep area.
Günther Nausch
The
hydrographic-hydrochemical state of the Baltic Sea in 2007
