The hydrographic-hydrochemical state of the Baltic Sea in 2007

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.

Dr. Günther Nausch