The hydrographic-hydrochemical state of the Baltic Sea in 2004

The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2004. Based on the meteorological situation, the horizontal and vertical distribution of temperature, salinity, oxygen, inorganic and organic nutrients are described on a seasonal scale.

The whole year 2004 was characterized by only low inflow activities. However, the major Baltic inflow from January 2003 continued to have effects. During the course of 2004 the oxygen content of the deep water in the Bornholm and eastern Gotland Basins decreased continuously. In December only 0,25 ml/l were measured. In the Gotland Deep, again anoxic conditions have restored below 200 m water depth indicating the beginning of a new stagnation period. In the western Gotland Basin the influence of the salt water inflow can be detected only delayed and with reduced intensity. In the Landsort Deep traces of oxygen were found only shortly and the Karlsö Deep remained anoxic throughout the whole year.

In the Gotland Deep, a continuous salinity increase took place in the 200 m level. The annual mean reached 12,7 psu. Such high values were observed for the last time in 1977 as a result of a huge major Baltic inflow. The reason can be seen in the warm baroclinic inflow of summer 2003. Warm water inflows as described already for 2002, seem to be an indication of a new quality in the longterm behaviour of the Baltic Sea.

The winter nitrate concentrations in the surface layer in the Arkona and Bornholm Sea as well as in the eastern and western Gotland Basin were very low. In contrast, the phosphate concentration was comparably high in the eastern but esp. in the western Gotland Sea causing very low N/P ratios. The reasons für this development are discussed.

Dr. Günther Nausch