The hydrographic-hydrochemical state of the Baltic Sea in 2001

The winter 2000/2001 was characterized by only minor ice coverage in the southern Baltic Sea and was the fourth mild winter after each other. The winter belongs to a period of relative mild winters which started 1987/1988. In the northern Baltic Sea the maximum ice coverage was normal. Because also the summer was warmer than the average the year 2001 was the eight warmest in northern Germany since 55 years. The strongest positive anomalies of the air temperature were observed in October making this month to the warmest October at least for the last 55 years.

Due to the mild winter the surface temperature in January was 2 K above the long-term mean. The minium temperature was observed not before March which occured only seldom since 1990. The warm summer resulted in positive water temperature anomalies up to 3 K in July. From August onwards the surface temperatures were on average.

The deep water temperatures of the central Baltic Sea were still high due to the inflows of warm water, especially in 1997. In the eastern Gotland Basin maximum values of the years before were not reached. In the Landsort and Karlsö Deep, however, temperatures measured in the 400 m and 100 m level respectively were with 5,24 ^oC and 4,66 ^oC the highest of the last years.

Again, in the year 2001 only low inflow activity across the sills was detected. Only in October/November a stronger inflow event occured. Within 16 days around 200 km³ were flowing into the central Baltic Sea causing a rapid increase of the oxygen content in the deep water of the Bornholm Basin up to 4,68 ml/l in December. In January 2002 the effects of this inflow were registered shortly in the eastern Gotland Basin.

Despite that, the stagnation period which started in 1995 continues in the central Baltic Sea deep basins. In the eastern Gotland Basin, the anoxic water covered the layer between 125 m and the bottom whereby the hydrogen sulphide concentrations in the bottom-near layer were comparable to the concentrations measured at the end of the long lasting stagnation period in 1992. This is also valid for the phosphate and ammonium concentrations.

The stagnation period continues also in the western Gotland Basin. In the Landsort Deep, hydrogen sulphide is present all the year round. In the Karlsö Deep low oxygen concentrations were found until May followed by anoxic conditions in the rest of the year. For the first time a negative annual mean for oxygen was calculated in the 100 m-level. The thicknesses of the hydrogen sulpide containing layers in November were 330 m and 20 m in the Landsort and Karlsö Deep, respectively.

Nutrient concentrations in the mixed winter surface layer are used as indicator for eutrophication. The inorganic nitrogen compounds are on a similar level since the beginning of the 1990s, no trends can be observed. For phosphate a decreasing trend was detected in many sea areas in the 2nd half of the 1990s. This trend was interrupted in the winter 1999/2000. In the winter 2000/2001 phosphate concentrations were, with the exception of the Bornholm Basin, again on a relative low level. However, a further decrease cannot be expected. Additionally the higher phosphate pool in the deep water due to the anoxic conditions has to be taken into account. This pool can lead under given hydrographic circumstances to an additional phosphate supply to the surface water.

For the first time, mean concentrations for different natural organic substances averaged over a longer period of time can be given. The mean content of particulate organic carbon (POC) in the Baltic Sea is 26,8 µmol/l (surface layer) and 24,6 µmol/l (bottom-near layer). In the surface layer a pronounced seasonality can be seen. For dissolved organic carbon (DOC) seasonal variability is comparatively low. The mean concentrations are 312 µmol/l in the surface layer and 272 µmol/l in the bottom-near layer.

Dr. Günther Nausch