The biological state of the Baltic Sea 1997

The results of the biological environmental monitoring of 1997 carried out by the Baltic Sea Research Institute (IOW) within the HELCOM programme are presented and discussed.

In 1997, the spring bloom of phytoplankton begun in Mecklenburg Bight and Arkona Sea already between early and mid-March and was produced mainly by diatoms. At later stages, the contribution of dinoflagellates increased continously. In the Bornholm Sea and the Gotland Sea, spring bloom (dinoflagellates) was in April/May. In summer, cyanobacteria and several flagellates were the dominant groups. In September/October a dinoflagellate bloom of Gymnodinium lohmannii was recorded which had never before been observed in the Arkona Sea and the Bornholm Sea. Subsequently, diatom abundance increased (Coscinodiscus granii). In the Gotland Sea, no autumn bloom was recorded.

The seaonal cycle of pelagic algae was also reflected in the sediment traps. However, so far, only the data collected in 1996 could be analysed. Beside the spring bloom, sedimentation of the cyanobacteria community in late summer was qunatitatively important for vertical particle flux.

Although nutrient concentrations remained at about the same level between 1979 and 1997, an increase of phytoplankton biomass was recorded in the Baltic Prober. The use of fertilizers in the catchment area of the Baltic Sea has decreased since about 1990. However, no reactions of the pelagic communities could be observed so far. The species composition of the phytoplankton has changed since 1989 due to a decrease of diatoms during spring in the southern Baltic Proper which was accompanied by an increase of dinoflagellates. These changes are related to winter temperatures, not to nutrient concentrations.

Throughout the entire year, copepods were the dominant mesozooplankton group in all regions under investigation, except in August in the Bornholm Sea when cladocerans (Bosmina) exhibited a higher abundance. When compared with results from previous years, abundance values of rotifers and cladocerans show a clear decrease in relation to those of copepods. The trend of increasing abundance of rotifers observed over the last years seems to be interrupted. Some zooplankton groups showed unusual values in August when rather high water temperatures were recorded.

Abundances and numbers of species of marcozoobenthos increased when compared with data from previous years, probably because of temporary inflow of salty and oxygen-rich water. Several species which were not found in recent years seem to have returned. Also, new species were encountered. The re-occurence of Astarte elliptica and Pontoporeia femorata indicates improved oxygen conditions. The strong increase of Diastylis rathkei is remarkable.

The impact of the Oder flood in July/August was limited to Pomeranian Bight and the southern Arkona Sea. In a region in front of the Island of Usedom, hydrogen sulphide was produced in the bottom water, leading to death of macrozoobenthos. A long-lasting effect on the ecosystem is, however, not expected.

Dr. Norbert Wasmund, Dr. Jürgen Alheit, Dr. Falk Pollehne, Dr. Herbert Siegel, Dr. Michael L. Zettler