The biological state of the Baltic Sea 2006

The time series on species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos in the Belt Sea and the Baltic proper, existing since 1979, was continued in 2006.

The phytoplankton growth started already in early February in Mecklenburg Bight and the Arkona Sea, probably caused by a shallow stratification. The spring bloom, however, could not be sampled because of cancellation of the March cruise. The late phase of the spring bloom (early May) was dominated by diatoms in the Bornholm Sea and by dinoflagellates in the Gotland Sea. A diatom summer bloom was not found. The cyanobacteria bloom was particularly strong in the western Baltic (exceptionally up to the Kattegat, see satellite images). The autumn bloom of Coscinodiscus was well developed in the entire Baltic Sea.

Data of sedimentation of organic matter in the Gotland Basin from spring and autumn 2005 are shown; the summer samples were defective due to clogging of the trap. The spring maximum occurred in mid of April and was dominated by diatoms. The main export of silica occurred in spring but the autumn diatom bloom was weak. Dinoflagellates played a minor role during the whole year. Mass flux (dry mass) amounted to 110 g a^-1.

Chlorophyll a data revealed a decreasing trend from1979-2006 in Mecklenburg Bight but an increase in the Baltic Proper if only the spring data were considered, whereas summer data did not show any trend.

Up to 27 mesozooplankton taxa (in July 2006) were identified. Some species indicate a higher saline phase in the western Baltic, others an increasing limnic influence in the eastern Baltic. Species with mass occurrences in summer (Bosmina spp.) were present even in January (prolongation of the vegetation period ?). Malacostraca–larvae probably return as a stable component of the crustacean community, while the microphagous larvaceans lost their year-round presence. The ctenophore Mnemiopsis leidyi were recorded in summer 2006 for the first time in the Baltic Sea. It over-wintered in the deep water of the Bornholm Sea (at 9 °C) as well as in coastal waters of the western Baltic Sea.

The 140 species found in the macrozoobenthos in 2006 mark one of the highest diversities during the last 6 years at the 9 monitoring stations sampled by the IOW. Favourable salinity and oxygen conditions in 2006 allowed a rapid recolonisation of the western Baltic after the oxygen depletions in 2002 and 2005. In the central Arkona Sea, species number, abundance and biomass resembled that of previous years. The Bornholm Sea (Stat. 213) was characterised by a very low oxygen concentration in the deep water (0.5 ml/l) and a very sparse colonisation by the polychaete Bylgides sarsi.

Dr. Norbert Wasmund, Dr. Falk Pollehne, Dr. Lutz Postel, Dr. Herbert Siegel, Dr. Michael L. Zettler