The biological state of the Baltic Sea 1998

The results of the biological environmental monitoring of 1998 carried out by the Baltic Sea Research Institute (IOW) within the HELCOM programme are presented and discussed.

In 1998, the spring bloom of phytoplankton started in Mecklenburg Bight in the mid of March and was produced mainly by diatoms (Chaetoceros sp.). At later stages, the contribution of dinoflagellates (Gymnodinium spp.) increased continuously. In the Arkona Sea, the spring bloom started approximately on 20 March (Chaetoceros sp.), again followed by Gymnodinium spp. Also in the Bornholm Sea, the spring bloom started at the end of March, but was dominated by Gymnodinium spp. from the beginning.

In summer, cyanobacteria and several flagellates were the dominant groups in the Baltic Proper. The diatom Dactyliosolen fragilissimus formed a bloom in Mecklenburg Bight in June/July 1998. A sample from 15 September 1998 from the Arkona Sea showed an emormous bloom of Prorocentrum micans. In late autumn, the diatom Coscinodiscus granii dominated in the Baltic Proper but the dinoflagellate Ceratium tripos in Mecklenburg Bight.

The statements on phytoplankton development are supported by chlorophyll data (gathered both by fluorometric analysis and remote sensing). The chlorophyll data are especially useful for long-term trend analyses. Different hypotheses for explaining the relatively late spring blooms in the eastern parts of the Baltic Sea and the long-term replacement of diatoms by dinoflagellates in the spring blooms are discussed.

The analysis of sedimenting particles for the year 1997 revealed the prevailance of the spring season for the vertical flux of particulate matter, in contrast to 1996, when high sedimentation occurred also in late summer/autumn. The qualitative analysis of sedimenting phytoplankton species showed, however, similar patterns in 1996 and 1997. The seasonal cycle of pelagic algae was well reflected in the sediment traps.

The abundance of rotatoria (Synchaeta sp.) increased strongly in comparison with the previous year (maximum in May, frequently a second maximum in October), but the abundance of copepods decreased. A seasonal shift in the copepod maxima was conspicuous: in 1997, the peak was reached only in August, but in 1998 already in May (exception: deep water in the Bornholm Sea and Eastern Gotland Sea). The abundance of cladocerans in 1998 was clearly lower than in 1997, especially in August. It has to be admitted that the real abundance peaks of the different zooplankton groups may have been missed due to a rather low sampling frequency. Methodological problems are discussed in detail.

The number of species of macrozoobenthos increased when compared with data from previous years. It amounted to 83 in 1998 and ranged from 3 in the Bornholm Sea to 49 in the Fehmarn Belt. The well-flushed, sandy or gravelly areas of Fehmarn Belt, Darss Sill and southern Arkona Sea were especially rich in species. At silty stations in Mecklenburg Bight, the stock of Euchone papillosa (Polychaeta) recovered. As in the previous year, the glacial relicts Pontoporeia femorata and Saduria entomon occurred at several stations. Also the mussels Astarte borealis and A. elliptica were noticed. Some species, absent for years or decades in Mecklenburg Bight, Arkona Sea and Pomeranian Bay, are still missing: Monoporeia affinis, Buccinum undatum, Nassarius reticulatus, Astarte montagui, Scrobicularia plana and Macoma calcarea. The density of individuals of macrozoobenthos ranged from 13 ind. m-2 (in Bornholm Sea) to 7.235 ind. m^-2 (southern Arkona Sea) in 1998. The biomass (as ash-free dry weight) was between 0,01 g m^-2 (Bornholm Sea) and 48,4 g m^-2 (east of Darss Sill).

For a better assessing of the macrozoobenthos, a dredge and video technique were applied besides the traditional van-Veen grab. Seven additional taxa could be found by these new methods. Moreover, the video images made an analysis of habitat structures, the substrate and the patchiness possible.

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