The biological state of the Baltic Sea 1999

Species composition and biomass or abundance of phyto- and mesozooplankton from the western parts of the Baltic Sea to the Eastern Gotland Sea were studied and discussed in comparison with the already existing 21-years data base of the HELCOM monitoring programme and in relation to physico-chemical and satellite data.

In 1999, the spring bloom of phytoplankton began in all open sea regions investigated almost simultaneously in the second half of March. In contrast to previous years, there was only about one week delay in the bloom in the Eastern Gotland Sea in comparison with the southern Baltic Proper. On the other hand, the bloom developed already in early March in shallow coastal regions (Lübeck Bay, Pomeranian Bay). As usual, the spring bloom began with a diatom development in Mecklenburg Bight. The early growth of a dinoflagellate bloom in the Arkona Sea is, however, new. In the Bornholm Sea and the Gotland Sea, the spring bloom was composed of dinoflagellates from the beginning. In contrast to previous years, the photoautotrophic ciliate Myrionecta rubra developed to the dominant species in the Baltic Proper in spring and summer.

Surprisingly, the diatom Coscinodiscus granii grew to high biomasses in the Bornholm Sea already in summer, whereas the summer bloom of the diatom Dactyliosolen fragilissimus in the Mecklenburg Bight was usual. Strong summer blooms of cyanobacteria (Nodularia spumigena) occurred only in the Gotland Sea.

The autumn bloom in the Mecklenburg Bight was no longer dominated by Ceratium tripos but by Gymnodinium sp. and the potentially toxic diatom Pseudo-nitzschia pungens.

Contrary to 1997, where sedimentation rates in the autumn period were low, sedimentation in the Gotland Sea in 1998 displayed the typical pattern of increased vertical particle flux towards the end of the year. The biogenic part of that sinking material consisted of filamentous and colonial cyanobacteria in September and a late diatom sedimentation period in November.

The change from diatom dominance to dinoflagellate dominance in spring blooms, noticed in the Bornholm Sea and Gotland Sea since 1989, is now also obvious in the Arkona Sea. It seems to be related to mild winter temperatures, not to nutrient concentrations. Coscinodiscus granii, which forms blooms normally in late autumn, developed already in summer in the Bornholm Sea. Special attention has to be paid to the future trends in the development of the ciliate Myrionecta rubra, which became dominant in spring and summer in the Baltic Proper and to Pseudo-nitzschia pungens, which formed a bloom in autumn in the Mecklenburg Bight.

The long-term increase in chlorophyll a concentrations in the Arkona Sea is still significant, whereas there is no longer a significant trend in the Bornholm Sea.

The abundance of copepods decreased in 1999 in comparison with previous years, whereas cladocerans increased, especially in August. Rotifers dominated in May but showed a decreasing tendency.

Detailed results are published inWasmund, N., J. Alheit, F. Pollehne und H. Siegel (2000): Der biologische Zustand der Ostsee im Jahre 1999 auf der Basis von Phytoplankton- und Zooplanktonuntersuchungen. Meereswiss. Ber., Warnemünde, 43, 65 pp.

Dr. Norbert Wasmund, Dr. Jürgen Alheit, Dr. Falk Pollehne, Dr. Herbert Siegel