The biological state of the Baltic Sea 2003

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 2003.

The phytoplankton (Skeletonema costatum) spring bloom developed exceptionally early (in February 2003) in Mecklenburg Bight and the Arkona Sea, perhaps owing to the inflow of saline bottom water which raised the pycnocline. In the Bornholm Sea and in the southern Gotland Sea it occurred only in the second half of March. The summer bloom of large diatoms was dominated by Guinardia flaccida and Dactyliosolen fragilissimus in Mecklenburg Bight. The typical cyanobacterial blooms (Nodularia spumigena, Aphanizomenon sp.) spread in the first half of July from the northern Gotland Sea towards the southern Arkona Sea and drifted into the Mecklenburg Bight by early August. The autumn bloom in Mecklenburg Bight is characterised by the coexistence of dinoflagellates (Ceratium tripos) and diatoms (Proboscia alata).

Whereas the share of diatoms in the spring blooms was very low in the 1990s in the southern Baltic proper, it increases since the year 2000 again. On the other hand, dinoflagellates and Mesodinium rubrum reduced their share in the spring bloom in this area.

In March, late summer and autumn 2003 the number of mesozooplankton taxonomic groups slightly increased in comparison to the last four years due to previous saltwater inflows. Some species like Oithona similis indicated the spreading of saline water in the basins of the central Baltic Sea. Polychaet and bivalve larvae reached highest concentration in March and October respectively. The average total abundance in 2003 significantly remained below the long-term mean at analysed stations in the central Arkona Sea and the eastern Gotland Sea caused by missing mass development of rotifers and cladocerans. Calanoid copepods never changed in total abundance in the last years but relative shifts occurred within taxa as usual.

The quick recovery of macrozoobenthos after the oxygen depletions in 2002 in the western Baltic and the re-colonisation of the Bornholm Sea after 5 years are remarkable. Since the beginning of the macrozoobenthos monitoring in 1991, 156 taxa were proved. Three species (Halicryptus spinulosus, Bylgides sarsi, Diastylis rathkei) showed high presence during all the years. The longterm tendencies in species number and Shannon-Wiener-Index since 1980 are displayed at one station in the Arkona Basin. The development of the abundance of Pygospio elegans and Macoma balthica in the south-eastern Arkona Sea from 1991 to 2003 is shown.

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