The biological state of the Baltic Sea 2000

Species composition and biomass or abundance of phyto- and zooplankton as well as macrozoobenthos from the western part of the Baltic Sea to the Eastern Gotland Sea in 2000 were studied and discussed in comparison with the already existing 22-years data base of the HELCOM monitoring programme and in relation to physico-chemical and satellite data. Data from sedimentation traps from 1999 were supplemented to complete the seasonal reflections of the phytoplankton development.

The Mecklenburg Bight in the western Baltic was different from the southern Baltic Proper (Arkona Sea and Bornholm Sea) concerning the timing of the spring bloom and the species composition of the phytoplankton in the different seasons. The border between these distinct waters was located not exactly at Darss Sill but in the western Arkona Sea. The western Baltic was characterised by a diatom bloom (Skeletonema costatum) at the end of March (i.e. earlier than in 1999), Gymnodinium cf. lohmannii (possibly also Dactyliosolen fragilissimus) in summer and Ceratium tripos in autumn. In the Arkona Sea and Bornholm Sea, the spring bloom (Skeletonema costatum) appeared at the beginning of April, Gymnodinium cf. lohmannii and diazotrophic cyanobacteria in summer and Coscinodiscus spp. in autumn. A succession from diatoms to dinoflagellates during the spring bloom, known from previous years in the Mecklenburg Bight, Arkona Sea and Bornholm Sea, was not noticed in 2000. The species of the autumn blooms developed already in summer at greater water depths (20 m) in Mecklenburg Bight and Arkona Sea. Gymnodinium sp. and Pseudo-nitzschia pungens, being the main species of the autumn blooms in Mecklenburg Bight in 1999 for the first time, were not present there in autumn 2000.

The spring bloom in the eastern Gotland Sea, having appeared only in May up to the year of 1998 (formed by Peridiniella catenata), started in 1999 and 2000 already in March by a mass development of the photoautotrophic ciliate Myrionecta rubra, followed by the usual growth of Peridiniella catenata in Mai. Obviously, Myrionecta rubra, being dominant already in winter, filled a gap in the phytoplankton development in early spring in the eastern Gotland Sea without interfering with the Peridiniella bloom in May. Peridiniella catenata occurred already in March in the western Baltic but was not able to reach high biomasses there. Myrionecta rubra became the dominant species in the eastern Gotland Sea at the beginning of August, again. A distinct cyanobacteria bloom at the water surface was visible during the July/August cruise only in the northern Baltic Proper at calm conditions. Satellite images showed, however, a growth of cyanobacterial blooms since 24 June 2000 between northern Gotland and the Swedish mainland and a spreading to the entire Gotland Sea up to the 30 July 2000. Afterwards, the bloom concentrated in the northern and western Gotland Sea until the 25 August 2000.

The long-term change from diatom dominance to dinoflagellate dominance in spring blooms, noticed in the Bornholm Sea and southern Gotland Sea since 1989 and the extension of this phenomenon into the Arkona Sea is stopped in the year 2000 for unknown reasons.

The long-term increase in chlorophyll a concentrations since 1979 in the Arkona Sea is still significant (p = 0.05), whereas the decrease in chlorophyll a concentrations in Mecklenburg Bight reached significance just when the data from the year 2000 were included.

The sedimentation patterns in the central Baltic Sea in 1999 were governed by high export rates of biogenic matter in late spring and late summer. A first analysis of the long-term data series on sedimentation shows, that the diatom- and dinoflagellate-based sedimentation peaks in spring and autumn are much more variable in terms of quantity and temporal succession than those in summer, which are connected to diazotrophic cyanobacterial communities. The quantitative aspect in this late summer phase is of special interest since the highest seasonal export of biogenic C, N, P and Si out of the mixed layer is the direct product of nitrogen fixation, which therefore evolves to a major control mechanism in the biogeochemistry of the central Baltic Sea.

All taxa of zooplankton, found in 2000, have been present at least since the beginning of the 1990s. The immigrant from the Ponto-Caspian region, Cercopages pengoi, did not occur in our samples up to now. The decrease of taxa from the western to the central Baltic Sea from 17 to 11 is caused by the regional decrease in salinity. 70% of all taxonomic categories occur throughout the year. In comparison with the first half of the 90's, the maximum abundance of mesozooplankton decreased to approximately 50 %. The seasonal peak in zooplankton abundance occurs in May, particularly on account of small, not quantitatively caught organisms like rotifers, nauplia and tintinnids. Due to the development of larger organisms, the biomass (except for rotifers) increases in the course of the year.

The species number of macrozoobenthos, amounting to 83, was constant in comparison with the preceding survey in 1998. The number of taxa has increased west of Darss Sill due to application of dredges and video technique, but has decreased in Arkona Sea and Bornholm Sea owing to anoxic conditions. No macrozoobenthos was traceable at station 213 in the year 2000.

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