Long-term dynamics of main mesozooplankton species in the central Baltic Sea

Long-term dynamics (1959–1997) of the copepod species Pseudocalanus elongatus, Temora longicornis, Acartia spp. and Centropages hamatus, as well as the taxonomic group of cladocerans, are described for the open sea areas of the central Baltic Sea. Differences between areas, i.e. Bornholm Basin, Gdansk Deep and Gotland Basin, as well as between 5 year periods, were investigated by means of Analysis of Variance (ANOVA). No significant differences in mesozooplankton biomass between areas were found. On the other hand, clear time-trends could be demonstrated and related to salinity and temperature, with P.elongatus biomass mainly dependent on salinity and T.longicornis, Acartia spp. and cladocerans biomasses dependent, to a large extent, on thermal conditions. Decreasing salinities since the early 1980s due to a lack of major inflows of highly saline water from the North Sea and increased river run-off, both triggered by meteorological conditions, obviously caused a decrease in biomass of P.elongatus. Contrarily, the standing stocks of the other abundant copepod species and cladocerans followed, to a large degree, the temperature development and showed, in general, an increase. The shift in species composition during this period is considered to be a reason for decreasing growth rates of Baltic herring (Clupea harengus) since the early 1980s, and for sprat (Sprattus sprattus) since the early 1990s. Generally, it is suggested that low mesozooplankton biomasses in the 1990s were caused, at least partially, by amplified predation by clupeid fish stocks.