Highlights of zooplankton dynamics in Estonian waters (Baltic Sea)

The ecological subsystems in Estonian waters differ in mesozooplankton structure. Euryhaline, eurytherm marine boreal species dominate in the NE Baltic Proper.Species adapted to lower salinities are abundant in the upper layers of the Gulf of Finland and Gulf of Riga, whereas glacial relicts and species of marine origin inhabit the deepwater layers. Freshwater species are more important in the easternmost parts. Large fluctuations in zooplankton composition, abundance, biomass, and spatial distribution are connected with the seasonal cycle. On the coastal slope, largest zooplankton biomasses and planktivorous fish concentrations were found during summer in the zones of intense vertical mixing with high nutrient supply. Available data suggest that predation by planktivorous fish may locally affect zooplankton abundance. Long-term dynamics of copepod species are mainly triggered by changes in salinity and temperature conditions, especially in marginal parts of their distribution areas. In coastal bays, there are some indications of local deviations in copepod abundance which may be induced by pollution. Cercopagis pengoi, a newcomer from the Ponto-Caspian region, has occupied an important niche in the pelagic food web in shallower areas of the Gulf of Riga and supports a significant component in the diet of planktivorous fish species during summer.