MESOZOOPLANKTON DYNAMICS IN THE NORTHERN BALTIC SEA - IMPLICATIONS OF VARIATIONS IN HYDROGRAPHY AND CLIMATE

An 18-year mesozooplankton time series from 1967 to 1984 from the SW coast of Finland, northern Baltic Sea, was studied in relation to climatic parameters (air temperature, air pressure, precipitation, and wind speed and direction), freshwater discharge and hydrographical factors (water temperature, salinity and water column stability). Climatic factors influenced mesozooplankton community through their effects on hydrography. The average water salinity responded to fluctuations in river discharge with a 1-2 year lag. River discharge was in turn regulated by variations in atmospheric pressure and precipitation, which oscillated on 3-5 year cycles. Shorter-term variations in salinity and temperature were related to changes in the dominating wind directions. The seasonally adjusted residuals of total mesozooplankton biomass, cladoceran biomass and those of many of the dominant mesozooplankton species were significantly negatively correlated with salinity. The annual peaks of the most abundant cladoceran species and one of the most abundant calanoid species co-varied with the seasonal fluctuation of surface water temperature. Canonical correspondence analysis identified the mesozooplankton species by their relationship to water stability: rotifers and cladocerans had their optima in warm and stabile conditions, while the neritic calanoid copepods were more abundant in relatively saline and mixed conditions.