Hydrophysical and hydrobiological processes in the Gulf of Finland in summer 1987: characterization and relationship

Abstract

On the basis of repeated CTD surveys and current measurements in June–July 1987 the generation and development of wind-induced upwelling (near the northern) and downwelling (near the southern) coasts, together with frontal zones caused by coastal water offshore transport, were observed. Strong coastal currents parallel to the coast occurred, together with downwelling near the southern coast of the Gulf of Finland. The formation of a “tongue-like” water mass with a sharp thermohaline “front” (TLF) was observed to extend from the southern coast, after the wind conditions changed. Hydrochemical and hydrobiological parameters were studied in both the upwelling and TLF, exposing the changes in biological parameters at different trophic levels in the fronts of different types. Higher values of nutrients, chlorophyll a content and primary production in the upwelling frontal zone were caused by the upwelled water. The increase in primary production in the surface layer within the upwelling area was caused by nutrient transport from the deeper layers, expressed only in the higher values of nitrogen. The absence of an increase in phosphorus, the limiting factor for primary production, can be explained by the high consuming rate. In the TLF, no increase in nutrients and only a slight increase in primary production took place. The increase in abundance of young herring and zooplankton in the TLF was caused by the distribution of the hydrophysical parameters. These organisms of the higher trophic level were carried by offshore currents, together with the less saline and warmer coastal waters. The increase in abundance of the young herring near the TLF is related also to the best feeding conditions near the front. It is concluded that temperature and salinity fronts are associated with changes in biological parameters of different trophic levels, depending upon the mechanisms of formation of the front.