HYDROPHYSICAL AND HYDROBIOLOGICAL PROCESSES IN THE GULF OF FINLAND INSUMMER 1987 - CHARACTERIZATION AND RELATIONSHIP

On the basis of repeated CTD surveys and current measurements in June- July 1987 the generation and development of wind-induced upwelling (ne ar the northern) and downwelling (near the southern) coasts, together with frontal zones caused by coastal water offshore transport, were ob served. Strong coastal currents parallel to the coast occurred, togeth er with downwelling near the southern coast of the Gulf of Finland. Th e 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 paramet ers were studied in both the upwelling and TLF, exposing the changes i n biological parameters at different trophic levels in the fronts of d ifferent types. Higher values of nutrients, chlorophyll a content and primary production in the upwelling frontal zone were caused by the up welled water. The increase in primary production in the surface layer within the upwelling area was caused by nutrient transport from the de eper layers, expressed only in the higher values of nitrogen. The abse nce of an increase in phosphorus, the limiting factor for primary prod uction, can be explained by the high consuming rate. In the TLF, no in crease in nutrients and only a slight increase in primary production t ook place. The increase in abundance of young herring and zooplankton in the TLF was caused by the distribution of the hydrophysical paramet ers. These organisms of the higher trophic level were carried by offsh ore currents, together with the less saline and warmer coastal waters. The increase in abundance of the young herring near the TLF is relate d also to the best feeding conditions near the front. It is concluded that temperature and salinity fronts are associated with changes in bi ological parameters of different trophic levels, depending upon the me chanisms of formation of the front.