The Gulf of Finland

Abstract

In this chapter, we describe the unique physical and biogeochemical conditions of the Gulf of Finland. Although morphologically a direct continuation of the Baltic Proper, several features distinguish the Gulf markedly from other parts of the Baltic Sea. Relative to its surface area, the Gulf of Finland has the largest catchment area and highest freshwater inflow within the Baltic Sea. The external loading of nitrogen (N) and phosphorus (P) relative to the surface area of the Gulf are about two and three times, respectively, those of the Baltic Sea. The Gulf of Finland has a distinct but unstable salinity stratification, which makes the sea area very sensitive to external perturbations. At times, this may lead to e.g. deep water anoxia/hypoxia, elevated benthic release of P and extensive cyanobacterial blooms. The poor oxygen conditions in the Gulf are caused primarily by the voluminous spring bloom and preceding sedimentation of detrital plankton material, which in turn is caused by high wintertime nutrient concentrations. Poor oxygen conditions and intensified sediment release of P have led to strongly increasing deep-water and winter concentrations of inorganic P over the last 10 years. In contrast, N concentrations have decreased since the late 1980s, suggesting the effect of decreased external N loading at the same time. This latter phenomenon probably contributed to the decreased magnitude of the phytoplankton spring blooms in the 1990s. The prevailing poor oxygen conditions have weakened the populations of benthic invertebrates and further decreased the nutrient retention and removal capacity of the sediments. The most important controlling factor in the recent adverse changes in fish stocks has been hydrographical changes. Especially in coastal waters, eutrophication and its associated consequences has shifted the biomass dominance from percids towards cyprinids. Summertime eutrophication has been going on in the Gulf of Finland since at least the 1970s. Since the mid-1990s the main reason for this development, both in the open Gulf and in the coastal waters, seems to have been the general development of eutrophication caused by internal loading, rather than any changes in land-based nutrient loads. The further reduction of the external nutrient load is, however, the only permanent way to counteract both sediment accumulation of labile organic matter and benthic release of nutrients, as well as summertime eutrophication of the Gulf of Finland.