Dynamics of phytoplankton in the brackish-water inlet Pojoviken,southern coast of Finland

The water exchange between the brackish-water firth Pojoviken and the Baltic Sea is restricted by a shallow sill (6 m). An outflowing, oligohaline surface layer is isolated from the nutrient-rich mesohaline deep water by a pycnocline at a depth of 6–10 m. During the ice-free period phytoplankton production is chiefly regulated by the river discharge regime. Contrary to the situation in the outer archipelago and the sea zone, in Pojoviken phytoplankton production continues until late autumn, because the stable salinity stratification prevents the phytoplankton from sinking below the critical depth for production. The phytoplankton composition seems to be regulated chiefly by salinity. The salinity interval 2–2.5 is apparently the critical range where brackish-water phytoplankton changes to an assemblage composed of typical freshwater species.