Oxygen conditions in the south eastern Baltic have improved thanks to an inflow of oxygen-rich water from the North Sea during November/December 2011. This inflow has also caused the concentrations of phosphate and silicate to increase in the surface waters, which could cause excessive algal blooms this summer.
As a result of the relatively large inflow from the Kattegat to the Baltic (about 150 km³ through the Sound and Belt Seas) which occurred during November/December 2011, oxygen concentrations in the southern Baltic Proper have improved markedly. Oxygen conditions in the Hanö Bight, Arkona and Bornholm Basins have been good since February. During the most recent measurements in April, the deep water had reached the south-eastern parts of the Baltic and could even be traced in the southern part of the Eastern Gotland Basin.
Concentrations of phosphate were above normal at all measurement stations, and considerably higher in the Hanö Bight and Arkona Basin. The high phosphate concentrations in these areas are most likely due to the upwelling of water from intermediate depths, as even salinity and silicate concentrations were higher than normal. This is a result of the inflow, which has replaced and lifted the old bottom water towards the surface.
High phosphate concentrations can cause excessive algal blooms this summer
High phosphate levels are one of the factors which can cause blue-green algae (so-called cyanobacteria) to bloom. If there is an excess of phosphate after the normal spring bloom, and if the summer weather is dominated by sunshine, warm weather and light winds, then excessive blooms of blue-green algae can result.