Cyanobacteria blooms in the Gulf of Finland triggered by saltwater inflow into the Baltic Sea

In the 1980s and 1990s prior to 1995, massive blooms of the diazotrophic cyanobacterium Nodularia spumigena occurred in the Baltic Sea Proper but never extended into the central and eastern Gulf of Finland. The absence of nitrogen-fixing cyanobacteria blooms in parts of the Baltic Sea with a high N:P ratio (e.g. Gulf of Finland) has been explained by their reduced competitive advantage in conditions of P limitation. Starting with the summer of 1995, massive blooms of N. spumigena occurred in the central and eastern Gulf of Finland, as detected by both satellite sensors and in situ monitoring. We propose that the eastward expansion of N. spumigena blooms was triggered by the 1993 saltwater inflow into the Baltic. With the arrival of the saline and oxygen-depleted waters in the Gulf of Finland in 1995, stratification in the bottom layers increased, oxygen concentrations decreased, and increased amounts of phosphate were released from the sediments. The subsequent decrease in the N:P ratio may have caused the reoccurring N. spumigena blooms.