Upwelling events may cause cyanobacteria blooms in the Baltic Sea

► Upwelling causes cyanobacteria blooms. ► Phosphorus input by upwelling is the decisive factor. ► Biomass is low in the intermediate water, but highly active if exposed to light. ► Inoculum of physiologically active “seeds” from surface water is a precondition. ► Phosphate input stimulates Nodularia and Anabaena more than Aphanizomenon.