The variability of parameters controlling the cyanobacteria bloom biomass in the Baltic Sea

The formation of wintertime excess dissolved inorganic phosphorus concentration (eDIP) in the surface layer and its influence on the late-summer diazotrophic cyanobacteria development have been evaluated on the base of 30 years nutrients and river runoff monitoring data in different Baltic Sea sub-basins, relatively short period satellite chlorophyll a (Chl a) concentration and modelled upper mixed layer depth (UMLD) data. The processes responsible for the formation of the surface and bottom phosphate pools in winter, the exchange processes between the pools and the amount of phosphate transported from the bottom pool to the surface are not fully understood. We did not find any significant correlation between the wintertime bottom layer (BL) DIP and upper mixed layer (UML) DIP for the deeper Baltic Sea sub-basins. Instead, we found a significant correlation between UML DIP and UMLD in winter. With the present considerable trend of decrease of the dissolved inorganic nitrogen concentration (DIN) in the Baltic Proper (BP) it is mainly the DIP variability determines the excess phosphate, and which is regarded as the main phosphate source for the late summer cyanobacterial blooms in the Baltic Sea. This conclusion is confirmed by the observed coherent behaviour of the satellite-derived mean Chl a concentration data in July (here considered as a proxy of the summertime cyanobacteria biomass concentration) and eDIP in the open Baltic Proper. Relying on the established here correlation between the July's mean Chl a concentration and the direct cyanobacteria biomass measurements in the Gulf of Finland, our study supports the following cause and effect chain: wintertime deep mixing → high wintertime eDIP → (potentially) strong cyanobacterial bloom in (late) summer.