Coupled biogeochemical, biological, and physical controls on nitrogen fixation during a cyanobacterial bloom in the Baltic Sea

Summer filamentous cyanobacterial blooms strongly influence the biogeochemistry of the Baltic Sea. In surface water, they sustain organic matter (OM) production via N 2 fixation despite the depletion of dissolved inorganic nitrogen (DIN). In deeper water, remineralization of this OM contributes to O 2 depletion and H 2 S formation, thereby promoting Baltic Sea hypoxia/anoxia. Primary production by cyanobacteria also drives a characteristic summer drawdown of total dissolved CO 2 (C T ). While this biogeochemical signal is thought to reflect N 2 fixation, direct observational evidence supporting this link remains limited. In addition, key questions remain regarding the factors regulating bloom initiation and intensity. This study applied a multi-method approach, combining continuous high-resolution surface water measurements of biogeochemical and biological variables made from the ship of opportunity Finnmaid traveling between Travemünde (Germany) and Helsinki (Finland), with modeled physical parameters, to investigate a cyanobacterial hotspot identified in summer 2023. During the 2.5-week C T drawdown (~1500 to ~1325 μmol/kg) in June, at the entrance to the Gulf of Finland, 70% of the decrease occurred in the final five days and coincided with a sharp increase in filamentous cyanobacterial biomass and pronounced N 2 undersaturation. Our observations indicate that once the cyanobacterial biomass has been accumulated during favorable environmental conditions, their impacts can be drastic to the carbon system. Our results link N 2 fixation with the C T decline in surface water, while also demonstrating that ecosystem productivity is enhanced by co-occurring phytoplankton species, which likely benefit from the nitrogen released by cyanobacterial N 2 fixation.