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

Kurzfassung

Summer filamentous cyanobacterial blooms strongly influence the biogeochemistry of the Baltic Sea. In surface water, they sustain organic matter (OM) production via N2 fixation despite the depletion of dissolved inorganic nitrogen (DIN). In deeper water, remineralization of this OM contributes to O2 depletion and H2S formation, thereby promoting Baltic Sea hypoxia/anoxia. Primary production by cyanobacteria also drives a characteristic summer drawdown of total dissolved CO2 (CT). While this biogeochemical signal is thought to reflect N2 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 CT 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 N2 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 N2 fixation with the CT 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 N2 fixation.