Nitrogen fixation by cyanobacteria is a common phenomenon in the Baltic Sea. It occurs in the absence of dissolved inorganic nitrogen (DIN) during mid-summer (June – August) mainly in the central Baltic Sea and the Gulf of Finland. The contribution to the N budget is significant and enhances the eutrophication of the Baltic Sea. Current input estimates range between 300 kt-N/yr and 800 kt-N/yr and are in the same order of magnitude as the sum of the riverine and airborne DIN input. The huge range of the different estimates is a consequence of both the considerable interannual variability of the N₂ fixation and huge uncertainties associated with the different approaches (¹⁵N incubation; total N budget; pCO₂ records; phosphate excess) for the quantification of the N₂ fixation and with extrapolating the results from local studies to entire basins.

Our approach is based on large-scale records of the surface water N₂ depletion during a cyanobacteria bloom, complemented by Ar measurements to account for the air-sea N₂ gas exchange. The N₂ and Ar concentrations will be determined semi-continuously by means of mass spectrometric analysis of N₂ and Ar in air equilibrated with a continuous flow of surface water (MIMS, Membrane Inlet Mass Spectroscopy). The measurement device will be attached to an established fully automated measurement system for the analysis of surface water trace gases (CO₂, CH₄, O₂, N₂O, CO) on a voluntary observation ship (VOS, “Finnmaid”). Through this the N₂ and Ar concentration between the Mecklenburg Bight and the Gulf of Finland will be obtained with a temporal resolution of 2 – 3 days.

Two intense measurement periods will be performed (2021 and 2022) in order to quantify the N₂ fixation during the cyanobacteria high season from June to August. Through this we are aiming to identify the factors which trigger and possibly limit the cyanobacteria growth such as temperature, P availability and meteorological/hydrographic conditions. Concurrent records of the pCO₂ by the existing measurement system will be used for independent estimations of the cyanobacteria biomass production and thus of the associated N₂ fixation. Likewise, measurements of total N and total P will be available through cooperation with the Finnish Environment Institute (SYKE) and facilitate nitrogen budget calculations for consistency tests with our direct N₂ fixation measurements.

To estimate the total N₂ fixation, the rates obtained for the upper surface layer must be integrated over depth. This will be achieved by numerical modelling (GETM) of the mixed layer depth which will be defined by different criteria. Extrapolation of the N₂ fixation obtained along the Finnmaid route to entire basins is a more challenging task. In cooperation with the remote sensing group of the Federal Maritime and Hydrographic Agency (BSH), we will make an attempt to use remote sensing data for an extrapolation procedure.