Continuous nitrogen and argon measurements for the quantification of nitrogen fixation in the Baltic Sea

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.                                                                               

To overcome some of these limitations, we have developed a new methodological approach to quantify the contribution of N₂ fixation to the N budget of the Baltic Sea. 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 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 Spectrometry). In our first pilot study, we have performed measurements on a voluntary observation ship (VOS, „Finnmaid“) for a period of six weeks in summer 2022 between the Mecklenburg Bight and the Gulf of Finland. The results demonstrate that our new approach is able to identify the areal distribution of cyanobacterial bloom events and to quantify the nitrogen fixation in a very precise way with a high temporal and spatial resolution (2 – 3 days, ~0.7 km).                                                                              

Within in future studies, we will perform further ship-based investigations and use the high-resolution dataset to set up a N budget for the Baltic Proper. In addition, we are aiming for the identification of the different factors which trigger and possibly limit the cyanobacteria growth such as temperature, phosphor availability and meteorological/hydrographic conditions.