Technical note: New approach for the determination of N₂ fixation rates by coupling a membrane equilibrator to a mass spectrometer on voluntary observing ships

Abstract. Nitrogen fixation by cyanobacteria plays an important role in the eutrophication of the Baltic Sea, since it promotes biomass production in the absence of dissolved inorganic nitrogen (DIN). Its contribution to the N budget is of the same order of magnitude as the combined sum of riverine and airborne DIN input, varying between 310 kt-N/yr and 792 kt-N/yr. The vast range is due to interannual variability, significant uncertainties in the various techniques used to determine N₂ fixation and in extrapolating local studies to entire basins. To overcome some of the limitations we introduce a new approach using a Gas Equilibrium – Membrane-Inlet Mass Spectrometer (GE-MIMS). A membrane contactor (Liquicel) is utilized to establish gas phase equilibrium for atmospheric gases dissolved in seawater. The mole fractions for N₂, Ar and O₂ in the gas phase are determined continuously by mass spectrometry and yield the concentration of these gases by multiplication with the total pressure and the respective solubility constants. The results from laboratory tests show that the accuracy (deviation from expected values): N₂: 0.20 %, Ar: 0.70 %, O₂: 0.20 % and the precision (2 times the absolute standard deviation) N₂: 0.05 %, Ar: 0.14 %, O₂: 0.11 % is sufficient enough to detect and quantify nitrogen fixation. The e-folding equilibration time is 4.8 min for N₂, 3.0 min for Ar and 3.2 min for O₂. The GE-MIMS is designed for deployment on a voluntary observing ship (VOS), enabling repeated transects along the same route and providing high temporal and spatial resolution data. Therefore, the method is suitable for offering large-scale records of the surface water N₂ depletion and of Ar to account for the air-sea gas exchange. Additional O₂ measurements will be utilized to estimate the net community production (NCP) triggered by N₂ fixation.