Technical note: Testing a new approach for the determination of N ₂ fixation rates by coupling a membrane equilibrator to a mass spectrometer for long-term observations

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). However, the estimates of the contribution of N2 fixation to the N budget show a wide range. This is due to interannual variability and 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 (Liqui-Cel) 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 concentrations of these gases by multiplication by the total pressure and the respective solubility constants. The results from laboratory tests show that the accuracies (deviations from expected values) of N2 (0.20 %), Ar (0.03 %) and O2 (0.20 %) and the precisions (2 times the absolute standard deviation) of N2 (0.05 %), Ar (0.14 %) and O2 (0.11 %) are sufficient enough to quantify the surface water N2 depletion caused by N2 fixation and to account for the interfering gas exchange on the basis of changes in the Ar concentration. The e-folding equilibration times are 4.8 min for N2, 3.0 min for Ar and 3.2 min for O2. Our GE-MIMS approach is designed for long-term observations on various platforms such as voluntary observing ships (VOSs). The latter are particularly suited to achieving the temporal and spatial resolutions necessary for studying large-scale N2 fixation in regions such as the Baltic Sea.