Development of a Gas-Equilibrium Membrane-Inlet Mass spectrometer (GE-MIMS) for continuous N2, Ar and O2 measurements to quantify nitrogen fixation in the Baltic Sea

Nitrogen fixation by cyanobacteria in the Baltic Sea plays a crucial role in the context of eutrophication, as it promotes biomass production in the absence of dissolved inorganic nitrogen (DIN). Its contribution to the N budget is comparable to the combined sum of riverine and airborne DIN input, ranging from 300 kt-N/yr to 800 kt-N/yr. The vast range is due to internal fluctuations and significant uncertainties in various techniques used to determine N₂ fixation and in extrapolate local studies to entire basins. To overcome some of the limitations we introduce a new approach based on large-scale records of the surface water N₂ depletion during summer.

For our studies we use a membrane contactor (Liquicel) to establish gas phase equilibrium for atmospheric gases dissolved in seawater. The mole fractions of N₂, Ar and O₂ in the gas phase are continuously determined by mass spectrometry, yielding the concentration of these gases by multiplication with the total pressure and the respective solubility constants.

Thorough laboratory tests demonstrated that our Gas Equilibrium–Membrane-Inlet Mass Spectrometer (GE-MIMS) has sufficient accuracy and precision to detect and quantify nitrogen fixation. In June/July 2023, the GE-MIMS was deployed (i) on a voluntary observing ship (VOS, “Finnmaid”) for surface water gas analyses and (ii) for vertical water column studies on RV Elisabeth Mann Borgese along the VOS route between Helsinki and Travemünde. The VOS campaign enabled repeated transects along the same route and providing high spatial and temporal resolution time series of N₂ concentration changes due to nitrogen fixation. First results clearly indicate regions and episodes where N₂ fixation was active. Concurrent records of pCO₂ obtained from a different measurement system, along with O₂ concentrations, will be used for an independent characterization of cyanobacterial biomass production and thus of the associated N₂ fixation. Additionally, Ar measurements are used to account for the air-sea gas exchange.

Our objectives are to identify factors initiating and limiting cyanobacteria growth, with the final goal of determining Baltic Proper's N₂ fixation capacity.