Preliminary Results

For our studies we developed a new measurement system that builds upon the established GE-MIMS approach (Gas Equilibrium – Membrane-Inlet Mass Spectrometer). It is specifically designed for deployment on a ferry line, enabling repeated transects along the same route and providing high temporal and spatial resolution data for N₂, Ar and O₂ gas concentrations in the surface water.

The fundamental principle of a GE-MIMS involves an equilibration process between the partial pressures of a water side and a gas side (headspace) that are separated by a gas-permeable membrane. We used the Liquicel mini-module cartridge containing porous hollow fibers, which create a membrane surface area of 0.9 m². The gas side of the membrane-equilibrator is connected with a pressure sensor and a mass spectrometer (MS). The latter is the commercially available Quadrupole-MS (QMS) GAM 2000 (InProcess Instruments, Bremen, Germany). Through calibration of the MS with a well-defined standard gas composition, the detected ion currents of the investigation gases (N₂: m/z = 28, O₂: m/z = 32 and Ar: m/z = 40) can be converted into their molar fractions. Together with the recorded total pressure in the headspace, the partial pressures of the respective gases and their concentration can be calculated.

Thorough laboratory tests demonstrated that our Gas Equilibrium–Membrane-Inlet Mass Spectrometer has sufficient accuracy and precision to detect and quantify nitrogen fixation. In June/July of 2023 the GE-MIMS was deployed on the voluntary observing ship (VOS) “Finnmaid”, where an automated measurement system for recording the surface partial pressures of CO₂, CH₄, O₂ and N₂O is already installed. Therefore, the existing infrastructure could be used to obtain a continuous water supply from a depth of approximately 3.5 meters. The VOS “Finnmaid” is travelling 2-3 times per week over a distance of about 1000 km between the Mecklenburg Bight and the Gulf of Finland. Therefore, we were able to create large scale concentration time series of N₂, O₂ and Ar with a temporal resolution up to 3 days and a spatial resolution of about 12 km.

Top-left: Deployment of the GE-MIMS measurement system on board VOS “Finnmaid”. Top-right: Route of the VOS “Finnmaid” between Lübeck-Travemünde and Helsinki in the Baltic Sea. Bottom-left: VOS “Finnmaid”. Bottom-right: RV “Elisabeth Mann Borgese”.

At the end of June 2023, a high productive phase of a cyanobacterial bloom occurred north of Gotland. First results confirm this and clearly indicate regions and episodes where N₂ fixation was active. Furthermore, Ar measurements are used to account for the air-sea gas exchange and O₂ measurements can be utilized to estimate the net community production (NCP) triggered by N₂ fixation. Additionally, we deployed the GE-MIMS for vertical water column studies on RV Elisabeth Mann Borgese along the the VOS route, in order to investigate whether the N₂ fixation signal is consistent throughout the mixed layer.

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

The project is coordinated by Oliver Schmale and Bernd Schneider. Our partners from the Finnish Environment Institute SYKE (Lumi Haraguchi and Jukka Seppälä) provide valuable support with plankton imaging, fluorescence analysis and nutrient data. Sören Iwe is the PhD student working in this project at the IOW.

Related publications:

Schmale, O., Karle, M., Glockzin, M., and Schneider, B.: Potential of Nitrogen/Argon Analysis in Surface Waters in the Examination of Areal Nitrogen Deficits Caused by Nitrogen Fixation, Environ. Sci. Technol., 53, 6869–6876, https://doi.org/10.1021/acs.est.8b06665, 2019