Technical note: Testing a new approach for the determination of N<sub>2</sub> fixation rates by coupling a membrane equilibrator to a mass spectrometer for long-term observations
<p>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 N<span class="inline-formula"><sub&...
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| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-04-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/22/1767/2025/bg-22-1767-2025.pdf |
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| Summary: | <p>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 N<span class="inline-formula"><sub>2</sub></span> 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 N<span class="inline-formula"><sub>2</sub></span> 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 N<span class="inline-formula"><sub>2</sub></span>, Ar and O<span class="inline-formula"><sub>2</sub></span> 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 N<span class="inline-formula"><sub>2</sub></span> (0.20 %), Ar (0.03 %) and O<span class="inline-formula"><sub>2</sub></span> (0.20 %) and the precisions (2 times the absolute standard deviation) of N<span class="inline-formula"><sub>2</sub></span> (0.05 %), Ar (0.14 %) and O<span class="inline-formula"><sub>2</sub></span> (0.11 %) are sufficient enough to quantify the surface water N<span class="inline-formula"><sub>2</sub></span> depletion caused by N<span class="inline-formula"><sub>2</sub></span> fixation and to account for the interfering gas exchange on the basis of changes in the Ar concentration. The <span class="inline-formula"><i>e</i></span>-folding equilibration times are 4.8 min for N<span class="inline-formula"><sub>2</sub></span>, 3.0 min for Ar and 3.2 min for O<span class="inline-formula"><sub>2</sub></span>. 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 N<span class="inline-formula"><sub>2</sub></span> fixation in regions such as the Baltic Sea.</p> |
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| ISSN: | 1726-4170 1726-4189 |