Research article - Peer-reviewed, 2022
Unraveling the Contribution of Turbulence and Bubbles to Air-Water Gas Exchange in Running WatersKlaus, M.; Labasque, T.; Botter, G.; Durighetto, N.; Schelker, J.
AbstractQuantifying air-water gas exchange is critical for estimating greenhouse gas fluxes and metabolism in aquatic ecosystems. In high-energy streams, the gas exchange rate k is poorly constrained, due to an incomplete understanding of turbulence and bubble contributions to k. We performed a flume experiment with air bubble additions to evaluate the combined effects of turbulence and bubbles on k for helium, argon, xenon, and methane. We created contrasting hydraulic conditions by varying channel slope, bed roughness, water discharge, and bubble flux. We found that k increased from 1-4 to 17-66 m d(-1) with increases in turbulence and bubble flux metrics. Mechanistic models that explicitly account for these metrics, as well as gas diffusivity and solubility, agreed well with the data and indicated that bubble-mediated gas exchange accounted for 64-93% of k. Bubble contributions increased with bubble flux but were independent of gas type, as bubbles did not equilibrate with the water. This was evident through modeled bubble life and equilibration times inferred from bubble size distributions obtained from underwater sound spectra. Sound spectral properties correlated well with turbulence and bubble flux metrics. Our results demonstrate that (a) mechanistic models can be applied to separate free surface- and bubble-mediated gas exchange in running waters, (b) bubble life and equilibration times are critical for accurate scaling of k between different gases, and (c) ambient sound spectra can be used to approximate contributions of turbulence and bubbles.
Keywordspiston velocity; channel; degassing; reaeration; hydrophone; bubble
Published inJournal of Geophysical Research: Biogeosciences
2022, volume: 127, number: 3, article number: e2021JG006520
Publisher: AMER GEOPHYSICAL UNION
Swedish University of Agricultural Sciences, Department of Forest Ecology and Management
Universite de Rennes 1
University of Padua
University of Padua
University of Vienna
UKÄ Subject classification
Oceanography, Hydrology, Water Resources
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