Unraveling the Contribution of Turbulence and Bubbles to Air-Water Gas Exchange in Running Waters

Abstract

Quantifying 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.

Keywords

piston velocity; channel; degassing; reaeration; hydrophone; bubble

Published in

Journal of Geophysical Research: Biogeosciences
2022, Volume: 127, number: 3, article number: e2021JG006520Publisher: AMER GEOPHYSICAL UNION

SLU Authors

• Klaus, Marcus

  • Department of Forest Ecology and Management, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Oceanography, Hydrology, Water Resources
Environmental Sciences


Publication identifier

DOI: https://doi.org/10.1029/2021JG006520

Permanent link to this page (URI)

https://res.slu.se/id/publ/116847