Distinct air–water gas exchange regimes in low- and high-energy streams


Gas exchange across the air–water interface drives the flux of climate-relevant gases and is critical for biogeochemical processes in aquatic ecosystems. Despite the presence of mountain streams worldwide, we lack basic understanding of gas exchange through their turbulent surfaces, making global estimates of outgassing from streams and rivers difficult to constrain. Here we combine new estimates of gas transfer velocities from tracer gas additions in mountain streams with published data to cover streams differing in geomorphology and hydraulics. We find two different scaling relationships between the turbulence-induced energy dissipation rate and gas transfer velocity for low- and high-channel slope streams, indicating that gas exchange in streams exists in two states. We suggest that turbulent diffusion drives gas transfer velocity in low-energy streams; whereas turbulence entrains air bubbles in high-energy streams, and the resulting bubble-mediated gas exchange accelerates with energy dissipation rate. Gas transfer velocities in the high-energy streams are among the highest reported. Our findings offer a framework to include mountain streams in future estimates of gas fluxes from streams and rivers at the global scale.

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Fig. 1: Data range of common gas exchange predictors and model fit based on low-sloped streams.
Fig. 2: k600 scaled against eD.
Fig. 3: Density distributions of stream characteristics for low- versus high-energy streams.
Fig. 4: Gas exchange increased with streambed roughness.

Data availability

The compiled data on k600, channel slope, velocity, depth, width and stream discharge, along with additional data and information on hydraulic scaling, a summary of data and statistics, and the code used to generate the results and figures presented here, can be found in the Supplementary Information.


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We thank F. Hammer, R. Romanens, L. Freund, F. Cuttat and V. Sahli for fieldwork conducting the argon gas tracer releases as well as measuring channel slopes for the Swiss study streams; S. Lane for helping with photogrammetry and the semivariance analysis; P. Raymond and co-authors for providing data from ref. 7; and D. McGinnis for discussions on bubble-mediated gas exchange. Financial support came from the Swiss Science Foundation (200021-163015) to T.J.B.

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A.J.U., R.O.H. Jr and T.J.B. conceived the idea of scaling. M.B.C. calculated streambed roughness. H.L.M. produced code to analyse Ar data. A.J.U. analysed the results. A.J.U., R.O.H., T.J.B. and A.N. wrote the manuscript. All authors reviewed the manuscript.

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Correspondence to Amber J. Ulseth.

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Ulseth, A.J., Hall, R.O., Boix Canadell, M. et al. Distinct air–water gas exchange regimes in low- and high-energy streams. Nat. Geosci. 12, 259–263 (2019). https://doi.org/10.1038/s41561-019-0324-8

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