Flume water air11/22/2023 ![]() ![]() Traditionally, variations of k for sparingly soluble gases have been assumed to result from irregular water motions near the water surface. However, these gas exchange estimates remain uncertain because k is inadequately constrained in mountain streams due to a poor understanding of the underlying physical drivers. Such streams are widespread across the earth (Larsen et al., 2014) and high k contributes to globally relevant GHG emissions (Horgby et al., 2019 Qu et al., 2017). Recent studies have highlighted exceptionally high k in steep mountain streams (Hall & Madinger, 2018 Ulseth et al., 2019). The k can be regarded as the water column depth that equilibrates with the atmosphere per unit time. The flux across the air‐water interface is described by Fick's first law of diffusion where F l u x = k C w − C e q, is the surface water gas concentration, C w is the air‐equilibrium gas concentration and k is the air–water gas exchange rate. 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.Īir‐water gas exchange is crucial for aquatic ecosystems as it affects metabolic fluxes, elemental cycling, and the exchange of greenhouse gases (GHGs) with the atmosphere (Laursen & Seitzinger, 2005 Likens, 2010 McCutchan et al., 1998 Raymond et al., 2013). Sound spectral properties correlated well with turbulence and bubble flux metrics. This was evident through modeled bubble life and equilibration times inferred from bubble size distributions obtained from underwater sound spectra. Bubble contributions increased with bubble flux but were independent of gas type, as bubbles did not equilibrate with the water. 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. We found that k increased from 1–4 to 17–66 m d −1 with increases in turbulence and bubble flux metrics. ![]() We created contrasting hydraulic conditions by varying channel slope, bed roughness, water discharge, and bubble flux. 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. In high‐energy streams, the gas exchange rate k is poorly constrained, due to an incomplete understanding of turbulence and bubble contributions to k. Quantifying air‐water gas exchange is critical for estimating greenhouse gas fluxes and metabolism in aquatic ecosystems. ![]()
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