Listening to air–water gas exchange in running waters

Abstract

Air–water gas exchange velocities (k) are critical components of many biogeochemical and ecological process studies in aquatic systems. However, their high spatiotemporal variability is difficult to capture with traditional methods, especially in turbulent flow. Here, we investigate the potential of sound spectral analysis to infer k in running waters, based on the rationale that both turbulence and entrained bubbles drive gas exchange and cause a characteristic sound. We explored the relationship between k and sound spectral properties using laboratory experiments and field observations under a wide range of turbulence and bubble conditions. We estimated k using flux chamber measurements of CO2 exchange and recorded sound above and below the water surface by microphones and hydrophones, respectively. We found a strong influence of turbulence and bubbles on sound pressure levels (SPLs) at octave bands of 31.5 Hz and 1000 Hz, respectively. The difference in SPLs at these bands and background noise bands showed a linear correlation with k both in the laboratory (R2 = 0.93–0.99) and in the field (median R2 = 0.42–0.90). Underwater sound indices outperformed aerial sound indices in general, and indices based on hydraulic parameters in particular, in turbulent and bubbly surface flow. The results highlight the unique potential of acoustic techniques to predict k, isolate mechanisms, and improve the spatiotemporal coverage of k estimates in bubbly flow.

A new method to quantify air–water gas exchanges in streams based on slug injection and semicontinuous measurement

“…The method relies on a slug injection and on a single measurement station. In that sense, it is simpler to setup than many other methods of gas exchange rate measurements at the stream‐scale (Wanninkhof et al 1990; Genereux and Hemond 1992; Morse et al 2007; Jin et al 2012; Benson et al 2014; Hall and Madinger 2018; Klaus et al 2019; Vautier et al 2020). Indeed, a slug injection is cheaper, faster, and easier to perform than a continuous injection (Jin et al 2012).…”

“…Gas exchanges can also be measured locally with domes (Borges et al 2004; Alin et al 2011). Recent studies alternatively proposed to correlate sound pressure levels with the turbulence and the bubbles content of the stream and so with gas exchange rates (Morse et al 2007; Klaus et al 2019). Many authors also developed empirical or semiempirical equations to estimate the gas exchange rate coefficient based on hydrodynamic characteristics of the stream such as the slope, the depth, the flow velocity, or the discharge (Churchill et al 1964; Tsivoglou and Neal 1976; Melching and Flores 1999; Gualtieri and Gualtieri 2000; Raymond et al 2012; Goncalves et al 2017).…”

Prediction of Acoustic Energy Radiated by Bubble Produced by Raindrops

“…e study of rainfall noise is important not only for ocean physicists studying (i) rainfall and wind over the ocean [4] and (ii) the usage of sonars [5][6][7] but also for biologists studying how anthropogenically generated sound impacts marine mammals [8]. Another important application of rainfall (bubble) noise is in the field of biogeochemistry, specifically in estimating the rate of exchange of gas between air and water [9] or sediments and water [10].…”

Corrigendum: Listening to air‐water gas exchange in running waters

“…In [1] the following errors were made: In equations 1‐4, as well as the captions for Table 4 and Table S3, the term “ k 600 ” should read “ln( k 600 )”. In other words, the logaritmus naturalis (ln) should be applied to k 600 in order to yield correct predictions. In Table S1, the correct unit of water depth is m , not cm , as indicated in the original table. …”

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In [1] the following errors were made:1. In equations 1-4, as well as the captions for Table 4 and Table S3, the term "k 600 " should read "ln(k 600 )".

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