Bubble size distributions in mountain streams

Mountain streams are hot spots for the exchange of gases such as oxygen or carbon dioxide with the atmosphere. Air-water gas exchange is accelerated by air bubbles entrained in turbulent flow and depends on bubble concentration and size. Yet, our understanding of gas exchange mechanisms and our ability to upscale gas fluxes is hampered by a severe lack of data on bubble size distributions in streams. Here, we measured bubble size distributions in 16 step-pool systems across six stream reaches in two mountain ranges (Dolomites, Italy; Vosges, France), combining Acoustic Bubble Spectrometry and ambient underwater sound recording. Bubble size distributions in our study streams were similar to those reported previously for ocean breaking waves: bubble concentrations decreased with bubble size following a power-law scaling, with a power exponent generally ranging from -2/3 to -10/3, increasing with bubble size, and varying both within and among step-pool systems. Total bubble concentrations exhibited a bilinear power law relationship with turbulent kinetic energy dissipation rates estimated from Acoustic Doppler Velocimetry, suggesting distinct bubble formation regimes under low and high turbulence. Bubble concentrations generally decreased with distance from steps, but invisible microbubbles still exhibited significant concentrations several meters downstream from steps where no visible macrobubbles were recorded. Our findings provide novel scaling laws for bubble size distributions, offering insights into the different gas exchange regimes observed in mountain streams. Additionally, they underscore the potential of bubble-mediated gas exchange even under absence of visible bubbles, highlighting the complexities involved in upscaling gas exchange mechanisms in such environments.