Hydrodynamic control of gas-exchange velocity in small streams

Gas exchange is a critical component of any biogeochemical mass balance model of dissolved gases in aquatic systems, yet the magnitude and drivers of spatial and temporal variations of air-water exchange rates in shallow streams are poorly understood. We investigated the relationships between gas exchange velocity of carbon dioxide and methane and flow hydraulics at different sections along a third order stream in Southwest Germany. To cover a wide range of different flow conditions, the sections were selected based on visual categorization of the dominant surface flow type. We found that in smooth and rippled flows, gas exchange velocities followed a universal dependence on turbulent dissipation rates predicted by the small-eddy and surface renewal models. For these surface flow types, the scaling applied to both, bulk-scale dissipation rates estimated from flow geometry and dissipation rates estimated from turbulence measurements. Turbulence was strongly anisotropic under rough flow conditions and gas exchange velocities were lower than predicted from measured dissipation rates. Nevertheless, near-surface turbulence and gas exchange velocities differed among surface flow type categories, indicating that quantitative assessment and mapping of surface flow type may facilitate improved parameterizations of gas exchange velocities at larger spatial scales. We further describe a novel instrument facilitating an objective assessment of surface flow by measuring the acceleration of a small floating sphere drifting freely on the water surface. In combination, our findings may open a new road for understanding, measuring and predicting spatial and temporal variability of gas exchange in streams.