Air–water gas exchange in lakes and reservoirs measured from a moving platform by underwater eddy covariance
Peter Berg, Michael L. Pace, Cal D. Buelo
Abstract
Abstract Air–water exchange rates of gasses, such as O 2 , CO 2 , and CH 4 , are widely used in ecosystem studies of lakes and reservoirs, but their magnitudes are often difficult to assess. In this proof‐of‐concept study, we measured gas exchange by underwater eddy covariance in such lentic systems from a moving platform. We used an Acoustic Doppler Velocimeter and a fast‐responding O 2 ‐temperature sensor mounted in the bow of a boat to measure water velocity, O 2 concentration, and temperature below the air–water interface (~ 10 cm) while the boat was propelled at constant speed (~ 25 cm s −1 ) by an electric trolling motor. Fluxes of O 2 and heat across the air–water interface and standard gas exchange coefficients, k 600 , were calculated for every 3 min of traveled distance (~ 45 m). All deployments were done under calm low‐wind conditions where empirical relationships for k 600 are most uncertain. Deployment averages of k 600 ranged from 0.070 to 0.39 m d −1 and were strongly correlated with both the heat flux and the water temperature. In one deployment, a > 20% variation in mean water column O 2 concentration was measured along a 1 km long transect of a reservoir. Given the typical size of O 2 concentration differences over the air–water interface that drive gas exchange, such lateral variations can, even at a near‐constant exchange coefficient, result in highly biased whole‐ecosystem fluxes if based on stationary single‐point O 2 measurements. “Mobile” aquatic eddy covariance measurements enable quantification of gas exchange in lakes and reservoirs under true in situ conditions and with high temporal and spatial resolution.