Surface gas exchange determined from an aquatic eddy covariance floating platform

We present a new approach to quantifying air-water flux and gas transfer velocity (k(600)) from underwater eddy covariance (EC) of dissolved oxygen. EC fluxes were measured 35 cm below the air-water interface using an acoustic Doppler velocimeter (ADV) coupled with a fast-responding dissolved oxygen optode probe, integrated on a floating platform. A micro-electro-mechanical system (MEMS)-based inertial motion unit integrated with the ADV enabled compensation of measured velocities for platform motion and changing sensor orientation. Deployments of 16 h and 30 h were conducted under low to moderate wind conditions in Sage Lot Pond, a small estuarine embayment. We evaluate air-sea flux parameterizations based on wind speed, current speed, and turbulent kinetic energy dissipation rate compared to our directly measured EC flux. Total kinetic energy was linearly correlated with EC-determined k(600), indicative of a more direct relationship to near surface turbulence compared to wind and current speed based parameterizations, which were subject to biases attributable to directional differences in fetch and low current speed. Our observations, which encompassed a period of low turbulent energy, suggest that existing parameterizations are not well constrained for these conditions. This new aquatic EC technique is highly advantageous for the determination of air-water exchange, especially in dynamic near-shore and inland systems. The system presented here has the potential for wide applicability for lake, riverine, and open-ocean air-water exchange and the results can be extrapolated for use with a wide range of biogeochemically relevant gases.