Interfacial gas exchange in vegetated channels under free surface waves*

Fluxes of dissolved oxygen (DO) at the air-water (AW) interface are crucial for availability of oxygen to aquatic biota. Turbulence generated by aquatic vegetation canopies is known to influence the exchange of dissolved gases with the atmosphere. In this study we performed experiments on a wave flume to determine the gas transfer rates under free surface waves of varied amplitude and frequencies through surrogate vegetation. We used arrays of rigid cylinders to mimic submerged and emergent vegetation canopies and used optical DO sensors to monitor the interfacial reaeration rates within and downstream of the canopy. We used Acoustic Doppler Velocimetry (ADV) to characterize the flow inside and outside the canopy and performed 2D planar Particle Image Velocimetry (PIV) to reveal the detailed dynamics inside the canopy. Data revealed enhanced gas transfer within the vegetation array once stem-scale turbulent wakes develop. Frequency of waves was observed to be a dominant factor enhancing gas transfer rate across all wave amplitudes. The results allow us to identify the critical parameters governing the interfacial gas transfer dynamics, and to develop theoretical predictors for surface gas transfer rates on vegetated oscillatory flows.