From substrate to surface: The effect of vegetation-generated turbulence on surficial gas transfer

Aquatic vegetation in streams generates turbulence at multiple temporal and spatial scales. These turbulent features alter mixing and transport processes in rivers, driving soil-water interactions at the river bed (such as sediment transport and resuspension), and water-air interactions at the free surface (such as gas transfer). This cascade of processes impacts water quality, and carbon and oxygen dynamics, which are fundamental for the health of aquatic ecosystems. We conducted a series of laboratory experiments, using rigid cylinders to mimic vegetation, using quantitative imaging to characterize the flow and sediment in suspension, and optical sensors to monitor dissolved oxygen levels in the water. As vegetation patches move from emergent to fully submerged, the significance of stem-scale eddies is surpassed by canopy-scale eddies resulting from the mixing layer atop the canopy, which can a) more effectively move sediment throughout the water column, and b) facilitate exchange processes throughout the water column. Our data, considering a range of submergence ratios and population densities, will allow us to develop a model to predict gas transfer rates based on turbulent kinetic energy production as a result of the vegetation-flow interactions, to directly relate the effect of aquatic vegetation on surface gas transfer, and to assess potential effects of suspended sediment into gas transfer predictions.