Effects of Aquatic Vegetation on Gas Exchange Process Across Air-Water and Sediment-Water Interface

Abstract:

Aquatic vegetation alters the hydrodynamics of natural waters, such as rivers, lakes, and estuaries. Plants can generate turbulence that propagates throughout the entire water column, which affects gas transfer mechanisms at both air-water and water-sediment interfaces, driving changes of dissolved oxygen (DO), an important indicator of water quality. We conducted a series of laboratory experiments with rigid cylinder arrays to mimic vegetation using a staggered configuration in a recirculating race-track flume. Walnut shells were chosen as the sediment substrate, which interacts with DO in water. 2D planar Particle Image Velocimetry was used to characterize the flow field under various submergence ratios, highlighting the effect of vegetation on turbulence quantities. Gas transfer rates were determined by measuring the DO concentration during the re-aeration process based on the methodology proposed by the American Society of Civil Engineers. Our data provide new insight on Air-Water-Vegetation-Sediment interactions in streams as a function of submergence ratio, array density, and plant stiffness. A modified surface renewal model using turbulence production as an indicator of gas transfer efficiency is used to predict surface gas transfer rates. Near-bed turbulent kinetic energy is used as an indicator of transfer processes at the sediment bed, in an effort to incorporate sediment oxygen demand in a substrate-to-surface transfer model.