Hydrodynamics of intertidal oyster reefs: The influence of boundary layer flow processes on sediment and oxygen exchange

An intertidal Crassostrea virginica oyster reef was instrumented to quantify processes affecting boundary layer flow, suspended sediment deposition and erosion, and the flux of oxygen to and from the benthos. Velocity and suspended sediment concentrations were measured at opposing sides of the reef and sediment fluxes, due to the combined effects of deposition, resuspension, and suspension feeding by the reef community, were computed from the difference between upstream and downstream suspended sediment concentrations. At the center of the reef, the flux of oxygen to and from the reef was measured using the eddy-correlation technique. While the reef was submerged, oxygen fluxes showed no significant correlation to light, and oxygen uptake increased linearly with velocity, ranging between 100 and 600 mmol m -2 d -1 . Sediment deposition to the reef also increased linearly for velocities between 0 and 10 cm s -1 , up to a maximum of 3500 g m -2 d -1 . For velocities .15 cm s -1 , sediment flux to the reef decreased as sediment resuspension occurred due to bed shear stresses that exceeded the critical threshold for erosion. At velocities .25 cm s -1 , there was net sediment erosion from the reef. Overall, during summertime and nonstorm conditions, mean oxygen uptake was 270 – 40 mmol m -2 d -1 and sediment deposition was 1100 – 390 g m -2 d -1 while the reef was submerged, indicating that oysters have a net positive effect on water clarity and that hydrodynamics exert a strong influence on benthic fluxes of oxygen and sediment to and from the reef.