A STUDY OF CONCENTRATION BOUNDARY-LAYER FORMATION OVER A BED OF MODEL BIVALVES

We conducted experiments in a laboratory flume to study the interaction of bivalve siphonal currents with the turbulent boundary layer, using a bed of model Tapes japonica (625 animals m-2). Refiltration of excurrent fluid, which represents a decrease in aggregate feeding efficiency, was measured for 225 siphon pairs oriented perpendicular to the cross-flow (free stream flow) direction. Refiltration was as high as 18% and related to position in the bed, pumping rate, and siphon height of the animals, as well as the cross-flow speed. Scaling analysis of refiltration data suggests that the velocity ratio, VR = u(j)/u*, where u(j) is the jet velocity and u* is the boundary shear velocity of the cross flow, is a good predictor of refiltration for VR > 20. Laser-induced fluorescence was used to visualize the flow. Analysis of the height of maximum phytoplankton depletion and spreading width of the plume obtained from quantitative concentration profiles show that changes in the vertical momentum of the excurrent jet become more important in the phytoplankton removal efficiency as VR decreases. The hydrodynamics of this complex interaction suggest that changes in pumping rate and siphon height will improve phytoplankton removal efficiency only for animals feeding in a strong cross flow.