THE EFFECT OF BIVALVE EXCURRENT JET DYNAMICS ON MASS-TRANSFER IN A BENTHIC BOUNDARY-LAYER

Predictions of phytoplankton depletion by benthic bivalves in shallow, tidally driven estuaries must account for the formation of concentration boundary layers resulting from the dynamic interaction of bivalve siphonal currents with the overlying turbulent boundary layer. To study the near-bed hydrodynamics of the benthic boundary layer, we conducted experiments in a laboratory flume using multiple jets and sinks to represent feeding by the siphonate species Tapes japonica and Potamocorbula amurensis. Refiltration fractions were determined by monitoring the concentration of dye ingested by incurrent siphons, and PLIF (planar laser-induced fluorescence) was used to characterize the concentration fields. Results show that refiltration fractions can be as high as 48% and are a function of several dimensionless parameters: animal spacing (S/d(o)), velocity ratio (u(j): u*), siphon height (h(s)/d(o)), and crossflow Reynolds number (Re-x). (S is the mean distance between animals, d(o) the excurrent siphon diameter, h the animal siphon height, u(j) the excurrent jet velocity, and u(j) the mean shear velocity.) We found that a good estimate of maximum refiltration (n(max)) based on animal spacing is (n(max)S/d(o)) approximate to 2-3 and have incorporated this result into a conceptual mass-transfer model. Differences in concentration profiles calculated from PLIF images are likely due to the relative influence of four sources of turbulence in the flow: boundary-layer shear, boundary roughness, jet in a crossflow, and multiple jet interactions.