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 concentrat ion boundary layers resulting from the dynamic interaction of bivalve siphonal currents with the overlying turbulent boundary layer. To stud y the near-bed hydrodynamics of the benthic boundary layer, we conduct ed experiments in a laboratory flume using multiple jets and sinks to represent feeding by the siphonate species Tapes japonica and Potamoco rbula amurensis. Refiltration fractions were determined by monitoring the concentration of dye ingested by incurrent siphons, and PLIF (plan ar laser-induced fluorescence) was used to characterize the concentrat ion fields. Results show that refiltration fractions can be as high as 48% and are a function of several dimensionless parameters: animal sp acing (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 heigh t, 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 incorp orated this result into a conceptual mass-transfer model. Differences in concentration profiles calculated from PLIF images are likely due t o the relative influence of four sources of turbulence in the flow: bo undary-layer shear, boundary roughness, jet in a crossflow, and multip le jet interactions.