FLUME EXPERIMENTS ON FOOD-SUPPLY TO THE BLUE MUSSEL MYTILUS-EDULIS-L AS A FUNCTION OF BOUNDARY-LAYER FLOW

A two-dimensional, advection-diffusion model of phytoplankton depletion above an assemblage of suspension feeders was tested in a laboratory flume containing a 6.1-m-long bed of the blue mussel Mytilus edulis L. Flows with near-surface velocities of similar to 5 and 15 cm s(-1) were tested in three experiments. Phytoplankton concentrations (as estimated by fluorescence) were measured just upstream of and near the end of the mussel bed. Vertical profiles of velocity indicated that turbulent stress, and thus turbulent mixing, were three (slow flow) to ten (fast how) times higher over the mussel bed compared to the smooth flume bottom. In two experiments that used natural seawater, initial fluorescence was low and there was little temporal depletion at either how speed. In the one experiment in which cultured phytoplankton were added to filtered seawater so that initial fluorescence was much higher, significant vertical and substantial horizontal gradients in fluorescence developed. In this experiment, depletion was enhanced in slow flow compared to fast flow in both the downstream and vertical directions, as qualitatively predicted by the model. Furthermore, with a how-speed-dependent mussel filtration velocity, modeled vertical profiles of fluorescence were consistent with the observations.