The importance of boundary‐layer flows in supplying phytoplankton to the benthic suspension feeder, Mytilus edulis L.

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Abstract
Measurements of vertical gradients in phytoplankton fluorescence over an intertidal mussel bed in the St. Lawrence River estuary (Québec) indicate a significant reduction in phytoplankton concentration close to the bed. Furthermore, measurements of near#x2010;bed fluorescence as a function of current speed, and of mussel consumption rate as a function of in situ fluorescence, suggest that consumption rate varies with flow speed. A steady, two#x2010;dimensional, finite#x2010;difference model was constructed to represent the balance between horizontal advection and vertical diffusion of phytoplankton over a mussel bed, in order to simulate the observed influence of mussels on the phytoplankton distribution. The vertical distribution of phytoplankton generated by the model shows a marked reduction close to the bed, consistent with the field measurements, and the model also confirms the dependence of consumption rate on flow speed. Enhanced vertical diffusive transport at higher current speeds results in a higher rate of replenishment of phytoplankton to food#x2010;impoverished near#x2010;bottom waters. Model runs with different values of the bottom roughness parameter indicate that roughness due to the mussels themselves may significantly increase the food supply to the bed, due to enhanced turbulent transport. In addition, since there is a strong vertical gradient in food concentration above the seabed, the time#x2010;averaged food concentration obtained by the mussels is a strong function of the height from which they ingest.