Turbulent mixing limits mussel feeding: Direct estimates of feeding rate and vertical diffusivity

Field measurements of physical and biological parameters, together with the estimation of mass transport and estimation of clearance rates (for feeding rate) using a biodeposition method (defecation) were combined to improve the understanding of the limiting factors that affect the feeding rate of blue mussels in an intertidal commercial re-laid benthic mussel bed. The study used an in situ feeding rate method to describe one of the mechanisms that underpins mussel bed self-patterning. The feeding rate of mussels measured using the in situ method (defecation) closely matched the rate of the measured turbulent transport of chl a. This relationship suggested that the supply rate of food limited the growth of mussels in this system. Vertical food depletion ([chl a] <1 μg l-1; height <6 cm) was measured above the mussel bed, and horizontal food depletion (at 5 cm above the bed) was measured in the water column at both mussel patches as well as at adjacent bare patches and in between. Mussel size mirrored the measured food gradient such that larger mussels occurred at the edge compared with the middle of the patches of mussels. In this system, the high mussel feeding rate (2.9 l ind.-1 h-1), despite food depletion near the mussel bed, is due to the interaction of 3 hydrodynamic processes: vertical turbulent mixing, advection and re-suspension. These processes are themselves altered through a positive feedback mechanism associated with the mussel bed morphology. We found that the highest feeding rates are achieved when mussel beds are interspersed with patches of bare mud; thus stocking strategies that promote the formation of self-patterning should be encouraged