Effects of mussel filtering activity on boundary layer structure

Abstract

The structure of the benthic boundary layer over a bed of mussels ( Mytilus edulis) was investigated in a large racetrack flume. Flow was observed to be modified both by the physical roughness of the mussel bed and by the momentum input of the exhalent jets of the mussels. Particularly when the mussels were closed, and filtering activity was reduced to a minimum, we observed an internal boundary layer, around 4 cm thick, within the log layer. This internal boundary layer was often masked when the mussels were filtering actively. The presence of an internal boundary layer indicates that the boundary layer is not only structured by friction drag, but that form drag due to roughness elements also plays an important role. Consequently, estimates of bed shear stress based on velocity or Reynolds stress measurements carried out more than a few cm above the bed may be inaccurate. Over inactive mussels the shear velocity in the internal boundary layer (the roughness sub-layer) is smaller and bed shear stress is consequently reduced. Filtration activity of the mussels increased the velocity gradient in the lower layer at low and intermediate velocities, but at higher flow rates velocity profiles were not affected. Clear effects of the exhalent jets on absolute levels of TKE could be measured at all ambient velocities, while the effect on the Reynolds stress was limited. Velocity normalised TKE and Reynolds stress also indicated that the effect of the siphonal currents was limited at high velocities. Our results indicate that mussel filtration activity may have an important effect on exchange processes at the sediment-water interface, but that the extent of the effect is highly dependent on the ambient flow conditions.