Effects of infaunal bivalve density and flow speed on clearance rates and near-bed hydrodynamics

The influence of bed density (from 0 to 1000 ind. M⁻²) and flow speed (2, 5, and 15 cm s⁻¹, i.e. “low”, “medium” and “high”, respectively) on clearance rates (CR) of an infaunal suspension-feeding bivalve (Austrovenus stutchburyi) and boundary layer dynamics were investigated in annular flumes. Near-bed flow speeds decreased and bed shear stresses increased with bivalve density due to the bed roughness generated by the shallow burrowers. At low densities (< 500 ind. m⁻²) there was no detectable effect of feeding currents on bed shear stress, but at high densities (1000 ind. m⁻²) and at the low flow speed bed shear stress was almost 80% higher when Austrovenus were feeding (0.0016 N m⁻²), compared to when they were not feeding (0.0009 N m⁻²). These results suggest that bed roughness generated by Austrovenus and their feeding currents are capable of influencing food supply to the bed. Individual CR were significantly (p < 0.01) greater at the high flow speed (0.88 L h⁻¹ ind. ⁻¹), compared to the medium and low flow speeds (0.34 to 0.54 L h− 1 ind.− 1). With increasing bed density, individual CR was significantly (p < 0.001) reduced; at low densities CR ranged from 0.6 to 1.4 L h⁻¹ ind. ⁻¹, but at high densities they were 0.1 to 0.8 L h⁻¹ ind. ⁻¹. Bed CR (L h⁻¹ m− 2) did not scale proportionally with density, initially increasing up to 500 ind. m− 2 but remaining relatively constant thereafter. This was in part due to the negative effect of density on individual CR but also because the proportion of bivalves feeding decreased with increasing density. Our results demonstrate that recording the number of bivalves feeding is necessary to interpret the effect of density on bed CR. Moreover, if the effects of both bivalve density and flow speed are not considered, then scaling up individual CR obtained using animals at low densities will over estimate population filtration capacity. As high (but equivalent to in situ) densities reduced both the number of animals feeding and individual CR other factors may offset this negative interaction, such as protection from wave disturbance or enhanced resuspension of benthic microalgae