Influence of current speed on clearance rate, algal cell depletion in the water column and resuspension of biodeposits of cockles (Cerastoderma edule)

The main objectives of this study were: 1) to determine the influence of water currents on the suspension feeding rate of cockles (Cerastoderma edule); 2) to quantify the interaction between cockle feeding and flow on algal cell depletion in the overlying water column, and 3) to measure the effect of flow on resuspension of their pseudofaeces and faeces. Flume experiments demonstrated that suspension feeding rate (i.e. clearance rate) of C. edule was not significantly affected by increasing current speed, at least between 5 and 35 cm s− 1. Measurement of vertical profiles in algal cell concentrations within the water column showed a marked depletion above the bed, and the size of this was inversely related to currents' speeds below 5 cm s− 1. At 2 cm s− 1 the algal cell depletion was maximum immediately above the bed. However, below currents of 1 cm s− 1 the maximum depletion was at 10 cm above the bed. This was a result of the exhalent jet of the cockle pumping filtered water (i.e. algal free) vertically into the water column and above the intake level of the inhalant siphon. Such stratification of the water column would appear to be beneficial to the cockle because it reduces the degree of re-filtration of algal cell depleted water at times of low flow, when there is poor mixing and thus poor replenishment of phytoplankton to the boundary layer. Critical erosion thresholds for cockle biodeposits, produced from a diet of silt and unicellular algae, were recorded at current velocities of 15 and 25 cm s− 1, or shear velocities of 0.6 and 1.0 cm s− 1, for pseudofaeces and faeces respectively.