Effect of Cerastoderma edule density on near-bed hydrodynamics and stability of cohesive muddy sediments

Flume experiments were performed to investigate the impact of cockle density (Cerastoderma edule - 0, 47, 141 and 312 individuals m− 2) on near-bed hydrodynamics and sediment erodability. Undisturbed cohesive muddy sediment was collected by coring at an intertidal site in the Tamar estuary (SW England) and placed in the annular flumes. Cockles at the required density were added to the surface sediments, allowed to bury, and were then exposed for 24 h to sinusoidal cycles of currents (3 to 18 cm s− 1) which simulated 6-h tidal cycles comparable to the Tamar site. During this period the cockles adopted their usual behaviour of burrowing, suspension feeding and shell valve adductions. After 24 h, current speeds were increased from 5 to 50 cm s− 1 in 11 steps, and current velocities, turbulent kinetic energy (TKE), and suspended sediment concentration (SSC) were measured. Current speeds decreased and turbulence increased within 3 cm of the sediment surface as a function of cockle density. Sediment resuspension increased with increasing cockle density: SSC at 0.5 m s− 1 = 156, 574, 1045 and 2253 mg L− 1 for 0, 47, 141 and 312 animals m− 2 respectively. Enhanced sediment erosion was due to increased bioturbation and bed roughness. Critical erosion velocity decreased from 26 to 8 cm s− 1 with increasing cockle density. Shear stress, measured in terms of TKE, increased at 0.5 and 1 cm above the bed for 141 and 312 animals m− 2 and up to 2 cm above the bed for 312 animals m− 2, reflecting the increased bed roughness due to bioturbation. However, the critical erosion shear stress was relatively independent of cockle density (0.225 and 0.151 N m− 2 for 0 and 312 animals m− 2 respectively). The valve adduction frequency of the cockles was also measured in response to increasing SSC. The frequency increased from 1.2 to 16.3 adductions h− 1 with increasing SSC from 13 to 308 mg L− 1. It represents a significant behavioural response, creating positive feedback with increased SSC further enhancing sediment disturbance and resuspension.