Effect of Nereis diversicolor density on the erodability of estuarine sediment

Nereis diversicolor is a widely distributed estuarine polychaete worm that is particularly abundant in the mid to upper parts of estuaries. Laboratory annular flumes were used to study the effects of N. diversicolor density (0 [control], 100, 300, 1000, 3000 individuals m–2) on erodability of cohesive muddy sediments. At the highest densities (1000 and 3000 ind. m–2) there was evidence of active sediment resuspension under low current speeds (U = 0.05 m s–1). In response to a step-wise increase in U there was a density-dependent increase in the sediment mass eroded (g m–2). At 3000 individuals m–2 and at a flow of 0.4 m s–1, sediment erosion increased 37-fold compared to the control. Erosion thresholds representing the onset of sediment resuspension (Type 1a erosion) were low and similar for all worm densities, including the control (critical erosion velocity Ucrit = 0.11 to 0.12 m s–1; critical bed shear stress for erosion [τe] = 0.02 to 0.036 Pa). However, at the lower densities (0 to 300 ind. m–2) there was little bed erosion until U = 0.4 to 0.45 m s–1 (bed shear stress = 0.16 to 0.2 Pa). The burrowing and surface feeding activity of the polychaete destabilised the bed and increased sediment erodability as a function of worm density. The influence of N. diversicolor (300 ind. m–2) on sediment erosion and deposition during sinusoidal tidal current cycles (0.05 to 0.35 m s–1) was also studied in the flumes. There was a consistent cycle of erosion and deposition with worms inducing ~2-fold higher erosion and deposition rates following an erosion threshold of U ≥ 0.11 m s–1 (τe = 0.02 Pa) and a deposition threshold of U ≤ 0.26 m s–1 (τd = 0.07 Pa). Maximum erosion and deposition rates occurred at 0.25 m s–1 and 0.11 m s–1, respectively. The results suggest that N. diversicolor can make a significant contribution to increased erosion of fine intertidal muddy sediments and to the overall turbidity in the upper estuary.