Influence of self-organised structures on near-bed hydrodynamics and sediment dynamics within a mussel (Mytilus edulis) bed in the Menai Strait

The impacts of natural mussel assemblages on near-bed hydrodynamics, sediment erosion and deposition were quantified in flume studies. Mussels were sampled from a site in the Menai Strait (North Wales), together with field measurements of hydrodynamics over the tidal cycle (water depth, current speeds, turbulent kinetic energy (TKE)). Bare sediment (within a mussel bed but 1.5 m from mussels) was very stable due to the influence of microphytobenthos (critical erosion velocity (U_crit) = 0.49 m s^{- 1}; critical bed shear stress (τ_e) = 0.4 Pa). Sediment associated with 55% and 95% mussel coverage was more easily eroded (U_crit = 0.18 and 0.21 m s^{- 1}). This was primarily due to the mussels enhancing near-bed TKE, and thus bed shear stress (τ₀), which resulted in τ_e values (0.5 and 0.7 Pa, respectively), slightly higher than the bare sediment (0% cover). Consequently, mussel assemblages induced resuspension of sediment and microphytobenthos at significantly lower current speeds, within the range recorded in the field. Flume simulated sinusoidal current cycles confirmed the influence of mussels on erosion thresholds for microphytobenthos and sediment with accelerating currents. Mussel suspension feeding activity enhanced the rapid clearance of particles from the water column (deposition) after the onset of decelerating currents. The results support the hypothesis that self-organised structures (mussel strings) enhance near-bed turbulence and the resuspension of microphytobenthos under moderate flows conditions (< 0.3 m s^{- 1}). Thus self-organised mussel beds enhance the provision of food through resuspension that serves to offset the depletion of phytoplankton in the water column immediately above mussel beds.