Hydrodynamics of discontinuous mussel beds: Laboratory flume simulations

Mussels, like all benthic filter feeders, are dependent on hydrodynamics (primarily horizontal advection and vertical turbulent mixing) for the delivery of seston from which they derive their food supply. Simultaneously, hydrodynamic forces are a major stress on mussel beds, which are often found in heterogeneous arrangements at a range of spatial scales. Little work has been reported that elucidates the influence of this spatial heterogeneity on hydrodynamics and the consequent implications for the mussels in terms of food supply and hydrodynamic stresses. Here, we address this issue by presenting results of laboratory flume experiments carried out to determine the influence of mussel bed patches on flow structure. Our experiments fall into two groups. The first used a single patch, and investigated the evolution of hydrodynamic profiles across it, and their dependence on the mean velocity of the incident flow (4.8–23.1 cm s− 1). The second compared the hydrodynamic profiles over 2- and 4-patch configurations at a single flow speed (18.4 cm s− 1). In the single patch experiments, the evolution of the profiles was dominated by events at the patch–gap boundaries, which were sites of enhanced turbulence production, and the Reynolds shear stress sagged near the patch centre, as observed in other studies. Comparisons of the two- and four-patch experiments suggest that both the depth-integrated turbulent kinetic energy, and the proportion of the turbulent kinetic energy close to the bed, are enhanced by increasing the number of patch–gap boundaries. Our results indicate that transitions in bed height and roughness that occur at patch edges have important impacts on the flow regime and hence on the food supply and abiotic stress, for attached organisms in these heterogeneous environments.