Effects of near-bed turbulence on the suspension and settlement of freshwater dreissenid mussel larvae

Larval transport and settlement in benthic invertebrates is theorised to be influenced by bottom roughness and the hydrodynamic forces that roughness generates near the bed. This study of freshwater Dreissena spp. bivalves examined the transport and suspension of pediveliger larvae and larval models in a laboratory flow chamber and larval settlement in Lake Erie. Particle image velocimetry (PIV) measurements in the laboratory, and acoustic Doppler velocimetry (ADV) measurements in the field, were used to estimate small-scale turbulence in the near-bed environment over differing roughness. Quadrant analysis was used to determine the frequency of turbulent sweeps and ejections, and the extent of roughness flow regime (skimming versus wake interference flow) was noted to understand the determinants of larval transport and settlement. Skimming flow generated above bottoms with high mussel densities had significantly lower suspended transport (i.e. suspension off the bottom in the flow chamber; 3.311.14%) and lower larval settlement in the field (1526 +/- 80 larvae m(-2) per day) compared to low mussel densities (6.63 +/- 1.54% and 1853 +/- 47 larvae m(-2) per day). Conversely, wake interference flow indicated by high frequencies of turbulent sweeps and ejections generated by the roughness due to mussel patches resulted in high suspended transport in the flow chamber (i.e. via ejections) and the highest larval settlement in the field (i.e. via sweeps; 1943 +/- 59 larvae m(-2) per day). The spatial configuration of mussel roughness influenced the creation and magnitude of skimming versus wake interference flow, which can inhibit or enhance larval settlement, respectively.