Turbulence levels in a flume compared to the field: Implications for larval settlement studies

The settlement stage of shellfish larvae is important in determining distribution patterns of adults. In order to reach the substratum, the larvae need to cross the benthic boundary layer near the sediment surface. Suspended larvae reach the substratum by a combination of passive sinking, turbulent advection and possibly active swimming. Subsequently the larvae need a certain amount of time in contact with the substratum to attach themselves to the bed. Flumes are commonly used experimental set-ups to study larval settlement in flowing water. Since turbulence plays an important role in the vertical transport of larvae and in the probability of resuspension, such experiments must take turbulence scaling into account. This study compares turbulence characteristics in the flume at the NIOO-CEME with field conditions encountered by intertidal bivalve larvae during their settlement stage. Turbulence in the flume was manipulated throughout the water column using a grid and in the benthic boundary layer by introducing roughness structures. Hydrodynamic parameters such as shear velocity (u⁎), roughness height (z0), turbulence intensity (TI) and Reynolds stress were determined. We used the dimensionless Rouse number to scale the ratio between advective motion and random (turbulent) motion. Relative particle concentrations (Rouse distribution) near the bottom, as calculated theoretically for the flume under normal conditions and in the field, show that the flume is biased towards sinking. In the Oosterschelde, where flow is tidally driven, the ratio between advective motion and random (turbulent) motion for flow velocities around 0.35 m s− 1 is balanced. In this situation a small change in directional movement (e.g. by swimming) may have an effect on settlement success of larvae. The grid has a modifying effect on mixing throughout the water column, but close to the bottom turbulence intensities are similar to the situation without a grid. Turbulence introduced by bottom roughness has a different effect from that introduced by a grid. Turbulence introduced by bottom roughness has only minor effects on mixing higher up in the water column, but due to the wake-flow has dramatic effects on near-bed turbulence. We discuss the effects of turbulence generated at the bottom on larval settlement as a case study. Both grids and bottom roughness used to induce turbulence in the flume are valuable tools in scaling turbulence parameters for quantitative research on larval settlement.