Measuring hydrodynamics and sediment resuspension in seagrass meadows

A number of studies have revealed how the large bottom roughness of coastal benthic ecosystems (collectively referred to as canopies) modify near-bed hydrodynamics, which in turn drive sediment suspension and transport. While a growing number of studies have investigated sediment dynamics within canopies in controlled laboratory experiments, field observations on the turbulence level within natural ecosystems still remain limited. In this study, we investigate how seagrass meadows (Posidonia Australis) that are abundant in southwestern Australia modify Suspended Sediment Concentrations (SSC) and fluxes over a wide range of current- and wave-dominated conditions. High frequency current profiles at the millimetre scale from 1 to 50 cm above bed and multi-frequency acoustic sounder measurements were taken from field-adapted instruments generally used in laboratory. Near-bed current values inside the canopy were then used to derive shear stress and understand SSC dynamics over time. Current data measured right above the meadow and higher up in the water column contributed to understand flow attenuation due to the seagrass characteristics. In-canopy wave velocities differed up to one order of magnitude from what was measured right above the canopy element. Different classes of sediment size distribution were analysed in terms of conventional formulations, which provided a general framework to understand the modification of sediment resuspension in terms of both concentration and sediment distribution within seagrass meadows in wave-dominated environments. The results highlight the importance the significant role that seagrass meadows play in regulating local rates of sediment resuspension and deposition through flow attenuation at the patch-scale within individual seagrass meadows.