Flow and particle distributions in a nearshore seagrass meadow before and after a storm

Fine-scale spatial effects of a seagrass meadow on suspended particle transport were assessed from current speeds, orbital wave velocities, turbulent Reynolds stress, in situ particle concentrations, and sedimentation rates for a horizontal grid in a coastal seagrass (Posidonia oceanica) meadow at 2 depths and during low- and high-energy periods. For the low-energy period, the vertical reduction of the total kinetic energy, from 100 cm to ≈10 cm above the bottom, was larger in the meadow (up to 95 %) than over the sand (35 to 75 %). Velocity maps suggest that a recirculating flow formed in the meadow with a higher Reynolds stress at the edge of the meadow. Near the bed, concentrations of small particles (<10 μm diameter) were lower inside the meadow than over barren sand, while concentrations of large particles (>10 μm) were lower over the barren sand. For the period of stronger current and wave activity following a storm, nearbed turbulence and orbital wave velocity were elevated, though still lower inside the meadow than over the sand. For this high energy period, particle concentrations increased over the whole study area, but were still lowest deep inside the meadow. Overall, the horizontal spatial distribution of plants in the study area had a profound effect on the flow field and on vertical transport, even during the high-energy period. The reduced nearbed turbulence and lower sedimentation rate below the canopy confirms it as a calm zone with lower mixing compared to unvegetated areas.