Abstract
Biotic and abiotic processes in estuaries are both strongly affected by the movement and flow of water, i.e., hydrodynamics. Seagrasses are highly productive and diverse ecosystems which provide many ecological services to organisms. Currents and waves dampened by seagrass create an hydrodynamically low-energy environment which can increase sediment deposition and reduce resuspension, thus enhancing habitat stability. The literature shows that field measurements of suspended matter have shown increased sedimentation and reduced resuspension, but, until now, particle resuspension has not been measured. There has been much study on the effect’s seagrass has on water flow and energy dissipation whilst quantitative estimates of deposition and resuspension are rare. Hence, in this study, we quantified and compared sediment resuspension and deposition rates in a Thalassia testudinum dominated seagrass bed during seasonal variations in Estero Bay, FL. Field measurements were collected within and outside such a seagrass bed in combination with laboratory experiments ran in a seawater flume which included transplanted plants and sediment from the field and with seawater running through it at various velocities. Understanding physical environmental processes are important to ecosystem functions. Quantitative measures of T. testudinum’s effects can be useful for improving numerical hydrodynamic model predictions that may be applicable to other seagrass species. Conservation and restoration efforts may be improved if we can better understand how seagrasses can reduce subtidal erosion and thus in return enhance or maintain the right conditions to survive or thrive in such conditions. T. testudinum abundances recorded at the vegetated site were greatest during December 2014. The median flux into the sediment traps ranged from 310 g DW m-2 d-1 to 7202 g DW m-2 d-1 at the barren site while the median flux into the sediment traps at the vegetated site ranged from 164 g DW m-2 d-1 to 799 g DW m-2 d-1. The median flux into the sediment plates at the barren site ranged from 41 g DW m-2 d-1 to 224g DW m-2 d-1 while the median flux into the plates at the vegetated site was similar in three out of the five deployments. When comparing the median flux into the sediment traps and plates at each site, more sediment was collected in the traps than the plates. Flume experiments on vegetated and unvegetated bottom exhibited a fully developed bottom boundary layer beginning at a flume velocity of 10 cm s-1. The flow over the vegetated bottom generally exhibited higher shear stress, and a thinner boundary layer that was elevated above the seagrass bed compared to the unvegetated bottom. The vegetated bottom effectively restricted the height of the water columns thus increasing the velocities above the seagrass compared to the same height in the unvegetated runs. The mean hydraulic roughness was greater on vegetated bottom than unvegetated bottom. The drag coefficients were calculated on both vegetated and unvegetated bottom and ranged from 0.02 - 0.18. The drag coefficients and hydraulic roughness calculated in this study can be used to improve numerical hydrodynamic model predictions. The seagrass effectively reduced water flow inside the bed when compared to bare sediment.
