The effects of waves and turbulence on sediment suspension and mixing within seagrass ecosystems

Seagrass meadows create significant benthic structure, which attenuates wave energy and near-bottom currents, altering mixing across the seagrass canopy and reducing bottom shear stresses. This enhances the deposition of suspended sediment and increases light availability to the benthos. To quantify how meadow structure influences the physical environment, spatial and temporal density gradients within Zostera marina and Thalassia testudinum meadows in South Bay, Virginia, and Florida Bay, Florida, were monitored. Results show that seagrass beds create a hydrodynamic shear layer with an inflection point of instability at the top of the canopy, which imparts substantial turbulent mixing. Seagrass beds were found to reduce near-bottom mean velocities by 25 to 85% depending on both canopy development and location within the meadow, while wave heights were reduced 10 to 70% compared to adjacent unvegetated regions. Attenuation of both velocities and waves increased with increasing seagrass canopy cover. Wave orbital velocities within the seagrass canopy were reduced by 20% compared to flow above the canopy, primarily acting as a low-pass filter by removing high-frequency wave motion. During maximum canopy development in the summer, bed shear stresses were above the critical threshold for initiating sediment suspension only 20 to 55% of the sampling time, compared to 80 to 85% in the winter and spring when seagrass canopy cover was lower. This compares to bed shear stresses that were greater than the critical threshold at the bare site > 90% of the sampling time. Overall, seagrass was found to stabilize sediment during most of the year (late spring to fall), creating a positive feedback for growth.;The Thalassia meadow in Florida had lower energy compared to the Z. marina meadow in Virginia, and oscillatory flows due to waves led to enhanced turbulence and mixing within the meadow as compared to unidirectional flows. In situ particle image velocimetry (PIV) was used to quantify turbulent momentum transport across the meadow. Turbulence was observed within the canopy at depths 2 to 3 times the penetration length-scale for shear layer vortices, suggesting stem-wake generated turbulence was formed within the canopy. This led to 3 times greater average Reynolds stress within the sparse canopy compared to ambient conditions above the canopy, while the dense meadow reduced Reynolds stress 16% across the canopy-water interface. Though turbulence penetration into the dense canopy was restricted, momentum transport was more efficient due to increased velocity shear at the top of the canopy, effectively increasing the canopy’s ability to exchange fluids (and likely nutrients and gases), with the overlying water column.