Quantifying Seasonal Seagrass Effects on Flow and Sediment Dynamics in a Back-Barrier Bay

Seagrass growth and senescence exert a strong influence on flow structure and sediment transport processes in coastal environments. However, most previous studies of seasonal seagrass effects either focused on small-scale field measurements or did not fully resolve the synergistic effects of flow-wave-vegetation-sediment interaction at a meadow scale. In this study, we applied a coupled Delft3D-FLOW and SWAN model that included effects of seagrass on flow, waves, and sediment resuspension in a shallow coastal bay to quantify seasonal seagrass impacts on bay dynamics. The model was extensively validated using seasonal field hydrodynamic and suspended sediment data within a seagrass meadow and a nearby unvegetated site. Our results show that seagrass meadows significantly attenuated flow (60%) and waves (20%) and reduced suspended sediment concentration (85%) during summer when its density reached a maximum. Probability density distributions of combined wave-current bed shear stress within the seagrass meadow indicate that significant reductions in sediment resuspension during summer were mainly caused by flow retardation rather than wave attenuation. Although low-density seagrass in winter resulted in much smaller reductions in flow and waves compared with summer meadows, small changes in winter seagrass density resulted in large differences in the magnitude of attenuation of flow and shear stress. Similarly, while high seagrass densities effectively trapped sediment during summer, small changes in winter density resulted in strong changes in net sediment flux into/out of the meadow. At our study site, low seagrass densities provided significant reductions in wintertime sediment loss compared to losses associated with completely unvegetated conditions. Plain Language Summary Seagrasses are valuable ecosystems that inhabit shallow coastal waters. In summertime, their dense canopies can significantly slow tidal currents and lower wave energy, thereby reducing sediment resuspension and improving light environments for seagrass growth. This strong seagrass control on bay dynamics diminishes during winter, however, when seagrass density is low. In order to better understand seasonal seagrass impacts on shallow coastal environments, we ran a coastal model that includes effects of seagrass on flow, waves, and sediment resuspension under both summer and winter conditions in a shallow coastal bay. We found that dense seagrass during summer can considerably lower the energy levels of the bay and effectively trap fine particles. The strong reductions in sediment resuspension are mainly due to flow reduction caused by seagrass rather than wave attenuation. During winter, although low densities of seagrass had relatively limited effects on flow and waves, vegetation was still very important to protect the seabed from erosion. Our model predicts a small change of seagrass density in winter could result in strong changes in sediment input/output of the meadows. This effect had not been well characterized before, and it is important to understand because it has a significant impact on seagrass ecosystems.

Automated summary

Seagrass growth and senescence have significant impacts on flow structure and sediment transport processes in coastal environments. Dense seagrass meadows during summer can effectively attenuate flow, waves, and reduce suspended sediment concentration, while low-density seagrass in winter still plays a crucial role in protecting the seabed from erosion. Changes in seagrass density can lead to significant alterations in sediment flux within the meadows, underscoring the importance of understanding seasonal seagrass impacts on coastal ecosystems.