The effects of oyster reef restoration on flow and turbulence characteristics in a microtidal estuary

Abstract:

While the ecological importance of intertidal oyster reefs has made them a focus of restoration efforts in recent years, hydrodynamic studies regarding flows above restored reefs remain sparse, and little is known about temporal changes in mean flow and turbulence with restoration age. In this study, we investigate hydrodynamic differences between restored (restoration age: 6m, 2y, 4y), degraded, and intact intertidal oyster reefs in the shallow waters of a microtidal estuary (Mosquito Lagoon) along the Atlantic coast of Florida (USA). Field experiments conducted at each reef were designed to characterize variability in mean flow, wave attenuation, and turbulence associated with differences in reef morphology and oyster canopy characteristics. Novel high-resolution laser scans collected during each experiment highlighted variations in reef roughness and solid volume fraction of oyster canopy between and within individual reefs. Channel-to-reef wave and velocity attenuation were consistent across all experiments, indicating that these parameters were more strongly associated with large scale bathymetric contours (i.e. reef slope) than small scale roughness enhancement (i.e. live vs. dead oysters). Observations of sub-canopy (2cm above bottom) turbulence (e.g. tke, dissipation, Reynolds stress) were similar for both live and degraded reefs, suggesting that very near bed mixing was likely a function of bed interaction rather than active oyster pumping or canopy-induced turbulence generation. In contrast, above canopy (9cmab) turbulence was significantly enhanced on live reefs when compared to our degraded control, with normalized dissipation and Reynolds stress estimates more than an order of magnitude higher in the presence of living oysters. These results have significant implications for food delivery and sediment transport above intertidal oyster reefs, motivating additional analysis of particle concentrations and benthic fluxes above restored reefs. Finally, comparisons between the intact and restored reefs examined in this study are used to conclude that properly restored reefs likely reach hydrodynamic similarity with historically healthy reefs within 6 months of restoration, with perceived differences in ecosystem services limited to biological and chemical characteristics.