Sediment Connectivity in the Coupled Tidal Flat-Seagrass-Marsh System

Seagrass and salt marsh are important sediment and carbon sinks in the global marine carbon cycle, yet are also among the most rapidly declining marine habitats. Their ability to sequester sediment and carbon depends on flow–sediment–vegetation interactions that promote sediment trapping and deposition, as well as high rates of primary production. Understanding sediment transport and the associated drivers within these ecosystems provides insight into how sediment and carbon accumulation in these systems responds to disturbance events and climate change. However, most previous studies of sediment transport within seagrass and saltmarsh ecosystems either have been limited in small spatial scale or mainly focused on processes relevant to one specific time scale. When submerged seagrass meadows occupy shallow tidal flats, very little is known about their effects on modulating sediment connectivity between the tidal flats and fringing intertidal marshes and the response of the coupled system to short-term disturbance events and longer-term drivers.
In this dissertation, I applied a process-based and spatially resolved hydrodynamic and sediment transport model Delft3D, in the shallow coastal bays within the Virginia Coast Reserve (VCR) on Virginia’s Atlantic coast, to quantify the sediment dynamics in the coupled tidal flat–seagrass–marsh system. The overarching research questions of this dissertation are: (1) what are the mechanisms that control sediment transport in the coupled tidal flat–seagrass–marsh system, and (2) how sediment accumulation rates and fluxes in this system respond to short-term events as well as seasonal wind patterns and seagrass growth cycle. I addressed the above questions in four research chapters. First, I applied the Delft3D model that couples flow–wave–vegetation–sediment interactions in South Bay, a successful seagrass restoration site with submerged seagrass meadows dominating the subtidal flats, to quantify seasonal seagrass impacts on bay dynamics during summer and winter conditions. Second, I extended the model simulation period to a complete annual cycle to examine the effects of seasonal and episodic variations in seagrass density on rates of sediment accumulation and carbon burial in the seagrass meadows. Third, I adapted the coupled Delft3D model to the unvegetated tidal flat–marsh system in Hog Bay, and investigated the impacts of episodic storm surge events on the coupled tidal flat–marsh system and the overall contribution of storm surge on marsh sediment deposition. Finally, I focused on analyzing annual simulation results in South Bay and examined the combined effects of seasonal wind patterns and seagrass density variations on sediment delivery and wave energy flux to an adjacent back-barrier marsh bordering the meadows in the bay.
My results show that the presence of submerged seagrass meadows on shallow tidal flats plays an important role in controlling sediment resuspension on the flats as well as sediment delivery to adjacent salt marshes. Sediment accumulation rates within seagrass meadows changed non-linearly between seasons as a function of seagrass density. While seagrass meadows effectively reduced sediment resuspension and trapped sediment at meadow edges during spring-summer growth seasons, during winter senescence low-density meadows (< 160 shoots m⁻²) were erosional with rates sensitive to density. Due to this nonlinear control of seagrass density on sediment accumulation, there was strong variability of sediment accumulation rates in the meadow in response to winter density variations and marine heatwave events. In addition, seagrass meadows also significantly altered the timing of sediment transport to the adjacent marsh platform (winter peak, density control) and reduced total annual sediment flux by 12% compared to the simulation with no seagrass (flux controlled by winds).
I also found that episodic storm surge events play an important role in transporting suspended sediment from unvegetated tidal flats to intertidal salt marshes. Although storm surge events only occurred 5% of the time at the study site, they disproportionately contributed 40% of marsh deposition during 2009–2020. While wind-driven waves control sediment resuspension on tidal flats, marsh deposition during storms was largely determined by tidal inundation associated with storm driven water levels and increased linearly with storm surge intensity, suggesting that marshes at the study site will likely be supplied with more sediment, primarily from eroding tidal flats, if storm magnitudes and/or frequencies increase in the future.
Overall, these findings highlight the strong control vegetation has in erosional and depositional processes in shallow coastal bays and the implications for the resilience of seagrass and marsh sediment accumulation under future climate change. The results of this dissertation also provide useful information for coastal managers to inform conservation and management strategies in coastal wetlands and practical guidelines for process-based modeling of flow–wave–vegetation–sediment interactions in shallow coastal environments.