Impacts of Seagrass Dynamics on the Coupled Long‐Term Evolution of Barrier‐Marsh‐Bay Systems
Seagrass provides a wide range of economically and ecologically valuable ecosystem services, with shoreline erosion control often listed as a key service. But seagrass can also alter the sediment dynamics and waves of back-barrier bays by reducing wave height and attenuating wave and current shear stresses acting on the sediment bed. This suggests that seagrass can play an important role in the evolution of the entire shallow coastal bay, back-barrier marsh, and barrier-island system, yet no study has previously examined these subsystems coupled together. Here we incorporate seagrass dynamics of the back-barrier bay into the existing coupled barrier-marsh model GEOMBEST+. In our new integrated model, bay depth and distance from the marsh edge determine the location of suitable seagrass habitat, and the presence or absence, size, and shoot density of seagrass meadows alters the bathymetry of the bay and wave power reaching the marsh edge. We use this model to run 3 sets of experiments to examine the coupled interactions of the back-barrier bay with both adjacent (marsh) and non-adjacent (barrier) subsystems. While seagrass reduces marsh edge erosion rates and increases progradation rates in many of our model simulations, seagrass surprisingly increases marsh edge erosion rates when sediment export from the back-barrier basin is negligible. Adding seagrass to the bay subsystem leads to increased deposition in the bay, reduced sediment available to the marsh, and enhanced marsh edge erosion until the bay reaches a new, shallower equilibrium depth. In contrast, removing seagrass liberates previously-sequestered sediment that is then delivered to the marsh, leading to enhanced marsh progradation. Lastly, we find that seagrass reduces barrier island migration rates in the absence of back-barrier marsh by filling accommodation space in the bay. These model observations suggest that seagrass meadows operate as dynamic sources and sinks of sediment that can influence the evolution of coupled marsh and barrier island landforms in unanticipated ways.