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 can also alter the sediment dynamics and waves within back‐barrier bays. Here we incorporate seagrass dynamics into an existing barrier‐marsh exploratory model, GEOMBEST++, to examine the coupled interactions of the back‐barrier bay with both adjacent (marsh) and nonadjacent (barrier island) subsystems. While seagrass reduces marsh edge erosion rates and increases progradation rates in many of our 288 model simulations, seagrass surprisingly increases marsh edge erosion rates when sediment export from the back‐barrier basin is negligible because the ability of seagrass to reduce the volume of marsh sediment eroded matters little for back‐barrier basins in which all sediment is conserved. Our model simulations also suggest that 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. Plain Language Summary Seagrass often grows in coastal bays sheltered behind barrier islands and salt marshes. While seagrass provides essential habitat for marine organisms, it also makes waves in the bay smaller and helps hold sediment in place. We use a barrier‐marsh‐bay computer model (GEOMBEST++Seagrass) to investigate how seagrass impacts the evolution of neighboring marsh and barrier island landforms. In our model simulations, we find that the presence of seagrass in the bay generally reduces the loss of marsh but under certain conditions may actually increase marsh loss. Additionally, we find that when seagrass is added to the bay, the marsh responds temporarily by eroding more rapidly because sediment that would otherwise be added to the marsh is instead held within the bay by seagrass. When seagrass is removed, in contrast, sediment that was once held within the bay