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Abstract Connectivity between adjacent ecosystems is thought to increase ecosystem resilience and function. In coastal ecosystems, the exchange of sediment and nutrients between mudflats and marshes is important for the long‐term dynamics of both systems. Mudflat morphodynamics are driven by the interaction of waves and sediment erodibility, which is a function of sediment type and the presence of biostabilizers such as microphytobenthos. However, there is a poor understanding about how the evolution of mudflats may impact the morphodynamics and function of adjacent salt marshes. Here, we use a Coastal Landscape Transect model connecting mudflats and marshes to investigate how microphytobenthos influence the coupled behavior of mudflats and marshes, and how that coupled behavior influences carbon storage. We find that biofilms reduce the connectivity between mudflats and marshes by reducing erodibility and sediment exchange. Reduced connectivity associated with microphytobenthos leads to a shallower mudflat and more carbon stored in the mudflat sediments, which in turn cascades to a higher combined marsh and mudflat carbon stock. Furthermore, our results highlight the role of connectivity across the coastal landscape and suggest that biostabilization leads to relatively small changes in morphodynamics but relatively large changes in ecosystem function. 

Abstract Sea level rise is leading to the rapid migration of marshes into coastal forests and other terrestrial ecosystems. Although complex biophysical interactions likely govern these ecosystem transitions, projections of sea level driven land conversion commonly rely on a simplified “threshold elevation” that represents the elevation of the marsh‐upland boundary based on tidal datums alone. To determine the influence of biophysical drivers on threshold elevations, and their implication for land conversion, we examined almost 100,000 high‐resolution marsh‐forest boundary elevation points, determined independently from tidal datums, alongside hydrologic, ecologic, and geomorphic data in the Chesapeake Bay, the largest estuary in the U.S. located along the mid‐Atlantic coast. We find five‐fold variations in threshold elevation across the entire estuary, driven not only by tidal range, but also salinity and slope. However, more than half of the variability is unexplained by these variables, which we attribute largely to uncaptured local factors including groundwater discharge, microtopography, and anthropogenic impacts. In the Chesapeake Bay, observed threshold elevations deviate from predicted elevations used to determine sea level driven land conversion by as much as the amount of projected regional sea level rise by 2050. These results suggest that local drivers strongly mediate coastal ecosystem transitions, and that predictions based on elevation and tidal datums alone may misrepresent future land conversion. 

Abstract 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.