Biogeomorphological Response of a Tidal Marsh to Salinity-Induced Vegetation Shifts

As salinity distributions in estuaries shift, often moving landward due to climate change or anthropogenic interventions, the environmental niches of marsh species shift accordingly. Present-day fresh and brackish marshes may experience increasing salinity stress, leading to changes in plant-community composition and ultimately shifts in dominant vegetation species. Building on prior work showing that vegetated intertidal areas modify estuarine hydrodynamics (Bootsma et al., 2025) and that managed realignment can alter salinity gradients, this study investigates how salinity-induced species shifts affect marsh morphology through biogeomorphological feedbacks. While salinity and inundation are recognized drivers of marsh zonation, the geomorphic consequences of salinity-driven species shifts remain understudied, particularly in interaction with changing inundation regimes. To address this gap, we use a 2D depth-averaged Delft3D-FM model coupled with a dynamic vegetation module, simulating a brackish tidal marsh at the marsh-mudflat scale (~1-10 km2) over a decadal timespan (~50 years). The model includes multiple species with distinct growth traits and salinity tolerances. We explore three environmental change scenarios: (1) a gradual increase in salinity, (2) a gradual increase in sea level (inundation frequency), and (3) both combined. This setup allows us to isolate and compare the geomorphic effects of salinity and inundation on vegetation distribution and marsh morphology. Because vegetation traits such as stem diameter, growth form and sediment trapping capacity vary across species, species shifts will modify marsh morphology such as altering total marsh extent, accretion of sediment within the marsh, vegetation patchiness and creek formation. This study provides new insight into the eco-geomorphic resilience of tidal wetlands under environmental change and highlights how vegetation shifts along salinity gradients may influence long-term marsh morphology.