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Storm surge has the potential to significantly increase suspended sediment flux to microtidal marshes. However, the overall effects of storm surge on microtidal marsh deposition have not been well quantified, with most modeling studies focusing on regular (astronomical) tidal flooding. Here we applied the Delft3D model to a microtidal bay‐marsh complex in Hog Bay, Virginia to quantify the contributions of storm surge to marsh deposition. We validated the model using spatially distributed hydrodynamic and suspended sediment data collected from the site and ran model simulations under different storm surge conditions with/without storm‐driven water level changes. Our results show that episodic storm surge events occurred 5% of the time at our study site, but contributed 40% of marsh deposition during 2009–2020. Our simulations illustrate that 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. A moderate storm surge event can double sediment flux to most marshes around the bay and deliver more sediment to the marsh interior compared to simulations that include wind waves but not storm surge variations in water levels. Simulations of bay and marsh response to different storm surge events with varying magnitude of storm surge intensity reveal that total marsh deposition around the bay increased linearly with storm surge intensity, suggesting that future changes to storm magnitude and/or frequency would have significant implications for sediment supply to marshes at our study site.

Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively 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. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change.

Seagrass growth and senescence exert a strong influence on flow structure and sediment transport processes in coastal environments. However, most previous studies of seasonal seagrass effects either focused on small‐scale field measurements or did not fully resolve the synergistic effects of flow‐wave‐vegetation‐sediment interaction at a meadow scale. In this study, we applied a coupled Delft3D‐FLOW and SWAN model that included effects of seagrass on flow, waves, and sediment resuspension in a shallow coastal bay to quantify seasonal seagrass impacts on bay dynamics. The model was extensively validated using seasonal field hydrodynamic and suspended sediment data within a seagrass meadow and a nearby unvegetated site. Our results show that seagrass meadows significantly attenuated flow (60%) and waves (20%) and reduced suspended sediment concentration (85%) during summer when its density reached a maximum. Probability density distributions of combined wave‐current bed shear stress within the seagrass meadow indicate that significant reductions in sediment resuspension during summer were mainly caused by flow retardation rather than wave attenuation. Although low‐density seagrass in winter resulted in much smaller reductions in flow and waves compared with summer meadows, small changes in winter seagrass density resulted in large differences in the magnitude of attenuation of flow and shear stress. Similarly, while high seagrass densities effectively trapped sediment during summer, small changes in winter density resulted in strong changes in net sediment flux into/out of the meadow. At our study site, low seagrass densities provided significant reductions in wintertime sediment loss compared to losses associated with completely unvegetated conditions.