Sediment enhancement and hydrological restoration impacts on salt marsh vegetation and carbon storage in coastal New England

Salt marsh restoration has become increasingly important in New England due to the impacts of sea level rise. Sea level rise within New England is accelerating at a rate faster than the global average. Therefore, these areas are particularly susceptible to salt marsh loss. Sediment enhancement and hydrological restoration (via dredged runnels) are two types of restoration techniques that have been implemented in Rhode Island to combat the effects of sea level rise. In this study, we investigated the impacts of these restoration treatments on belowground biomass, soil % organic matter, and vegetation composition. We also measured the greenhouse gas emissions (methane and carbon dioxide) to evaluate the impact of the restoration treatments on carbon cycling and storage. Runnel areas showed an increase in plant density over time as well as higher Spartina patens (high marsh plant) coverage than the Spartina alterniflora (low marsh plant) dominated control areas. The higher Spartina patens coverage in the runnel treatment suggests that additional drainage provided by the runnels is leading to the persistence of high marsh plants. After one year the runnel areas had an increase in the rate of carbon dioxide uptake, which is likely due to the increase in plant density post-restoration treatment. The sediment enhancement treatment resulted in low belowground biomass, soil % organic matter, and plant recolonization. This suggests that the added sediment material inhibited vegetation growth, and therefore belowground biomass and soil % organic matter. We have learned that larger grain size sediment (higher % sand) is important for successful sediment enhancement projects. Our results suggest that dredged runnels can be a potential method to enhance salt marsh resiliency, carbon storage, and mitigate sea level rise impacts. For future sediment enhancements projects, we have learned that large sediment grain size is essential for project success.