Assessing long-term, time-averaged (im)balance of erosional and depositional fluxes on a marsh platform and implications for management strategies: Grand Bay Estuarine System (GBES), Mississippi and Alabama.

Longevity of coastal salt marshes is inevitably controlled by the imbalance of processes driving vertical elevation change (e.g., organic and inorganic sediment accumulation) and spatial extent (e.g., shoreline erosion, upland migration). Short-term (days to years) observations and measurements of vertical and lateral change may not capture natural spatial variability. Decadal-to-centennial assessments of lateral and vertical change (i.e. sediment budgets) can aid in reducing spatial uncertainty. In this study, we present a long-term, time-averaged sediment budget developed for the GBES (Alabama/Mississippi) that aids in identifying the consequences of prolonged imbalances in vertical and lateral sediment fluxes and discuss factors contributing to these imbalances. Our data complement and support efforts by Grand Bay National Estuarine Research Reserve scientists and National Wildlife Refuge managers to assess and quantify habitat loss and ecosystem services. Geologic and geospatial data for GBES are used to evaluate and assess change in the marsh platform sediment budget. Depositional mass flux at the centennial time-scale was more than twice erosional mass flux along the marsh shoreline, suggesting a long-term net sediment surplus delivered to the marsh platform. Additionally, the sediment budgetary analysis was evaluated for two time-periods: 1917-1957 and 1992-2012. The mass flux ratio for deposition (jD) and erosion (jE) was approximately 4:1 for 1917-1957 but was unity (break-even) for 1992-2012. The difference in jD:jE is driven by the disproportionate increase in erosion flux (nine-fold) relative to depositional flux (four-fold). Change in geomorphology (loss of Grand Batture Island(s)) and sea-level likely explain the decreased mass flux ratio. Management implications of these data show the importance of evaluating sediment budgetary imbalances and interactions to ensure restoration actions to abate one component (e.g., erosion control) do not ignore or negatively influence another component (sediment supply).