Shifts in marsh erosion, migration, and wave exposure over nearly two centuries of sea-level rise in the Gulf of Mexico

Abstract

Coastal wetlands are economically important ecosystems, but are at risk to erosion from waves, storms, and sea level rise. However, marshes can persist under rising sea level through vertical accretion and migration into adjacent higher-elevation habitats. We measured rates of marsh shoreline change and migration and compared the results for historical and modern periods in a largely undeveloped and marine-dominated estuary within the Mississippi-Alabama coast of the Northern Gulf of Mexico. Mean shoreline change rate for the modern (post-1957) period was higher than historical rates (pre-1957) at −1.55 ± 0.11 m yr⁻¹ and -0.84 ± 0.07 m yr⁻¹, respectively. Shoreline change rates were highest in regions where exposure to wind-driven waves increased due to barrier island migration and land loss. Marsh migration (modern 1.25 ± 0.37 m yr⁻¹ and historical 1.01 ± 0.13 m yr⁻¹) occurred at similar rates as shoreline erosion except for the highest erosive shorelines, leading to an overall 15% marsh loss. Upland-to-marsh conversion occurred in forested areas and may have been encouraged by changes in management practices. In the Grand Bay estuary, our data show that marsh migration is not occurring at sufficient rates to compensate for marsh loss, resulting in overall loss in marsh habitat. Overall, migration may allow marsh to persist under rising sea levels but can lead to an overall reduction in forested or freshwater habitats if steep slopes or other barriers prevent their migration.