Changes in Sediment Organic Carbon Accumulation under Conditions of Historical Sea-Level Rise, Southeast Saline Everglades, Florida, USA

Mangroves can store more sediment organic carbon (SOC) than freshwater and salt marshes. Understanding how mangroves have responded to historical sea-level rise (SLR) is fundamental to assessing their resilience and capacity to store carbon as SLR accelerates. We quantified landscape-level temporal and spatial trends in historical coastal wetland sediment accumulation and associated SOC content (i.e., storage) along coastal-to-inland gradients in Southeast Florida. The observed trends were transgressive and attributed to the historical rise in sea level. Our results indicate an overall significant increase in the SOC content of the historic wetland sediment succession caused by the vertical accumulation and landward migration of carbon-rich mangrove-dominated plant communities (mean = 0.08 g cm−3) into and over carbon-poor wet prairie plant communities (mean = 0.02 g cm−3). The observed historical increase in SOC is predicted to diminish over time as the difference between rates of SLR and vertical sediment accumulation increases and because the landward migration of mangrove-dominated plant communities is now obstructed by a shore-parallel flood-control levee. These results are likely to unfold in other low-latitude coastal wetlands where they are sandwiched between rising seas and an urbanized landscape.