Coastal Blue Carbon Assessment of Mangroves, Salt Marshes, and Salt Barrens in Tampa Bay, Florida, USA

Compared to other terrestrial environments, coastal “blue carbon” habitats such as salt marshes and mangrove forests sequester disproportionately large amounts of carbon as standing plant biomass and peat deposits. This study quantified organic carbon stocks in 16 salt marshes, salt barrens, and mangrove forests in Tampa Bay, Florida, USA. The sites included natural, restored, and created wetlands of varying ages and degrees of anthropogenic impacts. Peat deposits were generally less than 30-cm deep and organic content rapidly decreased with depth in all habitats. The top 15 cm of mangrove soil contained an average of 11.0% organic carbon by weight, salt marshes contained 6.6%, and salt barrens contained 1.0%. Total organic carbon stock in mangroves was 133.6 ± 12.8 Mg ha−1, with 69.5% of that carbon stored belowground. Salt marshes contained 66.4 ± 25.0 Mg ha−1 (93.5% belowground carbon), and salt barrens contained 26.6 ± 8.3 Mg ha−1 (96.1% belowground carbon). These values were much lower than global averages for carbon stocks in mangroves and salt marshes, likely due to Tampa Bay’s location near the northern limit of mangrove habitat, sandy soil, young age of the restored wetlands, presence of mosquito ditches, and recent habitat conversion from salt marshes to mangroves. In the late 1800s, Tampa Bay’s coastal wetlands were dominated by salt marshes, but today they are dominated by mangroves. Based on the blue carbon values from the natural sites in this study, this habitat switching has led to the additional sequestration of 141,000 Mg of carbon in remaining wetlands in the Tampa Bay watershed.