Mangroves as a Source of Greenhouse Gases to the Atmosphere and Alkalinity and Dissolved Carbon to the Coastal Ocean: A Case Study from the Everglades National Park, Florida

Most research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial in sediments. However, the few studies that have quantified lateral exchange of carbon and alkalinity indicate that the dissolved carbon and alkalinity export may be several‐fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon and greenhouse gas fluxes of the mangrove‐dominated Shark River estuary located in the Everglades National Park in Florida, USA. Spatial surveys and 29‐hour time series were conducted to assess total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) dynamics. Dissolved carbon and greenhouse gas concentrations were coupled to porewater input, which was examined using radon‐222. Shark River was a source of CO₂ (92 mmol/m²/d), CH₄ (56 μmol/m²/d) and N₂O (2 μmol/m²/d) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m²/d, DOC = 39 mmol/m²/d, normalised to mangrove area) was several‐fold higher than previously reported carbon burial rates in the study area (~28 mmol/m²/d). The majority of the DIC was exported as TAlk (97 mmol/m²/d), which remains dissolved in the ocean for millennia and, therefore, represents a long‐term sink for atmospheric carbon. By integrating our results with previous studies, we argue that alkalinity, dissolved carbon and greenhouse gas fluxes should be considered in future blue carbon budgets.