Mangroves as source of 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. 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. Time series and spatial surveys were conducted to asses total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Dominant metabolic processes driving dissolved carbon and greenhouse gas dynamics varied along the estuary salinity gradient. Dissolved carbon and greenhouse gas concentrations were strongly coupled to porewater input, which was examined using Rn-222. Shark River was a source of CO₂ (92 mmol/m^­2/d), CH₄ (60 µmol/m^­2/d) and N₂O (2 µmol/m­²/d) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m­²/d, DOC = 39 mmol/m­²/d) was several-fold higher than burial (~28 mmol/m²/d) and represents an additional carbon sink. Furthermore, the estuary was a source of TAlk (97 mmol/m­²/d) to the coastal ocean, potentially buffering coastal acidification. Despite accounting for only a small share of TAlk, OAlk had a large effect on the estuarine pH. 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.