Mangroves as a Source of Alkalinity and Dissolved Carbon to the Coastal
Ocean: A Case Study from the Everglades National Park, Florida
Abstract
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 assess 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 estuarine salinity gradient.
Dissolved carbon and greenhouse gas concentrations were strongly coupled
to porewater input, which was examined using radon-222. Shark River was
a source of CO2 (92 mmol/m2/d), CH4 (60 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) 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, normalised to mangrove area)
to the coastal ocean, potentially buffering coastal acidification.
Organic alkalinity was also exported to the coastal ocean (1.9
mmol/m2/d, normalised to mangrove area). 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.