UC Davis

AbstractI investigated water temperature, dissolved oxygen (DO), clarity, salinity, stable oxygen isotopes (δ18O), pH, alkalinity, dissolved inorganic carbon (DIC), the stable carbon isotope of DIC (δ13CDIC), silica, and nitrate in the open water of a tropical mangrove estuary and rivers feeding the estuary. I aimed to document how anthropogenic pollution by nutrients drives carbon cycling in the open water of the estuary. Salinity-δ18O mixing modeling confirmed a two-endmember seawater-freshwater hydrologic mixing in the estuary. The spatial distribution of alkalinity and DIC was modeled by a two-endmember seawater-freshwater mixing. However, a salinity-DIC-δ13CDIC conservative mixing model revealed a mismatch in stations in the lower estuary because of isotopically lower-than-expected δ13CDIC. I attribute the isotopically lower than predicted δ13CDIC to nitrate-driven eutrophication and subsequent production of isotopically light CO2 from organic matter oxidation. The anthropogenic nitrate perturbation that drives the cycling of carbon near the mouth of this tropical estuary is not sourced from seawater or freshwater but generated in situ. My findings implicate anthropogenic pollution from shipping activities in the carbon cycling in this tropical mangrove estuary.