IMPACT OF NITRATE POLLUTION ON CARBON CYCLING IN THE OPEN WATER OF A TROPICAL MANGROVE ESTUARY

We investigated water temperature, dissolved oxygen (DO), clarity, salinity, stable oxygen isotopes (δ¹⁸O), pH, alkalinity, dissolved inorganic carbon (DIC), the stable carbon isotope of DIC (δ¹³C_DIC), silica, and nitrate in the open water of a tropical mangrove Wouri Estuary, Douala, Cameroon, and rivers feeding the estuary. We aimed to document how anthropogenic nitrate pollution 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 were consistent with a two-end-member seawater-freshwater mixing. However, a salinity-DIC-δ¹³C_DIC conservative mixing model revealed isotopically lower-than-expected δ¹³C_DIC in the lower estuary. We attribute the isotopically lower than predicted δ¹³C_DIC to nitrate-driven eutrophication and subsequent production of isotopically light CO₂ from organic matter oxidation. Assessment of two end members mixing of geogenic silica model show that nitrate perturbation driving the cycling of carbon in the lower estuary is not related to normal seawater and freshwater mixing. Our findings implicate anthropogenic nitrate pollution likely from shipping activities as a major driver of carbon cycling in this tropical mangrove estuary.