Nitrate perturbation drives carbon cycling in a tropical mangrove estuary

Douala, Cameroon, West Africa Tropical mangrove estuaries in sub-Saharan Africa receive far greater amounts of organic and inorganic carbon and pollutants compared to subtropical and temperate estuaries, yet they have not been investigated for their role in local, regional, or global cycling. The lack of data limits our understanding of the contributions of tropical mangrove estuaries in sub-Saharan Africa to global biogeochemical cycling. We document how anthropogenic pollution by nitrate drives carbon cycling in the open water of the tropical mangrove Wouri Estuary. We observed anomalously higher nitrate concentrations of 136–345 µmol/kg over a 15 km range from the estuary mouth. Whole estuary salinity-δ 18 O mixing is modeled as a two-endmember seawater-freshwater mixing. Although alkalinity and DIC were modeled as a two-endmember seawater-freshwater mixing, a three-component salinity-DIC-δ 13 C DIC conservative model revealed a mismatch in stations in the lower estuary because of isotopically lower-than-expected δ 13 C DIC . The isotopically lower than predicted δ 13 C DIC is from nitrate-driven eutrophication and subsequent production of isotopically light CO 2 from organic matter oxidation. Modeled pCO 2 more than doubled from 20 times atmospheric in the upper estuary to 50 times atmospheric, increasing the DIC exchanged with the atmosphere. We conclude that anthropogenic nitrate pollution from shipping activities drives carbon cycling near the mouth of this tropical mangrove estuary. • We measure high nitrate concentrations of 135–345 µmol/kg over a 15 km range from an estuary mouth. • Two-endmember mixing define salinity-δ 18 O, salinity-alkalinity and salinity-DIC relationships. • Hydrologic mixing controls the distribution of geogenic silica but not anthropogenic nitrate. • Estuarine nitrate pollution is in situ and not from a seawater or freshwater source. • Nitrate fueled organic carbon respiration cause a more than doubling of pCO 2 from 20 µatm to 50 µatm.