Nitrate perturbation drives carbon cycling in a tropical mangrove estuary

Study region

Douala, Cameroon, West Africa

Study focus

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.

New hydrologic insights

We observed anomalously higher nitrate concentrations of 136–345 µmol/kg over a 15 km range from the estuary mouth. Whole estuary salinity-δ¹⁸O 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-δ¹³C_DIC conservative model revealed a mismatch in stations in the lower estuary because of isotopically lower-than-expected δ¹³C_DIC. The isotopically lower than predicted δ¹³C_DIC is from nitrate-driven eutrophication and subsequent production of isotopically light CO₂ from organic matter oxidation. Modeled pCO₂ 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.