Investigating Anthropogenic Perturbations on Carbon Cycling in AN Urbanized Tropical West African Estuary

We conducted an axial survey of salinity, stable isotopes of oxygen (δ18O) of water, nitrates, pH, alkalinity, dissolved inorganic carbon (DIC) and stable isotopes of carbon (δ13C) of DIC in the Douala Estuary, Cameroon, West Africa. Our objective was to assess anthropogenic impact of pollution on carbon cycling in this urbanized tropical estuary. Salinity ranged from 0.03 to 23.95 and increased steeply from the head to 15 km, stayed nearly constant to 24 km, and then increased steadily to the mouth. The δ18O ranged from -0.4‰ to -5.0‰ and increased steadily from the head to13 km, then slowly to 24 km before increasing sharply to the mouth. The similar behaviour between salinity and δ18O was caused by dilution from the Mungo R. to the west and the Dibamba R. to the east. The nitrate concentrations ranged from 0 to 34 mg/L and behaved similarly to salinity, which was low at the head (0 - 4 km), increased steeply to 15 km and stayed nearly constant to 24 km before increasing sharply to mouth. The spatial distribution of nitrates indicate pollution from agricultural input through the Mungo R., sewage discharge along the urbanized Wouri R. and Dibamba R. and from industries and the port facility along the estuary. The alkalinity concentrations ranged from 12 to 60 mg/Kg and DIC concentrations ranged from 2.9 to 15.5 mg C/Kg, are both positively correlated (R2 = 0.94) and both increased from the estuary head towards the mouth. The salinity concentrations show a good positive correlation (R2 = 0.98) with DIC concentrations. The partial pressure of CO2 (pCO2) which was decreasing from the estuary head towards the mouth reverses at 10 km and increases down estuary towards the mouth. The pCO2 behaviour indicates that the axial increase in the DIC concentrations is from the addition of CO2, although mixing of ocean water with higher DIC concentrations can also explain this observation. The generation of CO2 in the water column is reflected in the pH which increased steadily from 6.3 to 7.7 from the estuary head to about 9 km, then decreased to 7.6 at 24 km and finally to 6.5 at the estuary mouth. The CO2 production is consistent with decreases in the δ13C of DIC. We conclude that nutrient pollution caused increases CO2 production and the pCO2 which is higher than atmospheric makes the Douala Estuary a source of CO2 to the atmosphere.