Biogenic gases and the oxidation and reduction of carbon in Amazon River and floodplain waters

Concentrations of CO 2 , O 2 , CH 4 , and N 2 O in the Amazon River system reflect an oxidation‐reduction sequence in combination with physical mixing between the floodplain and the mainstem. Concentrations of CO 2 ranged from 150 µ M in the Amazon mainstem to 200–300 µ M in aerobic environments and up to 1,000 µ M in oxygen‐depleted environments of the floodplain. Apparent oxygen utilization (AOU) ranged from 80 to 250 µ M. Methane was highly supersaturated with respect to atmospheric equilibrium. Concentrations ranged from 0.06 µ M in the mainstem to 100 µ M on the floodplain. Concentrations of N 2 O were slightly supersaturated in the mainstem (∼13 nM) but were undersaturated on the floodplain (averaging 9 nM). Fluxes calculated from these concentrations indicated decomposition of 1,600 g C m −2 yr −1 of organic carbon in Amazon floodplain waters. Analysis of relationships between CH 4 , O 2 , and CO 2 concentrations indicated that about 50% of carbon mineralization on the floodplain is anaerobic, with 20% lost to the atmosphere as CH 4 . The predominance of anaerobic metabolism leads to consumption of N 2 O on the floodplain. Elevated concentrations of CH 4 in the mainstem probably reflect input from the floodplain, while high levels of CO 2 in the mainstem are derived from a combination of floodplain drainage and in situ respiration.