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

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Abstract
Concentrations of CO2, O2, CH4, and N2O in the Amazon River system reflect an oxidation‐reduction sequence in combination with physical mixing between the floodplain and the mainstem. Concentrations of CO2 ranged from 150 µM in the Amazon mainstem to 200–300 µM in aerobic environments and up to 1,000 µM in oxygen‐depleted environments of the floodplain. Apparent oxygen utilization (AOU) ranged from 80 to 250 µM. Methane was highly supersaturated with respect to atmospheric equilibrium. Concentrations ranged from 0.06 µM in the mainstem to 100 µM on the floodplain. Concentrations of N2O 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 CH4, O2, and CO2 concentrations indicated that about 50% of carbon mineralization on the floodplain is anaerobic, with 20% lost to the atmosphere as CH4. The predominance of anaerobic metabolism leads to consumption of N2O on the floodplain. Elevated concentrations of CH4 in the mainstem probably reflect input from the floodplain, while high levels of CO2 in the mainstem are derived from a combination of floodplain drainage and in situ respiration.