Significance of pelagic aerobic methane oxidation in the methane and carbon budget of a tropical reservoir

The Petit Saut Reservoir (French Guiana), flooded in 1994, has an anoxic and methane‐rich (>400 μmol.L−1) hypolimnion and an oxycline located at around 5 meters depth. Waters passing through the turbines are enriched in CH4, which partly degasses downstream of the dam. Aerobic methane oxidation rates were measured in the waters of the reservoir epilimnion and, in river waters downstream of the dam, during the wet and dry seasons. A unique first order relationship between methane concentration (CCH4) and aerobic methane oxidation rates (CH4‐ox) was found for the reservoir and the river downstream of the dam (CH4‐ox = 0.11 ± 0.01 CCH4 + 1.18 ± 0.74) giving a specific CH4 oxidation rate of 0.11 ± 0.01 h−1. In the reservoir, depth‐integrated methane oxidation rates were calculated from oxidation rates and the distributions of CH4 and oxygen and ranged between 11 ± 6 and 310 ± 132 mmol.m−2.d−1, consistent with the turbulent vertical flux from the hypolimnion (38–321 mmol.m−2.d−1) and almost two orders of magnitude higher than the atmospheric fluxes (0.3 ± 0.2–1.9 ± 1.2 mmol.m−2.d−1). The average integrated methane oxidation rate in the epilimnion given by these two independent methods is 100 ± 80 mmol.m−2.d−1. Downstream of the dam, about 40% of the methane entering the river was oxidized (77 ± 40 mmol.m−2.d−1) and 50% was emitted to the atmosphere (98 ± 28 mmol.m−2.d−1). Our results show that total pelagic oxidation (67 ± 64 Gg C‐CH4.y−1) reduce atmospheric emissions by more than 85% for the whole lake‐river system and that this process is responsible for more than 25% of total CO2 emissions from the whole reservoir‐river system.