Lake Mai Ndombe emits multiple greenhouse gases year-round

What the study found: Lake Mai Ndombe was weakly to non-stratified and highly supersaturated with respect to atmospheric equilibrium for all measured greenhouse gases across seasons, stations, and water depths. The study also found that concentrations increased with depth.
Why the authors say this matters: The authors conclude that carbon inputs, total water depth, and/or dissolved oxygen saturation are major drivers of the flux and composition of greenhouse gases emitted from Lake Mai Ndombe and potentially other tropical humic lakes. They also note that mechanistic insight into greenhouse gas cycling in humic tropical lakes and wetlands remains limited, especially in central Africa.
What the researchers tested: The researchers measured seasonal concentrations and isotopic compositions of the major dissolved greenhouse gases in Africa's largest humic lake, Lake Mai Ndombe in the Democratic Republic of Congo. They sampled during high-, falling-, and low-water seasons and combined field measurements such as temperature and wind speed with extrapolation to estimate emissions.
What worked and what didn't: The lake emitted estimated amounts of carbon dioxide, methane, and nitrous oxide, with reported fluxes of 375 ± 32 Gg C yr−1 as carbon dioxide, 623 ± 136 Mg C yr−1 as methane, and 223 ± 20 Mg N yr−1 as nitrous oxide. Carbon-isotope signals suggested that carbon dioxide was largely sourced from respiration of bioavailable organic carbon, while methane reflected sedimentary methanogenesis followed by aerobic methanotrophy in the water column; nitrogen-isotope data indicated a nitrogen cycle dominated by sedimentary denitrification with near-quantitative reduction to nitrous oxide before outgassing.
What to keep in mind: The abstract does not describe specific limitations beyond noting that mechanistic insight in these systems is limited and that the findings are based on Lake Mai Ndombe. The emission estimates are derived from extrapolation and Monte Carlo uncertainty was reported.

Key points

• Lake Mai Ndombe was weakly to non-stratified and highly supersaturated with all measured greenhouse gases.
• Greenhouse gas concentrations increased steadily with water depth.
• The lake was estimated to emit carbon dioxide, methane, and nitrous oxide in substantial amounts.
• Isotope data suggested carbon dioxide came largely from respiration of organic carbon.
• Methane and nitrous oxide signals pointed to sedimentary processes, including methanogenesis and denitrification.