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Constraining Greenhouse Gas Cycling and Emissions in Africa's Largest Humic Lake.

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Bibliographic Details
Title:	Constraining Greenhouse Gas Cycling and Emissions in Africa's Largest Humic Lake.
Authors:	Barthel, M.¹ (AUTHOR), Drake, T. W.¹ (AUTHOR), de Clippele, A.¹ (AUTHOR), de Groot, L. W.¹ (AUTHOR), Engelhardt, M.² (AUTHOR), Haghipour, N.^2,3 (AUTHOR), Hou, Y.² (AUTHOR), Kueter, N.^2,4 (AUTHOR), Lewicka‐Szczebak, D.⁵ (AUTHOR), Ludjwera Bahati, A.⁶ (AUTHOR), Tschumbu, C. L.⁶ (AUTHOR), Van Oost, K.⁷ (AUTHOR), Wabakanghanzi, J. N.⁸ (AUTHOR), Werner, R. A.¹ (AUTHOR), Zambo Mandea, J.⁹ (AUTHOR), Six, J.¹ (AUTHOR), Hemingway, J. D.² (AUTHOR) jordon.hemingway@eaps.ethz.ch
Source:	Journal of Geophysical Research. Biogeosciences. Mar2026, Vol. 131 Issue 3, p1-20. 20p.
Subject Terms:	Greenhouse gases, Lakes, Nitrous oxide, Carbon content of water, Microbiology, Carbon dioxide, Methanotrophs
Geographic Terms:	Africa, Congo (Democratic Republic)
Abstract:	Humic tropical lakes and wetlands are globally important sources of atmospheric greenhouse gases (GHGs). However, mechanistic insight into GHG cycling in such systems remains limited—especially in understudied central Africa. To address this, here we measured high‐, falling‐, and low‐water seasonal concentrations and isotopic compositions of the major dissolved GHGs CO2 ${\text{CO}}_{2}$, CH4 ${\text{CH}}_{4}$, and N2O ${\mathrm{N}}_{2}\mathrm{O}$ in Africa's largest humic lake: Mai Ndombe, Democratic Republic of Congo. We find that the water column is weakly to non‐stratified and is highly supersaturated with respect to atmospheric equilibrium for all GHGs across all seasons, sampling stations, and water depths. Additionally, all GHG concentrations increase steadily with increasing water depth, reflecting atmospheric gas exchange due to physical mixing in the upper water column as well as biological processes. Extrapolating these results—combined with field measurements such as temperature and wind speed—we estimate that Lake Mai Ndombe emits 375 ± 32 Gg C yr−1 as CO2 ${\text{CO}}_{2}$, 623 ± 136 Mg C yr−1 as CH4 ${\text{CH}}_{4}$, and 223 ± $\pm $ 20 Mg N yr−1 as N2O ${\mathrm{N}}_{2}\mathrm{O}$ (μ±1σ $\mu \pm 1\sigma $ propagated Monte Carlo uncertainty). Furthermore, carbon‐isotope signals suggest that CO2 ${\text{CO}}_{2}$ is largely sourced from respiration of bioavailable organic carbon, whereas CH4 ${\text{CH}}_{4}$ reflects sedimentary methanogenesis followed by aerobic methanotrophy in the water column. Finally, N2O ${\mathrm{N}}_{2}\mathrm{O}$ bulk and position‐specific isotopic compositions reveal a nitrogen cycle dominated by sedimentary denitrification, with near‐quantitative reduction to N2 ${\mathrm{N}}_{2}$ prior to upward diffusion into the water column and eventual outgassing. Combined, these observations reveal that carbon inputs, total water depth, and/or dissolved oxygen saturation are the major drivers of the flux and composition of GHGs emitted from Lake Mai Ndombe and potentially other tropical humic lakes. Plain Language Summary: Constraining natural and anthropogenic emissions of greenhouse gases to the atmosphere is critical for understanding Earth's climate and carbon cycle. Beyond carbon dioxide, methane and nitrous oxide are powerful greenhouse gases that can be produced by microbes living in oxygen‐poor environments. Organic‐carbon rich ("humic") tropical lakes and wetlands are major natural sources of methane and nitrous oxide, but the underlying mechanisms and resulting importance of these emissions—particularly in understudied regions such as central Africa—are not well known. Here, we measured carbon dioxide, methane, and nitrous oxide concentrations and isotopic compositions in the largest humic lake in Africa, Lake Mai Ndombe, Democratic Republic of Congo. We show that the concentrations of all greenhouse gases in the lake water are higher than expected if in equilibrium with the atmosphere, indicating significant production within the lake system. Further, isotopic data imply that most methane and nitrous oxide that is originally produced in sediments is subsequently consumed by microbes before reaching the atmosphere. We nevertheless observe large outgassing emissions despite such high consumption rates. Combined, this implies that small perturbations to humic lake depth, dissolved oxygen content, and microbial communities could have profound impacts on the amount of greenhouse gases emitted from these systems. Key Points: Lake Mai Ndombe is supersaturated in CO2 ${\text{CO}}_{2}$, CH4 ${\text{CH}}_{4}$, and N2O ${\mathrm{N}}_{2}\mathrm{O}$, with outgassing fluxes of 375 ± 32 Gg C yr−1, 623 ± 136 Mg C yr−1, and 223 ± 20 Mg N yr−1 (μ±1σ $\mu \pm 1\sigma $)Up to 90% of pelagic methane is reoxidized by methanotrophic microorganismsNitrous oxide is mainly formed by denitrification, with an average of 97% subsequently reduced to N2 ${\mathrm{N}}_{2}$ prior to outgassing [ABSTRACT FROM AUTHOR]
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Database:	GreenFILE

Description
Abstract:	Humic tropical lakes and wetlands are globally important sources of atmospheric greenhouse gases (GHGs). However, mechanistic insight into GHG cycling in such systems remains limited—especially in understudied central Africa. To address this, here we measured high‐, falling‐, and low‐water seasonal concentrations and isotopic compositions of the major dissolved GHGs CO2 ${\text{CO}}_{2}$, CH4 ${\text{CH}}_{4}$, and N2O ${\mathrm{N}}_{2}\mathrm{O}$ in Africa's largest humic lake: Mai Ndombe, Democratic Republic of Congo. We find that the water column is weakly to non‐stratified and is highly supersaturated with respect to atmospheric equilibrium for all GHGs across all seasons, sampling stations, and water depths. Additionally, all GHG concentrations increase steadily with increasing water depth, reflecting atmospheric gas exchange due to physical mixing in the upper water column as well as biological processes. Extrapolating these results—combined with field measurements such as temperature and wind speed—we estimate that Lake Mai Ndombe emits 375 ± 32 Gg C yr−1 as CO2 ${\text{CO}}_{2}$, 623 ± 136 Mg C yr−1 as CH4 ${\text{CH}}_{4}$, and 223 ± $\pm $ 20 Mg N yr−1 as N2O ${\mathrm{N}}_{2}\mathrm{O}$ (μ±1σ $\mu \pm 1\sigma $ propagated Monte Carlo uncertainty). Furthermore, carbon‐isotope signals suggest that CO2 ${\text{CO}}_{2}$ is largely sourced from respiration of bioavailable organic carbon, whereas CH4 ${\text{CH}}_{4}$ reflects sedimentary methanogenesis followed by aerobic methanotrophy in the water column. Finally, N2O ${\mathrm{N}}_{2}\mathrm{O}$ bulk and position‐specific isotopic compositions reveal a nitrogen cycle dominated by sedimentary denitrification, with near‐quantitative reduction to N2 ${\mathrm{N}}_{2}$ prior to upward diffusion into the water column and eventual outgassing. Combined, these observations reveal that carbon inputs, total water depth, and/or dissolved oxygen saturation are the major drivers of the flux and composition of GHGs emitted from Lake Mai Ndombe and potentially other tropical humic lakes. Plain Language Summary: Constraining natural and anthropogenic emissions of greenhouse gases to the atmosphere is critical for understanding Earth's climate and carbon cycle. Beyond carbon dioxide, methane and nitrous oxide are powerful greenhouse gases that can be produced by microbes living in oxygen‐poor environments. Organic‐carbon rich ("humic") tropical lakes and wetlands are major natural sources of methane and nitrous oxide, but the underlying mechanisms and resulting importance of these emissions—particularly in understudied regions such as central Africa—are not well known. Here, we measured carbon dioxide, methane, and nitrous oxide concentrations and isotopic compositions in the largest humic lake in Africa, Lake Mai Ndombe, Democratic Republic of Congo. We show that the concentrations of all greenhouse gases in the lake water are higher than expected if in equilibrium with the atmosphere, indicating significant production within the lake system. Further, isotopic data imply that most methane and nitrous oxide that is originally produced in sediments is subsequently consumed by microbes before reaching the atmosphere. We nevertheless observe large outgassing emissions despite such high consumption rates. Combined, this implies that small perturbations to humic lake depth, dissolved oxygen content, and microbial communities could have profound impacts on the amount of greenhouse gases emitted from these systems. Key Points: Lake Mai Ndombe is supersaturated in CO2 ${\text{CO}}_{2}$, CH4 ${\text{CH}}_{4}$, and N2O ${\mathrm{N}}_{2}\mathrm{O}$, with outgassing fluxes of 375 ± 32 Gg C yr−1, 623 ± 136 Mg C yr−1, and 223 ± 20 Mg N yr−1 (μ±1σ $\mu \pm 1\sigma $)Up to 90% of pelagic methane is reoxidized by methanotrophic microorganismsNitrous oxide is mainly formed by denitrification, with an average of 97% subsequently reduced to N2 ${\mathrm{N}}_{2}$ prior to outgassing [ABSTRACT FROM AUTHOR]
ISSN:	21698953
DOI:	10.1029/2025JG009218