Tropical swamps and peatlands are critical players in Earth’s carbon cycle and, by extension, the global climate. In regions such as the Amazon Basin, the Congo Basin, and the wetlands of Southeast Asia, thick layers of partially decomposed plant material build up over time. Together, these ecosystems lock away roughly 100 gigatonnes of carbon.
At the center of Africa, the Congo Basin contains one of the largest and most significant of these carbon reserves. Although its peatlands and swamps cover just 0.3 percent of the planet’s land surface, they store about one third of all carbon held in tropical peatlands worldwide.
Despite their importance, these remote ecosystems have not been extensively studied. Large parts of the central Congo Basin are difficult to reach, and travel to isolated lakes and swamps often requires boats or traditional pirogues. As a result, their influence on the global climate has remained uncertain.
Surprising Discoveries in Congo Blackwater Lakes
Over the past decade, a team led by ETH Zurich has been investigating the Congo Basin more closely. Their work has already revealed unexpected findings, including the Ruki River, one of the darkest blackwater rivers on Earth (ETH News reported).
In a recent study published in Nature Geoscience, the researchers turned their attention to two blackwater lakes stained dark by plant material: Africa’s largest blackwater lake, Lac Mai Ndombe, and the smaller Lac Tumba. Once again, they encountered an unexpected result.
Lake Mai Ndombe is more than four times larger than Lake Constance, and its water resembles strong black tea. It is bordered by vast swamp forests and largely undisturbed lowland rainforest growing atop deep peat deposits. As plant debris and soil organic matter wash into the lake from surrounding forests, they tint the water a deep brown.
Ancient Carbon Released Into the Atmosphere
Measurements show that substantial amounts of carbon dioxide are escaping from both lakes into the atmosphere. However, the origin of that carbon was not what scientists anticipated.
While some of the emissions come from recently grown plant material, up to 40 percent of the carbon dioxide originates from peat that accumulated thousands of years ago in nearby ecosystems. Researchers determined this by analyzing the age of the dissolved CO2 using radiocarbon dating (radiocarbon dating).
“We were surprised to find that ancient carbon is being released via the lake,” explains lead author Travis Drake, a scientist in the Sustainable Agroecosystem (SAE) group led by ETH Professor Johan Six. “The carbon reservoir has a leak, so to speak, from which ancient carbon is escaping,” adds co-author Matti Barthel, research technician in SAE.
How Is the Carbon Being Mobilized?
Previously, scientists believed that carbon stored in Congo Basin peat remained locked away for extremely long periods and would only be released under specific conditions such as extended drought.
Exactly how this old carbon is being freed from undecomposed plant matter remains uncertain. Researchers also do not yet know the precise pathways that allow it to move from peat soils into lake water.
Understanding whether this release signals a destabilizing shift or reflects a natural balance offset by new peat formation is now a key research question.
Climate Change and the Risk of Peatland Drying
The escape of ancient carbon could point to a broader concern. Environmental changes driven by climate change may be triggering processes that increase carbon release.
If conditions become drier, peat soils may dry out more frequently and for longer periods. This allows oxygen to penetrate deeper into the peat layers, accelerating microbial breakdown of once stable organic material. As decomposition speeds up, more CO2 from this enormous carbon store could enter the atmosphere.
“Our results help to improve global climate models, because tropical lakes and wetlands have been underrepresented in these models so far,” as Six stated.
Water Levels and Methane Emissions
Beyond carbon dioxide, the team also studied emissions of nitrous oxide and methane from Lake Mai Ndombe. In a parallel study published in the Journal of Geophysical Research, they found that water levels strongly influence how much methane escapes.
When lake levels are high, microorganisms more effectively consume methane before it can reach the atmosphere. During the dry season, when water levels drop, methane is broken down less efficiently and larger amounts are released.
“Our fear is that climate change will also upset this balance. If droughts become longer and more intense, the blackwater lakes in this region could become significant sources of methane that impact on the global climate,” says ETH Professor Jordon Hemingway. “At present we do not know when the tipping point will be reached.”
Deforestation and Land Use Pressures
Climate is not the only factor that could disrupt this system. Land use changes may pose an even greater threat. The population of the Democratic Republic of Congo is projected to triple by 2050, increasing demand for farmland and leading to further forest clearing.
Deforestation can intensify drought conditions, potentially keeping lake levels persistently low. “We all know the analogy whereby forests are the green lungs of the Earth,” says Barthel. “They are not only responsible for gas exchange like our lungs, however, but they also evaporate water through their leaves, thereby enriching the atmosphere with water vapor. This promotes cloud formation and precipitation, which in turn feeds rivers and lakes.”
Why These Findings Matter
These results sharpen our understanding of how tropical peatlands and blackwater lakes influence global climate dynamics. They also highlight the urgency of protecting Congo Basin wetlands and limiting greenhouse gas emissions.
The research was conducted as part of the TropSEDs project led by ETH Zurich and funded by the Swiss National Science Foundation, in collaboration with scientists from the University of Louvain in Belgium and the Democratic Republic of Congo.
