Scientists have discovered that blackwater lakes in the Congo Basin are emitting carbon dioxide from peat stored for thousands of years. Up to 40 percent of the CO2 from lakes like Lac Mai Ndombe originates from ancient sources, challenging previous assumptions about carbon stability in tropical peatlands. This finding highlights potential risks from climate change and deforestation in the region.
Tropical peatlands, including those in the Congo Basin, play a vital role in the global carbon cycle by storing vast amounts of carbon. These ecosystems, covering just 0.3 percent of the Earth's land surface, hold about one third of the carbon in tropical peatlands worldwide, contributing to roughly 100 gigatonnes of stored carbon globally.
A team led by ETH Zurich has been studying the Congo Basin for the past decade. Their recent research, published in Nature Geoscience, focused on two blackwater lakes: Lac Mai Ndombe, Africa's largest and more than four times the size of Lake Constance, and the smaller Lac Tumba. These lakes, tinted dark brown by plant debris and soil organic matter from surrounding swamp forests and peat deposits, are releasing substantial amounts of carbon dioxide into the atmosphere.
Using radiocarbon dating, researchers found that while some emissions come from recent plant material, up to 40 percent of the CO2 derives from peat accumulated thousands of years ago. "We were surprised to find that ancient carbon is being released via the lake," said lead author Travis Drake, a scientist in the Sustainable Agroecosystem group at ETH Zurich. Co-author Matti Barthel added, "The carbon reservoir has a leak, so to speak, from which ancient carbon is escaping."
The exact mechanisms mobilizing this old carbon from peat soils into lake water remain unclear. Previously, scientists thought such carbon stayed locked away except during extended droughts. However, drier conditions from climate change could accelerate decomposition by allowing more oxygen into peat layers, potentially increasing emissions.
The study also examined other gases. A parallel paper in the Journal of Geophysical Research showed that water levels in Lac Mai Ndombe affect methane emissions: high levels promote microbial consumption of methane, while low levels during dry seasons allow more to escape. "Our fear is that climate change will also upset this balance," said ETH Professor Jordon Hemingway. "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."
Human activities add pressure. The Democratic Republic of Congo's population is projected to triple by 2050, driving deforestation for farmland. This could worsen droughts and lower lake levels. "Forests are not only responsible for gas exchange like our lungs, but they also evaporate water through their leaves, thereby enriching the atmosphere with water vapor," Barthel explained. "This promotes cloud formation and precipitation, which in turn feeds rivers and lakes."
The findings, from the TropSEDs project funded by the Swiss National Science Foundation and involving collaborators from the University of Louvain in Belgium and the Democratic Republic of Congo, underscore the need to include tropical lakes and wetlands in global climate models. ETH Professor Johan Six noted, "Our results help to improve global climate models, because tropical lakes and wetlands have been underrepresented in these models so far."