The Laboratory of Chemical Oceanography at the University of Liège has reevaluated the atmospheric emissions of powerful greenhouse gases carbon dioxide (CO2) and methane (CH4), from African lakes.
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Although it was once believed that these lakes were major producers of CO2, it has subsequently been found that they leak relatively little CO2 and a lot of methane. Results were published in the journal Science Advances.
Predicting potential changes in greenhouse gas (GHG) emissions from the planet’s natural ecosystems is one of the keys to predicting climate change. However, assessing them precisely and comprehend the underlying mechanics to achieve this is critical.
On Earth, there are around 1.5 million lakes. They contribute significantly to the release of greenhouse gases, unlike oceans. It took some time for the significance of continental seas as CO2 and CH4 emitters, the two primary GHGs, to be recognized.
Since they were not first investigated until the middle of the 1990s, there are no samples available.
This is problematic, because spatial heterogeneity is very important, both within a single lake or river and between different systems. If the heterogeneity is very high, very large amounts of data are required to obtain a robust estimate of GHG emissions. There are almost two million lakes on Earth.
Alberto Borges, Oceanographer, Laboratory of Chemical Oceanography, University of Liège
Researchers only had information up until now on lakes in North America and Scandinavia (boreal), and they had scant information on lakes in the tropics and Africa. These numbers were extended to include tropical lakes as well as all other lakes in the world.
However, in terms of emissions and GHG sequestration, these lakes “behave” differently. Researchers from the U Liège Chemical Oceanography Laboratory have demonstrated through a ten-year study that the data gathered for lakes in North America and Scandinavia is not applicable to lakes in Africa.
“The micro-algae that make up the phytoplankton are very fond of the warm and luminous conditions of the tropical ‘endless summer,’ which means that some of the African lakes that we have studied are extremely productive,” stated Borges.
Borges added, “However, through photosynthesis, phytoplankton remove CO2 from the water and these lakes therefore sequester CO2 in the form of organic matter buried at the bottom of the lakes in the sediments. They therefore act as carbon sinks, whereas until now it had always been assumed that lakes emitted CO2 in very large quantities into the atmosphere, as do boreal lakes.”
Boreal lakes cannot serve as sinks like African lakes do since they contain very little phytoplankton and are only able to “compost” plant waste that has been carried to the lakes by runoff from the surrounding forest.
However, there is a drawback to the year-round tropical warmth. Archaea, a kind of methane (CH4)-producing microorganism, thrive in warm environments.
As a result, methane concentrations in tropical lakes are much higher than in boreal lakes, particularly since the phytoplankton that settles at the bottom of tropical lakes offers a highly intriguing substrate from a methane-producing archaea’s perspective as a “nutrient.”
What was assumed to be “gained” in terms of CO2 sequestration in tropical lakes is really “lost” due to increased CH4 production.
Borges concluded, “Thanks to an understanding of the mechanisms underlying the production of CO2 and CH4 by lakes (depth and surrounding vegetation cover), we can now have a more informed and rigorous approach to the situation rather than a blind extrapolation based on a simple average of all the data, as has been done until now in the literature.”
By combining multiple geographical databases, the study conducted by the ULiège researchers enabled the projection of CO2 and CH4 emissions to 72,500 tropical lakes worldwide.
The measurements used in this study, which was recently published in Science Advances, were collected for more than ten years in 24 African lakes, including the largest of the African Rift (Victoria, Tanganyika, Albert, Kivu, and Edouard), during 17 field missions, as part of two BELSPO projects (EAGLES, HIPE), and five FNRS projects (TRANS-CONGO, LAVIGAS, TANGAGAS, KYBALGAS, MAITURIK).
Borges, A. V., et al. (2022) Greenhouse gas emissions from African lakes are no longer a blind spot. Science Advances. doi:10.1126/sciadv.abi8716