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Team reassesses greenhouse gas emissions from African lakes

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The emission of carbon dioxide (CO2) and methane (CH4) – the most potent greenhouse gases – released into the atmosphere from African lakes are being reassessed in a study conducted by the Laboratory of Chemical Oceanography (FOCUS Research Unit / Faculty of Science). Although these lakes were previously believed to contain significant CO2 sources, it has since been discovered that they are really very low in CO. expel2 but a lot of methane, which adds to the emissions burden. The study is published in scientific progress

One of the keys to predicting climate change is predicting how greenhouse gas (GHG) emissions from our planet’s natural ecosystems might change. But in order to do this, it is important to be able to estimate them as accurately as possible and to understand the underlying mechanisms. There are about 1.5 million lakes on Earth. Unlike the oceans, they play an important role in greenhouse gas emissions. Recognition of the important role of continental waters as emitters of carbon dioxide (CO2 ) and methane (CH4) – the two main greenhouse gases – came quite late. It was not until the mid-1990s that they were studied and are therefore relatively undersampled.

“This is problematic,” explains Alberto Borges, FNRS researcher at the Laboratory of Chemical Oceanography at ULiège, “because spatial heterogeneity is very important, both within a single lake or river and between different systems. If the heterogeneity is very high is, very large. Amounts of data are needed to get a robust estimate of greenhouse gas emissions. There are almost two million lakes on Earth.”

Until now, researchers only had data on North American and Scandinavian (boreal) lakes and very little on tropical lakes and none on African lakes. These values ​​have been extrapolated to all lakes worldwide, including tropical lakes. However, these lakes do not “behave” in the same way in terms of greenhouse gas sequestration and emissions. A study over a decade conducted by researchers at the ULiège Chemical Oceanography Laboratory has shown that the data collected for North American and Scandinavian lakes do not apply to African lakes.

“The microalgae that make up the phytoplankton really like the warm and bright conditions of the tropical ‘endless summer,'” says Alberto Borges, “meaning that some of the African lakes we studied are extremely productive. photosynthesis, removes phytoplankton CO2 from the water and these lakes therefore lay CO . fixed2 in the form of organic matter buried at the bottom of the lakes in the sediments. Thus, they act as carbon sinks, whereas until now lakes have always been assumed to contain CO. expel2 in very large quantities in the atmosphere, just like they do in boreal lakes.” Indeed, due to the colder and less luminous conditions, boreal lakes have very little phytoplankton and are limited to “composting” vegetable waste from the surrounding forest, which is transported to the lakes by runoff, and therefore cannot play the role of sink like African lakes.

But the warm tropical conditions all year round have a downside. The heat is very beneficial for the development of archaea, microorganisms that produce methane (CH4† This means that methane concentrations in tropical lakes are very high compared to boreal lakes, especially because the phytoplankton that sediment on the bottom of tropical lakes is a very interesting substrate from a ‘nutrient’ point of view for methane-producing archaea. What was thought to be ‘mined’ in tropical lakes in terms of CO2 sequestration, is in fact ‘lost’ by increased CH4 production.

“Thanks to an understanding of the mechanisms underlying the production of CO2 and CH4 by lakes (depth and surrounding vegetation), 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 data, as has been done so far in the literature,” concluded Alberto Borges.

The study by the ULiège researchers made the extrapolation of CO . possible2 and CH4 emissions to 72,500 tropical lakes around the world by integrating various spatial databases. This study synthesizes measurements obtained over more than 10 years in 24 African lakes, including the largest of the African Rift (Victoria, Tanganyika, Albert, Kivu, Edouard), during 17 field missions, under 2 BELSPO projects (EAGLES , HIPE ) and 5 FNRS projects (TRANS-CONGO, LAVIGAS, TANGAGAS, KYBALGAS, MAITURIK).

Tropical lakes may emit more methane

More information:
Alberto V. Borges et al, Greenhouse gas emissions from African lakes are no longer a blind spot, scientific progress (2022). DOI: 10.1126/sciaadv.abi8716

Provided by the University of Liège

Quote: Team assesses greenhouse gas emissions from African lakes (2022, June 29) retrieved June 29, 2022 from https://phys.org/news/2022-06-team-reassesses-greenhouse-gas-emissions.html

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