Reassessment of greenhouse gas emissions from African lakes

One of the keys to predict climate change is to predict how greenhouse gas (GHG) emissions from our planet's natural ecosystems might change. But 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 the emission of greenhouse gases. Recognition of the important role of continental waters as emitters of carbon dioxide (CO₂ ) and methane (CH₄ ) -- the two main GHGs -- came rather late. It was not until in the mid-1990s that they began to be studied and are therefore relatively little sampled. "This is problematic, explains Alberto Borges, oceanographer at the Laboratory of Chemical Oceanography at the University of Liège, "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."

Until now, researchers have only had data on North American and Scandinavian (boreal) lakes and very little on tropical lakes and none on African lakes. These values were extrapolated to all lakes worldwide, including tropical lakes. However, these lakes do not "behave" in the same way in terms of GHG sequestration and emissions. A study conducted over ten years by researchers from the ULiège Chemical Oceanography Laboratory has shown that the data collected for North American and Scandinavian lakes does not apply to African lakes.

"The micro-algae that make up the phytoplankton are very fond of the warm and luminous conditions of the tropical 'endless summer'," says Alberto Borges, "which means that some of the African lakes that we have studied are extremely productive. 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. Indeed, due to the colder and less luminous conditions, boreal lakes have very little phytoplankton and are limited to "composting" plant waste from the surrounding forest, which is transported to the lakes by runoff, and therefore cannot play the role of sink as African lakes do.

But the warm tropical conditions all year round have a downside. The heat is very favourable to the development of archaea, micro-organisms that produce methane (CH₄). This means that methane concentrations in tropical lakes are very high compared to boreal lakes, especially as the phytoplankton that sediment at the bottom of tropical lakes provide a very interesting substrate from a 'nutrient' point of view for methane-producing archaea. What was thought to be 'gained' in tropical lakes in terms of CO₂ sequestration, is in fact 'lost' through increased CH production₄ .

"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," concluded Alberto Borges. The study carried out by the ULiège researchers allowed for the extrapolation of CO2 and CH4 emissions to 72,500 tropical lakes around the world by integrating several spatial databases. This study, which has just been published in Science Advances, synthesises 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, in the framework of 2 BELSPO projects (EAGLES, HIPE) and 5 FNRS projects (TRANS-CONGO, LAVIGAS, TANGAGAS, KYBALGAS, MAITURIK).