African escapades by ULiège oceanographers
Hubert Damas and Alberto Borges, both marine biologists at the University of Liège, have conducted research on understanding the dynamics of the African lakes Kivu, Edward and Ndagala (among others). 80 years separate their studies, yet these lakes continue to deliver innovative and unexpected scientific results.
Pubert Damas, appointed head of the ULiège Zoology Department in 1945, is one of the researchers who made ULiège famous in the field of marine biology and oceanography. In particular through research he carried out following expeditions to Africa to describe the chemistry and ecology of lakes Kivu, Edouard and Ndagala. Alberto Borges, FNRS Research Director at the FOCUS Research Unit (Faculty of Science) of ULiège, has also carried out research in Lake Edouard.
At the time, Professor Damas described in Kivu a "sort of dead sea at the bottom of the lake" rich in hydrogen sulphide, carbon dioxide (CO2 ) and methane (CH4 )," explains Alberto Borges. His work on African lakes has been a reference for many decades. Alberto Borges' research on the dynamics of CO2 , CH4 and nitrous oxide (N2 O) in oceans, lakes and rivers coincidentally followed Damas's work when he started working on Lake Kivu in 2007, in collaboration with Professor Jean-Pierre Descy of the University of Namur. Since then, he has also sampled other large African lakes including Lake Ndagala and Lake Edward (2016-2019). In a recently published paper on Lake Edward, he even used in his analysis the temperature data obtained in this same lake by Hubert Damas more than 80 years earlier.
Comparing current temperature data with Damascus in 1930’s shows that the waters of Lake Edward have warmed at the rate expected from long-term global warming," says Alberto Borges. Although relevant, this is an anecdotal detail of this research, whose central theme is the dynamics of CO2 , CH4 and N2 O in Lakes Edward and George. On the one hand, this article shows that the values of CO2 and CH4 in tropical African lakes are very different from those in boreal and temperate lakes. On the other hand, this work shows that the emissions of CO2 , CH4 and N2 O have "hot moments" with peaks during the mixing of the waters of Lake Edward during storms (frequent in the tropics). "This is very different from what was known about temperate and boreal lakes. When the surface waters of a lake warm up in the spring and summer, this creates a layer of surface water in contact with the atmosphere (the so-called mixed layer) physically separated by what is called thermal stratification from the bottom layer of the lake, isolated from the atmosphere. In this bottom layer, gases such as CO2 , CH4 and N2 O accumulate. When this stratification is broken by the mixing of the water, the surface layer mixes with the bottom layer, and greenhouse gases are emitted to the atmosphere.
However, in temperate lakes (such as in Belgium) this stratification is very strong in summer, because the surface waters are strongly warmed by the sun, while the bottom waters keep the temperature they had at the end of winter. "The temperature difference between the surface and the bottom is very high, typically around 10°C. This creates a very strong physical barrier that can only be destroyed in autumn when the surface cools and autumn storms mix the waters. There is then a peak in the emission of CO2 , CH4 and N2 O, which is unique and only occurs in autumn. In boreal lakes, the freezing of the lake’s surface in winter causes an accumulation of gases under the ice. There is an additional peak in the emission of CO2 , CH4 , and N2 O when the ice melts in late winter.
"This dynamic is very different in African lakes, because they experience an endless tropical summer. So the temperature difference between the surface and bottom waters is much smaller, typically only about 1°C. In this case, the slightest storm, allows sufficient mixing to occur between the surface and bottom waters, and also causes a peak in the emission of CO2 , CH4 and N2 O. " But in this case, not just one or two peaks occur, but bursts of emission peaks during thunderstorms, which are frequent during the rainy season, as opposed to the "one shot" in temperate zones or the "two shots" in boreal zones. Alberto Borges' work is the first to show and quantify this phenomenon. This work is part of what seems to be a tradition of oceanographers from Liège who like to discover the oddities of African lakes, with a particular taste for greenhouse gases.
This research was funded by the BRAIN-Be programme of the Belgian Federal Science Policy (BELSPO) within the framework of the HIPE project (Human impacts on ecosystem health and resources of Lake Edward, 2015-2021).
Borges AV, W Okello, S Bouillon, L Deirmendjian, A Nankabirwa, E Nabafu, T. Lambert, J-P Descy & C Morana (2023) Spatial and temporal variations of dissolved CO2 , CH4 and N2 O in Lakes Edward and George (East Africa). Journal of Great Lakes Research, doi: 10.1016/j.jglr.2022.11.010q.
References for Hubert Damas' work in the Great Lakes of Africa
Damas, H., 1937. Hydrobiological research in lakes Kivu, Edouard, and Ndalaga. Explor. Part Natl. Albert, Mission H. Damas (1935-36), Fast. I., 128 p. Inst. Parts Natl. Congo Belge.
Damas, H., 1938. Thermal and chemical stratification of lakes Kivu Edouard and Ndalaga (Belgian Congo), SIL Proceedings, 1922-2010, 8:3, 51-68.