A complex and unique ecosystem

The EAGLES project shed light on other grey areas, some of them rather unusual. “Kivu is one of the lakes that releases the least methane into the atmosphere. A contrast with its deep waters, which store enormous quantities of this gas”, Alberto Borges continues. “One of the reasons is the lake’s stratification, which allows methane to accumulate at the bottom, significantly slowing down the transfer of methane to the surface. This transfer is so slow that the bacteria break down the methane, so when the water comes into contact with the atmosphere, the concentrations of methane are extremely low and, consequently, very little is released into the atmosphere. In fact, any small pond, even the one in your garden, emits more methane than Lake Kivu!” The transportation of the bottom waters to the surface is caused by sources beneath the lake bottom bringing waters loaded with mineral salts and gas (essentially CO₂) to various depths, provoking what is known as upwelling, i.e. a vertical transport of the waters. At a rate of one centimetre a year, this is very slow, but these waters bring with them all the substances dissolved therein. The methane, CO₂ and nutrients will therefore eventually come to the surface. However, at the interface of the anoxic deep waters and the oxygenated surface waters, varied microbial communities develop, which use the resources of the oxic/anoxic gradient. By studying these communities, the researchers were able to satisfy their curiosity. “Methane is a high energy molecule. That’s why it’s burnt to produce electricity. In a similar way, bacteria metabolise it to live: these methanotrophs form a very active community. Since the methane diffuses very slowly, it has broken down well before reaching the surface.”

Another particularity which was interesting to understand within the framework of global warming, was the quantification of its CO2 emissions into the atmosphere. “It’s normal for a small lake to produce CO₂ because it receives a great amount of organic matter in its watershed (area of land that drains rainwater and supplies the lake with water and mineral and organic substances). But at the same time, this gas is also incorporated to the organic matter through the photosynthesis of the phytoplankton, which eventually sediments. This process allows the absorption of the CO₂ present in the atmosphere when it comes into with the surface. In the great lakes, the fixation of CO₂ is greater than the production of CO2, and they act as a carbon sink. In Lake Kivu, the geological sources of CO2 are added to the water column. Subsequently, a significant quantity of gas ends up on the surface and is then released into the atmosphere, making the lake a source of CO₂ rather than a sink”, Alberto Borges concludes.

An abrupt end?

For the team, the EAGLES project marks the end of more than twelve years of research on Lake Kivu. The challenge, which was largely met, was to establish the present state of knowledge as well as predictions ensuring a fine balance between the exploitation of methane, climate changes, the protection of an ecosystem and the maintenance of local fishing. However, while the databases were mainly completed during these campaigns, many of the fields of research encountered difficulties and various hitches, which would be worth fine tuning. This is especially the case regarding paleolimnology, fishing estimates, and the development of regional forecasting models. “This was one of the initial objectives”, Jean-Pierre Descy tells us. “To develop a model that allows you to simulate the functioning of the body of water according to the weather, conditioning the ecosystem’s productivity. We can simulate mixing phenomena but we haven’t been able to properly simulate the ecological response to these climate changes, for a variety of reasons. In particular, the modelling team had to collect a series of meteorological data itself, which it hoped already existed in order to be able to calibrate its tools. Unfortunately, we ran out of time and we still don’t know whether we can continue our research.” What will continue, within the framework of bringing methane extraction platforms into operation, is a minimum surveillance of the impact of the methane exploitation. The acquisition of all this biological, ecological and geochemical information offers in-depth knowledge on how the lake functions and provides local teams with an important tool to assess potential disturbances. However, the lake’s supervision campaigns only include the routine monitoring of different indicators, making it difficult to expand our understanding of the history of Lake Kivu, how it functions and its future.