“The lake is separated in two by this "lid” 250 metres down, which allows the gas to accumulate deep down. Releasing these deep denser waters on the surface, could break the lake’s stratification and create a lake with homogenous waters. The gas would be diluted in a greater volume of water, so less concentrated, and more difficult to exploit. But the main risk would be the eutrophication of the lake. The deep waters don’t only retain gas. They also retain everything that sinks from the surface and are therefore extremely rich in nutrients, especially phosphate and ammonium. What we fear is that if the waters are released on the surface, there will be a sudden excess of nutrients in the ecosystem, causing an excessive development of phytoplankton. At first sight, this may appear beneficial for the entire food chain of the lake which is very poor in nutrients. But this eutrophication could create harmful effects. When the algae die, they are subject to bacterial decomposition, a process that consumes a large amount of oxygen.” A greater production of algae would therefore generate higher oxygen consumption, which could lead to problems of anoxia, i.e. the absence of oxygen in certain surface water zones. As it is, aerobic life only develops in the first 60 metres of the lake, which isn’t very much. Simulations based on ecological models have shown that in the case of the lake’s eutrophication, this oxygenated zone could be reduced to the first 20 metres of the lake, which would be catastrophic for all the species that develop there.

The main issue for industrialists will therefore be to find the best compromise between the economic profitability of the gas extraction, maintaining the lake’s stability and preserving the integrity of the ecosystem. Because beyond its ecological interest, the preservation of the lake’s fauna is also of economic importance.

The surface waters mixed on a seasonal basis

While the deep waters are blocked below 275 metres and accumulate a high quantity of gas and nutrients, the surface waters are no less interesting and have also revealed secrets that make Lake Kivu a unique ecosystem. The first 60 metres are subject to a high level of seasonal mixing, considerably influencing the variations in this system. “An observation that goes against what the scientific community previously thought as regards the tropical and equatorial aquatic system", François Darchambeau is glad to say. Until recently, we thought that they were stable in time, enjoying a continuous summer and producing constant biological activity. We helped to show that this was not the case at all.”

During the rainy season, stratification in several layers is established. Humidity is greater, the winds are less strong, and the conditions cause the mixing of the surface waters to slow down. These waters then warm up on the surface leading to stratification.  During the dry season, the air is dryer, the winds stronger, and the water evaporates more easily, thus encouraging the rupture of the seasonal stratification. Hence, the water mixes up to a depth of 60 metres.

The direct consequence is that the biomass of the phytoplankton evolves according to these variations. “There isn’t a constant availability of nutrients for the phytoplankton during the year.  In the rainy season, the stratification of the water blocks the diffusion from deep waters of these nutrients and therefore limits the abundance of food for the phytoplankton that lives in the first 30 metres of the surface. Some of them die and sink and therefore can no longer feed the rest of the food chain. On the other hand, during the period when the waters mix, the accumulated nutrients at a depth of 30 to 60 metres, rise to the surface and can once again participate in the growth of the biomass.” Thus, during the dry season, there is a peak in the phytoplanktonic biomass within the lake. A greater production of algae, more zooplankton that feeds on the first ones, and then probably the fish who feed on the zooplankton.