A larger food chain uncovered

Lake Kivu is an oligotrophic lake (i.e. particularly poor in nutrients). François Darchambeau remembers the words of Henri Damas, an oceanographer and chemist from the University of Liège who, during an expedition in the 1930s, said that "these clear and transparent waters are an absolute desert". This is partly true. Very few nutrients arrive in the lake, and rapidly sink to the deep waters, never to be seen again. “Furthermore, there was very little fish in the 1930s. Just a few species along the coastline and, in principle, no pelagic species compared with other similar lakes with a very developed fauna".

The current dearth of species could be the result of the gaseous eruption 5 000 years ago, revealed by the study of sediments, which could have destroyed all forms of life in the lake. It would seem that it was only recolonised with species from tributaries. To solve this problem and encourage fish production, Belgian agronomic engineers, among them Alphonse Collart, decided in the 1950s to introduce Stolothrissa tanganicae, a species of sardine endemic to Lake Tanganyika that feeds on zooplankton. The idea then was to introduce a predator to this sardine in order to be able to fish the biggest fish. “However, after several fruitless attempts, the transfer of living sardines from Lake Tanganyika finally succeeded in 1959. A year later, the war of independence broke out in the Congo, and the Belgians there had to abandon their work.”

It was only in 1985 that a professor in ecology from Namur found some small freshly-caught sardines from Lake Kivu in a local market in Bukavu.  So, a sardine had indeed developed there, but it wasn’t Stolothrissa tanganicae as hoped, but Limnothrissa miodon, a species close to S. tanganicae and also endemic to Lake Tanganyika (it was undoubtedly difficult to distinguish between them when they were transferred to Lake Kivu). L. miodon seems to have adapted perfectly to life in Lake Kivu, for instance, by broadening its diet. Indeed, recent studies have shown that during the rainy season, when small zooplanktonic crustaceans are less abundant in the pelagic waters, the fish comes closer to the coast and adapts its diet by feeding on insect larvae, a behaviour that it doesn’t seem to have adopted in Lake Tanganyika where it continues to live in the pelagic zone.

Life in Lake Kivu has therefore evolved significantly since the middle of the 20^th century. Fishing is relatively widespread and more than 10 000 tons of fish are caught a year, which is economically very good for the inhabitants around the lake. Following the appearance of the sardines, the zooplankton biomass was divided by eight compared with the estimation made before the 1950s. This can be explained mainly by the predation caused by the introduction of the sardines. “However, this reduction brings the biomass of the zooplankton back to levels comparable to those observed in lakes Tanganyika and Malawi, which makes us believe that the situation before 1950 was abnormal.” Other studies published in the work also present a large microbial community, which has introduced itself into the food chain between the phytoplankton and the zooplankton and which also participates in the production of the lake’s food chain. Among other things, this microbial community feeds on the CH₄ that constantly rises up from the bottom of the lake, transforming it into CO₂. This bacterial oxidisation of the CH₄, or methanotrophy, is extremely efficient, which means there are only very modest concentrations of CH₄ on the surface of the lake, and lower than the great majority of other lakes elsewhere in the world. Methanotrophy is therefore behind the paradox that the lake which contains the most CH4 in its deep waters, is also the one that emits the least into the atmosphere.
 
The work proposed by this team of international researchers therefore serves to raise awareness among its readers, scientific colleagues, students in limnology as well as political decision-makers, concerning the ecological workings of a completely unique tropical lake. This arrival of this timely work reveals the stakes of a well-thought-out and measured extraction of the riches that currently lie dormant at the bottom of Lake Kivu.