Lake Kivu - Biology

Biology

The fish fauna in Lake Kivu is relatively poor (28 species). There are several endemic species of Haplochromis (cichlids) and a small clupeid, Limnothrissa miodon. Its exploitable stock was estimated at 2000 - 4000 tons per year. The sardine Limnothrissa miodon was introduced to Lake Kivu in the late 1950s by a Belgian Engineer A. Collart. At present, Lake Kivu is the sole natural lake in which Limnothrissa miodon, an endemic sardine originally from Lake Tanganyika, has been introduced initially to fill an empty niche. Indeed, prior to the introduction, no planktivorous fish was present in the pelagic waters of Lake Kivu.

Following this introduction, the sardine has gained substantial economic and nutritional importance for the lakeside human population but from an ecosystem standpoint, the introduction of planktivorous fish may result in important modifications of plankton community structure. Recent observations showed the disappearance during the last decades of a large grazer, Daphnia curvirostris, and the dominance of mesozooplankton community by three species of cyclopoid copepod: Thermocyclops consimilis, Mesocyclops aequatorialis and Tropocyclops confinis.

The first comprehensive phytoplankton survey was released in 2006. With an annual average chlorophyll a in the mixed layer of 2.2 mg m-3 and low nutrient levels in the euphotic zone, the lake is clearly oligotrophic. Diatoms are the dominant group in the lake, particularly during the dry season episodes of deep mixing. During the rainy season, the stratified water column, with high light and lower nutrient availability, favour dominance of cyanobacteria with high numbers of phototrophic picoplankton. The actual primary production is 0.71 g C m-2 d-1 (~ 260 g C m-2 y-1).

A study of evolutionary genetics showed that the cichlids from lakes in northern Virunga (e.g., Edward, George, Victoria) would have evolved in a "proto-lake Kivu", much older than the intense volcanic activity (20,000-25,000 years ago) which cut the connection. The elevation of the mountains west of the lake (which is currently the Kahuzi-Biega National Park, one of the largest reserves of mountain gorillas in the world), combined with the elevation of the eastern rift (located in eastern Rwanda) would be responsible for drainage of water from central Rwanda in the actual Lake Kivu. This concept of "proto-lake Kivu" was challenged by lack of consistent geological evidence, although the cichlid's molecular clock suggests the existence of a lake much older than the commonly cited 15,000 years.