Lake Kivu is a fascinating lake. An exceptional amount of methane and carbon dioxide has accumulated in its deep waters.
Age: ~ 1 to 5 million years old
Surface area: 2370 km2
Maximum depth: 485 m
Volume: 560 km3
Location: East African Rift Valley
Riparian countries: Democratic Republic of Congo, Government of Rwanda
Altitude: 1463 m above sea level
Uniqueness: more methane than any other lake
Gases in Lake Kivu
Carbon dioxide: ~300 km3 STP, 80% of the gases volume, only 20% of the total gas pressure
Methane: ~60 km3 STP, 20% of the gases volume, but 80% of the total gas pressure.
These gases are dissolved (like in a bottle of Coke) and trapped below 260 m depth, where they accumulated over ~800 years. This unusual accumulation can be explained by the lake’s permanent stratification and the input of carbon dioxide from the surrounding volcanoes.
CO2 has a magmatic origin. CH4 is generated biologically by the reduction of magmatic CO2 (~65%) and the mineralisation of organic carbon (~35%). CH4 concentrations seemed to have increased by 15% in the last 30 years.
Structure of Lake Kivu
The water column is characterized by increasing temperature, salinity and gas concentrations with depth and by several sharp gradients resulting from subaquatic springs.
The annual deep mixing occurs during the dry season (June to September). The mixing depth varies annually but is limited to a maximum depth of 63 m. Below this oxygenated zone, the different layers are homogenous and stable. Lake Kivu is therefore permanently stratified.
Lake Kivu is an ancient rift lake. Around 12 000 years B.P., lavas flows from the Virunga volcanoes dammed its former outflow to the Nile. By 9 500 years B.P., Lake Kivu had become a deep lake with an overflow via the Ruzizi River to Lake Tanganyika. The lake became strongly stratified about 5 000 years B.P. when volcanism and hydrothermal activities began.
Catchment area: 5097 km2 (excluding the lake), relatively small compared to the lake surface.
5 Basins around Idjwi Island: Main Basin, Kalehe Basin, Ishungu Basin, Bukavu Bay and Kabuno Bay.
Rivers: ~200 small inflowing rivers, but no surface water in the volcanic region of Goma Internal sub-aquatic springs: in the north, at different depths mainly at 180 and 250 m.
Annual lake level fluctuations: 0.17 to 1.17 m
Biology of Lake Kivu
Only 31 fishes live in Lake Kivu. The Tanganika sardine (Isambaza) was introduced in the 1960s, to fill the empty pelagic zone. The Isambaza sustain traditional fisheries and is an important source of protein for the local population.
The food web of Lake Kivu is simple: phytoplankton are eaten by zooplankton, which are fed by Isambaza fish.