| Lake Kivu | |
|---|---|
| Satellite image of Lake Kivu courtesy of NASA. | |
| Lake type | Rift Valley lakes, Meromictic |
| Primary outflows | Ruzizi River |
| Catchment area | 7,000 km2 (2,700 sq mi) |
| Basin countries | Rwanda, Democratic Republic of the Congo |
| Max. length | 89 km (55 mi)[1] |
| Max. width | 48 km (30 mi)[1] |
| Surface area | 2,700 km2 (1,040 sq mi)[1] |
| Average depth | 240 m (787 ft) |
| Max. depth | 480 m (1,575 ft) |
| Water volume | 500 km3 (120 cu mi) |
| Surface elevation | 1,460 m (4,790 ft) |
| Islands | Idjwi |
| Settlements | Goma, Congo Bukavu, Congo Kibuye, Rwanda Cyangugu, Rwanda |
Lake Kivu is one of the African Great Lakes. It lies on the border between the Democratic Republic of the Congo and Rwanda, and is in the Albertine Rift, the western branch of the East African Rift. Lake Kivu empties into the Ruzizi River, which flows southwards into Lake Tanganyika. The name comes from kivu which means "lake" in Bantu language, just like the words tanganyika or nyanza.
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The lake covers a total surface area of some 2,700 km2 (1,040 sq mi) and stands at a height of 1,460 metres (4,790 ft) above sea level. Some 1 370 km2 or 58% of the lake's waters lie within DRC borders. The lake bed sits upon a rift valley that is slowly being pulled apart, causing volcanic activity in the area, and making it particularly deep: its maximum depth of 480 m (1,575 ft) is ranked eighteenth in the world. The lake is surrounded by majestic mountains.
The world's tenth-largest inland island, Idjwi, lies in Lake Kivu, as does the tiny island of Tshegera, which also lies within the boundaries of Virunga National Park; while settlements on its shore include Bukavu, Kabare, Kalehe, Sake and Goma in Congo and Gisenyi, Kibuye and Cyangugu in Rwanda.
Native fish include species of Barbus, Clarias, and Haplochromis, as well as Nile Tilapia. Limnothrissa miodon, one of two species known as the Tanganyika sardine, was introduced in 1959 and formed the basis of a new pelagic zone fishery. In the early 1990s, the number of fishers on the lake was 6,563, of which 3,027 were associated with the pelagic fishery and 3,536 with the traditional fishery. Widespread armed conflict in the surrounding region from the mid-1990s resulted in a decline in the fisheries harvest.[2]
Lake Kivu is one of three known exploding lakes, along with Cameroonian Lake Nyos and Lake Monoun, that experience violent lake overturns. Analysis of Lake Kivu's geological history indicates sporadic massive biological extinction on millennial timescales. The trigger for lake overturns in Lake Kivu's case is unknown but volcanic activity is suspected. The gaseous chemical composition of exploding lakes is unique to each lake; in Lake Kivu's case, methane and carbon dioxide due to lake water interaction with a volcano. The amount of methane is estimated to be 65 cubic kilometers (if burnt over one year, it would give an average power of about 100 gigawatts for the whole period). There is also an estimated 256 cubic kilometers of carbon dioxide. The methane is reported to be produced by microbial reduction of the volcanic CO2.[3] The risk from a possible Lake Kivu overturn is catastrophic, dwarfing other documented lake overturns at Lakes Nyos and Monoun, because of the approximately two million people living in the lake basin.
Cores from the Bukavu Bay area of the lake reveal that the bottom has layered deposits of the rare mineral monohydrocalcite interlain with diatoms, on top of sapropelic sediments with high pyrite content. These are found at three different intervals. The sapropelic layers are believed to be related to hydrothermal discharge and the diatoms to a bloom which reduced the carbon dioxide levels low enough to precipitiate monohydrocalcite.[4]
Scientists hypothesize that sufficient volcanic interaction with the lake's bottom water that has high gas concentrations would heat water, force the methane out of the water, spark a methane explosion, and trigger a nearly simultaneous release of carbon dioxide.[5][6] The carbon dioxide would then suffocate large numbers of people in the lake basin as the gases roll off the lake surface. It is also possible that the lake could spawn lake tsunamis as gas explodes out of it.[7][8]
The risk posed by Lake Kivu began to be understood during the analysis of more recent events at Lake Nyos. Lake Kivu's methane was originally thought to be merely a cheap natural resource for export, and for the generation of cheap power. Once the mechanisms that caused lake overturns began to be understood, so did awareness of the risk the lake posed to the local population.
An experimental vent pipe was installed at Lake Nyos in 2001 to remove gas from the deep water, but such a solution for the much larger Lake Kivu would be considerably more expensive. No plan has been initiated to reduce the risk posed by Lake Kivu. The about 500 million tonnes of carbon dioxide in the lake is a little under 2 percent of the amount released annually by human fossil fuel burning. Therefore the process of releasing it could potentially have costs beyond building and operating the system.
Lake Kivu has recently been found to contain approximately 55 billion cubic metres (72 billion cubic yards) of dissolved methane gas at a depth of 300 metres (1,000 ft). Until 2004, extraction of the gas was done on a small scale, with the extracted gas being used to run boilers at a brewery, the Bralirwa brewery in Gisenyi.[9][10] As far as large-scale exploitation of this resource is concerned, the Rwandan government has negotiated with a number of parties to produce methane from the lake.
In 2011 Contour Global, a U.S. based energy company focused on emerging markets, secured project financing to initiate a large-scale methane extraction project. The project will be run through a local Rwandan entity called KivuWatt, using an offshore barge platform to extract, separate and clean the gasses obtained from the lake bed before pumping purified methane via an underwater pipeline to on-shore gas turbines ("gensets"). Stage one of the project aims to build and supply three "gensets" along the lake shore, totaling 25MW of electrical capacity. Initial project operations are scheduled to commence in 2012. [11] In addition to managing gas extraction, KivuWatt will also manage the electrical generation plants and on-sell the electrical power to the Rwandan government under the terms of a long-term Purchase Power Agreement (PPA). This allows KivuWatt to control a vertically integrated energy offering from point of extraction to point of sale into the local grid. Extraction is said to be cost-effective and relatively simple because once the gas-rich water is pumped up, the dissolved gases (primarily carbon dioxide, hydrogen sulphide and methane) begin to bubble out as the water pressure gets lower. This project is expected to increase Rwanda's energy generation capability by as much as 20 times and will enable Rwanda to sell electricity to neighboring African countries.[10]
A problem associated with the prevalence of methane is that of mazuku.
The fish fauna in Lake Kivu is relatively poor (28 species[12]). 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.[13] 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.[14][15]
The first comprehensive phytoplankton survey was released in 2006.[16] 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, favoure dominance of cyanobacteria with high numbers of phototrophic picoplankton.[16][17][18][19] The actual primary production is 0.71 g C m-2 d-1 (~ 260 g C m-2 y-1).[20]
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.[21] 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,[22] although the cichlid's molecular clock suggests the existence of a lake much older than the commonly cited 15,000 years.
The first European to visit the lake was German Count Adolf von Götzen in 1894. Since then it has been caught up in the conflict between Hutu and Tutsi people in Rwanda, and their allies in DR Congo, which led to the 1994 Rwandan Genocide and the First and Second Congo Wars. Lake Kivu gained notoriety as a place where many of the victims of the genocide were dumped.