Lake Tanganyika

Category: Inland Aquatic Biomes.

Geographic Location: Africa.

Summary: Lake Tanganyika is the world’s second-deepest freshwater lake, and supports an amazing variety of fish species.

Lake Tanganyika is the second-deepest lake in the world, at 4,823 feet (1,470 meters), after Lake Baikal in Siberia. It is also the second-largest freshwater lake in the world by volume. It is located within the East African Great Rift Valley and divided among four countries: Burundi, Democratic Republic of the Congo (DRC), Tanzania, and Zambia. Most of the lake is within the DRC (45 percent) and Tanzania (41 percent). The water of the lake flows into the Congo River system and ultimately into the Atlantic Ocean.

Lake Tanganyika is narrow; it extends about 404 miles (650 kilometers) in length, by 31 miles (50 kilometers) wide, on average. It covers 12,703 square miles (32,900 square kilometers), with a shoreline of 1,136 miles (1,828 kilometers), a mean depth of 1,870 feet (570 meters), and a maximum depth of 4,823 feet (1,470 meters) in the northern basin. It holds an estimated 4,534 cubic miles (18,900 cubic kilometers) of water.

The lake has an average surface temperature of 77 degrees F (25 degrees C) and a pH averaging 8.4, certainly a bit on the alkaline side. Studies show that beneath the 1,640 feet (500 meters) of water are about 14,764 feet (4,500 meters) of sediment lying over the rock floor.

Setting and Hydrology

The lake catchment is approximately 89,190 square miles (231,000 square kilometers), with the Rusizi River entering from Lake Kivu in the north and the Malagarasi River entering from the east side of the lake, as well as numerous smaller rivers and streams.

The lake is surrounded by mountainous areas. The eastern side has poorly developed coastal plains; on the western coast, the steep side walls of the Great Rift Valley reach 6,562 feet (2,000 meters) in relative height from the shoreline. Its sole effluent river, the Lukuga, starts from the middle part of the western coast and flows westward to join the Lualaba, a tributary of the Congo River, which flows on to the South Atlantic Ocean.

The water balance of the lake is largely determined by rainfall and evaporation. A total of 31–47 inches (78–119 centimeters) per year of rain falls in the vicinity of the lake, while evaporation averages 60 inches (152 centimeters) per year. The many rivers and streams that enter the lake, including its major tributaries, the Rusizi and the Malagarasi, play small roles in the lake’s water balance. The flushing time of water in the lake (lake volume divided by river outflow) is up to 7,000 years because the rivers have such a small influence over the lake.

Due to the lake’s enormous depth and location within the tropics, there is little or no mixing of the shallower and deeper waters, which qualifies the lake as meromictic: The noncirculating hypolimnion (bottom layer) does not mix with the circulating upper layer (epilimnion). The mixed oxygenated layer extends to about 164–820 feet (50–250 meters), influenced by the seasons. The distribution of aquatic life is limited to this depth, as beyond this depth the environment is anaerobic, and accumulated particulate matter makes the water much denser. Permanent stratification keeps much of the nutrients in the noncirculating hypolimnion.

Biodiversity

The lake has some of the richest lake fauna on Earth, making it an important biological resource for the study of speciation in evolution. The lake’s great depth is thought to have given its fauna an evolutionary advantage. Over its long history, estimated to be 9 million to 12 million years, Lake Tanganyika must have served as a refuge for aquatic organisms during extremely dry periods when other water bodies desiccated.

The split into two or three separate basins during low lake levels seems to have had important effects on the evolution and distribution of the ichthyodiversity, and facilitated allopatric speciation. The age of the lake has also facilitated further differentiation of the fish fauna compared with the fauna of lakes Malawi and Victoria. The Lake Tanganyika cichlids, for example, are considered to have evolved from eight ancestral lineages, more than in Lake Malawi or Victoria.

The lake’s biodiversity exhibits high levels of endemism (species found exclusively here) within several taxonomic groups at the species and genus levels. It holds at least 250 species of cichlid fish and 150 noncichlid species, most of which live along the shoreline to a depth of approximately 591 feet (180 meters).

The fish fauna of Lake Tanganyika have strong affinities with the Congo basin, and the two systems are still connected hydrologically. In the late Miocene-early Pliocene, the Congo basin contained a large internal lake that covered the Cuvette Centrale.

It is believed that Lake Tanganyika’s fauna originated in this ancient environment, although the details and timeline are not complete. The 23 fish families of Lake Tanganyika are all present in the Congo basin fauna.

Human Activities

It is estimated that 60 percent of the protein in the diet of the approximately 1 million people living around the lake comes from lake fish. Fishery products, especially the Tanganyika sardine (Stolothrissa tanganikae), are vital for the local economy. Currently, around 100,000 people are directly involved in the fisheries, operating from almost 800 sites. The lake is also vital to the estimated 10 million people living in the greater basin area.

Lake Tanganyika fish are exported throughout eastern Africa. Commercial fishing began in the mid-1950s and has had an extremely heavy effect on the pelagic fish species. In the mid-2010s, the total annual catch was about 200,000 tons (181,000 metric tons). However, the lake’s fish production has since begun a rapid decline, mostly due to climate change and the movement of refugees caused by regional conflicts in Burundi, Rwanda, and the Democratic Republic of the Congo.

The lake also serves as a transport gateway to the larger East Africa region. Regular ship lines connect Kigoma, Tanzania; Kalemie, Zaire; and other coastal towns as an essential part of the inland traffic system of the region. Two ferries carry passengers and cargo along the eastern shore of the lake: the MV Liemba, which runs between Kigoma and Mpulungu, and the MV Mwongozo, which runs between Kigoma and Bujumbura.

Agriculture, livestock, and the processing of their related products, as well as mining (tin, copper, and coal, mainly) are still the main industries in the drainage basin of Lake Tanganyika.

Environmental Threats

Some of the major environmental threats are deforestation, resulting from agricultural expansion, and overgrazing due to an influx of livestock, mostly from Sukuma-land in Tanzania. These activities have led to increased erosion, causing more flooding in coastal areas and the siltation of the lake. There are also destructive fishing practices; the use of beach seines is rampant, especially in the inshore areas of the lake.

Artisanal, small-scale gold mining is another challenge to the lake ecosystem, particularly the use of mercury for binding the gold. In many cases, mercury residues escape to pollute the rivers used by the miners to process their gold, not to mention the neurotoxic effect of mercury on the miners themselves.

Climate change impacts upon Lake Tanganyika have been the subject of numerous studies. The lake’s waters are warming—rising by about 2.3 degrees Fahrenheit (1.3 Celsius) from 1913 to the mid-2010s. Warmer water is lighter and limits the mixing of colder, deeper waters with surface waters. This, in turn, lessens the amount of nutrients that can churn up to the surface, reducing the prime food supply for the lake’s fish. Heavy rains caused by climate change have also raised the lake level, leading to flooding that has displaced many of those living near its shores. In Burundi alone, the rising levels of Lake Tanganyika were responsible for 84 percent of internal migration in the late 2010s and early 2020s.

Bibliography

Cohen, Andrew. “Lake Tanganyika Is Changing, and the Fate of Millions Lie in the Balance.” CNN, 18 May 2017, www.cnn.com/2017/05/18/africa/lake-tanganyika-under-threat-the-conversation. Accessed 30 Aug. 2022.

Coulter, G. W. Lake Tanganyika and Its Life. Oxford, UK: Oxford University Press, 1991.

Lowe-McConnell, Rosemary H. “Fish Faunas of the African Great Lakes: Origins, Diversity and Vulnerability.” Conservation Biology 7 (1993).

Patterson, G. and J. Makin. The State of Biodiversity in Lake Tanganyika: A Literature Review. Chatham, UK: Natural Resources Institute, 1998.

Paul, Madhumita. “Most Internal Displacement in East Africa’s Burundi Due to Rise of Lake Tanganyika.” Down to Earth, 30 Sept. 2021, www.downtoearth.org.in/news/climate-change/most-internal-displacement-in-east-africa-s-burundi-due-to-rise-of-lake-tanganyika-79106. Accessed 30 Aug. 2022.

Roberts, T. R. “Geographical Distribution of African Freshwater Fishes.” Zoological Journal of the Linnean Society 57 (1975).

Salzburger, W., A. Meyer, et al. “Phylogeny of the Lake Tanganyika Cichlid Species Flock and Its Relationship to the Central and East African Haplochromine Cichlid Fish Faunas.” Systematic Biology 51, no. 1 (2002).