Lake Tanganyika

Lake Tanganyika is a massive African Great Lake. It is a vital part of the Earth's water system. This lake is the longest freshwater lake in the world. It is also the second deepest and second largest by volume. Only Lake Baikal in Siberia is larger in these ways.

The lake sits within the Albertine Rift. This is the western branch of the East African Rift. High, mountainous walls confine the valley where the lake lies. It is the largest rift lake on the African continent.

Four different countries share the waters of Lake Tanganyika. These nations are Tanzania, the Democratic Republic of the Congo (DRC), Burundi, and Zambia. Tanzania possesses 46% of the lake. The DRC holds 40% of the water. The lake drains through the Lukuga River. This river flows into the Congo River system. Eventually, that water reaches the Atlantic Ocean at Banana.

The lake's water levels determine if it can reach the sea. When levels are low, sand bars and weeds block the Lukuga River. In those times, the river relies on its own tributaries like the Niemba River.

Water movement in the lake is a complex process. Most of the water comes from rain. At least 90% of the water influx is from precipitation. At least 90% of the water loss is from direct evaporation. Two major rivers flow into the lake. The Ruzizi River enters from the north. It was formed about 10,000 years ago from Lake Kivu. The Malagarasi River enters from the east side. This river is Tanzania's second largest river. It is older than Lake Tanganyika itself.

Lake Tanganyika is an ancient feature of the Earth. It is one of only twenty lakes older than one million years. The lake is made of three distinct basins. These basins have different ages. The central basin began forming 9 to 12 million years ago. The northern basin formed 7 to 8 million years ago. The southern basin is the youngest at 2 to 4 million years old. In the past, these basins may have been separate lakes. The lake's history includes many changing flow patterns due to volcanic activity.

The chemistry and temperature of the water are very specific. The water is alkaline, meaning it has a high pH. At depths of 100 meters, the pH is around 9. In the deepest parts, it decreases to between 8.3 and 8.5.

Surface temperatures change with the seasons. In the south, temperatures range from 24°C in August to 28°C in March. Below 200 meters, the temperature is very stable at 22°C. The lake is stratified, which means it has distinct layers. Seasonal mixing usually does not go deeper than 200 meters. This leaves the deep sections with "fossil water."

This stratification creates a difficult environment for life. The deep sections are anoxic, meaning they have no oxygen. These deep areas contain high levels of toxic hydrogen sulfide. Consequently, these sections are essentially lifeless except for bacteria. Most fish and aerobic organisms are limited to the upper layers. In the north, this limit is typically around 100 meters. In the south, the limit is similar.

Some deep-water cichlid genera live in places with almost no oxygen. Scientists are still unsure how they survive there.

The lake is a massive center for biological diversity. It is home to at least 250 endemic species of cichlids. Endemic means they are found nowhere else on Earth. About 98% of the lake's cichlid species are unique to Tanganyika.

These fish show incredible adaptive radiation. This means they have evolved many different forms to fill different roles. Some eat plants, while others eat insects or even scales. Some are tiny, measuring only 2 centimeters. Others, like the giant cichlid, reach 60 centimeters.

The lake also hosts over 80 non-cichlid fish species. About 60% of these are also endemic. This makes the lake a precious resource for studying evolution.