One rift lake holds more fish species than any other
Lake Malawi—Nyasa in Tanzania, Niassa in Mozambique—ranks fourth among freshwater lakes by volume and ninth by area, yet leads the planet in fish diversity with at least 700 cichlid species. Meromictic layering keeps deep water from mixing with the sunlit surface.
The southernmost East African Rift lake stretches 560–580 kilometres long and about 75 wide across roughly 29,600 square kilometres, plunging to 706 metres. Malawi, Mozambique, and Tanzania share the shores; the Ruhuhu is the largest inflow, and the Shire River exits south toward the Zambezi. Evaporation removes more than four-fifths of the water that leaves the basin. Lake Malawi National Park guards the southern tip.
The water is alkaline, pH 7.7 to 8.6, with a surface usually between 24 and 29 °C and depths near 22 °C. A thermocline sits 40 to 100 metres down, and below about 250 metres there is no oxygen, so fish are confined to the upper layer. Small chemical and thermal gradients lock an oxic–anoxic boundary in place, so nutrients and oxygen stay stratified. Visibility can reach 20 metres, though under half that is typical and muddy river water clouds it from January to March.
Long dated at one to two million years, the basin now looks far older, beginning to form about 8.6 million years ago with deep water by 4.5 million. About 350 kilometres southeast of Tanganyika, the lake has swung historically from nearly 600 metres below today’s level to tens of metres above, sometimes shrinking to salty remnant pools. Chemistry like today’s appeared only about 60,000 years ago, and between 1390 and 1860 the level may have sat 120 to 150 metres lower.
The Portuguese trader Candido José da Costa Cardoso reached it in 1846, and David Livingstone arrived in 1859, calling it Lake Nyasa as well as the Lake of Stars, for fishermen’s lanterns, and the Lake of Storms, for its violent gales. On 16 August 1914 the British gunboat Gwendolen crippled the German Hermann von Wissmann with a single shot, a clash The Times hailed as Britain’s first naval win of the First World War.
Source: Lake Malawi
