Lake Victoria

Located on the border between Uganda, Kenya, and Tanzania, Lake Victoria is the world's second-largest lake. It is both the biggest tropical lake on Earth and Africa's largest freshwater body, with a surface of 69,000 square kilometers. Lake Victoria straddles the equator and forms the source of the Nile, Africa's longest river. The lake probably was formed 400,000 years ago over, ancient bedrock in a high, broad depression between East Africa's two great rift valleys.

Lake Victoria flowed westward until the Pleistocene epoch (which began 1.6 million years ago), when tectonic uplift turned its flow to the north. The lake level appears to have fluctuated significantly since then, and at the end of the last glaciation, between fifteen thousand and seventeen thousand years ago, the lake may have dried up entirely.

This is not surprising because it is shallow, with a maximum depth of 68 meters and a mean depth of only 40 meters. Even during the twentieth century the surface of Lake Victoria has varied by about 2 meters as a result of changes in precipitation and evaporation. The lake receives about 80 percent of its water as a result of rainfall directly onto the surface.

Lake Victoria has also been the site of a spectacular evolutionary event: the adaptive radiation of cichlid fishes from a single common ancestor into three hundred to four hundred species. Cichlids are small, . colorful fish frequently bred for aquariums.

Mitochondrial DNA evidence indicates that the cichlid species of Lake Victoria are all closely related and that many would even interbreed if they did not segregate themselves by habitat and sexual selection, which is usually based on physical characteristics such as color. Several studies have examined how so many species could have evolved from one progenitor in a single lake.

One hypothesis held that populations of cichlids had become isolated in small ponds in the past when the lake level declined, giving them time to diverge from their relatives through genetic drift (random changes in gene frequency, especially in small populations when leading to preservation or extinction of particular genes).

Another hypothesis held that the lake had not dried up, that it had been fairly stable over a long period of time, and that cichlids had diversified through sympatric (located in the same area) speciation. Recent evidence indicates that neither hypothesis is correct. It now appears that the cichlids of Lake Victoria radiated into their diverse modern forms only since the lake refilled after the end of the Pleistocene epoch, twelve thousand to fourteen thousand years ago. If correct, this would certainly be a world speed record for vertebrate evolution.

In the twentieth century Lake Victoria and its cichlids have fared poorly. Since the 1920s the formerly clear lake has become increasingly murky and anoxic (oxygen poor) as a result of eutrophication (the process by which a body of water becomes enriched in dissolved nutrients). Eutrophication is caused when nutrients enter an ecosystem and encourage algal blooms, which in turn deprive the water of its dissolved oxygen content.

Because sexual selection is largely based on coloration, many cichlid fishes may require clear waters in order to properly identify adequate mates. The water hyacinth, a noxious South American plant, has also colonized Lake Victoria, thriving in nutrient rich waters and creating dense mats of vegetation that destroy habitat, block sunlight, and render travel impossible. Finally, the enormous and predatory Nile perch was introduced in the 1950s to increase the marketable fishery resources of the lake.

The perch's population exploded in the 1980s, and its appetite for cichlids seems insatiable. Over two hundred species of Lake Victoria cichlids have probably become extinct as a result, and many others are considered endangered. In the future scientists, policymakers, and a rapidly growing local population will have to make difficult choices about how best to achieve a healthy and sustainable Lake Victoria.