The Holocene History of Lake Victoria
Johnson, Thomas C
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Recent investigations by the International Decade for the East African Lakes (IDEAL) have significantly advanced our knowledge on the history of Lake Victoria. Seismic reflection profiles confirm the origin of the lake as a result of regional tilting and provide an estimated age of 400 000 years for the lake basin. Three major desiccation events are recorded in the seismic records that may reflect the 100 000 year Milankovitch cycle of climate forcing. The most recent arid period resulted in complete desiccation of the pre-existing lake. Lake Victoria arose from a dry landscape 14 600 calendar years ago (14.6 ka). Primary production was extremely high as lake level rose in its first 500 years, nourished by the high input of nutrients from the flooded landscape. A few species of cichlids and other fish swam out of their fluvial refugia to colonize the new lake, generating hundreds of new endemic species over the ensuing 14 000 years. Lake level rose until a brief overflow event at about 14.2 ka to 13.6 ka. Closed-basin conditions returned during the Younger Dryas until 11.2 ka, when major outflow and open-basin conditions were permanently established. The lake experienced progressively stronger stratification and water-column stability through the first half of the Holocene, and diatom productivity dropped to a minimum from 9.8 to about 7.5 ka. This period coincides with, but is much longer than, the 8.2 ka climatic event that has been observed in many Holocene records from throughout the world. The degree of water-column mixing appears to have steadily increased over the last 6000 years. Short cores from the open lake, document a shift in lake conditions beginning in the 1930s that progressed to the major ecosystem collapse of the early 1980s. The coincidence of the shift in sediment properties in the 1930s with the beginning of rapid expansion of human population and agricultural activity suggests cause and effect. It is conceivable that the lake experienced similar conditions due to natural causes between about 9800 and 7500 years ago.