The Limnology, Climatology and Paleoclimatology of the East African Lakes

Abstract

The large lakes of the East African Rift Valley are among the oldest on Earth, and are vital resources for the people of their basins. They are unique among the large lakes of the world in terms of their sensitivity to climatic change, rich and diverse populations of endemic species, circulation dynamics and water-column chemistry, and long, continuous records of past climatic change. More than four kilometers of sediment underlie Lakes Tanganyika and Malawi, and their ages are estimated to be on the order of 10-15 million years based on models of sedimentation and compaction in rift basins. Some of the lakes of the Rift Valley are closed basins (i.e., without outlets) and their surface levels fluctuate dramatically both seasonally and interannually in esponse to rainfall variability. Even the open-basin lakes such as Tanganyika, Victoria, and Malawi lose 80-90% of their water by evaporation. These lakes have fluctuated between closed- and open-basin status frequently in response to varying rainfall and evaporation, causing their levels and water chemistry and biota to shift significantly with climatic change. The largest lakes contain hundreds of endemic species of fish and invertebrates, providing ample opportunity for the study of evolutionary biology within confined systems. The African Great Lakes have unique physical qualities that affect water circulation. Coriolis Force is weaker in equatorial lakes than in lakes at higher latitudes. The Rift Valley channels the trade winds so effectively that wind forcing of the lake circulation is remarkably unidirectional. Temperature profiles are nearly isothermal, yet contain fascinating but subtle structure that greatly impacts vertical circulation. Water chemistry reflects a variety of biological, physical, and chemical processes. The deep lakes are anoxic in their hypolimnions ; nutrient cycling is complex; and inorganic reverse weathering reactions may occur in early diagenesis of lake floor sediments that have major impact on the major ion composition of the overlying water. Changes in sediment composition in response to changing climate and lake level are easily discerned on a time scale resolvable to decades, if not individual years, because of rapid sedimentation rates and a lack of bioturbation in the deep basins.