Instrumental observations suggest that Lake Tanganyika, the largest rift lake in East Africa, has become warmer, increasingly stratified and less productive over the past 90 years (refs1,2). These trends have been attributed to anthropogenic climate change. However, it remains unclear whether the decrease in productivity is linked to the temperature rise, and whether the twentieth-century trends are anomalous within the context of longer-term variability. Here, we use the TEX 86 temperature proxy, the weight per cent of biogenic silica and charcoal abundance from Lake Tanganyika sediment cores to reconstruct lake-surface temperature, productivity and regional wildfire frequency, respectively, for the past 1,500 years. We detect a negative correlation between lake-surface temperature and primary productivity, and our estimates of fire frequency, and hence humidity, preclude decreased nutrient input through runoff as a cause for observed periods of low productivity. We suggest that, throughout the past 1,500 years, rising lake-surface temperatures increased the stratification of the lake water column, preventing nutrient recharge from below and limiting primary productivity. Our records indicate that changes in the temperature of Lake Tanganyika in the past few decades exceed previous natural variability. We conclude that these unprecedented temperatures and a corresponding decrease in productivity can be attributed to anthropogenic global warming, with potentially important implications for the Lake Tanganyika fishery.
Bibliographical noteFunding Information:
We thank Y. Huang and M. Alexandre for laboratory analytical assistance. We also thank the Tanzania Fisheries Research Institute (TAFIRI), the University of Dar es Salaam and the crew of the M/V Maman Benita for assistance in the field. This research was supported by NSF-EAR 0639474 to J.M.R. and the Nyanza Project (NSF-ATM 0223920 to A.S.C.).
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