维多利亚湖的全新世历史
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摘要
"国际东非湖泊十年计划"(IDEAL)近年的调查工作,大大加深了我们对维多利亚湖演化历史的认识。地震反射剖面证实了湖泊成因是区域掀斜的结果,并提出了湖盆形成的估算年龄为40万年。地震剖面记录的三次主要干旱事件,反映了影响气候变化的10万年的米兰科维奇周期。最近一次干旱导致了原湖泊完全干涸。维多利亚湖水位由14600年前的干湖底上升了。由于洪水输入了许多高营养物质,湖泊水位上升的前500年其初级生产力很高。一些丽鱼科鱼和其它的鱼种游出它们在河中的隐蔽所来到新的湖泊,在随后的14000年中产生了几百个新的特有种。湖泊水位上升一直持续到约14200～13600年前的一次大泛滥。直到11200年前的新仙女木时期湖泊重新封闭,此时形成了永久的外流、开放湖盆的条件。在全新世前半段,湖泊分层逐渐增强,水体相对稳定,硅藻的生产力在距今9800～7500年降到了最低值。这一时期是与全球范围内许多全新世记录中观察到的距今8200年气候事件是一致的,但要长得多。最近6000年间,水体混合程度稳步增强。从开放湖盆所取的短岩芯证明,湖泊条件的变化开始于20世纪30年代,它导致了80年代早期主要生态系统的崩溃。30年代沉积物性质的变化与人口剧增和农业活动加剧的开始相一致,说明这两者的因果关系。可以想象,此湖泊在约9800～7500年前经历了自然条件造成的类似状况。
Recent investigations by the International Decade for the East African Lakes (I DEAL) 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.
Recent investigations by the International Decade for the East African Lakes (I DEAL) 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.
引文
1． Talling, J. F. 1965． The photosynthetic activity of phytoplankton in East African lakes. Int. Rev. Hydrobiol. 50, 1-32．
2． Talling, J. F. 1966． The annual cycle of stratification and phytoplankton growth in Lake Victoria (East Africa). Int. Revi. Hydrobiol. 51,545-621．
3． Hecky, R. E. 1993． The eutrophication of Lake Victoria. Proc. Int. Ass. Theor. Appl. Limnol. 25,39-48．
4． Mugidde, R. 1993． The increase in phytoplankton productivity and biomass in Lake Victoria (Uganda). Int. Ass. Theore. Appl. Limnol. Proc. 25, 846-849．
5． Ogutu-Ohwayo, R. 1990． The decline of the native fishes of Lake Victoria and Kyoga (East Africa) and the impact of introduced species, especially the Nile perch, Lates niloticus and the Nile tilapia, Oreochromis niloticus. Euviron. Biol. Fisheries 27, 81-86．
6． Ogutu-Ohwayo, R. 1992． The purpose, costs and benefits of fish introductions: with specific reference to the Great Lakes of Africa. Proc. Int. Ass. Theor. Appl. Limnol. 23,34-44．
7． Witte, F., Goldschmidt, T., Wanink. J. M., Oijen, M. J. P. van, Witte-Maas, E. L. M. and Bouton, N. 1992． The destruction of an endemic species flock: quantitative data on the decline of the haplochromine cichlids of Lake Victoria. Environ. Biol. Fisheries 29, 1-28．
8． Johnson. T. C. And Odada, E. O. (eds.). 1996, The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach, Amsterdam, the Netherlands, 664 pp.
9． Lehman, D. 1998． History and ontogeny of IDEAL. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 1-6．
10． Johnson, T. C. 1993． IDEAL: An International Decode for the East African Lakes. Science and Implementation Plan. PAGES Workshop Report, Series 93-2． PAGES Core Project Office, Bern, Switzerland. 40 pp.
11． Herdendorf, C. E. 1990． Distribution of the world's largest lakes. In: Large Lakes: Ecological Structure and Fundction. Tilzer, M. M. And Serruya, C. (Eds.). Springer-Verlag, Berlin, Germany, pp. 3-38．
12． Bishop, W. W. and Trendall, A. F. 1967． Erosion surfaces, tectonics and volcanic activity in Uganda. Quart. J. Geol. Soc. London 122． 385-420．
13． Scholz, C. A., Johnson, T. C., Cattaneo, P., Malinga, H. and Shana, S. 1998． Initial results of 1995 seismic reflection survey of Lake Victoria, Uganda and Tanzania. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers. Dordrecht, Germany, pp. 47-58．
14． Kendall, R. L. 1969． An ecological history of the Lake Victoria basin. Ecol. Monogr. 39, 121-176．
15． Stager, J. C., Reinthall, P. N. and Livingstone, D. A. 1986． A 25,000 year history for Lake Victoria, East Africa and some comments on its significance for the evolution of cichlid fishes. Freshwater Biol. 6, 15-19．
16． Talbot, M. R. and Livingstone, D. A. 1989． Hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators. Palaeogeogr., Palaeoclimatol.,Palaeoecol. 70, 121-137．
17． Johnson, T. C., Scholz, C. A., Talbot, M. R., Kelts, K., Ricketts, R. D., Ngobi, G., Beuning, K. R. M., Ssemmanda, I. and McGill, J. W. 1996． Late Pleistocene desiccation of Lake Victoria and rapid evolution of cichlid fishes. Science 273, 1091-1093．
18． Broecker, W. S., Peteet, D., Hajdas, I., Lin, J. and Clark, E. 1998． Antiphasing between rainfall in Africa's Rift Valley and North America's Great Basin. Quart, Res.50, 12-20．
19． Greenwood, P. H. 1980． Towards a phyletic classification of the "genus" Haplochromus (Pisces, Cichlidae) and related taxa. Part Ⅱ: The species from Lakes Victoria, Nabugabo, Edward, George and Kivu. Bull. British Museum Nat. Hist.(Zoology) 39,1-101．
20． Kaufman, L. and Ochumba, P. 1993． Evolutionary and conserv, biology of cichlid fishes as revealed by faunal remnants in northern Lake Victoria. Conservation Biol. 7, 719-730．
21． Cohen, A., Soreghan, M. and Scholz, C.A. 1993． Estimating the age of formation of lakes: an example from Lake Tanganyika, East African rift system. Geolagy 21, 511-514．
22． Meyer, A., Montero, C. M. and Spreinat, A. 1996． Molecular phylogenetic inferences about the evolutionary history of East African cichlid fish radiations. In: The Limnology, Climatology and Paleoclimatology of the East African Lakes. Johnson, T. C. and Odada, E.O.(eds.). Gordon and Breach, Amsterdam, the Netherlands, pp, 303-323．
23． Meyer, A., Kocher, T. D., Basasibwaki, P. and Wilson, A. C. 1990． Monophyletic origin of Lake Victoria cichlid fishes suggested by mitochondrial DNA sequences. Nature 347． 550-553．
24． Galis, F. and Metz, J. A. J. 1998． Why are ther so many cichlid species? Tree 13, 1-2．
25． Stiassny, M, L. J. and Meyer, A. 1999． Cichlids of the rift lakes. Sci. Amer. 280, 64-69．
26． Owen, R. B., Crossley, R., Johnson, T. C., Tweddle, D., Kornfield, I.Davidson, S., Eccles, D. H. and Engstrom, D. E. 1990． Major low levels of Lake Malawi and implications for speciation rates in cichlid fishes. Proc. Roy. Soc. London B 240, 519-553．
27． Fryer, G. 1997． Biological implications of a suggested Late Pleistocene desiccation of Lake Victoria. Hydrobiologia 354． 177-182．
28． Beuning, K. R. M., Talbot, M. R. and Kelts, K. 1997: A revised 30000-year paleoclimatic and paleohydrologic history of Lake Albert, East Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol. 136, 259-279．
29． Talbot, M. R. and Laerdal, T. 1999, The Late Pleistocene-Holocene palaeolimnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter. J. Paleolimnol. (in press.)
30． Le Bas. 1997． (Please complete)
31． Ngobi, G. N., Kelts, K., Johnson, T. C. and Silheid, P. A. 1998． Environmental magnetism of Late Pleistocene/Holocene sequences from Lake Victoria, East Africa. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 59-74．
32． Meyers, P.A. and Ishiwitari, R. 1995． Organic matter accumulation records in lake sediments. In: Physics and Chemistry of Lakes. Lerman, A., Imboden, D. M. and Gat, J. R. (Eds.). Springer-Verlag, Berlin, Germany, pp. 279-328．
33． Tyson, R. V. 1995． Sedimentary Organic Matter: Organic Facies and Palynofacies. Chapman and Hall, London. United Kingdom, 615 pp.
34． Hecky, R. E. and Hesslein, R. H. 1995． Contributions of benthic algae to lake food webs as revealed by stable isotope analysis. J. N. Amer. Benthol, Soc. 14, 631-653．
35． Bootsma, H. A., Hecky, R. E., Hesslein, R. H, and Turner, G. F. 1996． Food partitioning among Lake Malawi nearshore fishes as revealed by stable isotope analyses. Ecology 77,1286-1290．
36． Kidd. K. A., Lockhart, L., Wilkinson, P. and Muir, D. C. G. 1998． Metals, pesticides and other persistent contaminants in water, sediments and biota from Lake Malawi/Nyasa. In: Water Quality Report. Bootsma, H. A. And Hecky, R. E. (eds.). Lake Malawi/Nyasa Biodiversity Conservation Project, Senga Bay, Malawi. Chapter 9．
37． Sirocko, F. 1997． The evolution of the monsoon climate over the Arabian Sea during the last 24 000 years. Palaeoecol. Afr. 24, 53-70．
38． Blunier, T., Chappelaz, J., Schwander, J., Stauffer, B. and Raynaud, D. 1995． Variations in atmospheric methane concentration during the Holocene epoch. Nature 374, 46-49．
39． Alley, R. B., Mayewski. P. A., Sowers, T., Stuiver, M., Taylor, K. C. And Clark, P. U. 1997． Holocene climatic instability: a prominent, widespread event 8200 yr ago. Geology 25, 483-486．
40． Indermuhle, A., Stocker, T. F., Joos, F., Fischer, H., Smith. H. J., Wahlen, M., Deck, B., Mastroianni, D., Tschumi, .J., Blunier, T., Meyer, R. and Stauffer, B. 1999． Holocene carbon cycle dynamics based on CO_2 trapped in ice at Taylor Dome, Antarctica. Nature 398, 12-126．
41． Johnson, T. C., Chan, Y., Beuning, K. R. M.. Kelts, K., Ngobi, G. and Verschuren, D. 1998． Biogenic silica profiles in Holocene cores from Lake Victoria: implications for lake level history and initiation of the Victoria Nile. In: Environmental Change and Response in East African Lakes. Lehman. J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 75-88．
42． Beuning, K. R. M.. Kelts K., Ito, E. And Johnson, T. C. 1997． Paleohydrology of Lake Victoria, East Africa, inferred from ~(18) O~(16) O ratios in sediment cellulose. Geology 25． 1083-1086．
43． Roberts, N., Taieb, M., Barker, P., Damnati, B. Icole, M. and Williamson, D. 1993． Timing of the Younger Dryas event in East Africa from lake-level changes. Nature 366, 146-148．
44． Beuning, K. R. M., Kelts, K. and Stager, J. C. 1998． Abrupt climatic changes associated with the arid Younger Dryas interval in Africa. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 147-156．
45． Stager, J. C. and Johnson, T. C. 1999． A 12,400 ~(14) C year offshore diatom record from east central Lake Victoria, East Africa. J. Paleolimnol., (in press.)
46． Barber, D. C., Dyke, A., Hillaire-Marcel, C., Jennings, A. E., Andrews, J. T., Kerwin, M. W., Bilodeau, G., McNeely, R., Southon, J., Morehead, M. D. and Gagnon, J.-M. 1999． Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes. Nature 400, 344-348．
47． Street-Perrott and Roberts 1983． (Please complete).
48． Edwards, T. W. D., Wolfe, B. B.and MacDonald, G. M. 1996． Influence of changing atmospheric circulation on precipitation ~(18) O-temperature relations in Canada during the Holocene. Quat. Res. 46, 211-218．
49． Verschuren, D., Edgington, D. N., Kling, H. And Johnson, T. C. 1998． Silica depletion in Lake Victoria: sedimentary signals at offshore stations. J. Great Lakes Res. 24,118130． 50． Ochumba, P. B. O. And Kibaara, D. I. 1989． Observations on blue-green algal blooms in the open waters of Lake Victoria, Kenya. Afri. J. Ecol. 27, 23-34．
51． Seehausen, O., van Alphen, J. J. M. and Witte, F. 1997． Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 277, 1808-1811．
52． Holtzman, J. And Lehman, J. T. 1998． Role of apatite weathering in the eutrophication of Lake Victoria. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 89-98．
53． Stager, J. C. 1998． Ancient analogues for recent environmental changes at Lake Victoria, East Africa. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 37-46．
54． We thank the governments of Tanzania and Uganda for providing IDEAL researchers access to their territorial waters of Lake Victoria in 1995 and 1996, and the scientists and staff of the Ugandan Fisheries Research Institute in Jinja, under the direction of Dr. Fred Bugenyi, for logistical support. This manuscript benefited from the thoughtful reviews by Henry F. Lamb and Michael R. Talbot. Funding was provided by grants from the U.S. National Science Foundation to T. C. Johnson and K. Kelts. This is publication number 78 of the International Decade for the East African Lakes (IDEAL).
2． Talling, J. F. 1966． The annual cycle of stratification and phytoplankton growth in Lake Victoria (East Africa). Int. Revi. Hydrobiol. 51,545-621．
3． Hecky, R. E. 1993． The eutrophication of Lake Victoria. Proc. Int. Ass. Theor. Appl. Limnol. 25,39-48．
4． Mugidde, R. 1993． The increase in phytoplankton productivity and biomass in Lake Victoria (Uganda). Int. Ass. Theore. Appl. Limnol. Proc. 25, 846-849．
5． Ogutu-Ohwayo, R. 1990． The decline of the native fishes of Lake Victoria and Kyoga (East Africa) and the impact of introduced species, especially the Nile perch, Lates niloticus and the Nile tilapia, Oreochromis niloticus. Euviron. Biol. Fisheries 27, 81-86．
6． Ogutu-Ohwayo, R. 1992． The purpose, costs and benefits of fish introductions: with specific reference to the Great Lakes of Africa. Proc. Int. Ass. Theor. Appl. Limnol. 23,34-44．
7． Witte, F., Goldschmidt, T., Wanink. J. M., Oijen, M. J. P. van, Witte-Maas, E. L. M. and Bouton, N. 1992． The destruction of an endemic species flock: quantitative data on the decline of the haplochromine cichlids of Lake Victoria. Environ. Biol. Fisheries 29, 1-28．
8． Johnson. T. C. And Odada, E. O. (eds.). 1996, The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach, Amsterdam, the Netherlands, 664 pp.
9． Lehman, D. 1998． History and ontogeny of IDEAL. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 1-6．
10． Johnson, T. C. 1993． IDEAL: An International Decode for the East African Lakes. Science and Implementation Plan. PAGES Workshop Report, Series 93-2． PAGES Core Project Office, Bern, Switzerland. 40 pp.
11． Herdendorf, C. E. 1990． Distribution of the world's largest lakes. In: Large Lakes: Ecological Structure and Fundction. Tilzer, M. M. And Serruya, C. (Eds.). Springer-Verlag, Berlin, Germany, pp. 3-38．
12． Bishop, W. W. and Trendall, A. F. 1967． Erosion surfaces, tectonics and volcanic activity in Uganda. Quart. J. Geol. Soc. London 122． 385-420．
13． Scholz, C. A., Johnson, T. C., Cattaneo, P., Malinga, H. and Shana, S. 1998． Initial results of 1995 seismic reflection survey of Lake Victoria, Uganda and Tanzania. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers. Dordrecht, Germany, pp. 47-58．
14． Kendall, R. L. 1969． An ecological history of the Lake Victoria basin. Ecol. Monogr. 39, 121-176．
15． Stager, J. C., Reinthall, P. N. and Livingstone, D. A. 1986． A 25,000 year history for Lake Victoria, East Africa and some comments on its significance for the evolution of cichlid fishes. Freshwater Biol. 6, 15-19．
16． Talbot, M. R. and Livingstone, D. A. 1989． Hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators. Palaeogeogr., Palaeoclimatol.,Palaeoecol. 70, 121-137．
17． Johnson, T. C., Scholz, C. A., Talbot, M. R., Kelts, K., Ricketts, R. D., Ngobi, G., Beuning, K. R. M., Ssemmanda, I. and McGill, J. W. 1996． Late Pleistocene desiccation of Lake Victoria and rapid evolution of cichlid fishes. Science 273, 1091-1093．
18． Broecker, W. S., Peteet, D., Hajdas, I., Lin, J. and Clark, E. 1998． Antiphasing between rainfall in Africa's Rift Valley and North America's Great Basin. Quart, Res.50, 12-20．
19． Greenwood, P. H. 1980． Towards a phyletic classification of the "genus" Haplochromus (Pisces, Cichlidae) and related taxa. Part Ⅱ: The species from Lakes Victoria, Nabugabo, Edward, George and Kivu. Bull. British Museum Nat. Hist.(Zoology) 39,1-101．
20． Kaufman, L. and Ochumba, P. 1993． Evolutionary and conserv, biology of cichlid fishes as revealed by faunal remnants in northern Lake Victoria. Conservation Biol. 7, 719-730．
21． Cohen, A., Soreghan, M. and Scholz, C.A. 1993． Estimating the age of formation of lakes: an example from Lake Tanganyika, East African rift system. Geolagy 21, 511-514．
22． Meyer, A., Montero, C. M. and Spreinat, A. 1996． Molecular phylogenetic inferences about the evolutionary history of East African cichlid fish radiations. In: The Limnology, Climatology and Paleoclimatology of the East African Lakes. Johnson, T. C. and Odada, E.O.(eds.). Gordon and Breach, Amsterdam, the Netherlands, pp, 303-323．
23． Meyer, A., Kocher, T. D., Basasibwaki, P. and Wilson, A. C. 1990． Monophyletic origin of Lake Victoria cichlid fishes suggested by mitochondrial DNA sequences. Nature 347． 550-553．
24． Galis, F. and Metz, J. A. J. 1998． Why are ther so many cichlid species? Tree 13, 1-2．
25． Stiassny, M, L. J. and Meyer, A. 1999． Cichlids of the rift lakes. Sci. Amer. 280, 64-69．
26． Owen, R. B., Crossley, R., Johnson, T. C., Tweddle, D., Kornfield, I.Davidson, S., Eccles, D. H. and Engstrom, D. E. 1990． Major low levels of Lake Malawi and implications for speciation rates in cichlid fishes. Proc. Roy. Soc. London B 240, 519-553．
27． Fryer, G. 1997． Biological implications of a suggested Late Pleistocene desiccation of Lake Victoria. Hydrobiologia 354． 177-182．
28． Beuning, K. R. M., Talbot, M. R. and Kelts, K. 1997: A revised 30000-year paleoclimatic and paleohydrologic history of Lake Albert, East Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol. 136, 259-279．
29． Talbot, M. R. and Laerdal, T. 1999, The Late Pleistocene-Holocene palaeolimnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter. J. Paleolimnol. (in press.)
30． Le Bas. 1997． (Please complete)
31． Ngobi, G. N., Kelts, K., Johnson, T. C. and Silheid, P. A. 1998． Environmental magnetism of Late Pleistocene/Holocene sequences from Lake Victoria, East Africa. In: Environmental Change and Response in East African Lakes. Lehman, J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 59-74．
32． Meyers, P.A. and Ishiwitari, R. 1995． Organic matter accumulation records in lake sediments. In: Physics and Chemistry of Lakes. Lerman, A., Imboden, D. M. and Gat, J. R. (Eds.). Springer-Verlag, Berlin, Germany, pp. 279-328．
33． Tyson, R. V. 1995． Sedimentary Organic Matter: Organic Facies and Palynofacies. Chapman and Hall, London. United Kingdom, 615 pp.
34． Hecky, R. E. and Hesslein, R. H. 1995． Contributions of benthic algae to lake food webs as revealed by stable isotope analysis. J. N. Amer. Benthol, Soc. 14, 631-653．
35． Bootsma, H. A., Hecky, R. E., Hesslein, R. H, and Turner, G. F. 1996． Food partitioning among Lake Malawi nearshore fishes as revealed by stable isotope analyses. Ecology 77,1286-1290．
36． Kidd. K. A., Lockhart, L., Wilkinson, P. and Muir, D. C. G. 1998． Metals, pesticides and other persistent contaminants in water, sediments and biota from Lake Malawi/Nyasa. In: Water Quality Report. Bootsma, H. A. And Hecky, R. E. (eds.). Lake Malawi/Nyasa Biodiversity Conservation Project, Senga Bay, Malawi. Chapter 9．
37． Sirocko, F. 1997． The evolution of the monsoon climate over the Arabian Sea during the last 24 000 years. Palaeoecol. Afr. 24, 53-70．
38． Blunier, T., Chappelaz, J., Schwander, J., Stauffer, B. and Raynaud, D. 1995． Variations in atmospheric methane concentration during the Holocene epoch. Nature 374, 46-49．
39． Alley, R. B., Mayewski. P. A., Sowers, T., Stuiver, M., Taylor, K. C. And Clark, P. U. 1997． Holocene climatic instability: a prominent, widespread event 8200 yr ago. Geology 25, 483-486．
40． Indermuhle, A., Stocker, T. F., Joos, F., Fischer, H., Smith. H. J., Wahlen, M., Deck, B., Mastroianni, D., Tschumi, .J., Blunier, T., Meyer, R. and Stauffer, B. 1999． Holocene carbon cycle dynamics based on CO_2 trapped in ice at Taylor Dome, Antarctica. Nature 398, 12-126．
41． Johnson, T. C., Chan, Y., Beuning, K. R. M.. Kelts, K., Ngobi, G. and Verschuren, D. 1998． Biogenic silica profiles in Holocene cores from Lake Victoria: implications for lake level history and initiation of the Victoria Nile. In: Environmental Change and Response in East African Lakes. Lehman. J. T. (ed.). Kluwer Academic Publishers, Dordrecht, Germany, pp. 75-88．
42． Beuning, K. R. M.. Kelts K., Ito, E. And Johnson, T. C. 1997． Paleohydrology of Lake Victoria, East Africa, inferred from ~(18) O~(16) O ratios in sediment cellulose. Geology 25． 1083-1086．
43． Roberts, N., Taieb, M., Barker, P., Damnati, B. Icole, M. and Williamson, D. 1993． Timing of the Younger Dryas event in East Africa from lake-level changes. Nature 366, 146-148．
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54． We thank the governments of Tanzania and Uganda for providing IDEAL researchers access to their territorial waters of Lake Victoria in 1995 and 1996, and the scientists and staff of the Ugandan Fisheries Research Institute in Jinja, under the direction of Dr. Fred Bugenyi, for logistical support. This manuscript benefited from the thoughtful reviews by Henry F. Lamb and Michael R. Talbot. Funding was provided by grants from the U.S. National Science Foundation to T. C. Johnson and K. Kelts. This is publication number 78 of the International Decade for the East African Lakes (IDEAL).
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