Global Warming Effects Around the World

Lake Tanganyika, Tanzania

Top Impact

Temperature (Water)

Other Impacts

Ecosystems (Lakes and Rivers)

People (Food)

Fishing in Lake Tanganyika threatened by warming water.

Fishing in Lake Tanganyika is a critical source of both nutrition and income for the surrounding population. Rising water temperatures are causing the lake's ecosystems to decline, threatening the sustainability of the fishing industry.1

Key Facts

People in the four countries bordering Lake Tanganyika rely heavily on fishing for both food and income. Some 1 million people earn their livelihood by fishing,3,4 and lake fish account for 25 to 40 percent of the animal protein supply for people living nearby.2 As a result of warming temperatures over the last century, biological activity in the lake has declined, threatening the sustainability of Lake Tanganyika's fishing industry.

  • Since 1913, the surface waters of Lake Tanganyika have warmed by 1.6 to 2.3° F (0.9 to 1.3° C).5,6
  • Rising temperatures are associated with a decrease in primary productivity—life activity at the base of the food chain—of 20 percent or more.5,9,10,11
  • Given a mid–level scenario for future emissions of heat–trapping gases—primarily from the burning of coal, oil, gas, and trees—East Africa could warm by 5.4 to 6.3° F (3.0 to 3.5° C) by the end of this century, causing further declines in Lake Tanganyika's ecosystems.15,16

Details

Lake Tanganyika is one of the African Great Lakes, and the second–largest lake in the world. Bordered by Burundi, Tanzania, Zambia, and the Democratic Republic of Congo, the lake is a critical source of both food and income for local people. Fish accounts for 25 to 40 percent of the animal protein supply for people in countries surrounding the lake,2 and demand for fish has been growing as chronic civil unrest disrupts agricultural production.3 The lake's fisheries provide a livelihood for about 1 million people, and some 10 million people rely on the lake for food, income, drinking water, or transportation.3,4

Over the last century, warming water temperatures have caused changes to the lake's ecosystems that threaten the fish species people depend on for food. Since 1913, the surface waters of Lake Tanganyika have warmed by 1.6 to 2.3° F (0.9 to 1.3° C).5,6 By analyzing the chemistry of lake sediments, scientists have determined that the lake is warmer now than at any point in more than 1,500 years.7

As the surface layer of the lake warms, it becomes less dense and floats on top of the deeper waters like icing on a cake. This layering, or stratification, prevents surface and deep waters from mixing. Such mixing is important, because it brings oxygen–rich waters from the surface down into deeper portions of the lake, and brings nutrient–rich deeper waters up to the surface.

These nutrients fuel biological activity at the base of the food chain—also known as primary production—in surface water. The amount of mixing between surface and deep waters, and the resulting amount of primary production, are critical determinants of the fish catch, because primary producers provide food for animals higher up on the food chain.8

Lake Tanganyika has historically been stratified, with very little mixing.5,6 However, the warming over the last 100 years has meant that even less mixing is occurring.5,9 Because less nutrient–rich water is mixing into the surface layer of the lake, primary production is declining.5,9,10,11 Several studies have found that primary production has fallen by 15 to 20 percent since the 1950s,5,10,11 while other studies suggest an even larger drop.9

The decline of primary production is troublesome for Lake Tanganyika's fish. Fisheries in Lake Tanganyika are dominated by just three species: two in the herring and sardine family, and one perch–like species.8 The sardine species feed on plankton, so higher catches of these fish are linked to higher plankton abundance and greater primary production.8 Catches of these sardine species have declined by 30 to 50 percent since the late 1970s, with changes in the lake's mixing patterns contributing to the decline.5

Part of a Larger Pattern

Since 1962, fish catches per unit of effort have been dropping, with the decline since 1985 particularly sharp.6,12 This change is consistent with the climate–induced changes in mixing and primary productivity.5,6 However, total fishing effort has also been rising, so the declines in catch per unit of effort may also reflect growing pressure on the lake's fish stocks.12 Further studies are needed to determine how much of the decline in fisheries is attributable to climate change and how much is attributable to overfishing.

Lakes are often viewed as sentinels of climate change, because they integrate shifts in their catchment regions and respond quickly to environmental change.13 While the mechanisms linking changes in climate to changes in lake ecosystems vary from lake to lake—depending on factors such as location, biology, and geography—clear signals of climate change are emerging from lakes throughout the world.13

At Lake Tanganyika, the stress of climate change is adding to stresses from other human activities to spur significant changes in fish abundance. At other lakes, the effects of climate change manifest in different ways. Warming water temperatures in the English Lake District, for example, mean that less mixing occurs between warm surface waters and cooler deep waters. That decline, in turn, is limiting the amount of oxygen available for animals living in the deeper waters of the district's lakes.14

What the Future Holds

Because heat–trapping emissions are rising as we continue to burn coal, oil, gas, and trees, temperatures in East Africa are projected to rise significantly.15 Given a mid–level scenario for future global warming emissions,16 East Africa could warm by 5.4 to 6.3° F (3.0 to 3.5° C) by the end of this century.15

Such warming would likely worsen the ecosystem changes under way in Lake Tanganyika, and further threaten food security in this vulnerable region. Policies that protect the lake's fish from overexploitation while ensuring access for people who need it could help foster sustainable fishing on the lake in the face of a changing environment.17

Credits

Endnotes

  1. Photograph used with permission of David Bygott. Online at http://www.flickr.com/photos/davidbygott/5091918973/. Accessed January 9, 2013.
  2. Greboval, D., M. Bellemans, and M. Fryd. 1994. Fisheries characteristics of the shared lakes of the East African rift. CIFA Technical Paper 24. Rome: Food and Agriculture Organization of the United Nations. Online at http://www.fao.org/docrep/008/v3470e/v3470e00.htm. Accessed September 14, 2011.
  3. Mölsä, H., J.E. Reynolds, E.J. Coenen, and O.V. Lindqvist. 1999. Fisheries research towards resource management of Lake Tanganyika. Hydrobiologia 407:1–24.
  4. Magnet, C., J.E. Reynolds, and H. Bru. 2000. Lake Tanganyika Regional Fisheries Programme (TREFIP): A proposal for implementation of the Lake Tanganyika Framework Fisheries Management Plan. Rome: Food and Agriculture Organization of the United Nations.
  5. O'Reilly, C.M., S.R. Alin, P-D. Plisnier, A.S. Cohen, and B.A. McKee. 2003. Climate change decreases aquatic ecosystem productivity of Lake Tanganyika, Africa. Nature 424:766–768.
  6. Verburg, P., and R.E. Hecky. 2009. The physics of the warming of Lake Tanganyika by climate change. Limnology and Oceanography 54(6, part 2):2418–2430.
  7. Tierney, J.E., M.T. Mayes, N. Meyer, C. Johnson, P.W. Swarzenski, A.S. Cohen, and J.M. Russell. 2010. Late–twentieth-century warming in Lake Tanganyika unprecedented since AD 500. Nature Geoscience 3:422–425.
  8. Plisnier, P.–D., H. Mgana, I. Kimirei, A. Chande, L. Makasa, J. Chimanga, F. Zulu, C. Cocquyt, S. Horion, N. Bergamino, J. Naithani, E. Deleersnijder, L. André, J.-P. Descy, and Y. Cornet. 2009. Limnological variability and pelagic fish abundance (Stolothrissa tanganicae and Lates stappersii) in Lake Tanganyika. Hydrobiologia 625:117–134.
  9. Verburg, P., R.E. Hecky, and H. Kling. 2003. Ecological consequences of a century of warming in Lake Tanganyika. Science 301:505–507.
  10. Stenuite, S., S. Pirlot, M–A. Hardy, H. Sarmento, A–L. Tarbe, B. Leporcq, and J-P. Descy. 2007. Phytoplankton production and growth rate in Lake Tanganyika: evidence of a decline in primary productivity in recent decades. Freshwater Biology 52:2226–2239.
  11. Bergamino, N., S. Horion, S. Stenuite, Y. Cornet, S. Loiselle, P.–D. Plisnier, J–P. Descy. 2010. Spatio–temporal dynamics of phytoplankton and primary production in Lake Tanganyika using a MODIS based bio-optical time series. Remote Sensing of Environment 114:772–780.
  12. Sarvala, J., V.T. Langenberg, K. Salonen, D. Chitamwebwa, G.W. Coulter, T. Huttula, R. Kanyaru, P. Kotilainen, S. Makasa, N. Mulimbwa, and H. Molsa. 2006. Fish catches from Lake Tanganyika mainly reflect changes in fishery practices, not climate. Verhandlungen Internationale Vereinigung f�r Theoretische und Angewandte Limnologie 29:1182�1188. Abstract online at http://library.wur.nl/WebQuery/wurpubs/348774. Accessed September 17, 2011.
  13. Adrian, R., C.M. O'Reilly, H. Zagarese, S.B. Baines, D.O. Hessen, W. Keller, D.M. Livingstone, R. Sommaruga, D. Straile, E. Van Donk, G.A. Weyhenmeyer, and M. Winder. 2009. Lakes as sentinels of climate change. Limnology and Oceanography 54(6):2283–2297.
  14. Foley, B., I.D. Jones, S.C. Maberly, and B. Rippey. 2011. Long–term changes in oxygen depletion in a small temperate lake: effects of climate change and eutrophication. Freshwater Biology doi:10.1111/j.1365–2427.2011.02662.x.
  15. Christensen, J.H., B. Hewitson, A. Busuioc, A. Chen, X. Gao, I. Held, R. Jones, R.K. Kolli, W.–T. Kwon, R. Laprise, V. Magaña Rueda, L. Mearns, C.G. Menéndez, J. Räisänen, A. Rinke, A. Sarr, and P. Whetton. 2007. Regional climate projections. In: Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller, eds. Cambridge, UK, and New York, NY: Cambridge University Press.
  16. The scenario referred to here is the middle–emissions pathway known as A1B from the Intergovernmental Panel on Climate Change.
  17. Smith, L.E.D., S.N. Khoa, and K. Lorenzen. 2005. Livelihood functions of inland fisheries: policy implications in developing countries. Water Policy 7:359–383.
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