HR: 1330h
AN: GC12A-0160    [PDF]
TI: Climate Effect on Circulation in Lake Tanganyika: Increase of the Anoxic Hypolimnion and Loss of Productivity
AU: * Verburg, P
EM: pverburg@scimail.uwaterloo.ca
AF: University of Waterloo, 200 University Avenue West, Waterloo, on n2l 3g1 Canada
AU: Hecky, B
EM: rehecky@scimail.uwaterloo.ca
AF: University of Waterloo, 200 University Avenue West, Waterloo, on n2l 3g1 Canada
AB: Lake Tanganyika, largest by volume of the East African Great Lakes (maximum depth 1470 m), has warmed up over the past century. Heating rates, the density depth gradient and the oxygen distributions were examined. The amount of energy absorbed by Lake Tanganyika is substantial on a global heat budget scale and is in the same order as that absorbed by the melting of Arctic sea-ice in the past century. At higher temperatures density decreases. Because the surface warmed up more than deep water the difference in density between shallow and deep water increased. The increased density gradient has slowed down vertical mixing and circulation and as a result oxygen concentrations and the maximum depth of oxygen penetration have decreased. SO4, introduced to the lake by river inflow, is now almost completely (98 %) lost from the lake by reduction at the oxic-anoxic interface. As a result the depth at which H2S is detectable has become much more shallow, from 300 m in 1938 to 120 m in 2000. At the south end where mixing is traditionally deepest in the lake, driven by south east trade winds, organisms which lived at 100-300 m a century ago are now forced into a more shallow distribution. The increase of the density gradient from deep nutrient rich to shallow nutrient poor water and the reduced mixing capacity of the lake has substantially impacted the offshore ecosystem. Primary production by phytoplankton has decreased as shown by increased silica concentrations and lower algal biomass, probably by reduced availability of essential macro and micro nutrients in epilimnetic water. The epilimnetic dissolved silica concentration tripled as diatom production and sedimentation of biogenic silica dropped in the last decades of the past century. Blooms of cyanobacteria in the stratified season may have been more common earlier in the century compared with the present and the lake is now much more transparent. Temperature is an important parameter in tropical lakes and climate warming has changed the ecosystem in Lake Tanganyika over the past century. Both reduced productivity and a reduced oxygen penetration will threaten the persistence of some of the hundreds of endemic species in Lake Tanganyika, as their habitats contract spatially and become more nutrient poor.
DE: 0400 Biogeosciences
DE: 1615 Biogeochemical processes (4805)
DE: 1620 Climate dynamics (3309)
DE: 1845 Limnology
SC: Global Climate Change [GC]
MN: 2003 Fall Meeting