PARENT SESSION
Oral Session #44: Global Change and Climate Change.
Presiding: C. Wessman
Tuesday, August 6. 1:00 PM to 4:45 PM. Grand Ballroom West, Radisson.

Climate changing a tropical Rift Valley Lake.

VERBURG, PIET^*,1, HECKY, ROBERT¹, KLING, HEDY², ¹ University of Waterloo, Waterloo, Ontario, Canada² Algal Taxonomy and Ecology Inc. R3T 2X8, Winnipeg, MB, Canada

ABSTRACT- Lake Tanganyika is the largest by volume of the East African Great Lakes (length 670 km, maximum depth 1.5 km). A comparison of historic records (since 1913) and temperature data collected in 1993-2001 illustrates the warming of the lake over the past century. Lake Tanganyika is an oligotrophic lake. Inputs of nutrients to the lake are low and recycling of nutrients is an important mechanism for the maintenance of production. Especially seasonal upwelling driven by south east trade winds is important to the recycling of nutrients. However one of the effects of climate warming on the lake is the increase of the density gradient from deep (nutrient rich), to shallow water. This has reduced the mixing capacity of the lake. To investigate the effect of the warming on the offshore ecosystem, nutrient levels and phytoplankton were examined in the two main basins of the lake. Silica, now less utilized by diatoms than in the past, was found in 2000-2001 to accumulate in the upper 100 m of the water column. Oxydation of H₂S mixed through the oxic-anoxic boundary (c. 150 m depth) decreased. Phytoplankton biomass decreased between the early seventies and 2000. The phytoplankton community during the upwelling season changed from one dominated by diatoms to a chlorophyte community.

KEY WORDS: tanganyika, East-Africa, warming