On the causal factors and mechanisms for ecosystem change in Lake Victoria

117 Striking ecosystem changes have occurred in Lake Victoria that, though not completely understood, are attributed to eutrophication and climate change. To improve understanding of these changes, carbon and nitrogen elemental and isotopic analyses were performed on three short cores taken from Lake Victoria. Sediment age was determined by radiometric ( 210 Pb, 137 Cs) dating. The results show that many aspects of the current perceptions on the causal factors and timing of events are correct. A marked increase in the deposition of TOC and TN with concomitant decrease in C/N ratio and δ 13 C values occurred starting from ca. 1886-1920 that signified an increase in lake primary productivity in response to increasing nutrient loading. There were three areas of discrepancy with current perceptions. First, it was found that the organic matter in the lake was not predominantly of algal origin but comprised of mixtures of land and swamp plants, and phytoplankton. Second, there was no lake-wide synchrony in ecosystem changes, which is presumed to indicate spatial heterogeneity in morphometric, hydrodynamic and water quality conditions. Third, Infixing cyanobacteria are not dominant over non-N-fixing phytoplankton in far offshore waters. Considering that signal excursions started earlier than the commencement of large-scale anthropogenic disturbance of the watershed, it is argued that eutrophication only amplified a shift in lake conditions that was already underway from climate change. Comparison of ecosystem trends in Lakes Victoria and Tanganyika shows that universal generalizations on the impacts of climate change on aquatic ecosystems may be misleading as contrasting outcomes in lacustrine primary productivity may result from the interaction between impacts of climate change, eutrophication and hydrodynamic regime.