Lake Victoria: Structure and Function of a Tropical Ecosystem

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9318085 Kling Lake Victoria serves as the basic food and economic resource for millions of people. The last 30 years have produced remarkable and alarming changes in the ecosystem. Phytoplankton productivity has increased 2 to 3-fold and the biomass has increased 10-fold. Water clarity, oxygen content, and silica concentrations have been dramatically reduced. The tremendous diversity of endemic fishes (300 species) has been all lost, due in part to the components that have contributed to these changes: (1) alteration of the food web by the introduction of Nile perch, (2) eutrophication caused by nutrient inputs from the basin or atmosphere, and (3) recent changes in climate that have modified water-column structure and caused mixing patterns to favor development of bluegrass algae and the loss of oxygen in bottom waters. %%% Although there is evidence that each of these components has contributed to the changes in Victoria, it is impossible to determine their relative importance and how they interact die to our complete understanding of ecosystem function. Theory predicts a weak trophic cascade in a tropical lake, a forecast clearly at odds with the observations in Lake Victoria. The phosphorus-driven eutrophication models fail to explain the change in productivity of the lake. Is it because the outcome of biogeographical control is different at high and uninterrupted temperatures, or is there an unrecognized feedback through trophic interactions or variations in climate? In this research we will explore these apparent contradictions and test these basic but unproved hypotheses. %%% A survey and experimental approach will be used to deduce the specificity and intensity of nutrient limitation, and to estimate the rates of important biological and chemical processes. A modeling approach will be used to identify the important predator-prey interactions, and to synthesize these results into quantitative estimates of herbivore impacts. Food web interaction models will be employed to evaluate previous, current, and future trophic states of the lake. We will use the paleolimnological record of change in diatoms, pigments, crustaceans, and element deposition as a benchmark in testing the mechanisms of process regulation and their integration at the ecosystem scale.