Lake Victoria in East Africa underwent a series of dramatic ecological changes between 1960 and 1990. The population explosion of an introduced predator, the Nile perch in the 1980s led to the extinction of dozens of species of native haplochromine cichlids. In the lower food web, phytoplankton biomass increased 3-5 fold, the dominant diatom Aulacoseira was replaced by Nitzschia, and several species of crustacean zooplankton were lost. These events were investigated using fossil remains of diatoms, Bosmina, chydorids, and the invertebrate predator Chaoborus in sediment cores from 3 locations (2 nearshore, 1 offshore) to assemble a 50-100 year record of changes in the plankton community. The most dramatic changes occurred in the 1980s, when the number of chydorid carapaces declined from approximately 5000 carapaces/g (dry weight) to <100 carapaces/g at all 3 sites. Bosminid remains declined at the two nearshore locations from a maximum of approximately 18000 individuals/g to less than 1000 individuals/g. Bosminid remains at the offshore location did not decline. The loss of cladocerans from the nearshore plankton corresponded to a shift in the diatom community from dominance by Aulacoseira sp. to Nitszchia sp., suggesting that changing environmental conditions played a role in the decline of cladoceran species in nearshore areas. Over the same interval that cladoceran species declined, the density of Chaoborus fossils increased in nearshore areas, largely due to the expansion of an offshore species, Chaoborus edulis into nearshore areas. The expansion of C. edulis into nearshore areas indicates increased hypoxia of bottom waters.

A historical study of the diet of a newly abundant planktivore Rastrineobola argentea indicates that these fish preyed upon all species of crustacean zooplankton in the past and may have contributed to the decline of Bosmina, Chydorus, Ceriodaphnia, Moina and Daphnia species in nearshore areas. Enclosure experiments indicate that presently, R. argentea has a larger effect on populations of calanoid copepods and Tropocyclops sp. than on the predominate copepods, Thermocyclops sp. Grazing by the present zooplankton community does not suppress primary production, probably because of high phytoplankton growth rates and low zooplankton community biomass.

E-mail: Thomas.bridgeman@utoledo.edu