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The response of phytoplankton assemblages isolated in enclosures for short periods of time was examined in hyper-eutrophic Lake Chivero (Harare, Zimbabwe), to determine the factors that influenced the structure of the phytoplankton community, after noticing a marked decline in the dominance of Microcystis aeruginosa in recent years. The phytoplankton assemblage in the lake during summer, winter and the end of winter was dominated by Cryptomonas sp. and Cyclotella sp., with an average relative abundance of > 95%, based on phytoplankton biomass estimations. Isolation in summer resulted in the exclusion of Cyclotella sp., a decline of Cryptomonas sp. and an increase in M. aeruginosa and Anabaena sp. In winter, when M. aeruginosa was absent in the inoculum, isolation resulted in an increase in Cryptomonas sp. biomass and a decline of Cyclotella sp. At the end of winter Cryptomonas sp. initially increased but later declined following the increase in chlorophytes. The non-equilibrium state in Lake Chivero caused pioneer species to dominate rather than M. aeruginosa.
Keywords: Microcystis aeruginosa, non-equilibrium state, pioneer species, turbulence