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SUMMARY
The zooplankton biomass characteristics of 11 South African impoundments were investigated to assess the value of zooplankton biomass as a trophic status index. Mean annual zooplankton biomasses ranged from 0,38 to 5,32 g m−2, compared with the mean annual chlorophyll a concentrations in the euphotic zone of 1,1 to 27,8 mg l −1. Highly significant correlations were obtained between zooplankton biomass, chlorophyll a and inorganic nitrogen and orthophosphate suggesting that zooplankton biomass is acceptable as an index of trophic status. It is proposed that impoundments with a mean annual zooplankton biomass of less than 0,5 g m−2 may be considered oligotrophic, those with more than 1,0 g m−2, eutrophic and those intermediate, mesotrophic.