ZOOPLANKTON BIOMASS OF SOME TRANSVAAL IMPOUNDMENTS AND ITS RELATIONSHIP TO TROPHIC STATUS

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.