SUMMARY. 1. Changes in zoopiankton composition and abundance in Lake le Roux, a turbid subtropical reservoir on the Orange River in South Africa, were correlated with changes in water transparency (related to suspended sediment levels) during a 7 year field study. Results of radiotracer studies of the effect of mineral turbidity on zooplankton feeding rates which potentially influence competitive ability, and thus community structure, are reported here.
2. Feeding rates of five zooplankters were very variable, but consistently declined with rising turbidity: rates of decline differed between species. A regression estimate of the critical turbidity threshold at which food intake matched the estimated respiratory need was derived for each species. This yielded the following “turbidity-tolerance” ranking: Moina brachiata Jurine > Metadiaptomus meridianus (van Douwe) ×Daphnia gibba Methuen > D. barbata Weltner > D. longispina O. F. Muller. The consistency between this ranking and one based upon abundance-transparency relationships in the field study suggests that community structure is related to differential feeding capabilities, although other influences are not excluded.
3. Tests on D. gibba and M. meridianus failed to reveal any detectable feeding rate saturation (incipient limiting food level) below 1.2mg 1−1 C. The relative reduction in feeding rates at elevated turbidity was nearly 3 times greater for the daphnid than the copepod over a range of food concentrations, and considerably reduces the competitive ability of this (and other) daphnids. The turbidity tolerance disparity between Moina and the daphnids demonstrates a more complex situation than a simple copepod/cladoceran dichotomy. These findings and their implieations are discussed in relation to wider features of zooplankton ecology.