ZOOPLANKTON ABUNDANCE, COMMUNITY STRUCTURE AND DYNAMICS IN RELATION TO INORGANIC TURBIDITY, AND THEIR IMPLICATIONS FOR A POTENTIAL FISHERY IN SUBTROPICAL LAKE LE-ROUX, SOUTH-AFRICA

1. The abundance and composition of entomostracan zooplankton were studied between 1977 and 1984 in relation to abiotic and biotic conditions in Lake le Roux (LLR), a large silt-laden reservoir on the Orange River formed in 1976. The community consisted of Metadiaptomus meridianus, Lovenula excellens (Calanoida), Daphnia gibba, D. barbata, D. longispina and Moina brachiata (Cladocera), and various Cyclopoida. 2. Zooplankton biomass varied seasonally from winter lows below 50 mg m-2 dry wt at temperatures of 9-10.degree. C to summer peaks generally above 1000 mg m-2 at 21-23.degree. C. It correlated strongly and positively with prevailing water temperature and transparency but only weakly with chlorophyll concentration. 3. Mean annual zooplankton biomass, which varied four-fold (97-408 mg m-2) in 7 years, increased with annual mean transparency (23-75 cm Secchi depth) and especially with summer heat content (33-320 degree-days above 20.degree. C) which was itself positively correlated with transparency. 4. Daphnid species showed most annual variation in abundance. Large forms (D. gibba and D. barbata) were scarce or absent in two years of very low transparency and low planktivore abundance. The smaller D. longispina developed once during clearer conditions when more fish were present. Copepod biomass also varied inversely with turbidity, but was not as severely reduced at high turbidities. 5. Smallmouth yellowfish (Barbus aeneus: Cyprinidae) is the principal planktivore and candidate fishery species in LLR. It feeds selectively on Lovenula and large daphnids. Catches and growth rates of yellowfish varied directly with the abundance of zooplankton, particularly large food forms, and with water transparency. High turbidity and associated food limitation seems to reduce standing stocks especially of the daphnid zooplankton more than the effects of fish predation. 6. Spawning of yellowfish depends upon the release of water from an upstream reservoir which concurrently reduces transparency and thus zooplankton availability in LLR. Transparency values above 30-35 cm SD appear necessary for the development of sufficient and suitable zooplankton to benefit the fishery.