'n Ondersoek na die rol van soöplankton in Roodeplaatdam

English: Roodeplaat Dam is a warm monomictic and eutrophic waterbody, which is situated in a summer rainfall area. A definite thermocline and hypolimnion are found in summer, together with a high primary production, which gives rise to a large anaerobic hypolimnion during summer. The Pienaars River/Hartbees Spruit arm of the dam was much more turbid than the Edendale Spruit arm and the main basin of the dam. Large water inflows during summer had a dilutionary effect on the electrical conductivity, nitrate, nitrite, and phosphate concentrations. High chlorophill values and dominance for extended periods by the blue-green algae, Microcystis and Anabaena, were indications of the degree of eutrophication in the dam. Five fish species were found in Roodeplaat Dam, of which Oreochromis mossambicus and Chetia flaviventris were dominant. The fry of these species are known to be predators of zooplankton. Low total zooplankton biomass was found in summer because of decreased living space, due to a large anaerobic hypolimnion. The high zooplankton numbers during summer, in spite of low total biomass, resulted from a change of dominance from the large Daphnia spp. to the smaller Thermocyclops and Ceriodaphnia spp. There were marked increases in zooplankton numbers with the change of· phytoplankton dominance from blue-green algae to green algae and cryptomonads. Twenty two zooplankton species in numbers warranting examination were found in Roodeplaat Dam. By the use of correlation coefficients, Cluster Analysis and correlation with environmental conditions, the zooplankton were separated into eight groups. The most influential conditions governing the density of zooplankton taxa in Roodeplaat Dam were those relating to food type and temperature, and to a lesser extent pH and electrical conductivity. Thermocyclops oblongatus was present throughout the study period and dominated the zooplankton community. Extremely high annual p/B ratios of 82,63 and 88 were found by two production-determination methods. Correspondingly an annual production of 589,24 kgC.annum-1 was derived for the total zooplarikton population. In comparison with the zooplankton communities of other South African systems, Roodeplaat Dam's zooplankton corresponded most closely with that of Hartbeespoort Dam, which is also a warm monomictic and eutrophic system.