Sekere aspekte van die abioties-biotiese verwantskappe van 'n troebeldam

English: Wuras Dam is a shallow, turbid polymictic impoundment. The turbidity of the water is influenced by the silt-laden flood water entering the dam from the catchment area, and by re-suspension of silt as a result of wind action and consequent water agitation. The dam has a maximum depth of 5 m and an average depth of 2,8 m. Average chorophyll α values indicate that Wuras Dam falls into the late mesotrophic to early eutrophic category. Temperature stratification did not occur and there was little temperature difference between bottom and surface water. More significant were the differences in oxygen concentration between surface and bottom water. Anaerobic conditions as such, however, did not occur and the lowest oxygen concentration registered was 2 mg l-1. The pH, alkalinity and mineral salts gradually increase as the water level falls, reaching a maximum during the dry season. This may be as a result of evaporation from the dam. The inflow of mineral-rich ground water from the catchment area may also be a contributing factor. Flood water which is low in mineral concent has a diluting effect on the water in the dam and consequently results in a decrease in the pH, alkalinity and mineral concentration. Furthermore growth in Wuras Dam appears to be limited to nitrogen, whereas the low Si-concentrations may from time to time inhibit diatom growth. The orthophosphate concentration (PO4-P) fluctuates largely and could possibly be attributed to biological assimilation and release and/or chemical reactions and to input from flood water. Five species of fish, all breeding during the summer months. inhabit Wuras Dam. The dam yields approximately 900 fish ha-1 and the fish production amoutns to approximately 452,5 kg ha-1. The figures must be regarded as provisional as the study on the biomass of fish has not yet been completed. Twenty seven zooplankton species were found in Wuras Dam, sixteen of which are more common. Mesocyclops schuurmanae and Asplanchna were the two main predators, whereas Chaoborus larvae were seldom encountered. The zooplankton is dominated by Mesocyclops schuurmanae which constitutes more than 20% of the total zooplankton population. Five other species, Metadiaptomus meridianus (Copepoda), Daphnia longispina/puler and Moina dubia (Cladocera) and Brachionus calyciflorus and Hexarthra (Rotifera), make with Mesocyclops schuurmanae the greater part of the zooplankton. According to the linear and logartihmic classifications in terms of correspondence analysis five zooplankton groups can be identified. The classification of the various groups depends more on reaction of the zooplankton to environmental factors than on their feeding habits. The two statistical procedures that were applied to the results, viz. the correlation coefficient matrix and the correspondence analysis led to the same results. Correspondence analysis appears to be a very suitable procedure for dividing the zooplankton into groups. Four groups of environmental parameters influenced the density of the zooplankton, namely: silt content of the water, water temperature, chlorophyll α concentration and pH and conductivity of the water. Whereas most chemical parameters were related to the distribution of zooplankton, the others were not as significant as the abovementioned. Furthermore it is clear that the organisms in the dam are subject to wide fluctuations in the concentrations of minerals, plant nutrients, pH, conductivity, alkalinity and suspensoids in the water. Consequently, if they are to survive, the organisms must be adapted to these fluctuations or have the ability to recover rapidly from catastrophical population declines. From the results of this study it can be determined whether the silt content of the water does in fact influence the distribution of zooplankton. Although the results are suggestive, the precise influence of silt on the zooplankton has not yet been established and consequently warrants further research.