'n Studie op aspekte van die soöbentos van Wurasdam

[摘要] English: Wuras Dam is a shallow, suspensoid-rich polymictic impoundment. The turbidity results from the inflow of turbid water from the cathment, as well as from resuspension due to wind action and consequent water movement. On the basis of its chemical composition Wuras Dam can be considered a mesotrophic to eutrophic system.Environmental factors such as temperature, dissolved oxygen concentration, pH, turbidity and conductivity did not vary greatly over the study period, while the water level did.The sediment had a relatively high organic content and the particle size varied from fine silt to coarse sand. In the areas where the sediment was very fine, an ooze layer was indicated.For the benthic fauna, three macrohabitats were studies, viz. the macrophytes, the stones and the sediment.It was found that the high suspensoid load in the water column reduced light penetration and algal growth, resulting in a vertical zonation of organisms on the macrophytes and stones. A large number of taxons were found, with the Molllusca, Chironomidae and Ephemeroptera being numerically dominant. Colonisation on the sprouting macrophytes occured after the winter, and periphyton growth appears to have been the most important factor controlling the presence and distribution of the epifauna on the macrophytes and stones.The epifauna on the macrophytes comprised approximately half of the benthic standing crop of the dam (3260 kg a-1), while the stone epifauna made an insignificant contribution to the total biomass.The dominant organisms in the sediment were the Oligochaeta, Nematoda and Chironomidae. Their numbers and production were largely influenced by the decaying organic material derived from the macrophyte beds. Other factors influenced numbers and production such that a sediment with an organic content of 4 to 8 per cent had the highest zoobenthic production. Sand harboured a higher production than finer particles and a water depth of 60 to 120 cm appeared to be the most productive zone. Periphyton growth in the shallower areas resulted in the development of a different community in the sediments of that region, which was marked by Chironomid community of specific composition.The presence of an ooze layer, particularly in the deeper parts of the dam, affected the occurrence of most organisms adversely, effectively excluding all but Chaoborus, Nematoda, Tubificidae and 3 genera of the Chironomidae.Suspensoids appear to have both direct and indirect effects on all three zoobenthic macrohabitats. In spite of this the production of zoobenthos per ha in Wuras Dam was found to be much higher than that of other South African standing waters.

[发布机构] University of the Free State