Limnology of hartbeespoort dam

Abstract

Hartbeespoort Dam is a hypertrophic, warm, monomictic impoundment. With a mean depth of 9.6 m and a surface area of 20 km2, the system demonstrates that hypertrophy is not confined to small shallow lakes as concluded by Barica (1981). The combination of high nutrient loading, high incident solar radiation, low wind speeds and warm water makes it the ideal environment for the prolific growth of the buoyant blue-green alga, Miarooystis aeruginosa Photosynthetic production of this organism is regulated mainly by the penetration of light through the water column. The Miorooystis population grows to such large levels that the population self-shades itself, that is., the alga is usually the dominant factor regulating light penetration in the water. Unlike most hypertrophic lakes, wide spread anoxia resulting from the die-off of the large Miovoaystis population does not occur in Hartbeespoort Dam. Rather, Microcystis tends to accumulate in large, floating mats termed hyperscums and then is redistributed throughout the dam under favourable wind conditions. However, the anaerobic hypolimnion, which may extend upward to within about 8 m of the lake surface, contains large concentrations of reduced compounds and it is the release and distribution of these throughout the water which may result in almost total lake anoxia at overturn