Hartbeespoort Dam: A Case Study of a Hypertrophic, Warm, Monomictic Impoundment

Hartbeespoort Dam is a hypertrophic impoundment in South Africa, with large blooms of the potentially toxic blue-green alga Microcystis aeruginosa and an extensive anaerobic zone of up to 40% of the total volume during summer months. The aerial total phosphorus and total nitrogen loading rates are in the order of 20 g/m2/y and 128 g/m2/y respectively. Monthly mean chlorophyll concentrations measured over two years in the impoundment were higher than 30 mg/m3 for at least half of the year. Based on the Vollenweider model the standing crop of phytoplankton is much lower than would be expected in an equivalent north-temperate lake. This can be attributed to limitation of algal production by temperature and available light, as well as a flush-out of phytoplankton during summer flow-in. During 1976 and 1977 up to 60% of the water surface was covered by water hyacinth, Eichhomia crassipes, before removal by chemical spraying.