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Towards the assessment of the trophic status of South African impoundments for management purposes: Bon Accord Dam

CE van Ginkel
BC Hohls
E Vermaak


The 1mg/l phosphate effluent standard legislated in South Africa in 1980 in seven sensitive catchments, and ad hoc eutrophication-related requests, necessitated the monitoring of 53 impoundments throughout the country. Variables that were monitored, and are still being monitored in some cases, included nutrients, chlorophyll a, algal identification, suspended solids and other major inorganic constituents. The measurement of temperature and oxygen profiles was conducted on an ad hoc basis when the sites were visited. The results, as determined from data collected between 1989 and 1998, indicated a range of trophic states from oligotrophic to hyper-eutrophic in the sensitive catchments. The catchments that were covered in the survey are: the Vaal River, Crocodile River (North West Province), Pienaars River, Olifants River (Mpumalanga Province), Letaba River, Buffalo River, Berg River, White Mfolozi River, Mgeni River, Mlazi River and the Molopo River catchments.

The objective of this paper is to discuss the methods, variables and presentation for determining the trophic status of an impoundment for management purposes by using findings in the Bon Accord Dam, a small reservoir near Pretoria. The trophic status of selected impoundments was determined by using the mean total phosphorus (TP) concentrations, the nitrogen to phosphorus ratio, the mean chlorophyll a concentration, the presence of cyanobacteria and the turbidity (measured by means of Secchi disc readings) of the system.

The mean annual phosphorus concentrations dropped from more than 800µg/l to less than 100µg/l in three years, and have remained at that level since 1987. The authors suggest that further reductions in phosphorus input are necessary to prevent eutrophication in the reservoir.

Keywords: eutrophication; trophic status; phosphorus; nitrogen; phytoplankton; cyanobacteria; Bon Accord Dam; South Africa

(Afr J Aqua Sci: 2000 25: 211-218)

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eISSN: 1727-9364
print ISSN: 1608-5914