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

Abstract

The 1 mg/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. Introduction

Living with eutrophication in South Africa: a review of realities and challenges

“…The derivation of the South African guidelines is unclear, as is the use of 130 μg l -1 TP as the hypertrophic boundary, this being attributed to two anonymous reports produced by the then Department of Water Affairs (cited in Van Ginkel et al, 2001). Herein it is reported that the "target water quality aimed for to control eutrophication was set to maintain mean chlorophyll concentrations in the receiving water bodies [= reservoirs] at such levels that severe nuisance conditions would not occur for more than 20% of the time".…”

The effects of insect–insect interactions on the performance of three biological control agents released against water hyacinth

“…The damage inflicted on the plants by the two Neochetina weevils did have an effect on plant parameters under high nutrients (1.6 mg NO 3 -N L (1 and 1.0 mg PO 4 -P L (1 ), but not as pronounced as on the plants grown under medium nutrients (0.4 mg NO 3 -N L (1 and 0.025 mg PO 4 -P L (1 ) (Heard and Winterton 2000). The nutrient conditions used for the current experiment were chosen to best represent the conditions from Coetzee et al (2007), where the total nitrogen levels were extremely high at 50.5 mg N L (1 , and represent the level at which maximum nitrogen storage occurs in water hyacinth (Reddy et al 1989) and are indicative of natural South African conditions (Van Ginkel, Hohls, and Vermaak 2000). …”

Using Multiple Indices for the Water Resource Management of a Monomictic Man-Made Dam in Southern Africa

“…Several authors [21,26,47] have identified TP as a good forecaster of algal growth, Chl-α as a reliable algal biomass indicator, and SD as the best measure for water clarity. Therefore, for the current study, samples for determining the trophic state analysis were collected at sampling Site C at the dam wall.…”

“…Therefore, for the current study, samples for determining the trophic state analysis were collected at sampling Site C at the dam wall. The dam wall is regarded as the most preferable position [21,45] for determining the relationships between Chl-α, TP, and trophic status because this is where potable water is abstracted for transfers and treatments and/or where the sample is deemed more likely to be representative of the greater water volume [48]. Meanwhile, samples for the overall water status of the dam were collected across the longitudinal gradient of the dam at sampling sites A-D to represent the overall physical, chemical, and biological characteristics of the dam.…”

“…The project was designed for job creation and to promote socio-economic development. The status quo assessment of the project noted that the area has high natural resource quality in terms of species richness due to the convergence of various biomes [20,21]. Water users such as commercial farmers who rely on potable water from this dam for irrigation purposes, locals who are dependent on this dam, as well as businesses who use this dam for recreational activities will be affected if no actions are put in place to curb the deteriorating water quality of this dam.…”