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Department of Water Affairs and Forestry |
| Institute for Water Quality Studies |
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The Roodeplaat Dam has shown increased incidences of toxic cyanobacterial blooms. Eichhornia crassipes (the water hyacinth is a macrophyte) was also found in the Roodeplaat Dam for the first time during 1999.
Figure 9. Per cent of the time that chlorophyll-a falls within a specified range of concentrations in the Roodeplaat Dam, at Site 1
The data from the Roodeplaat Dam are quite extensive since the IWQS conducts weekly sampling at the impoundment. The ranges of chlorophyll-a concentrations at Site 1 in the Roodeplaat Dam are shown in Figure 9. These results indicate that the Roodeplaat Dam experienced algal blooms for more than 20 per cent of the time during all of the hydrological years during the assessment period, except during the 1995 to 1996 year. This is as expected, since the nutrient concentrations are very high and the Roodeplaat is a clear system with low light limitation to algal growth. The mean annual dominance of the cyanobacteria (Figure 10) indicate that the Roodeplaat Dam is subject to potential toxic algal blooms, as was confirmed during the summers of 1998 to 1999 and 1999 to 2000.
Another alarming phenomenon is the fact that the chlorophyll-a concentrations at Site 1 during the last two hydrological years were never below the level of 20 µg/l (See Table 1), indicating that the system does not recuperate at any time during these years. The minimum, maximum and mean chlorophyll-a concentrations at Site 1 were respectively, 1.7 µg/l , 243 µg/l and 35.3 µg/l for the period 1989 to 2000. At Site 7 the minimum, maximum and mean chlorophyll-a concentrations were respectively, 6.1 µg/l , 136.0 µg/l and 47.3 µg/l , for the period 1997 to 2000.
Figure 10. Mean annual
composition of algal groups in the Roodeplaat Dam at Site 1.
The cyanobacteria (especially Microcystis and Anabaena genera) formed the dominant algal group for most of the study period (Figure 10). The cyanobacteria form more than 30 per cent of the phytoplankton population annually. This is an indication that the Roodeplaat Dam is in a serious state of eutrophication and management measures should be taken to improve the situation. Chlorophyta (e.g. Staurastrum and Scenedesmus), Chrysophyta (Melosira and Cyclotella) and Cryptophyta (Cryptomonas) formed significant parts of the phytoplankton population throughout the study period (Van Ginkel et al., 2000). It is interesting to note that the Pyrrhophyta and the Cryptophyta seem to have disappeared and decreased, respectively, from the phytoplankton population during the last eight years. This might be indicative of the extremely enriched state of the impoundment that is reflected in the phytoplankton population.
The Chlorophyta and the Chrysophyta still form a considerable proportion of the phytoplankton population, but might only reflect the seasonal differences in the phytoplankton population.
Microbiological pollution is also potentially a great threat to the fresh water resources of South Africa.
According to Hohls et al. (1999) the overall risk of contracting gastrointestinal illness (as determined by geometric mean of faecal coliform counts) within the dam is low – in the acceptable class for long-term exposure. Isolated samples do, however, reflect that there can be a risk of gastrointestinal illness for the occasional recreational water user (Hohls et al., 1999). This is due to the fact that the Zeekoegat WCW discharges into the impoundment in this section of the Roodeplaat Dam and the Baviaanspoort WCW discharges into the Pienaars River upstream of the impoundment. Informal settlements and livestock agricultural practices may also contribute to microbiological pollution during periods of higher rainfall when the surface wash-off could be a major contributor to microbiological and nutrient loads into the impoundment.