Friends of Rietvlei
Member of the Wildlife & Environment Society of South Africa Western Cape Region
Rietvlei Water Quality
The Annual Water Quality Report for Rietvlei and the Diep River was presented to the Rietvlei Management Working Group on 29 May 2013. The report is in pdf format and can be viewed here.
» Click here to open report.
Water Quality in Rietvlei North Lake
Monthly records of the water quality in Rietvlei date back to the year 2000, prior to that there are results from sporadic sampling. The results are utilised for both environmental health purposes (i.e. human health) and to assess the ecological health of the system.
In May Candice Haskins of the City’s Catchment, Stormwater and River Management Department presented her annual Water Quality Report to the Rietvlei Management Working Group. Here we have extracted results from her report for information about the current situation in Rietvlei North Lake, the watersports area. The graphs below are all from the Water Quality Report.
E. coli trend
E. coli (Escherichia coli) is an anaerobic, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless, but some serotypes can be harmful to humans. The harmless strains are part of the normal flora of the gut, and can benefit their hosts by producing vitamin K2, and by preventing the establishment of pathogenic bacteria within the intestine.
E coli is a good indicator of pollution levels, the number of bacteria in a 100ml of water is counted - high levels can be harmful to people using the water.
E coli
The full contact and intermediate contact guidelines are applied in recreational areas and indicate the suitability of the water for recreational use. Full contact is significant and lengthy full body immersion associated with swimming and diving; while intermediate contact is partial contact that would take place during paddling, splashing and brief immersion such as when a vessel capsizes. The guideline for full contact is the E.coli counts must be less than 130/100ml of water; intermediate contact guideline is less than 1000 counts/100ml of water.
In the graph above it can be seen that, while the E.coli counts are mostly below the full contact guideline, it does at times exceed 130/100ml and occasionally the intermediate contact guideline of 1,000/100ml. What is significant is that the trend indicates that the E.coli counts in North Lake have been increasing since 2000.
While the water quality in North Lake is currently suitable for full contact recreational use most of the time, it is strongly recommended that full contact be avoided – this means no swimming or diving. Intermediate contact recreational use is reasonably safe, but we advise people not to stay in the water for long periods.
Inorganic Nitrogen and Total Phosphate
Nutrient levels in rivers, lakes, estuaries and the sea are indicated by measuring inorganic nitrogen and the total phosphate in the water. Nutrients are essential for living organisms, in a natural unimpacted system the levels of nutrients are finely balanced to ensure optimum productivity. The absence of inorganic nitrogen or phosphates means that the system is sterile and devoid of most life forms, but if the levels are too high excessive plant growth leads to rapid turnover of plant material, particularly phytoplankton, decomposing plant material results in anoxic conditions.
Inorganic Nitrogen
Nitrogen levels of about 0.25 mg/l N are natural, levels up to 4 mg/l N are still acceptable. Total phosphate levels up to 1.3 mg/l P are acceptable, with the optimum levels being about 0.01 mg/l P. In Rietvlei nitrogen levels are in the lower ranges with only occasional peaks above 0.25 mg/l N; the trend since 1999 shows a steady decline in nitrogen levels with lower and less numerous peaks in the last 3 years.
Total Phosphate
The total phosphate in Rietvlei is quite high, there was a steady increase from 1999 to early 2011, however since then there has been a marked improvement with phosphate levels dropping below 1.3 mg/l P.
Algal blooms are a common occurrence in Rietvlei, the quantity of algae is determined by measuring chlorophyll-a. Algal blooms often result in anoxic conditions, i.e. dissolved oxygen in the water is used up by the algae and this seriously affects aquatic life in the system. In extreme cases the algae form a thick unsightly and smelly scum on the surface; fish deaths can occur which further adds to the smell. At times the algae produce toxins that can cause skin irritations and, if swallowed, gastro-enteritis and impaired liver function.
The graph shows that the levels of algae in Rietvlei have been steadily increasing since 1999, but the last two years shows an improvement with significantly lower chlorophyll-a levels (red square in graph).
Fortunately all blooms are not toxic, it is only when Microsystis blooms that one must be extra cautious. The last Microsystis bloom in Rietvlei was in January 2007 just after the fish kill the previous month. Even though most algal blooms are not harmful, the advice to recreational users is to keep clear of algal blooms in the water – most are fairly patchy and will cover relatively small areas before dissipating. If the situation really becomes bad, the authorities will timeously close Rietvlei to recreational use until conditions return to normal.
Dissolved oxygen
Dissolved oxygen is another indicator of the ecological health of the system. Low oxygen levels are often the result of algal blooms, but can also be the result of pollutants in the system when high organic loads deplete the dissolved oxygen.
Dissolved oxygen
In December 2006 the oxygen levels in Rietvlei dropped dramatically, resulting in a severe fish kill and subsequent bloom of toxic Microsystis algae (red circle in graph). The cause for the drop in oxygen was a period of hot windless summer days (when the water temperature increases the dissolved oxygen levels drop); this was prime conditions for algal blooms which further decreased the oxygen levels.
The oxygen levels in Rietvlei are mostly above 6mg/l and often above 8 mg/l which is good; oxygen levels above 8 mg/l are found in natural unimpacted systems. The occasional dip to lower levels of dissolved oxygen is most probably due to natural conditions (like hot calm days) and not as the result of polluting factors.
The results of the water sampling may appear to be conflicting, some parameters indicate high pollution levels, while others indicate a healthy system. This merely illustrates the complexity of wetland systems.
The water quality results are provided by the City of Cape Town and is used here with permission by the City. Our appreciation to Candice Haskins for allowing us to use the graphs in her annual Water Quality Report.
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