Nature Conservation: Rietvlei Nature Reserve
The Rietvlei Wetland Reserve is a 663-hectare (1,640-acre) nature reserve situated in Table View, Western Cape, South Africa. It is managed by the City of Cape Town’s Environmental Resource Management Department. The Rietvlei Wetland Reserve forms part of the greater 880 hectares (2,200 acres) Table Bay Nature Reserve.
The Milnerton Aquatic Club is located along the shores of the north lake (permanent freshwater lake) within the Rietvlei Wetland Reserve. Rietvlei forms part of the Table Bay Nature Reserve and offers a range of natural and semi-natural habitats.
While you enjoy your sport on the water or while relaxing on MAC’s lawn, you will not only be able to enjoy the beautiful view on Table Mountain but also the various animals. This is a birder’s paradise, especially in spring and summer, when migrant birds arrive from the northern hemisphere. Some 180 bird species are listed, including pelicans, flamingos, ducks, coots, herons, plovers, weavers and swallows. Many small mammals (e.g. Cape Grysbok, Cape Grey Mongoose, African Clawless Otters and Caracals), reptiles and insects live here, along with several frog species.
The Friends of Rietvlei provide assistance and support for the management of the Table Bay Nature Reserve, Cape Town.
SANCCOB, the coastal bird rehabilitation centre, where oiled penguins and gannets are cleaned, is adjacent to the reserve.
As one of the custodians of Rietvlei Nature Reserve, we ask all our members to protect the nature surrounding us. Please adhere to the nature conservation rules so that it can persist for future generations. Please assist in the protection and conservation of our Rietvlei Nature Reserve.
Although Rietvlei (the lake) was closed for many months in 2019 (blue green algae bloom), 2021 and January 2022 due to apparently high E.coli bacteria readings, MAC was able to prove that the previous measurements of the City of Cape Town were wrong and that E.coli bacteria are minimal within the lake. Since then, the water quality is being tested twice a month for various bacteria and chemical parts. In order to keep the water source clean and safe, MAC is engaging on a regular basis with the City of Cape Town as well as the Protected Area Advisory Committee of Table Bay Nature Reserve.
Data has been kindly provided by Scientific Services Labs (SCSI), City of Cape Town. The latest water quality data is available to the public via the following link: https://odp-cctegis.opendata.arcgis.com/documents/inland-water-quality/about
City of Cape Town conducts usually bi-monthly tests regarding E.coli and monthly tests regarding chlorophyll-a and the main blue-green algae toxin microcystin. At times of elevated levels, additional tests might be conducted.
Water samples usually take several days to be analysed and test results are only being made available to the public with a 1-2 months delay. Hence, results can only be used as an indication of the water quality. The situation on the day when the test results are released might be different to the situation on the sampling date.
Test results can be influenced by various factors, for example, storm water inflow, sewage pipe or pump overflows, bird droplets, wind speed and direction, etc. Very often, with the delayed test results it is not possible to determine the exact reason for an isolated peak or increased test value.
‘Full body contact recreation’ applies to swimming and diving where the full body is immersed in water for a long time. Please note that swimming and diving are not allowed at Rietvlei nor in any inland lake or dam across South Africa.
‘Intermediate contact recreation’ applies to sports like windsurfing, sailing, water-skiing and canoeing and assumes partial contact that would take place during paddling, splashing and brief immersion such as when a vessel capsizes.
Relevant testing points are MAC’s jetty (MAC) and MAC’s slipway (RTV02).
FAECAL COLIFORMS AND ESCHERICHIA COLI (E.COLI)
Recreational SA target guideline for intermediate contact recreation is <=1000 colony-forming units per 100ml (3 month median in fortnightly samples).
The City defines anything below 1000 CFU/100ml as Low Risk. Medium Risk (acceptable) is 1001-4000 CFU (faecal coliform count incl. Ecoli)/100ml, High Risk (unacceptable) >4000 CFU/100ml. Source: City’s S&P for closing and re-opening of Rietvlei, Zandvlei and Zeekoevlei.
Adverse effects on humans:
Most E. coli strains do not cause disease, naturally living in the gut, but virulent strains can cause gastroenteritis, urinary tract infections, neonatal meningitis, hemorrhagic colitis, and Crohn’s disease. Common signs and symptoms include severe abdominal cramps, diarrhea, hemorrhagic colitis, vomiting, and sometimes fever. (Source: Wikipedia)
Faecal contamination levels in the Rietvlei water sports area is low, and generally meet the intermediate contact guideline. Local sources of contamination could include run-off from residential properties bordering the water body, overflows from pump stations and Bayside Canal. It is noted however that flow from the latter canal first passes through dense shoreline reed beds before entering the deep water lake area which assists with water quality improvement. Isolated peaks may be due to sample anomalies e.g. collection of bird faeces in the water sample due to the resident bird population near the sample collection site (jetty). (Source: Water and Sanitation Department, ‘Analysis of ambient water quality in the Diep (N) Catchment within the City of Cape Town area’, 9. June 2020)
Historical Test Results (Source: Water and Sanitation Department, ‘Analysis of ambient water quality in the Diep (N) Catchment within the City of Cape Town area’, 9. June 2020)
BLUE-GREEN ALGAE AND MICROCYSTIN
The shoreline area of Rietvlei is characterised by a narrow shallowish band inhabited by pond weed and filamentous algae. The green coloured algae is harmless and a natural component of many open water bodies and wetlands. Both pondweed and filamentous algae thrive along the shoreline. The blue-green algae (cyanobacteria) also thrive in this nutrient enriched zone and can produce a class of toxins called microcystins. Demonstrations of toxicity of the cyanobacterial population in a given lake do not necessarily imply an environmental or human hazard as long as the cells remain thinly dispersed. Mass developments, so called ‘blooms’ and especially surface scums (thick, gelatinous substance) pose the risks.
Adverse effects on humans:
Health impairments from cyanobacteria in recreational waters must be differentiated between the chiefly irritative, allergic dermal symptoms caused by unknown cyanobacterial substances and the potentially more severe hazard of exposure to high concentrations of known cyanotoxins, particularly microcystins. Most documented cases of human injury through cyanotoxins involved exposure through drinking-water, and they demonstrate that humans have become ill (in some cases seriously ill) through ingestion or aspiration of toxic cyanobacteria. Symptoms reported include abdominal pain, nausea, vomiting, diarrhoea, sore throat, dry cough, headache, blistering of the mouth, atypical pneumonia and elevated liver enzymes in the serum, as well as hay fever symptoms, dizziness, fatigue, and skin and eye irritations.
(Source: WHO guidelines for recreational water use 2021)
Guidelines Used by City of Cape Town:
Usually, chlorophyll—a concentration is analysed once a month. Algal cell counts and species identification are undertaken. If blue-green cell counts are > 20 000 cells per ml, a toxin test (microcystin) is undertaken. Step-up sampling (more frequent and more locations) is undertaken when toxin levels are above 10μg/l microcystin.
According to CoCT (Vlei System and Procedure (SP), JULY 2022) and the City’sunderstanding of the WHO guidelines for recreational water use (2021) and WHO 2020b_Cyanobacterial toxins_microcystins_background info (2020) the following applies:
Low risk <10μg/l microcystins, Moderate risk 10-20μg/l microcystin, High risk 20-30μg/l microcystins, Very high risk > 30 µg/L and Extreme risk
is relevant to ≥ 40µg/L. https://Microcystic-blooms-producing-microcystin-toxins-and-relevant-risk-levels.jpg
Please note: MAC’s understanding of the WHO recreational water guideline is different to City of Cape Town’s (COCT):
In general, the weight of the person as well as the daily incidental intake are relevant. Based on a MAC water users’ survey in January 2022, the majority only swallow up to one teaspoon per day, i.e. 5ml per day compared to the WHO assumption of 250ml.
12μg microcystin/L is considered safe as short term DRINKING WATER ( for 14 days, 2 litres per day).
Recreational water with 24 μg microcystin/L is considered safe for a 15kg toddler with a daily incidental intake of 250ml (e.g. from bathing).
The threshold for a 60kg adult with an incidental daily intake of 250ml is 96μg microcystin/L.
Hence, any grown-up with less daily incidental intake than 250ml (for example during intermediate recreational sports), can have a higher threshold. For example: Adult, 60kg, 250ml intake: GV recreation = 96μg microcystin/L or Adult, 60kg, 10ml* intake: GV recreation = 2.400μg microcystin/L
We encourage all members to make themselves familiar with the WHO guidelines.
Historical Test Results (Source: Water and Sanitation Department, ‘Diep Catchment – a water quality overview’, presented to PAAC, 17 January 2019)
LATEST TEST RESULTS MICROCYSTIN and Ecoli
CITY OF CAPE TOWN RESULTS – Microcystin (Blue green algae)
CITY OF CAPE TOWN RESULTS – E-coli