Friends of Rietvlei
Newsletter Oct/Nov/Dec 2007
Another year is coming to its end which gives us a chance to reflect back. Our major achievement this year is the placement of Sandiso Kraai as conservator in the Diep River area. This came about as a collaborative effort with City of Cape Town, WESSA and ourselves, with the enabling funding from Table Mountain Fund. Some of you have hopefully been on one or both of our walks in the Diep River area with Sandiso, and will have seen, firsthand, what Sandiso has been achieving along the Diep River.
ACTIVITIES - the last for the year
Our final EVENING function for 2007 is on Thursday 15 November at the Education Centre. The Conservation Students will present their completed projects for 2007 and Sandoso Kraai will give us a short update on his work in the Diep River area. As this is our Year End Function please come at 19h30 and bring a plate of eats to share. All members, friends and relatives are welcome!
OPEN DAY on Saturday 17 November. Milnerton Aquatic Club is hosting a day in which the interested public can visit Rietvlei to see what goes on from a recreational perspective. We will be opening our Education Centre from 11h00, and we will lead a walk down to the birdhide at 15h00. Please invite friends and family to come for an outing to Rietvlei.
Queries? Phone Pauline on 021 557 6920 (a/h) or 083 255 2537 OR check our website.
Those of you who regularly drive past the vlei would have noticed the flamingos in all their glory, as well as our regular pelicans. If you live near the Gie Road end of Table View, you will have become accustomed to large flocks of pelicans returning to the vlei in the evening, and are able to hear the swoosh of their wings as they pass overhead. Margaret McIver reported that saw two Hottentot Teals were seen at Rietvlei, on the edge of the back pan, in August. These are rare in the Western Cape, and Margaret says that she saw her first one at Rietvlei ten years ago and has not seen them there since. Koos Retief has recently photographed the Greyheaded Gull from the birdhide, in its breeding plumage – a distinctive grey head! Spoonbills have been seen foraging near the birdhide. They have been nesting abundantly at Intaka Island, Century City, and are presumably using Rietvlei as their restaurant!
The CWCBR is a unique “estuary to estuary” biosphere reserve along our west coast. The Milnerton Lagoon forms the southern boundary, with the northern boundary formed by the Berg River estuary. A stakeholder consultation process to prepare a Spatial Development Plan is underway. A Background Information Document and a Status Quo Report are available for viewing at Milnerton and Atlantis libraries. You can also contact Dudley Janeke on 021 851 0900 for more information. An informative CWCBR newsletter has just been mailed out and we urge you to have a look at the website
Water Quality in Rietvlei  (by Niel van Wyk)
Water quality has been monitored in Rietvlei on a regular basis since 2002. A large number of determinands are monitored, but to most of us the tables with the results do not make much sense. To try and understand what is happening in the system, we have looked at some of the more critical indicators and transfered the data to graphs so that one can visualise the situation. These are available on this website, click here.
In our first analyses of the data, we looked at the situation in North Lake where sampling takes place at the end of the Milnerton Aquatic Club (MAC) jetty, results of samples taken in the Bayside Canal which is the main feeder 'stream' into North Lake is included as comparison.
Bacteriological monitoring (where total faecal coliforms are counted) indicates that the water quality at the MAC jetty is well within the limits allowed by the Water Quality Guidelines. Regular faecal coliform counts of over 1000/100ml of water means that the water is not suitable for swimming, but the highest count at MAC since April 2002 is 540/100ml. In fact the average monthly count over the last five years is 84/100ml. On the other hand, the results of Bayside Canal shows that the 1000/100ml level is often exceeded; the highest count was 270 000/100ml in October 2006, and it often exceeds 10 000/100ml.
Chemical monitoring of the water involves up to 19 determinands sampled on a monthly basis. Interpreting all this data is a complex task, and for easier understanding of what is happening, we chose two as indicators of the ecological viability of the system.
The first is Dissolved Oxygen – this is measured in mg/liter, the maximum levels can vary from about 8mg/l to about 16mg/l; temperatures, dissolved solids, and a few other elements has an effect on the saturation levels of oxygen in water (eg. higher temperatures mean lower saturation levels). However, internationally it is accepted that if the dissolved oxygen level drops below 2mg/l, oxygen-dependent life cannot survive.
Oxygen levels at the MAC jetty from April 2002 to August 2007 has been acceptably high and never dropped below 4.3mg/l; but in the Bayside Canal it has dropped below 2mg/l on several occasions. You may ask: "What about December 2006 when large numbers of fish died as result of low oxygen levels?" The sample for December 2006 was taken on 21 December (the reading was 7.8mg/l), but 5 days later oxygen levels dropped drastically. This just illustrates how rapid oxygen levels can change if the conditions are conducive to such change (eg. hot windless days). At the end of January 2007 the lowest dissolved oxygen level at the MAC jetty was measured at 4.3mg/l.
The other indicator we use is Total Nitrogen. Nitrogen is essential for several of the processes that generate life in an ecosystem. It is important in primary production, without it life cannot exist, but too much can also lead to excess production which results in oxygen levels dropping and thus affecting oxygen-dependent organisms.
Without going into details here, the results indicate that the total nitrogen at the MAC jetty was well within the acceptable range. On a few occasions levels rose over 2.5mg/l (which means eutrophic conditions), but most of the time it was within the 0.5-2.5mg/l mesotrophic range which is the sign of a healthy ecosystem. Levels over 10mg/l means hypertrophic conditions leading to runaway production and the system virtually “choking up”.
Again, the results of December 2006, and February/March 2007, did not give any indication of eutrophic conditions giving rise to fish deaths and blue-green algal blooms. This merely shows that total nitrogen levels can rise and fall quite rapidly and that the monthly sampling do not necessarily show whether such events are going to happen.
In conclusion, water sampling indicates that the water quality in north lake is within acceptable parameters. There does not appear to be any deterioration in water quality since 2002; and at present it also seems that water from the Bayside Canal is adequately filtered by the reedbeds, as well as being diluted when entering the lake, so as not to affect the water quality.
However, recent events have also shown that water sampling may not necessarily pick up any potential changes that lead to severe eutrophication, fish deaths and blue-green algae. The nature of this system is such that oxygen and nutrient levels can change very rapidly; this in itself is not strange, it is simply a further indication that Rietvlei is a very viable and dynamic ecosystem in which one can expect rapid fluctuations in many of the components contributing to life in the system.
We wish you all a safe and enjoyable festive season and we look forward to your active participation in 2008.
Thanks go to Chevron Refinery for sponsoring envelopes for the posted newsletter.
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