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.
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ACTIVITIES - the last
for the year |
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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! |
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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.
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SOME BIRDING
NEWS |
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!
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CAPE WEST COAST
BIOSPHERE RESERVE (CWCBR) |
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
www.capebiosphere.co.za.
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Water Quality in
Rietvlei (by Niel van Wyk) |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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”. |
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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. |
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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. |
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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. |
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We wish you all a safe and enjoyable festive season and we
look forward to your active participation in 2008. |
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| Thanks go to Chevron Refinery for
sponsoring envelopes for the posted newsletter. |
