Is there proof for the perceived effect of Rietvlei’s water mixers?

8 April 2012

Rietvlei Dam (Pretoria, South Africa) (Photo: Bill Harding)

In recent weeks several articles have been published on the apparent beneficial effects of installing a particular make of water circulators in South Africa’s Rietvlei Dam, a very-polluted reservoir situated south-east of Pretoria.  Some of these articles, e.g. this link issued by the US Embassy, or placed in journals including that of a national research organization, suggest an endorsement of the technology – which is odd because such a step should only follow a process of exhaustive, rigorous research.

Circulation and mixing are vital forcing-functions in lakes.  In most cases, nature provides sufficient energy through wind-action but, where eutrophication predominates, this may not been enough to offset the growth of problem-causing algae.  There are many different types of water-circulation devices on the market, but very few with scientific proof underpinning how they work.  They are not a ‘cure-all’.  As far as I can see, these [the Rietvlei] mixers were installed on faith, changes have been detected – which may or may not be due to the mixers (this conceded by those overseeing the project), and research will now be retrospectively undertaken to validate a vast, uncontrolled experiment.  If so, then these prior announcements of success may well be premature.  Is there pressure to produce some justification for a very large capital expenditure?

Now don’t get me wrong: I am the last person to stand in the way of a technology that may assist with attenuating the ravages of eutrophication.  Where my concerns lie, however, is the near-total absence of any scientific proof.  A colleague and I have just completed 30 months of quite detailed investigation of the various components of the Rietvlei food web.  When our project was planned we had no idea of the planned installation of the mixers and they arrived, in batches, pretty much from the time we started our work.

Our findings, although not specifically-tailored to try and track circulation-induced changes in the lake’s hydrodynamics, did not reflect any impacts that we could attribute to the claimed-changes in lake circulation.  Moreover, changes  in algal composition, suggested as being due to the mixers, were apparent in the lake ecosystem before the physical intervention.  We have shared our findings with those involved with the mixer project, but without any acknowledgement or response as yet.

Anyone who works in nature and particularly in the aquatic environment, will be aware of the vast number of interlinked factors that have to be confidently-excluded before any perceived response can be conclusively attributed to a man-induced change.  Rietvlei is a enigmatic system that evidences atypical responses to the high levels of pollution, i.e. even more difficult to exclude “nature”.  Given these complexities, I can only find it strange that claims are being made so soon, without any mention of solid, scientific proof.

If these systems do work, we need to understand how and why.  Or is science to become driven by commercial claims?

(Bill Harding is an internationally-recognized lake biologist, specializing in cyanobacteria – blue-green algae – and the responses of waterbodies to eutrophication.  His CV may be found on the website  His Rietvlei research, undertaken with Emeritus Professor Rob Hart,  will shortly be published by the Water Research Commission as WRC K5/1918, Elucidation of Foodweb Interactions in South African Reservoirs using Stable Isotopes.  The findings will also be published in various peer-reviewed journals).


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