‘Who eats whom and where?’ research project for SA dams completed

22 May 2012

Foodweb schematic for Rietvlei (Harding and Hart, 2012)

South African limnologists Bill Harding and Rob Hart have just completed thirty months of research testing a new (for South Africa at least) approach for determining the structure of the foodwebs in our dams.  The work was conducted at Rietvlei, east of Pretoria.  The work was funded by the Water Research Commission.

The method makes use of stable isotopes of carbon and nitrogen, using a method known as Stable Isotope Analysis (SIA).  These isotopes provide very stable signatures in the tissue of plants and animals, making it possible to determine where something has been feeding, or on what – the so-called “who eats whom and where” approach.

The basis for the study was two-fold: firstly to test the method against more conventional or “classical” means of mapping lake food webs, and secondly to assess whether or not the deliberate harvesting of fish from polluted (eutrophic) dams would result in reduced levels of problematical algae.  The work was successful on both scores.

Rob Hart, South Africa’s guru on zooplankton, had carefully postulated that the proposed fish management theory did not hold water – and the 30-month project proved him to be entirely correct.  This finding nullifies a recommendation arising from an earlier WRC study and work undertaken at Hartbeespoort Dam.

Serious lake studies have not been undertaken on any South African dams for many years.  The work undertaken at Rietvlei provided a refreshing return to what should be a commonality in a country dependent on dams.

A source of samples. Fishermen in Massingir Dam, Mozambique (Photo: Bill Harding)

Bill and Rob have also used the SIA method in the Kruger National Park, where they assessed foodweb structures in samples taken down the Olifants River from Phalaborwa to the outlet of the Massingir megadam in Mozambique.  This work was linked to the mysterious deaths of crocodiles in the park during the recent past and showed the presence of the dangerous, toxic cyanobacterium, Cylindrospermopsis, in Massingir.

African wild dogs, en route to Massingir (Photo: Bill Harding)



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