Many of our rivers and dams in South Africa have become eutrophic. While travelling through the countryside we often see dams with an unnatural green colour. According to a well-known water scientist, Prof Anthony Turton, South Africa’s water quality is the biggest challenge and not our water quantity, although the water shortage is a serious problem. What bothers him the most is the contamination of our drinking water reservoirs with toxic cyanobacteria or blue green algae, a unicellular organism that is 3.5 billion years old. It is estimated that between 50-70% of our dam reservoirs have already been contaminated. Hartbeespoort Dam is a good example.


Eutrophication is the enrichment of an ecosystem with plant nutrients, especially compounds containing nitrogen and phosphorous. Eutrophication is a natural process in dams and rivers, occurring as they age through geological time. However, human activities can accelerate the rate at which nutrients enter ecosystems. Fertilisers used on farms, golf courses and lawns often run-off into dams, rivers and streams after a shower of rain to feed hungry algae and phytoplankton. Concentrated animal feeding operations (feeding pens) is another source of contamination to augment eutrophication by sewage run-offs into dams and rivers.

The effect eutrophication has on the environment

The gas ratio in freshwater (O2 – CO2 – relation) is sustained by photosynthesis of plants and algae – using carbon dioxide and releasing oxygen. When plant nutrients such as nitrate and phosphate from sewage or fertilizer contaminate ground and surface water it causes plants such as algae and phytoplankton to flourish to such an extent that the whole surface of the dam or river can become covered with them. This thick green algae plume, which mainly forms at the top of the water, blocks the sunlight from reaching aquatic plants in the water. This prevents aquatic plants from photosynthesising to produce oxygen in the water and they die.

The algae also start to die when all the nutrients are consumed and there is no more food for them to thrive on. Dead algae sink to the bottom and aerobic organisms, like saprophyte bacteria, flourish. The pH of the water drops (becoming more acidic) leaving an impassable environment for fish and other living organisms to survive. Bacteria start to break down (decompose) the dead algae and plants and they release more nutrients back into the water, continuing the algae bloom cycle and consuming more oxygen. The bacteria multiply much faster in the water and the gas balance of the water is affected. The water becomes anoxic (oxygen depleted) and all non-bacterial life in the water, including fish and other animals die. The water becomes an awful, smelly, dead green soup.

South Africa

In South Africa, the drought and heat wave aggravated the problem and caused cyanobacteria to flourish.  Cyanobacteria blooms are found in 26 of our 50 dams, including the Vaal Dam. The problem with cyanobacteria blooms is that it produces a toxin known as microcystin which is released when the cells break open during water treatment or when chlorine gets into contact with it. The cells stress and release the toxin into the water. This toxin is similar to snake venom. It gives the water a bad earthy taste. Researches have linked the toxin to different cancers, Alzheimer’s and motor neuron disease (MND).

If you want to know more or protect your family by purifying your drinking and cooking water, please contact Perfect Water on