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Potentially toxic red algal blooms will increasingly become the norm in South Africa

Anthony Turton
By Anthony Turton
15 Apr 2025
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Rivers in South Africa are generally in distress. Dams are a critical part of the infrastructure needed to create water security by sustaining the jobs needed for social stability.

We have become accustomed to seeing large water bodies covered in floating aquatic weeds. Where open water is seen, it is often a luminous green colour, commonly caused by blue-green algae, known as a harmful algal bloom.

Both floating aquatic weeds and harmful algal blooms are classic symptoms of nutrient-enriched water. Known technically as eutrophication, it simply means that excess nutrients are present in the water. The more nutrients available, the greater the biomass that the system can sustain, resulting in the proliferation of floating aquatic weeds and submerged algae.     

In March last year, I reported in Daily Maverick on the discovery of a species that has the capacity to turn rivers, ponds and lakes red. This species differs from blue-green algae that now infest most of our watercourses.

It is a single-celled photosynthesising organism that moves in the water by flailing a whiplike appendage, known as a flagellum. So, it is a cell that is mobile (which is an animal-like trait), but with the ability to photosynthesise (which is a plant-like trait).

Defining feature


A defining feature is the presence of a protein-based structure beneath the cell membrane that forms the outer body of the creature. These can be thought of as primitive muscles because they enable the organism to move using an inching motion, like a slug.

A few weeks ago, I was contacted by the owner of a farm in the Western Cape asking if I could advise on the reason their dam had suddenly turned red. I immediately connected the farmer with Professor Sanet Janse van Vuuren from the Unit for Environmental Sciences and Management at North-West University because her team has the technical capacity to correctly identify the species causing the red colouration.

This fed into the research programme at North-West University and has resulted in the recent submission of a technical paper to the African Journal of Aquatic Science.

This paper reports the following: “Euglena sanguinea is a red-pigmented euglenoid species known for producing the ichthyotoxin (fish toxin) euglenophycin, which has been linked to fish mortalities in various aquatic systems. First reported in the Kruger National Park in South Africa during 2016, this species has since rapidly expanded its range across multiple provinces, forming persistent dense blooms in several freshwater bodies. This study documents its spread, potential ecological impacts, and the need for further monitoring and management strategies.”

The red pigmentation is caused by the presence of haemachrome pigments. This can be thought of as a natural form of sunblock, enabling the species to survive in warm water under extremely bright sunlight.

They are extremophiles, because of their ability to survive under the harshest of environmental conditions. They produce a range of substances, including a toxin that is specific to fish, but no fish kills due to the presence of this organism are yet on record in South Africa. They also produce a substance that would potentially be useful in cancer treatment.

Limited data


Of particular interest to management practices, the limited data available suggests that Euglenoids (the collective name given to several genera having similar characteristics) can tolerate mildly saline water. This is tentative evidence of long-term changes in the ecology of South African rivers, where salinisation is a documented trend.

For example, the Vaal River Salinity Study conducted by the Department of Water and Sanitation in 2006 reported that 34% of the salt entering the system came from a combination of acid mine water draining from the mine void and sewage return flows. An additional 39% originated from diffuse sources including mine tailings dams.

The recommendation from the department was that two actions need to be taken as a matter of urgency. The first is dilution to minimise the impact of salinity, and the second is mitigation to remove salts from their point of origin. 

Dilution is currently managed by releasing water from strategic storage in Sterkfontein Dam. Mitigation is not being implemented, but a good place to start would be to improve the process engineering in the 32 wastewater treatment works discharging into the system. (See Understanding the sewage challenge facing Gauteng’s water supply).

A separate study completed by the Department of Water and Sanitation in 2012 indicated that 400 tons of salt enter the rivers draining the Witwatersrand Goldfields every day under normal low flow conditions. 

This number jumps to almost 1,000 tons a day under flood conditions when runoff from the tailings dams is at its peak. To illustrate what this means, we can think of a 10-ton truck in convoy carrying salt down a highway. During drought, this convoy would be 40 trucks long, but in times of abundance it would be 100 trucks long. This convoy would deliver its load into the river every day for as long as those flow conditions persist.

Red coloured algae belonging to the Euglenoid family were rare in South Africa until recently. Their rapid proliferation confirms that aquatic ecosystems associated with major rivers and dams are dynamic in nature.

Change can occur rapidly, resulting in new challenges for water resource management. This is important for water boards like Rand Water, Magalies Water, Berg Water and Umgeni Water.

During May 2024 the first blooming of E sanguinea was recorded at Pecanwood Estate on the banks of Hartbeespoort Dam. (Photo: Gill Ledger)



It is vitally important that adequate funding be provided to research institutions working on aquatic ecosystem health. It is also important for those research institutions to work closely with water boards responsible for the provision of safe potable water produced from eutrophic rivers, so that sensible decisions can be made about changes to process design.

Ultimately public trust must be maintained, placing greater demands on the communication of science in the service of society.

As our rivers become warmer, and as attempts to neutralise acidic mine waters change the pH of effluent discharge, Euglenoids start to gain a comparative advantage over blue-green algae.

It is therefore becoming increasingly likely that we will see rivers flowing red during hot dry summer months. This is not an apocalyptic message, because the process is generally well understood by scientists.

We can therefore predict with confidence that red, potentially toxic blooms will increasingly become the norm in South Africa as a result of, among other things, global warming.

Congratulations to North-West University’s Unit for Environmental Sciences and Management for leading the research into hazardous algal blooms. You are indeed a national treasure.  DM

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