Deadly, vivid-green mass sprawls across South African reservoir — Earth from space
QUICK FACTS
Where is he? Hartbeespoort Dam, North West Province, South Africa [-25.74486936, 27.858152164]
What’s in the photo? A bloom of bright green toxic algae and planets spreading in a tank
Which satellite took the photo? Landsat 8
When was it taken? August 10, 2022
This striking satellite photo shows a verdant mass of algae and other aquatic plants spreading across the surface of a recreational reservoir in South Africa. This common phenomenon releases toxic chemicals into the water and creates hidden “dead zones” that can cause many fish to die.
Most large bodies of water experience seasonal blooms, during which photosynthetic algae, or cyanobacteria, multiply and spread rapidly on their surface due to climate change or nutrient input. However, Hartbeespoort Reservoir is in a constant state of “hypereutrophication,” meaning it has an overabundance of nutrients, such as nitrogen and phosphorus. As a result, flowering is much more frequent and intense, depending on NASA Earth Observatory.
“On a clear day…satellite images often reveal a reservoir with shades of deep blue interrupted by drifting patches of bright green,” Earth Observatory representatives wrote.
But algae is not the only thing blooming on the surface of the reservoir. The lush masses also include aquatic plants, including water hyacinths (Crassipes of Pontederia) And Salvinia minimaboth of which are invasive species and grow on the surface of the tank as large intertwined mats.
Eutrophication occurs when too many nutrients are added to a body of water, often due to human influences, and can lead to unusually large algae blooms. “It’s like having a garden” Bridget Seegersan oceanographer at the Goddard Space Flight Center in Maryland who leads NASA’s Cyanobacteria Assessment Network, told the Earth Observatory. “If you add a lot of nutrients, you’ll get a lot of growth.” (Hypereutrophication is a more extreme version of this phenomenon.)
However, unlike a garden, you can’t just cut off flowers you don’t want, and the consequences of not being able to contain plant life are much more serious. For example, some algae in Hartbeespoort Reservoir can produce toxins that must be filtered out of the water for it to be drinkable.
However, the bigger problem is that excessive plant life growth on the surface sucks oxygen from the water below, making it toxic to fish that need to extract oxygen from the water through their gills to breathe. These areas of oxygen-poor water are often nicknamed “dead zones,” and when blooms spread widely, there are few safe spaces for fish to move.
In April 2023, a lack of oxygen in part of the reservoir was linked to a mass die-off of fish, including hundreds of large carp, according to a study. government report. And in November 2025, another bloom likely triggered a similar die-off and left fish “gasping” on the reservoir’s surface, according to a South Africa-based study. News 24. These blooms eventually fade away, but the next one is rarely far behind.
Hartbeespoort Reservoir has experienced near-continuous hypereutrophication since the 1970s, except for a brief period in the 1990s during which a successful bioremediation program was introduced but was then quickly abandoned due to high costs. As a result, algae has been constantly blooming, spreading and dying on the surface of the reservoir for decades.
You can see it in the animation below, made from aerial images collected by the Landsat 8, Landsat 9 and Sentinel satellites between June 2022 and July 2023.
A Study 2023 which analyzed data from the previous four decades found that the main culprit was the Crocodile River, which had become extremely polluted by runoff from farms and golf courses that use nutrient-rich fertilizers.
In addition to threatening wildlife, toxins in water are known to cause skin rashes in water sports enthusiasts and sick dogs.
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