Deadly bright green blob spreads across South Africa’s reservoir — Earth seen from space

This impressive satellite photo shows lush clumps of algae and other aquatic plants spread across the surface of a South African recreational reservoir. This frequent event creates hidden “dead zones” where toxic chemicals can be released into the water and cause widespread fish mortality.

The reservoir is located at Hartbeespoort Dam, built in the 1920s, about 25 miles (40 km) west of Pretoria. This man-made water source is located just south of the Magaliesberg Mountains, partially visible at the top of the image, and is fed by the Crocodile River (spelled Krokodyillivier in Afrikaans), which runs through the nearby mountains. It is used not only to provide fresh water to nearby cities and farms, but also for recreational activities such as fishing and water sports.

Most large bodies of water experience seasonal blooms, in which photosynthetic algae or cyanobacteria rapidly multiply and spread across the water surface due to changes in weather or nutrient inputs. However, Hartbeesport Reservoir is always in a state of ‘hypereutrophication’, meaning that nutrients such as nitrogen and phosphorus are present in excess. As a result, blooms there are becoming more frequent and intense, according to NASA’s Earth Observatory.

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“On clear days…satellite images clearly show reservoirs with deep blue hues punctuated by bright green drifting spots,” Earth Observatory representatives wrote.

However, algae is not the only thing that grows on the surface of the reservoir. The lush community also includes aquatic plants such as water hyacinth (Pontederia crassipes) and Salvinia minima. Both are invasive species and grow in large tangled mats across the surface of the reservoir.

Eutrophication occurs when too many nutrients are added to a body of water, often due to human influence, and can lead to unnaturally large algae blooms. “It’s like having a garden,” Bridget Segers, an oceanographer at Maryland’s Goddard Space Flight Center who heads NASA’s Cyanobacteria Assessment Network, told the Earth Observatory. “The more nutrients you give, the bigger you grow.” (Hypertrophication is a more extreme version of this phenomenon.)

However, unlike in gardens, unwanted flowers cannot be easily cut away, and the consequences of failing to suppress plant life are even more severe. For example, some algae in Hartbeesport Reservoir can produce toxins that must be removed by filtering the water to make it safe to drink.

But the bigger problem is that excessive plant growth on the surface sucks in oxygen from the water below and becomes toxic to fish, which need to extract oxygen from the water through their gills in order to breathe. These oxygen-poor waters are often referred to as “dead zones,” and when a bloom spreads over a large area, there is little safe space for fish to go.

According to a government report, in April 2023, a lack of oxygen occurred in parts of the reservoir, leading to mass die-offs of fish, including hundreds of large carp. And in November 2025, another bloom caused similar deaths, likely leaving the fish gasping for air “for air” on the surface of the reservoir, South Africa-based News24 reported. These blooms eventually disappear, but the next one rarely appears very late.

Hartbeesport Reservoir has experienced almost continuous hypereutrophication since the 1970s, with the exception of a brief period in the 1990s when a successful bioremediation program was introduced, but then quickly discontinued due to high costs. As a result, algae bloomed continuously for decades, spreading over the entire surface of the reservoir and continuing to dry it up.

This can be seen in the animation below created from aerial imagery collected by Landsat 8, Landsat 9, and Sentinel satellites between June 2022 and July 2023.

A 2023 study that analyzed data from the past 40 years found that the main culprit was the Crocodile River, which was heavily polluted by runoff from farms and golf courses that used nutrient-rich fertilizers.

In addition to threatening wildlife, toxins in the water are known to cause rashes in water sports enthusiasts and sick dogs.