This striking satellite snap shows a verdant mass of algae and other aquatic plants sprawling across the surface of a recreational reservoir in South Africa. This frequent phenomenon releases toxic chemicals into the water and gives rise to hidden "dead zones" that can cause widespread fish fatalities.

Most large bodies of water experience seasonal blooms, where photosynthetic algae, or cyanobacteria, rapidly multiply and spread across their surfaces due to changes in weather or nutrient inflow. However, the Hartbeespoort reservoir is in a constant state of "hypereutrophication," which means it has an overabundance of nutrients, such as nitrogen and phosphorus. As a result, the blooms there are much more frequent and intense, according to NASA's Earth Observatory.

"On clear days … satellite images often reveal a reservoir with shades of deep blue interrupted by drifting patches of vivid green," Earth Observatory representatives wrote.

However, it is not just algae that bloom across the reservoir's surface. The verdant masses also include aquatic plants, including water hyacinths (Pontederia crassipes) and Salvinia minima, both of which are invasive species and grow across the reservoir's surface in 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 unnaturally large algal blooms. "It's like having a garden," Bridget Seegers, an oceanographer at 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're going to have a lot of growth." (Hypereutrophication is a more extreme version of this phenomenon.)

However, unlike in a garden, you cannot simply cut back the blooms you don't want, and the consequences of not being able to contain the plant life are much more severe. For example, some of the algae within the Hartbeespoort reservoir can produce toxins that must be filtered out of the water for it to be safe to drink.

However, the bigger issue is that the excessive growth of plant life on the surface sucks up the oxygen from the water below, making it toxic to fish that need to extract oxygen from water via their gills to breathe. These areas of oxygen-poor water are often dubbed "dead zones," and when the blooms spread widely, there are few safe spaces for the fish to go.

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 government report. And in November 2025, another bloom likely triggered a similar die-off and left fish "gasping for air" at the reservoir's surface, according to South Africa-based News 24. These blooms eventually disappear, but the next one is rarely far behind.

The Hartbeespoort reservoir has experienced near-continuous hypereutrophication since the 1970s, excluding a brief period in the 1990s when a successful bioremediation program was introduced but then was quickly scrapped due to high costs. As a result, algae have been blooming constantly, spreading and dying across the reservoir's surface for decades.

You can see this in the animation below, which was made from aerial images collected by the Landsat 8, Landsat 9 and Sentinel satellites between June 2022 and July 2023.

A 2023 study that analyzed data from the previous four decades found that the main culprit was the Crocodile River, which has become extremely polluted with runoff from farms and golf courses that use nutrient-rich fertilizers.

In addition to threatening wildlife, the toxins in the water have been known to cause rashes on water-sports enthusiasts and sicken dogs.

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Harry is a U.K.-based senior staff writer at Live Science. He studied marine biology at the University of Exeter before training to become a journalist. He covers a wide range of topics including space exploration, planetary science, space weather, climate change, animal behavior and paleontology. His recent work on the solar maximum won "best space submission" at the 2024 Aerospace Media Awards and was shortlisted in the "top scoop" category at the NCTJ Awards for Excellence in 2023. He also writes Live Science's weekly Earth from space series.