On clear days in Hartbeespoort, South Africa, satellite images often reveal a reservoir with shades of deep blue interrupted by drifting patches of vivid green. These shifting features indicate algae blooms, which can affect water quality, ecosystems, and nearby human communities.
In this animation, from June 2022 to July 2023, an algal bloom grows, moves around the reservoir, and then fades. The animation is composed of images from Harmonized Landsat and Sentinel-2 (HLS), a NASA product that combines imagery from the NASA/USGS Landsat 8 and Landsat 9 satellites and the European Space Agency’s Sentinel-2A, 2B, and 2C satellites.
Algae is an umbrella term for photosynthetic organisms that live in water, encompassing everything from single-celled cyanobacteria to seaweed. They play a vital role in maintaining healthy ecosystems. But when colonies of algae spread too widely or release harmful toxins, they can threaten the very environments they support. These colonies are known as harmful algal blooms, or HABs.
Some HABs are toxic and often are part of a process called eutrophication. Eutrophication begins when there are too many nutrients in an ecosystem—because of agricultural runoff and other inputs—leading to a rapid growth of algae.
“It’s like having a garden,” said Bridget Seegers, a NASA scientist who studies cyanobacteria in freshwater ecosystems. “If you add a lot of nutrients, you’re going to have a lot of growth.”
Eventually, the algae die off. As decomposers break down the dead algae, they consume oxygen, which can lead to hypoxia and the formation of dead zones.
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