On clear days in Hartbeespoort, South Africa, satellite imagery often shows a body of water with shades of deep blue, interrupted by floating patches of bright green. These changes indicate algal blooms that can affect water quality, ecosystems and nearby human communities.
In this animation, from June 2022 to July 2023, an algal bloom grows, moves across a body of water, and then subsides. The animation consists of images from Harmonized Landsat and Sentinel-2 (HLS), a NASA product combining NASA/USGS imagery. Landsat 8 And Landsat 9 satellites and the European Space Agency's Sentinel-2A, 2B and 2C satellites.
Algae is a general term for photosynthetic organisms that live in water, covering everything from single-celled cyanobacteria to seaweed. They play a vital role in maintaining healthy ecosystems. But when algae colonies spread too widely or release harmful toxins, they can threaten the very environment they support. These colonies are known as harmful algal bloomor HAB.
Some HABs are toxic and are often part of a process called eutrophication. Eutrophication begins when there are too many nutrients in an ecosystem (due to agricultural runoff and other factors), leading to rapid algae growth.
“It's like a garden,” said Bridget Seegers, a NASA scientist who studies cyanobacteria in freshwater ecosystems. “If you add a lot of nutrients, you will have good growth.”
Over time, the algae die off. By decomposing dead algae, decomposers consume oxygen, which can lead to hypoxia and the formation of dead zones.
These conditions were recorded at Hartbeespoort Dam, located approximately 25 kilometers (16 miles) west of Pretoria and used primarily for recreation and irrigation. Harmful algal blooms containing cyanobacteria regularly occur in the reservoir. It is also home to large mats of invasive water hyacinths. Although hyacinths do not produce toxins, they contribute to eutrophication when they die and decompose.
Harmful algal blooms can affect the health of ecosystems and the lives and livelihoods of people. In April 2023, the South African authorities linked the death of large fish in Hartbeespoort due to low oxygen levels caused by excessive algae growth. More broadly, HABs in drinking water reservoirs can reduce water availability and increase water treatment costs, while swimming in waters contaminated with HABs can cause a rash, and pets or livestock that drink them can get sick or die.
One 2022 paper published in the journal Remote Sensing, examined algae in the pond from 1980 to 2020 using Landsat data. “This reservoir has always been carefully monitored by the local water department,” said Adam Ali, the paper's lead author. The study used satellite data to provide an overall picture of conditions across the reservoir over long-term timescales. Using 40 years of Landsat data, the researchers found that the main drivers of algae growth were total phosphorus (a nutrient found in runoff) and water temperature, with blooms typically expanding in the warm summer months and dying down in winter.
They also identified key trends across space and time. Seagrass productivity was higher near tributaries of the Crocodile River (Crocodile River) and in the western part of the reservoir due to golf course runoff and limited water circulation, demonstrating how HABs are influenced by river flow and inflow. Large blooms occurred between 1982 and 1986 when total phosphorus levels were high. A bioremediation program in the late 1980s succeeded in limiting algae growth, but after funding ran out in the late 1990s, harmful algal blooms surged again in the early 2000s.
To track the algae from space, the researchers analyzed the color of the water by measuring different wavelengths of light. From this, they estimated the concentration of chlorophyll-a, a common algal pigment, and used these values to estimate the algal biomass over time. While water samples are still needed to confirm the harmfulness of algal blooms, satellite data can help scientists understand the causes of harmful algal blooms, especially in remote regions where regular ground-based monitoring is expensive and time-consuming.
New and upcoming NASA missions promise to improve space-based water quality monitoring. The next Landsat satellite is expected to measure wavelengths specifically designed to detect HABs. NASA STEP The (Plankton, Aerosol, Cloud, Ocean Ecosystem) mission, launched in 2024, collects data in hundreds of precise wavelength bands in the visible spectrum, which could help scientists identify the type of algae causing a particular bloom—a key factor in determining toxicity. Given the spatial resolution of PACE, the data are most useful in coastal areas or large inland bodies of water. Ali is working with researchers at NASA Ames to integrate PACE into future research.
Animation by Ross Walter/Landsat Science Office support, using data from Harmonized Landsat and Sentinel-2 (HLS) product. Photo by Lauren Dauphin/NASA Earth Observatory. using Landsat data US Geological Survey. Story by Madeleine Gregory / Landsat Science Support.
- Ali, K., etc. (2022) Integrating in situ and current generation satellite data for temporal and spatial analysis of harmful algal blooms at Hartbeespoort Dam, Crocodile River Basin, South Africa. Remote sensing14(17), 4277.
- NOAA (April 27, 2016) What are harmful algal blooms? As of January 6, 2026
- Government of South Africa (26 April 2023) The Water and Sanitation Service has published an investigation report into the causes of fish kills at Hartbeespoort Dam.. As of January 6, 2026
