Algae swirls across a South African reservoir.

Satellite imagery shows an algal bloom in the Hartbeespoortdam reservoir growing, shifting, and fading between June 2022 and July 2023; researchers link blooms mainly to phosphorus in runoff and warmer water temperatures.

Satellite images and an animation show an algal bloom appearing, moving, and fading in the Hartbeespoortdam reservoir from June 2022 to July 2023. The animation uses Harmonized Landsat and Sentinel-2 (HLS) data, which combine NASA/USGS Landsat imagery with ESA Sentinel-2 images. Algae includes a wide range of photosynthetic organisms and can be beneficial, but dense blooms called harmful algal blooms (HABs) can release toxins or reduce oxygen. Eutrophication, often driven by nutrient runoff such as phosphorus, and warmer temperatures commonly promote large blooms. Key observations: - The animation covers June 2022 through July 2023 and highlights shifting green patches that indicate algal growth. - HLS combines Landsat 8 and 9 with Sentinel-2A, 2B, and 2C to provide regular, landscape-scale water images. - Hartbeespoortdam is about 25 km west of Pretoria and is used mainly for recreation and irrigation; it has recurring cyanobacterial HABs and mats of invasive water hyacinth. - A 2022 study using 1980–2020 Landsat data found that total phosphorus and water temperature were the main drivers of algal productivity, with higher values near river inflows and in the reservoir’s west. - South African authorities reported a large fish kill in April 2023 linked to low oxygen from excessive algal growth. - New and recent NASA missions are expected to improve space-based water monitoring: the PACE mission (launched 2024) provides many visible-wavelength bands useful for identifying algal types, and a future Landsat satellite is planned to include wavelengths aimed at detecting HABs. Summary: The observed blooms can affect water quality, oxygen levels, ecosystems, and local uses such as recreation and irrigation. Long-term satellite records help scientists identify patterns and drivers like nutrient inputs and temperature. Ongoing and planned satellite missions, and integration of PACE data into studies, are described as part of efforts to better monitor and understand HABs in reservoirs.