The summers of 2021 and 2022 were tough seasons for Colorado’s Blue Mesa Reservoir. A severe drought gripped much of the western U.S., prompting emergency water releases that brought the reservoir to its lowest level since 1984. Marinas and boat ramps closed, remnants of a ghost town emerged from the muck, and parts of the reservoir turned greenish and swirled with toxic cyanobacteria blooms.
Research conducted by scientists at the U.S. Geological Survey and the National Park Service analyzed decades of Blue Mesa Reservoir data and found a connection between low water levels, warm water temperatures, and harmful blooms.
“Algal blooms were more common when water levels were below 7,470 feet and water temperatures were above approximately 19.5 degrees Celsius (67.1 degrees Fahrenheit),” said Tyler King, a research hydrologist with U.S. Geological Survey. Water levels that low are relatively common and have occurred every few years in recent decades.
While some cyanobacteria, also called blue-green algae, are always present in the reservoir in small numbers, problems occur when certain types proliferate. Aphanizomenon, Dolichospermum, and Woronichinia, for instance, thrive when the reservoir’s waters become warm and stagnant, releasing a toxin called microcystin that can cause skin and eye irritation, respiratory problems, and liver damage. Children and pets are particularly vulnerable to microcystin poisoning because of their size and tendency to ingest more water than adults.
King and colleagues analyzed in situ water samples and satellite observations from the European Space Agency’s Sentinel-2 mission and the NASA/U.S. Geological Survey Landsat satellites. A Sentinel-2 sensor that detects the light-harvesting pigment chlorophyll was particularly useful for mapping the blooms, while Landsat sensors were used to map water temperatures over time.
The National Park Service and U.S. Geological Survey launched the project in 2021 after anecdotal reports and water sampling suggested elevated cyanobacteria conc …