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Using Landsat 5 data acquired from 1984 to 2013, NASA contractor and LST member Nima Pahlevan and his colleagues Jeff Ho and Anna Michalak at the Carnegie Institute for Science uncovered long-term trends in summer algal blooms for 71 large lakes in 33 countries across six continents.

More specifically, they found that the peak intensity of summertime algal blooms increased in more than two-thirds of those freshwater bodies, while a decrease in bloom intensity occurred in only six lakes.

“It’s a big problem,” Pahlevan said. “It’s not just North America and the U.S. This trend is occurring widely across the globe, but we just don’t necessarily hear about it.”

A concern about intensifying algal blooms is the poisonous microcystin toxins that may form in blooms comprised of microscopic bacteria called cyanobacteria. Blooms that erupt in freshwater lakes put people at risk when their drinking water comes from those lakes, or when they swim, ski, and fish in them. If ingested, microcystins can cause adverse health effects in people and animals—from skin rashes to serious illnesses to even death.

In 2014, Toledo, OH, became the first large U.S. city where tap water was rendered unsafe by microcystins when Lake Erie—Toledo’s water source—was choked by a huge algal bloom triggered by runoff from industrial agriculture operations. A half million people in Toledo were warned not to drink or bathe in their tap water for three days.

In 2018, the threat went coast to coast. Officials in Salem, OR, Rushville, NY, and Greenfield, IA, all warned residents not to use tap water after the detection of microcystins in their water supplies. Like Toledo, all those cities rely on surface water, like lakes, rivers and reservoirs, for drinking. Studies have indicated that just in the U.S. alone, freshwater blooms have resulted in $4 billion in economic losses a year, Pahlevan said.

Until he and his colleagues completed their research, which has been published in Nature, it was unclear whether such freshwater blooms were intensifying on a global scale. Using 30 years of Landsat 5 at 30-meter resolution, the researchers found the long-term trends in intensification in part by partnering with Google Earth Engine to process and analyze more than 72 billion Landsat 5 image pixels.

What their study did not reveal was how much such human activities as agriculture, urban development, or even climate change, for that matter, were contributing to the problem.

“That’s something that potentially could be evaluated in the future,” Pahlevan said. “We looked at temperature, precipitation, fertilizer use trends, and we couldn’t see significant correlation between the occurrence of these blooms and these variables.”

Interestingly enough, the six lakes that experienced the least amount of warming over the 30 years also saw sustained improvement in bloom conditions. For Pahlevan, that points to a potential impact of climate warming. “They warmed less and seemed to be cooler. That’s an indication of changing climate,” he said. “According to our study, climate change does definitely have a role and shows up as a player in the lakes that seem to be improving in time.”

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Leif Olmanson, a research associate at the University of Minnesota who has worked on water quality in that state for 20 years, called the study by Pahlevan and his colleagues “the tip of the iceberg” considering that there are 117 million freshwater bodies across the globe. While Pahlevan and his colleagues didn’t include any Minnesota lakes in their study, Olmanson said he has been using remotely-sensed imagery in a July 15-to-September 15 window in late summer “to catch the days of algal maximum and water clarity minimum so we can identify where the blooms are.”

In fact, he said he has been using Landsat imagery for two decades now to make water clarity assessments for over 10,000 lakes in Minnesota that can then be viewed online at lakes.rs.umn.edu.

These days, Olmanson and his colleagues are working on a system that is gathering clear imagery from Landsat and the European Space Agency’s (ESA) Sentinel-2 mission. All those images, processed on a supercomputer at the Minnesota Supercomputing Institute, will hopefully result in clear imagery every two weeks for the entire open water season in the state, “enabling us to get a better handle on the dynamics” of algal blooms and other water quality issues, Olmanson said.

Pahlevan’s hope is that other water managers and decision makers, whether at regional, state, national, or international levels, will chip away at that tip of the iceberg as well.

“What we found is quite widespread, in various freshwater systems, across latitudes and longitudes ... it’s really all over the place,” he said. “So, I can see this study as being a major driver for future studies of how to use satellite data to improve the understanding of impacts of climate change, and the anthropogenic activities on global aquatic ecosystems in general.”

Along with the USGS LST Award, the research by Pahlevan and his colleagues was supported by the U.S. National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, Google Earth Engine, and a NASA ROSES grant.

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