lgal blooms have become a familiar sight on Lake Erie, as common during the summer as island-hopping, scenic cruises, and roller coasters. In 2024, the earliest bloom of blue-green algae recorded by NOAA began forming in the lake’s western basin on June 24. This marked the earliest detection since NOAA started monitoring these events in 2002. By early September, the bloom was still present, and bloom season can extend into October, depending on how frequently winds mix the lake's waters in the fall.
When the OLI-2 (Operational Land Imager-2) on Landsat 9 acquired this image on August 13, the bloom covered approximately 320 square miles (830 square kilometers). Since that date, which was the last time Landsat satellites got a clear look at this part of the lake, the bloom would more than double in area to the season’s likely largest extent of 660 square miles (1,700 square kilometers) on August 22.
Phytoplankton blooms carry implications for the lake ecosystem, human health, the local economy, and even municipal water supplies. The dominant organism in this bloom, a Microcystis cyanobacteria, produces the toxin microcystin, which can cause liver damage, numbness, dizziness, and vomiting. NOAA’s Great Lakes Environmental Research Laboratory measured toxins at concentrations above the recreational limit the week of August 12. The agency noted that toxins can be concentrated in scums, advising that people and their pets stay out of the water near scums.
NOAA and its research partners had forecasted a moderate to above-moderate harmful algal bloom (HAB) in western Lake Erie this summer. Blooms are classified based on their biomass, and a moderate-severity bloom will produce noticeable areas of scum. However, the agency noted, a bloom’s size does not necessarily correlate with its toxicity.
“Nutrient input from the Maumee River is the dominant driver of HAB variability from year to year,” said Brice Grunert, a professor in the department of Biological, Geological, and Environmental Sciences at Cleveland State University. Other factors such as temperature, mixing of the water column, and water movement also influence the extent and duration of blooms, he said. Precipitation can increase the load of nutrients such as nitrogen and phosphorous in runoff to the lake, and warmer, more stratified water can amplify blooms. In 2024, the bloom followed a period of record April rainfall and an intense heatwave, according to news reports.
Satellite imagery plays an important role in helping scientists understand the nuances of phytoplankton blooms, which in turn can aid those charged with monitoring and forecasting the events. Grunert has been working in Lake Erie’s western basin for the past three years to better understand phosphorous cycles within the lake. His team is investigating how satellite imagery, combined with data from sediment sampling and chemical tracers, relates to the amount of algae-producing phosphorous in the water column.
He and other scientists studying aquatic ecosystems will soon have a new tool at their disposal in the form of the OCI (Ocean Color Instrument) aboard NASA’s PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) satellite. This instrument measures waterbodies in hundreds of wavelengths across a spectrum of ultraviolet, visible, and near-infrared light. When fully calibrated, the data will enable scientists to track the distribution of phytoplankton and—for the first time from space—identify which communities of these organisms are present on daily, global scales.
Despite the presence of the word “ocean” in the mission title, PACE also opens new lines of inquiry in the freshwater realm. “There are a lot of interesting questions that can be addressed using PACE imagery in the Great Lakes,” Grunert said. For example, hyperspectral data will be able to reveal phytoplankton pigments that could previously only be estimated with the limited number of spectral bands, he said. And a more detailed perspective of blooms over space and time is expected to help scientists decipher how HABs in Lake Erie develop and why cyanobacterial blooms in Lake Superior are starting to occur. “This unlocks a whole new level of information that can be used to describe the unique and changing ecosystems and biogeochemistry within the Great Lakes,” he said.
Grunert is currently working on a PACE Validation Science Team project, taking field measurements in parallel to observations being collected by PACE’s OCI. These include water-surface color and the optical properties of phytoplankton, sediment, and other substances in the water column.
On July 28, 2015, the Operational Land Imager (OLI) on Landsat 8 captured these images of algal blooms around the Great Lakes. The bloom is visible as swirls of green in western Lake Erie (top) and in Lake St. Clair (bottom).
Earlier in July, NOAA scientists predicted that the 2015 season for harmful algal blooms would be severe in western Lake Erie. They suggest that algae growth in western Lake Erie could rival the blooms of 2011. Algae in this basin thrive when there is an abundance of nutrients (many from agricultural runoff) and sunlight, as well as warm water temperatures. The season runs through summer and peaks in September.
Research confirmed that in 2011, phosphorus from farm runoff combined with favorable weather and lake conditions to produce a bloom three times larger than previously observed. The researchers noted that if land management practices and climate change trends continue, the lake is likely to see more blooms like the 2011 event.
Harmful algal blooms can lead to fish kills. They also can affect the safety of water for recreation and for consumption (as was the case in Toledo, Ohio, and southeast Michigan during a 2014 bloom). As of July 30, 2015, drinking water was reported to be safe in these areas.
In April 2015, NASA and several partners announced a new multi-agency effort to develop an early warning indicator for harmful algal blooms in fresh water. The system is expected to make ocean color satellite imagery more easily available to environmental and water quality managers.
NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey. Caption by Kathryn Hansen.
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