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Scientists aboard the U.S. Environmental Protection Agency’s research vessel Lake Guardian look for clues about Great Lakes health by repeatedly measuring water, aquatic life and sediments—not by taking a single snapshot and declaring a lake healthy or unhealthy. EPA’s routine spring and summer surveys sample all five lakes, capturing different seasonal conditions that help agencies track ecosystem changes and inform decisions about water quality, fisheries, drinking water and recreation.
What does the Lake Guardian monitor?
The Lake Guardian is used by EPA’s Great Lakes National Program Office to monitor and report on ecosystem status and trends in Lakes Superior, Michigan, Huron, Erie and Ontario. EPA describes it as the largest research vessel in its fleet and the largest research vessel operating on the Great Lakes; the Great Lakes Restoration Initiative funds the vessel.
Its work extends beyond water samples. EPA says the ship gathers data on water, aquatic life, sediments and air. In routine surveys, scientists collect water, plankton and benthos—the organisms living on or in lake-bottom material. These observations can help assess stressors such as invasive species and harmful algal blooms, and contribute to water-quality management and fisheries decisions.
The Great Lakes contain a large share of the region’s surface freshwater. EPA’s December 2025 Research Vessel Lake Guardian 2021 and 2022 Annual Report describes them as holding “over 20% of the world’s surface freshwater” and “84% of North America’s surface freshwater.” Those are the report’s stated figures and scope; they are not a measure of all freshwater on Earth or in North America.
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Why sample the lakes in spring and summer?
Seasonal sampling helps scientists interpret measurements in context. Spring and summer can present markedly different conditions, so the two surveys provide complementary views rather than duplicate checks.
| What changes | Spring survey | Summer survey |
|---|---|---|
| Water-column structure | EPA characterizes spring waters as generally cold and well mixed. | Warm surface water can sit above colder bottom water, creating stratification. |
| Biological activity | Generally a period of lower biological activity. | Generally a period of higher biological activity. |
| Nutrients and oxygen | EPA’s April 2026 release says nutrient concentrations are generally highest for the season. | Stratification influences nutrient and oxygen levels, as well as temperature patterns. |
| What the snapshot helps show | A broad view of mixed-water conditions and seasonal nutrient context. | Conditions associated with algal growth, organism distribution and fish habitat. |
These are seasonal patterns, not guarantees that every station or lake behaves identically. For example, stratification affects how conditions vary with depth, which is why depth-specific measurements matter.
How do scientists collect samples aboard the ship?
The ship has wet, chemistry and biology laboratories for handling and analyzing samples. At designated stations, crews use different equipment depending on what they need to measure:
- Water and water-column conditions: A Rosette sampler carries twelve Niskin bottles and a multiparameter profiler. The bottles collect water from selected depths, while the profiler records water-quality readings through the water column.
- Sediments: PONAR samplers, a box corer and a multicorer collect lake-bottom material.
- Bottom organisms: A benthic sled supports video work and collection of bottom life.
- Small aquatic organisms: Nets collect zooplankton and Mysis, a small shrimp-like organism important in the food web.
Using more than one method gives researchers evidence about different parts of the ecosystem: the water itself, organisms living in it, and conditions at the bottom. Laboratory analysis can then add detail that a shipboard reading alone cannot provide.
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What do the 2026 surveys cover?
EPA announced that the 2026 spring survey began April 4 and published its announcement on April 7. The month-long survey was scheduled to visit all five lakes and collect water and small aquatic organisms, including phytoplankton and zooplankton. Participating institutions included Oak Ridge Institute for Science and Education, Cornell University, University of Minnesota-Duluth, Michigan Technological University and Research Institute, University of Chicago, University of Michigan and Cleveland State University.
EPA announced that the 2026 summer survey began August 3 and would run for a month across all five lakes. Its emphasis was water quality and food-web conditions during summer stratification.
Other ship work described by EPA in August 2026 is separate from those two annual all-lake surveys. It included May–July Lake Michigan surveys with the U.S. Geological Survey and partners on water quality, zooplankton and fish larvae; May–September monitoring of oxygen and temperature profiles in Lake Erie’s central basin; and a July Lake Superior sampling exercise involving educators and NOAA Sea Grant.
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EPA says biannual sampling surveys have been conducted since 1983 and characterizes the resulting dataset as the longest-running and most extensive monitoring dataset for Great Lakes water quality and aquatic biota. Repeated observations matter because a result from one cruise is a snapshot; trends emerge by comparing measurements across seasons and years.
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Chlorophyll-a offers one clue, not a complete health score
Chlorophyll-a is a pigment used by photosynthetic organisms. EPA uses it as an indicator of photosynthetic activity and algal biomass. The agency measures it in filtered water samples and also uses satellite imagery, where color intensity is related to chlorophyll-a concentration.
EPA’s indicator page reports decreasing open-water chlorophyll-a concentrations in Lakes Michigan, Huron and Ontario, relatively constant concentrations in Lake Superior, and an increasing trend in Lake Erie. These patterns describe this particular indicator; they do not rank the lakes’ overall health. Chlorophyll-a needs to be considered alongside other measurements, including oxygen, nutrients, aquatic organisms and habitat conditions.
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A survey can document conditions at sampled locations and depths, and repeated surveys can contribute to a picture of ecosystem change. That evidence helps EPA and partner agencies assess concerns such as harmful algal blooms and invasive species, and supports management related to fisheries and water used for drinking, fishing and recreation.
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A cruise does not, by itself, deliver a final verdict on the health of an entire lake. The value lies in connecting carefully collected samples and measurements with other observations over time. Spring and summer surveys, focused studies such as Lake Erie oxygen monitoring, and long-running records each answer different parts of the larger question.
Quick Recap
Sources
- EPA: “EPA’s Research Vessel Lake Guardian Begins Annual Sampling of Great Lakes,” April 7, 2026
- EPA: “The Lake Guardian”
- EPA: “EPA’s Research Vessel Lake Guardian Begins Summer Sampling of Great Lakes,” August 3, 2026
- EPA: “Great Lakes Monitoring”
- EPA: “Great Lakes Chlorophyll-a Monitoring”
- EPA: Research Vessel Lake Guardian 2021 and 2022 Annual Report, December 2025
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