Scientists aboard the U.S. Environmental Protection Agency’s research vessel Lake Guardian sample water and aquatic life across all five Great Lakes to track water quality and ecosystem conditions over time. Their measurements—from oxygen and temperature at different depths to plankton and bottom-dwelling organisms—help agencies identify changes and assess concerns. A cruise is a set of observations, not a stand-alone verdict on an entire lake’s health.
What do scientists measure aboard the Lake Guardian?
EPA’s Great Lakes National Program Office uses the ship to monitor and report on the status and trends of the Great Lakes ecosystem: Superior, Michigan, Huron, Erie and Ontario. Routine surveys collect water, plankton and benthos—the organisms living on or in lake-bottom sediments. EPA also describes broader monitoring of sediments and air. The work is supported by laboratories aboard the vessel for wet, chemistry and biology analyses. EPA’s Lake Guardian overview explains the ship and its equipment.
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Water at different depths
A Rosette sampler carries twelve Niskin bottles and a multiparameter profiler. Together, they collect water and readings from selected depths, rather than treating the lake as one uniform surface sample. These profiles can show how temperature, oxygen and other water-quality conditions vary through the water column.
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Scientists use nets to collect zooplankton and Mysis, small shrimp-like animals that are part of the food web. Sediment gear includes PONAR samplers, box corers and mult corers; a benthic sled supports video and collection of bottom organisms. These samples add evidence about the life and habitat beneath the visible surface.
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Why sample the lakes in spring and summer?
The two routine survey seasons capture different conditions, so their results provide complementary snapshots rather than duplicate measurements. EPA characterizes spring as a period of low biological activity when the lakes are generally cold and well mixed; nutrient concentrations are generally highest for the season. Summer brings greater biological activity and stratification, when warmer surface water lies above colder deep water. This layering affects oxygen and nutrients, algal growth, where organisms are found and the habitat available to fish. EPA’s April 7, 2026 release describes that year’s spring survey, while its August 3, 2026 release outlines the summer conditions and survey.
| Season | Typical lake conditions | What the snapshot helps show |
|---|---|---|
| Spring | Generally cold, well mixed; nutrient concentrations generally highest for the season, according to EPA | Water quality and aquatic life during a lower-activity period |
| Summer | Stratified water, with warmer surface layers over colder deep water; higher biological activity | How layering relates to oxygen, nutrients, algae, organism distribution and fish habitat |
What the 2026 surveys covered
EPA said the 2026 spring survey began April 4 and would spend a month visiting all five lakes, collecting water and small aquatic organisms including phytoplankton and zooplankton. The agency named 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 among the participating institutions.
The 2026 summer all-lake survey began August 3 and was also planned to run for a month, with an emphasis on water quality and food-web conditions during summer stratification. EPA’s August account also described other ship work that year, which should be understood as separate projects rather than part of the routine all-lake spring and summer surveys:
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems- May–July Lake Michigan surveys with the U.S. Geological Survey and partners examined water quality, zooplankton and fish larvae.
- May–September work in Lake Erie’s central basin tracked oxygen and temperature profiles.
- In July, educators joined a Lake Superior sampling exercise with NOAA Sea Grant.
How do the measurements help assess lake health?
Repeated observations help agencies distinguish long-term trends from conditions at a single station or during one season. EPA says the data can support assessment of ecosystem stressors such as invasive species and harmful algal blooms, inform fisheries and water-quality management, and help protect water used for drinking, fishing and recreation. The EPA Great Lakes Monitoring overview describes the broader program and the kinds of environmental data it gathers.
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Chlorophyll-a is one clue, not a health score
Chlorophyll-a is a pigment used as an indicator of photosynthetic activity and algal biomass. EPA measures it in filtered water samples and also uses satellite imagery, where color intensity is related to chlorophyll-a concentration. EPA reports decreasing open-water concentrations in Lakes Michigan, Huron and Ontario, relatively constant concentrations in Superior, and an increasing trend in Erie. Those directions describe this particular indicator; they do not rank overall lake health or by themselves establish the cause of a change. See EPA’s chlorophyll-a methods and trend page.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How long has the monitoring record run?
EPA says biannual sampling surveys have been conducted since 1983 and characterizes the resulting data as the longest-running and most extensive monitoring dataset for Great Lakes water quality and aquatic biota. A longer record gives scientists context for interpreting new readings, but it does not remove the need to consider location, season, sampling method and the particular indicator being measured.
The Lake Guardian also supports the Cooperative Science and Monitoring Initiative, which thoroughly samples one Great Lake per year on a five-year rotation, as well as Lake Erie dissolved-oxygen surveys. These are additional programs, not substitutes for or alternate names for the routine spring and summer surveys. EPA says the vessel is funded by the Great Lakes Restoration Initiative and is both the largest research vessel in its fleet and the largest research vessel operating on the Great Lakes. Its 2025 Research Vessel Lake Guardian 2021 and 2022 Annual Report says the lakes contain “over 20% of the world’s surface freshwater” and “84% of North America’s surface freshwater”; those are EPA’s quoted descriptions in a report about vessel activity in 2021–2022, not fresh measurements made in 2025.
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