An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Great Lakes Phytoplankton Monitoring

Phytoplankton are a major piece of the Great Lakes food web. They are food for zooplankton and small and young fish. EPA compares phytoplankton numbers to determine year-to-year and long-term differences.


Researchers collect many types of phytoplankton samples twice per year, in early spring and in summer throughout the Great Lakes.  Scientists identify and measure the samples to determine the total numbers of each type of algae as well as the total mass of the phytoplankton. 


  • EPA's research vessel Lake Guardian is used to gather samples from eight to twenty sites in each of the Great Lakes annually.
  • The sites are generally far away from areas that experience rapid changes in water quality and phytoplankton.


  • The Rosette Sampler is used to collect phytoplankton by moving down through the water from the surface of the lake to the bottom.
  • Several samples, at various depths, are put in bottles, that are later mixed together to form a combined sample.
  • A microscope is used to count and identify the samples.
  • Data from the identified samples are analyzed and usually available in 1.5 - 2 year.
  • New and updated data analyses occurs at least yearly.
  • The updates show phytoplankton trends in the lakes, or offer answers to outstanding research questions.

Phytoplankton Trends

Journal of Great Lakes Research: Phytoplankton Changes in the Great Lakes, 2001 - 2011 Exit