U.S. Environmental Protection Agency's (USEPA) Great Lakes National Program Office (GLNPO) has a water quality monitoring program in the offshore waters (water greater than 30 meters in depth or greater than 3 miles from shore) of the Laurentian Great Lakes. Samples are taken from set stations in each lake twice a year, once in the spring and once in the summer. Samples have been collected from some stations since the 1980s.
The following parameters from GLNPO's long-term monitoring stations are available on this site: