The Guardian is operated by the U.S. Environmental Protection Agency's Region V office in Chicago, and used to dock in Bay City's Wenonah Park.
The ship moved out about eight years ago, and now spends its summers working throughout the five Great Lakes and wintering in Milwaukee, according to federal officials.
Paul Horvatin, program manager at the EPA's Great Lakes National Program Office in Chicago, said the Lake Guardian recently finished up a round of testing in the Bay City area as part of an international research mission studying the food web in Lake Huron and the outer reaches of the bay.
The 180-foot-long, 40-foot-wide floating laboratory hosted researchers from Canadian environmental agencies, the U.S. Fish and Wildlife Service, U.S. Geological Survey, National Oceanic and Atmospheric Administration, Michigan Department of Environmental Quality and numerous universities. The work took place from April through Oct. 1, he said.
''Basically, we were looking at nutrients, looking at the lower food web,'' Horvatin said.
''The lower food web in Lake Huron has been changing quite drastically. The base of the food chain for Lake Huron has changed, not necessarily for the better. We were just trying to get a better understanding of what the food chain conditions are.''
That included measuring phytoplankton and phosphorus levels and looking for diporeia, a shrimp-like animal that has nearly disappeared from Lake Huron in recent years, contributing to a collapse of alewives, the principal food for Chinook salmon.
Horvatin said scientists on the ship study a different Great Lake every year, monitoring pollutant concentrations in the water, air, fish and sediment. They also measure the effects of pollution and other natural and human stresses on the Great Lakes ecosystem, according to other EPA officials.
Next year, the Lake Guardian will be doing research in Lake Ontario.
The ship called Bay City its home port during the 1990s, but lost exclusive use of its dock in 2001 due to plans for a nearby hotel-conference center, which opened in June 2004, according to Times files.