In an unusual move, Gov. Gretchen Whitmer of Michigan, citing environmental concerns, is shutting down an underwater pipeline that carries oil to refineries in her state and Canada.

Pipeline operations normally fall under federal jurisdiction. Governor Whitmer, a Democrat, is acting under the state’s public trust doctrine, which requires state authorities to protect the Great Lakes. The pipeline in question, known as Line 5, has been in operation since the 1950s.

The decision, announced on Friday, requires the pipeline operator Enbridge to cease operations on a specific section of Line 5 by May 2021, but it will have the effect of curtailing the entire pipeline, which runs between Superior, Wis., and Sarnia, Ontario.

“Enbridge has routinely refused to take action to protect our Great Lakes and the millions of Americans who depend on them for clean drinking water and good jobs,” Governor Whitmer said in a statement.

While the line moves a relatively small quantity of oil — about 540,000 barrels of light crude oil and liquid natural gas each day, compared with national average consumption of 20 million barrels of crude oil per day — environmentalists applauded the move. While it was not clear that the legal strategy could easily be applied to other pipelines, they also said it was significant in that it focused on an older pipeline rather than a new project.

“I think this Line 5 decision is going to spark some interest in existing pipelines,” said Jared Margolis, a senior attorney with the Center for Biological Diversity. “I think, at some point, we do need to turn to pipelines that are in the ground that are dangerous, that are posing a serious risk.”

Governor Whitmer’s action will revoke the 1953 easement that allows Enbridge to operate pipelines through the Straits of Mackinac, a narrow waterway that connects Lake Michigan and Lake Huron.

“Enbridge remains confident that Line 5 continues to operate safely and that there is no credible basis for terminating the 1953 easement allowing the Dual Line 5 Pipelines to cross the Straits of Mackinac,” the spokesman, Michael Barnes, said. “Line 5 is an essential source of energy for not only Michigan but for the entire region including Wisconsin, Indiana, Ohio, Pennsylvania, Ontario, and Quebec.”

Opponents of the move also said they worried that an abrupt shutdown would disrupt oil supplies, forcing nearby refineries out of business and disrupting the propane supplies to Michigan’s Upper Peninsula, where many rural residents rely on propane to heat their homes.

“Shutting down Line 5 would kill thousands of jobs in Michigan, severely impact manufacturing and cost our economy hundreds of millions of dollars when the state is already reeling from a pandemic,” said Mike Johnson, the vice president of government affairs at the Michigan Manufacturers Association.

A study commissioned by the National Wildlife Federation, however, found that the economic impact for Michigan oil producers of shutting down Line 5 would be minimal and that other shipping methods like trucking would keep refineries running.

“There are alternatives to moving that oil,” said Margrethe Kearney, a senior attorney at the Environmental Law and Policy Center. “I think that we should be really careful and really skeptical of arguments by Enbridge that shutting this pipeline down will result in some dire circumstance and that the sky will fall.”

The decision came after nearly a decade of political pressure. According to Ms. Kearney, a 2010 oil spill on another Enbridge line in Michigan, which poured nearly 850,000 gallons of tar sand into the Kalamazoo River, piqued public interest in Line 5 and other pipelines.

“It’s a very iconic part of Michigan’s culture,” Ms. Kearney said of the area. “So when people realized there’s this oil pipeline down there, it was a really big deal.”