HARTFORD, Conn. — Connecticut’s nuclear power plant shut one of two units on Sunday because seawater used to cool down the plant is too warm.

Unit 2 of Millstone Power Station has occasionally shut for maintenance or other issues, but in its 37-year history it has never gone down due to excessively warm water, spokesman Ken Holt said on Monday.

Water from Long Island Sound is used to cool key components of the plant and is discharged back into the sound. The water cannot be warmer than 75 degrees and following the hottest July on record has been averaging 1.7 degrees above the limit, the Nuclear Regulatory Commission said.

The federal agency issued an “emergency license amendment” last week, allowing Millstone, a subsidiary of Dominion Resources Inc., to use an average temperature of several readings.

“It wasn’t enough to prevent us from shutting down,” Holt said.

In addition to the extreme heat last month, the mild winter didn’t help because it kept Long Island Sound water unusually mild, Holt said.

Robert Wilson, a professor at Stony Brook University’s School of Marine and Atmospheric Sciences, said readings show temperatures in central Long Island Sound are nearly 80 degrees, much higher than the more typical 74 degrees.

He blamed weather patterns, beginning with the mild winter and little wind that allows heat to hang around.

“If you start from warm winters, then have sustained persistent surface heating without wind stirring you get very high temperatures,” Wilson said.

Millstone provides half of all power used in Connecticut and 12 percent in New England. Its two units produce 2,100 megawatts of electricity, which is reduced by 40 percent with Unit 2 down, Holt said.

Richmond, Va.-based Dominion, which operates Millstone, does not have an estimate of when the unit will restart, he said.

Marcia Blomberg, a spokeswoman for regional grid operator ISO-New England, said the loss of electricity will not be a major problem. The Holyoke, Mass.-based agency generally operates with a margin of reserve and plans for the possibility of lost resources, she said.

“Generators are big machines,” she said. “It happens frequently that resources are unable to start up or have to power down.”

Dave Lochbaum, director of the Union of Concerned Scientists’ nuclear safety project, said he believes the partial Millstone shutdown is the first involving a nuclear plant pulling water from an open body of water. A few nuclear plants that draw water from inland sources have powered down due to excessively warm water, he said.

Browns Ferry nuclear plant in Alabama, for example, reduced power for 50 days in the summer of 2010 and fewer than 10 days last year, said Ray Golden, spokesman for the Tennessee Valley Authority, which operates the plant.

No power reductions were needed this year because the plant cools the water, he said.

Lochbaum said the Union of Concerned Scientists believes climate change is the reason why rivers, lakes and Long Island Sound are hotter.

“It is evidence of global warming with problems both obvious and subtle,” he said.

But Krista Lopykinski, a spokeswoman for Exelon Corp., which operates six nuclear plants in Illinois, said seeking state authorization to operate at an unchanged or higher level in response to elevated lake and river temperatures is “pretty common.”

“It happens every summer,” she said.

Still, Exelon asked for federal approval — the first time in 12 years — to continue operating when water in its cooling pond at an Illinois nuclear plant topped 100 degrees last month.