WASHINGTON — The Supreme Court grappled yesterday with the classic dilemma of federal regulators: balancing the cost of a government rule with the common good — in this case, the good of protecting marine life from the machinery of a power plant.

The case of a New York power complex has potentially costly ramifications for utilities in New England and nationwide. During yesterday’s oral arguments, several justices agreed with environmentalists that it’s difficult, and perhaps impermissible, to pit the value of an environmental rule directly against its economic cost.

Justice David H. Souter characterized this as a conflict between “a few baby clam larvae and so on” versus the “millions of dollars” it might cost to save them. “Are a thousand plankton worth a million dollars?” Souter asked. “I don’t know.” Therefore, he suggested, straightforward attempts at cost-benefit analysis may be fruitless. Indeed, environmentalists argued that Congress specifically forbade regulators to use direct cost-benefit analysis when the Clean Air Act was written.

But some justices also agreed with the electric-power business that federal officials are obliged to give some consideration to the price of the environmental regulations that they impose on industry — especially when human health and lives are not at stake.

At issue in the case — concerning efforts to retool the aging Indian Point nuclear power plants on New York’s Hudson River — was how much weight should be given to cost, and how such cost concerns can fairly be factored into environmental rules.

“I mean, we’d spend trillions to make America secure so not 50 thousand people die, but we won’t spend trillions for a road accident. And of course you take those things into account,” said Justice Stephen G. Breyer, arguing that rulemakers continually make judgments about the dimensions of their policy goals and what it costs to achieve them. The trick, as Breyer described it, is not to make the balance precise but to make it reasonable.

Lawyers for the Bush administration and the energy business argued that an adverse ruling on this case could force Indian Point and many other plants across the country to upgrade their cooling systems at a cost of billions.

Rhode Island and several other Northeastern states have joined environmentalists in this case in fighting federal regulators who want to balance the environmental benefits of clean-water rules squarely against the cost to the electrical generating plant and its customers.

The Bush administration asked the high court to overturn a lower court ruling that would force the owners of Indian Point to build a new cooling system which, according to environmentalists, would greatly reduce damage to the river’s marine life. Indian Point’s owner, New Orleans-based Entergy, disputes the extent to which the plant harms fish populations in the surrounding waters.

The case focuses on action by the Environmental Protection Agency that made it easier to weigh against its benefits. The EPA did that by allowing Indian Point to seek a variance from rules that otherwise would have forced it to build an expensive new cooling system to reduce the damage to fish in nearby waters. The company argued for the variance on cost grounds but the Second U.S. Circuit Court of Appeals ruled that the variance procedure was a grant of cost-benefits analysis that is not permitted under the Clean Air Act.

Tricia O’Hare Jedele, special assistant attorney general representing Rhode Island as an observer, said after the arguments that the collapse of the winter flounder population in Mount Hope Bay is a vivid example of the damage that a power plant cooling system can do to the surrounding marine habitat.

Environmentalists and commercial fishing interests blamed the cooling system in the generating plant at Brayton Point in Somerset, which sucks huge amounts of water into the plant and sends it back into the bay 30 degrees hotter. Some fish are killed by striking screens designed to keep them out of machinery, others by being drawn into the system. About a year ago, Dominion Resources, the owners of Brayton Point, agreed to replace its old cooling system with two large cooling towers, settling a long-standing dispute with the EPA.

The old mechanism, known as an open-cycle cooling system, cools the coal-fired electricity plant with as much as a billion gallons of water a day. Besides destroying fish eggs and small fish in the action of the system, the plant may also drive away marine life by cycling the heated water back into the bay. The new, “closed cycle” system will cut the water intake by 96 percent, using a pair of hourglass-shaped towers — similar in appearance to the signature towers at nuclear plants, and comparable in size to the 450-foot tall smokestacks that have long made Brayton Point a landmark on the region’s horizon.

Dominion’s spokesman, Jim Norville, said the new system will cost the company about $500 million. Norville said the company’s decision to build the new cooling towers was largely based on its desire to conclude a dispute that it inherited when Dominion bought Brayton Point in 2005. Experts disagreed about the damage to the fish, he said, but “we needed to get this behind us.”

The origins of the Brayton Point case predate by many years the Bush administration rule-making at issue in the Indian Point case, but both cases involve the issue of power-plant cooling systems and the expense involved in limiting the damage they can do to fish.

The timing of the case underlines the power of a president to shape the nation’s laws — through the Supreme Court — long after his term is done. President Bush, whose nominees have made the current court more conservative and, in practical terms, often more friendly to business concerns, will leave office in just over a month. But the Supreme Court’s ruling on this Bush administration regulatory case could have much more lasting effects.

jmulligan@belo-dc.com