The U.S. Environmental Protection Agency is proposing a multibillion-dollar effort to limit pollution from sewage treatment plants that feed into the Chesapeake Bay, turning to strict regulation in an attempt to end massive "dead zones" in the water.

Most of the 368 large sewage plants in the Chesapeake watershed could be forced to reduce their output of nitrogen and phosphorus under a draft proposal issued last week.

The move would reflect a shift from carrot to stick: In the past, the EPA has relied on plants to make voluntary improvements in many cases.

The first permits with new limits could be issued next year. Scientists say it would be the most important move against sewage pollution since the 1980s.

"It's the one technical, affordable solution that will get us part of the way" toward goals of cleaning up the bay by 2010, said William C. Baker, president of the Chesapeake Bay Foundation.

But the costs could be staggering: Many treatment plants would need renovation, and the price tag is expected to exceed $1 billion in Maryland alone. And those costs would likely be borne directly or indirectly by the homeowners and businesses that rely on the sewer system.

Nitrogen and phosphorus in sewage, though they are not poisonous on their own, have emerged as some of the Chesapeake's worst pollutants because they provide food for "blooms" of harmful algae.

These algae can cloud bay water, choking off light to underwater plants and using up underwater oxygen. Nearly every familiar species of bay life is affected by the dead zones created when oxygen and plants are taken away.

"When you lose the plants, you lose the habitat that protects the small fish, the oyster spat, the small crabs," said Clifford W. Randall, a professor emeritus at Virginia Tech. "The whole system just starts collapsing."

Though scientists made the connection between nitrogen and phosphorus and dead zones more than a decade ago, environmentalists have said the EPA was slow to act.

The agency did not require all sewage plants in the watershed to limit their nitrogen and phosphorus pollution; instead it used grants and other encouragements to coax them to make improvements. Some states did impose limits on the pollutants -- though usually with the needs of local streams, not the Chesapeake, in mind.

Now, about a dozen plants in the watershed have limits on nitrogen built into their permits, and about half have phosphorus limits, according to the EPA.

Environmentalists stepped up their campaign to change this situation after the largest-ever dead zone appeared in the bay last summer. There was oxygen-poor water stretching from Baltimore to the York River near Hampton, Va., and watermen reported seeing blue crabs leap from the water in a desperate attempt to breathe.

"Nero fiddles; Rome burns," said Baker, the bay foundation president. "Here's EPA fiddling, and the Chesapeake Bay is continuing to die a slow death." Even with the new proposal, Baker said, the EPA's language is vague enough that some plants could avoid improvements to meet the standards.

When the EPA acted last week, some observers said the agency must have been prompted by 2003's dead zone or by the Maryland General Assembly's passage of a "flush tax" that made it a leader for cleaning up sewage plants.

But the EPA has rejected these suggestions.

"We were starting well before the more recent problems with the dead zone," said Robert Koroncai, the EPA's chief of water protection for Maryland, Virginia and the District. "Because of the science, this does not happen overnight."

Over four years, the EPA said, scientists threw out the old standard for judging the right amount of dissolved oxygen in the bay -- set decades ago -- and drew up a map showing what plants and fish in various areas need to live. One scientist called it the bay's "local zoning map."

With these new water-quality standards nearing completion, the EPA issued a draft "permitting approach" July 16.

In torturous and tentative bureaucratic language, it laid out a proposal that could radically alter the state of the bay.

Sewage treatment plants would be held to permits capping their outputs of nitrogen and phosphorus. In many cases, experts say, the standards would require plants to be pushed to the limits of technology.

"It's like having a plain old, regular car. . . . They decided that you need to have a Ferrari or a Maserati," said Lawrence Jaworski, president of the Water Environment Federation, an Alexandria-based trade association that represents water-treatment plants.

The scale of the project is huge: The plants stretch from the Hampton Roads region in Virginia to the town of Campbell in New York's Finger Lakes region. Virginia has calculated that nearly a billion would be spent to improve its plants, and West Virginia has estimated $133 million.

In Maryland, the flush tax -- fees on sewage users -- would help pay for many of the $1.07 billion worth of anticipated improvements. The source of funding is not as clear in other states. Jaworski said he worried that costs would eventually be paid in higher sewage bills.

"This is going to come from the ratepayers, you and me," he said.

Plants probably would have several years to make the changes required by the new permits.

Still, as slow as the process could be, scientists say it could make one of the speediest improvements in the plodding history of efforts to "save the bay."

By contrast, the efforts to stop nitrogen and phosphorus pollution by agriculture -- which accounts for about twice the pollution of sewage plants -- will take much longer because they will require changes at thousands of small farms.

Meanwhile, the dead zone has begun to grow again. On Thursday, John Page Williams, a senior naturalist for the bay foundation, stuck monitoring equipment into a 50-foot-deep hole in the Severn River near Annapolis.

Many times of the year, he would find fish at the bottom. But Thursday, they were crowded into the top 15 feet of water because below that there was not enough oxygen.

"Everything's bare down below," he said. "I mean, it's lost habitat."