Stamford, Conn. - Agreeing for the first time to make deep cuts in nitrogen pollution in Long Island Sound, representatives of New York City and the states of New York and Connecticut joined federal officials, environmentalists and scientists yesterday in approving a 15-year cleanup plan that will cost an estimated $450 million.

The unanimous vote by the management committee of the Long Island Sound Study had seemed in doubt just days ago, but a flurry of last-minute phone calls between city and state officials resulted in compromise language that would ease the agreed-upon 58.5 percent cut in nitrogen discharges if costs soar, as city officials predict they will.

The plan still needs final approval from another committee of high-level state and federal officials, set to meet Feb. 5 in New York City. But that's expected to be a mere formality.

Jubilant environmentalists and relieved state officials said New York City's last-minute decision to support the accord was a watershed event in the history of the troubled estuary because more than 75 percent of the nitrogen that enters the Sound from point sources comes from the city's six giant sewage-treatment plants, which are in Queens and the Bronx.

New York City's decision to vote yes leaves four small Long Island sewer districts - Oyster Bay, Port Washington, Great Neck and Great Neck Village - as the only vocal critics of the cleanup plan. Sewer districts are not voting members of the study committee but could attempt to torpedo the plan through a lawsuit. A lawyer for the four dissenting districts, however, said he was hopeful that ongoing talks with the state would resolve the group's concern that the plan is too rigid and prohibitively expensive.

Researchers say achieving the nitrogen-reduction goals would dramatically improve the Sound, but will require huge outlays by taxpayers. Precisely how much is still unclear - the state's official estimate for New York state is $200 million - but the money would come from a mix of state grants, sewer district taxes and possibly federal aid.

The city's approval, along with the $200 million earmarked for Long Island Sound in the bond act New York voters approved in 1996, has made longtime Sound-watchers more optimistic than ever of eventually ending the nitrogen-induced oxygen deprivation that every August turns large portions of the Western Sound into a dead sea.

"We've been at this for more than 10 years, and the vote today is one of the most historic events in the history of the Long Island Sound Study because New York City has come around and said `yes' to signficiant nitrogen reductions," said David Miller, executive director of the National Audubon Society of New York state.

The plan calls for a 23 percent cut by 2004, a 44 percent cut by 2009, and a 58.5 percent cut by 2014. By mid-1999, state officials are to develop customized plans for each region of the Sound to allow the nitrogen cuts to be made in the most cost-effective way.

Researchers say a reduction that large would eliminate 97 percent of nitrogen's adverse impact on fish abundance in the far western Sound, and end the problem entirely in the central and eastern Sound. Almost all of the reductions would come from sewage plants but there would also be some efforts to curb stormwater runoff - a less cost-effective way to control nitrogen.

The states' official estimate is that meeting the targets will cost $200 million in New York and $250 million in Connecticut. But New York City estimates that the city alone will have to spend about $200 million to achieve the 44 percent cut, and more than $1.5 billion to meet the 58.5 percent reduction, said Robert Gaffoglio, deputy commissioner of the city Department of Environmental Protection.

Still, Gaffoglio said the city decided to sign on to the plan after the rest of the committee agreed to insert language explicitly stating that the 10and 15-year targets would be reconsidered if new computer models and ongoing pilot projects under way at several city treatment plants show the state has underestimated the plan's true costs.

"We want to deal with the perception out there that New York City is the problem," said Gaffoglio, explaining the city's decision to drop its opposition. The revised language, he said, "is a compromise, and the essence of compromise is that no one's ever completely happy."

Committee members said the change was minor because the plan had already required revisting the targets every five years as new information surfaces.