Federal environmental officials are proposing a new nitrogen-reduction strategy for Long Island Sound, warning New York and four other states that their current efforts to reduce the pollutant won’t be enough.

The Environmental Protection Agency described its proposal in a Dec. 23 letter to the heads of environmental departments for New York, Connecticut, Massachusetts, New Hampshire and Vermont — all of which either border the Sound or have tributaries that feed into it.

The effort would be in conjunction with previous actions by the EPA and the states to reduce nitrogen through requiring changes at wastewater-treatment plants that discharge to the Sound, and would look at more than water quality in measuring progress.

The strategy would focus on three groupings: coastal watersheds, tributary watersheds, and coastal watersheds in the western Long Island Sound that have large wastewater-treatment plants that discharge directly into the Sound.

While those plants have made progress in upgrading their systems to reduce the amount of nitrogen in their effluent, nitrogen from other sources remains a problem, according to the letter, which was signed by EPA regional administrators H. Curtis Spalding and Judith Enck.

Rather than just focusing on decreased oxygen levels, the EPA’s plan would focus on other indicators of excess nitrogen — such as the loss of eelgrass beds, which particularly affect the Sound’s bays and coasts.

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A threshold for nitrogen would be developed as a numeric target for each state to assess how they could reduce the nutrient in each of the three groupings.

“Developing and implementing this strategy will provide a strong technical foundation for taking actions both at the state and federal level to protect Long Island Sound and its embayments,” the letter said, adding that states could use the technical foundation to develop their own plans on nitrogen reduction.

The letter named stormwater runoff, septic and cesspool systems and turf fertilizers — adding that the amount of nitrogen from all these sources “have remained steady or increased.”

Excess nitrogen in water bodies can lead to negative effects such as harmful algal blooms, decreased oxygen, shellfishing closures and the loss of eelgrass — a submerged grass that provides a critical habitat for marine life.

“Now is the time to meet these challenges,” the letter reads, proposing a “holistic framework” in which the EPA would work with the states to reduce nitrogen in the Sound.

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Mark Tedesco, director of the EPA’s Long Island Sound office, said the federal agency plans to meet with New York officials in a few weeks — but that New York, and especially Long Island, already has taken definitive steps to address the problem.

“In many ways, from the Long Island perspective, we’ve gotten a head start,” Tedesco said. “We’ve gotten a big down payment on controlling nitrogen pollution.”

He said the agency was “very gratified” to see the state’s Long Island Nitrogen Action Plan, a $5 million effort in the draft phase that is designed to address the nutrient in Long Island’s waterways. The state held three public meetings on that plan this month.

But, Tedesco said, “despite progress, we clearly believe there’s more to do.” The DEC did not respond to requests for comment.

Tedesco said the agency has not yet determined a time frame for its plan.

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“I think the key thing is it’s near-term,” he said. “The preference is to move quickly, even if it’s in setting some interim targets that can always be refined later.”