NEW YORK - The scene from Dan Mundy's living room window is worlds away from the normal urban views of New York City. The sky is a brilliant blue, and the waters lapping at the stone wall just a few feet away are clear and calm. A duck paddles off, and even a jellyfish looks more peaceful than dangerous as it undulates near Mundy's dock. Welcome to Jamaica Bay, a wildlife haven just next door to John F. Kennedy International Airport, reachable by subway from Manhattan's skyscrapers some 15 miles away.

The tranquility hides a truth well-known to Mundy and others who have spent their lives here - the salt marsh islands dotting Jamaica Bay are disappearing. The loss of the islands could have huge ramifications for the environment because a quarter of the country's bird population makes its way through Jamaica Bay.

Marsh loss has always been part of life in the bay, but it has been accelerating in the past decade or so, said Mundy, a retired firefighter who advocates for the marshes.

Records show Jamaica Bay averaged a loss of 26 acres a year from the mid-1970s until the mid-1990s, but the pace picked up to more than 40 acres a year by 1999, the last time a comprehensive look was taken, said Brad Sewell of the Natural Resources Defense Council, who serves as co-chairman of an advisory committee for the bay.

Anecdotal evidence indicates the situation has probably gotten worse in the last couple of years. There are around 1,000 acres of salt marsh islands in the bay. If the disappearance continues at its current rate or accelerates, the islands could be gone in less than 20 years, Sewell said.

No one knows for sure why the marshes are disappearing. Several possibilities are being considered, including the rise in sea level and the lack of sediment to renew the marshes flowing into the bay.

"Research has looked at a handful of contributing factors, none of them have emerged as a clear cause," said Steve Zahn, a program manager for a marine resources unit for the state Department of Environmental Conservation.

Some are convinced that excessive nitrogen from the city's four wastewater treatment plants is a factor. The nitrogen - a byproduct from the water treatment process _ feeds algae blooms, which die off and are decomposed by bacteria that use a lot of oxygen, leaving less in the system.

The city acknowledges that more nitrogen than the system can handle is being discharged into the bay, but also says there is no definitive scientific evidence that the nitrogen is the main cause of the marsh loss.

The scientific model for water quality doesn't show that making the significant financial investment into reducing the amount of nitrogen coming out of the wastewater treatment plants will significantly raise the dissolved oxygen level, said Angela Licata, deputy commissioner of environmental planning and analysis at the city Department of Environmental Protection.

The water quality model did show an improvement in Long Island Sound, so the city invested in reducing nitrogen output there, she said.

Restoration is one possible way to fix the problem, but restoration projects are expensive.

A couple of marsh islands have been replanted, but the cost needs to come down before more projects are done, said Douglas Adamo, chief of the division of natural resources for Gateway National Recreation Area, of which Jamaica Bay is a part.

Otherwise, "the cost will be so prohibitive that we're not going to get many acres for the dollar," he said.

But the price of a bay without the marsh islands is higher than anyone would want to pay, said Mundy and others.

Mundy pointed out that the islands act as a buffer for waves coming across the bay. Without them, the waves would roll in several feet higher than they already do. "It's like a disaster waiting to happen here," he said.

And the role the marshes play in the ecosystem can't be overstated, with so many fish and fowl in the bay, Sewell said.

Plus, he said, it's a resource for urban dwellers. "It's the only unit of the National Park Service that's accessible by subway," he said.