Intertidal (saltwater) marshes of a coastal watershed along the Gulf of Mexico, Louisiana. (U.S. Fish and Wildlife Service/Handout)

Over a four-year span, the United States lost more than 360,000 acres of freshwater and saltwater wetlands to fierce storms, sea-level rise and booming development along the coasts, according to a newly released federal study.

The disappearance of so much grass and forest marsh on the edge of waterways is a disturbing sign that government projects to restore wetlands are failing to keep pace, environmentalists said, as storms intensify, the sea level creeps up and development paves the way for rising coastal populations.

Saltwater wetlands help buffer sea surges that cause flooding during powerful storms along the coasts — such as Hurricane Sandy last year — and freshwater wetlands soak up storm-water runoff that often causes sewers to overflow.

They also serve as nurseries for numerous species of fish and assorted marine life, while providing habitat for three-quarters of the nation’s waterfowl and migrating birds. Nearly half of endangered species depend on them to live.

“They are getting it from all directions,” said Tom Dahl, lead author of the study funded by the U.S. Fish and Wildlife Service, the National Oceanic and Atmospheric Administration and the U.S. Environmental Protection Agency.

Study areas include the Gulf of Mexico, Atlantic and Pacific coasts, the Great Lakes and other fresh inland waters.

In the Chesapeake Bay region, Maryland has lost 60,000 acres of wetlands since the 1940s because of population growth and farming, and in 1997 it launched a bid to restore them, according to the state’s Department of the Environment.

Virginia estimates that half of the wetlands that existed in the colonial period have been lost over time to farming and development. There is an ongoing effort in the state to restore and add to the 1 million acres that remain, according to the Virginia Department of Environmental Quality.

Across the nation, wetlands have been converted to open water in some places and to mud in others. They include mangrove swamps, salt marshes, freshwater forested swamps, shrub depressions and wetlands floating on the edges of rivers.

The disappearance of marshes during the period covered in the study — between 2004 and 2009 — represented a 25 percent increase in the rate of loss in the same areas from the previous survey , which covered the six-year period between 1998 to 2004.

Storms and wetlands have waged an epic struggle on the coasts for eons. What’s relatively new, and detrimental to the wetlands, is an explosion of coastal residential and business development, along with coastal farming, that drain water from the wetlands or fill them with dirt for agriculture, parking lots, housing and retail stores.

As a result, sizeable chunks of wetlands die. Surviving wetlands are battered by rainwater runoff pouring from newly built surfaces such as driveways and roads, and much of that water is polluted with garbage, toxins and fine particle sediment. Wetlands can’t handle the added deluge.

“The plumbing of the whole system is altered,” said Dahl, a senior scientist for wetlands status and trends for the U.S. Fish and Wildlife Service.

Dahl and his co-author, Susan-Marie Stedman, a fisheries biologist for NOAA, were clear about the potential effects of such a massive decline.

“You lose places for those organisms to breed, feed, rest,” Dahl said. “You’re losing some capability for other environmental functions like filtering pollutants, providing some protection from storm damage.

“You’re losing recreational opportunities for bird-watching and canoeing. You’re affecting hydrology. The areas are no longer able to retain water. The hydrology is changing and we don’t recognize what the full implications are,” he said.

Coastal counties generated $6.6 trillion largely on warm-weather recreation and tourism, slightly less than half the nation’s gross domestic product in 2011, according to a report by NOAA.

“Some estimates indicate that development will cover one quarter of the land area of the nation’s coastal watersheds by 2015,” according to the study, “Status and Trends of Wetlands in the Coastal Watersheds of the Conterminous United States.”

It mentions but does not fully address the effects of sea-level rise , which is increasing 4 millimeters per year for various reasons in Virginia alone.

“With sea-level rise, we’re going to lose a lot of our coastal wetlands . . . and it’s surprising to me they didn’t tease that out,” said Steve Bunker, director of conservation programs for the nonprofit Nature Conservancy in Maryland.

At the Blackwater National Wildlife Refuge on Maryland’s eastern shore, “They’ve lost a lot of wetlands,” Bunker said. “It’s not clear why that’s happening, but sea-level rise is certainly a concern, and it’s thought that wetlands will not be able to keep pace.”

Sea-level rise is a concern, Dahl said, but it happens over decades, and its effects couldn’t be estimated in the short period of the study. “It would really be going out on a limb,” he said.

In order to gauge the status of wetlands, the scientists randomly selected four-square-mile areas and used high-resolution digital images to assess changes. More than 2,600 such areas were studied.

By researching wetland trends and other data, Dahl and Stedman determined that about 80,000 acres of wetlands disappeared each year in the years of the study, compared with 60,000 acres per year in the previous study.

The starkest decline was in the Gulf, which was roiled by several massive hurricanes, including Katrina, Rita and Ike, Dahl said. “They contributed to washing away some coastal salt-marsh area, piling sand on wetland, debris. It was the number of storms and intensity of storms.”

But human development was a major factor, especially in places such as coastal North Carolina on the Atlantic Coast, where a development boom is sending storm-water runoff cascading into wetlands that cannot handle it.

Inland, the situation isn’t that much better. Urban and suburban development, road building and tree harvesting muddied wetlands. Wetland loss along the Pacific Coast was comparatively minor because development is light and there are fewer storms, Dahl said.

“The overall message of this report is one of concern and that we need to take it seriously to address wetland loss,” said Bill Kittrell, director of conservation for The Nature Conservancy in Virginia.

Kittrell said programs aimed at restoring wetlands have had moderate success, and the study’s authors agreed. They attributed an increase of 50,000 acres in wetlands in the Great Lakes region to such efforts.

Gains also were also reported in South Carolina, Georgia and central Florida.

Kittrell said the conservancy received a $700,000 grant from the National Fish and Wildlife Foundation to help the large environmental nonprofit organizationovercome barriers to wetlands projects. But compared to the billions of dollars needed, it’s a drop the bucket.

“I think the programs and policies we’ve had in place for a number of years have been very effective,” Kittrell said, but there are too few of them for wetlands facing a triple threat of storms, human development and rising seas.