SHELL BEACH, La. (AP) — Battered by hurricanes, weakened by erosion and flood-control projects, the sprawling wetlands that nurture Gulf of Mexico marine life and buffer coastal sites from storm surges now face another stern test as a monster oil slick creeps ever closer.

About 40 percent of the nation’s coastal wetlands are clumped along southern Louisiana, directly in the path of oil that was still gushing Monday from a ruptured underwater well. Roughly 3.5 million gallons has escaped in the three weeks since an oil rig explosion, and some is bearing down on the marshes as workers rush to lay protective boom.

‘‘No question we will see some widespread impacts,’’ Garret Graves, chairman of the Coastal Protection and Restoration Authority of Louisiana, said after an observation flight. ‘‘If we allow this oil to get into our coastal areas and fundamentally change the ecosystem, the consequences are profound.’’

Removing oil from wetlands is a huge challenge. Bulldozers can’t simply scrape away contaminated soil, as they do on beaches. Cutting and removing oil-soaked vegetation could further weaken the fragile vegetation that holds the marshes together. Absorbent materials and detergents have limited effectiveness, Graves said.

If a thick enough layer of oil coats hardy swamp grasses and shrubs, scientists say it could shut down their equivalent of breathing — absorbing carbon dioxide and releasing oxygen.

‘‘You could literally suffocate the marsh,’’ said Alex Kolker, a coastal systems specialist with the Louisiana Universities Marine Consortium.

Even worse, the oil could soak into the ground and poison roots, killing entire plants. With nothing to anchor it, the soil would wash away, accelerating a process that has cost Louisiana about 2,300 square miles of coastal marshes and barrier islands the last 80 years — an area bigger than Delaware.

A spill-related loss of wetlands would ripple through the food chain they support, from tiny organisms to fish and birds.

‘‘It’s like you pull a thread on the shirt and it all comes apart,’’ said Mark LaSalle, an ecologist at the Pascagoula River Audubon Center in Moss Point, Miss.

Or the damage could be less severe and the ecosystem could survive yet again.

‘‘It’s like when you get pneumonia,’’ Kolker said. ‘‘There’s a certain amount you can handle and bounce back, and there’s a certain amount that will make you miserable but you’ll survive, and there’s a certain amount that will kill you.’’

All hinges on how much oil reaches the wetlands, and how soon workers can plug the leak from the stricken well pouring at least 200,000 gallons daily into the Gulf since the rig exploded and sank April 20.

From Texas to Florida, the Gulf region is laced with wetlands. But Louisiana’s are most directly threatened by the encroaching oil and by far the most plentiful, even after the state has suffered 80 percent of U.S. coastal wetland loss.

Wetlands feed and provide nesting and spawning grounds for multitudes of waterfowl and fish. Menhaden, the top commercial fish species in the lower 48 states and an ingredient in products ranging from insecticide to chicken feed, spends its crucial first months of life nibbling decomposed marsh grass.

‘‘Lose the marshes and we lose menhaden,’’ said Andy Nyman, a wetland and wildlife ecologist at the Louisiana State University Agricultural Center.

Wetlands perform the kidney-like function of filtering chemicals and other pollutants from waters, and they prevent floods by soaking up excessive waters like sponges and releasing them when levels recede. Historically, they have shielded inland cities such as New Orleans from the worst of the Gulf’s tidal surges during hurricanes and tropical storms.

As land area of wetlands has declined over the years, so has their effectiveness.

Beginning in the 1930s, levees built along the Mississippi River to ward off flooding curtailed flow of fresh water into estuaries, killing off plants unable to live year-round in salt water. That accelerated erosion and converted former wetlands into open water.

The river previously deposited layers of new mud to replenish the marshes. Now, the levees prevent that.

Yet another setback: Canals were dredged to benefit shippers and the oil and gas industry, allowing still more salt water to intrude.

Perhaps the most notorious such project was the Mississippi River Gulf Outlet, a 76-mile-long channel carved through marshes in the early 1960s to create a shipping shortcut between the Gulf and New Orleans. The Army Corps of Engineers closed the waterway to navigation after it provided a pathway to the city for the Hurricane Katrina storm surge in 2005.

Piloting his 24-foot fishing boat this week near the settlement of Shell Beach, Ed ‘‘Mopey’’ Schaumburg gestured toward open waters that were thriving marshlands before the channel was built.

‘‘It was the most devastating man-made disaster that ever happened to the ecological system in this part of Louisiana,’’ Schaumburg said.

Charlie Thomason, a 39-year-old charter boat operator in Hopedale, said he feared the oil spill would finally doom the marshlands.

‘‘Once it gets into that porous sediment you’ll never get it out,’’ Thomason said while buying fuel at Robert Campo’s family marina in Shell Beach. ‘‘It may take a long time, but it’ll kill everything — the grass, the plankton, the shrimp, the little organisms everything feeds on. As a business owner, my life’s in limbo right now.’’

State and federal agencies are trying to replace some of what has been lost, rebuilding barrier islands, beaches and marshlands. About $1.7 billion has been committed to the initiative over the next four years.

But they have a long way to go, and if the oil pollution is severe enough it could overwhelm their efforts and leave the wetlands less able to cope with vicious storms. Starting with Katrina in 2005, a series of hurricanes has swept away 340 square miles.

‘‘When the marshlands are not being mutilated by man, they’re pretty resilient,’’ said LaSalle, the Mississippi ecologist. ‘‘A healthy marsh can tolerate a hurricane; it’s natural. But an oil spill is not natural, and the marshes have never seen oil at this level. We just have to hope they can shut this thing off quickly.’’

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Foster reported from New Orleans.