Of the oil that did reach shore in the gulf, most has remained at the fringe of the dense marshes. That is largely good news for hundreds of thousands of acres of wilderness inside the fringe.

“The huge expanses of marshes that occur interiorly to the shoreline have been spared,” said Irving A. Mendelssohn, a professor of oceanography and coastal plant ecology at Louisiana State University.

Mr. Hales and other scientists say that even the oiled part of the marsh appears to be recovering.

Mark Kulp, an associate professor of coastal geology at the University of New Orleans, who also does research for a contractor for BP, says his observations of vegetation, from Spartina grasses to black mangrove forests, reveal a surprising rebirth. “There are places that I’ve seen where the stalks were laid over with oil and there is now new vegetation coming up through,” he said. “It is a pervasive thing.”

Marshes and wetlands are particularly sensitive to oil. If it enters the soil, it can kill grasses and their roots, leading to erosion. In some spills, including the Exxon Valdez, residual oil has been found in marshes decades later. Dr. Kulp suggests that will not be true for the gulf. “One thing working in our favor is the nature of the substrate,” he said, referring to the marsh bottom. “It is made dominantly of muddy sediment and is relatively impenetrable.”

Others are less sanguine. John W. Day Jr., a retired professor of oceanography and coastal sciences at Louisiana State, has worked with a private firm that developed a complex sensor for aerial detection of oil and gas. Its measurements show that plants in the marsh interior which appear healthy are, in fact, stressed and will probably soon die. Dr. DiPinto of NOAA is also concerned. She said that marsh creatures that dig burrows, like crabs, may eventually bring oil deeper into the soils.

And Wilma Subra, a chemist who provides technical assistance to the Louisiana Environmental Action Network and who has found substantial residual oil in marshes and estuaries near the mouth of the Mississippi River, the Atchafalaya River and Terrebonne Bay, said: “The government and BP continue to say it is very much improved out there, but there is still a lot of oil. Any fisherman could tell you that.”

Ms. Subra said the government needs to do more to assess the situation. “I am not saying their people in the field are not doing a good job, but there is a lot we are hearing about from fishermen and seeing ourselves that is not being investigated,” she said. “I’ve seen a lot of heavily oiled areas where the vegetation is not coming back.”