Scientists said the rapid dissipation of the surface oil was probably due to a combination of factors. The gulf has an immense natural capacity to break down oil, which leaks into it at a steady rate from thousands of natural seeps. Though none of the seeps is anywhere near the size of the Deepwater Horizon leak, they do mean that the gulf is swarming with bacteria that can eat oil.

The winds from two storms that blew through the gulf in recent weeks, including a storm over the weekend that disintegrated before making landfall, also appear to have contributed to a rapid dispersion of the oil. Then there was the response mounted by BP and the government, the largest in history, involving more than 4,000 boats attacking the oil with skimming equipment, controlled surface burns and other tactics.

Some of the compounds in the oil evaporate, reducing their impact on the environment. Jeffrey W. Short, a former government scientist who studied oil spills and now works for the environmental advocacy group Oceana, said that as much as 40 percent of the oil in the gulf might have simply evaporated once it reached the surface.

An unknown percentage of the oil would have been eaten by bacteria, essentially rendering the compounds harmless and incorporating them into the food chain. But other components of the oil have most likely turned into floating tar balls that could continue to gum up beaches and marshes, and may represent a continuing threat to some sea life. A three-mile by four-mile band of tar balls was discovered off the Louisiana coast on Tuesday.

“Less oil on the surface does not mean that there isn’t oil beneath the surface, however, or that our beaches and marshes are not still at risk,” Jane Lubchenco, administrator of the National Oceanic and Atmospheric Administration, said in a briefing on Tuesday. “We are extremely concerned about the short-term and long-term impacts to the gulf ecosystem.”

Thad W. Allen, the retired Coast Guard admiral who leads the government’s response, has emphasized that boats are still skimming some oil at the surface. Admiral Allen said the risk of shoreline oiling might continue for at least several more weeks.

“While we would all like to see the area come back as quickly as it can,” he said, “I think we all need to understand that we, at least in the history of this country, we’ve never put this much oil into the water. And we need to take this very seriously.”