The oil slick in the Gulf of Mexico appears to be dissolving far more rapidly than anyone expected, a piece of good news that raises tricky new questions about how fast the government should scale back its response to the Deepwater Horizon disaster.

The immense patches of surface oil that covered thousands of square miles of the gulf after the April 20 oil rig explosion are largely gone, though there continue to be sightings of tar balls and emulsified oil here and there.

Reporters flying over the area Sunday spotted only a few patches of sheen and an occasional streak of thicker oil, and radar images taken since then suggest that these few remaining patches are quickly breaking down in the warm surface waters of the gulf.

John Amos, president of SkyTruth, an advocacy group that sharply criticized the early, low estimates of the size of the BP leak, noted that no oil had gushed from the well for nearly two weeks.

“Oil has a finite life span at the surface,” Mr. Amos said Tuesday, after examining fresh radar images of the slick. “At this point, that oil slick is really starting to dissipate pretty rapidly.”

Feds won't walk away
The dissolution of the slick should reduce the risk of oil killing more animals or hitting shorelines. But it does not end the many problems and scientific uncertainties associated with the spill, and federal leaders emphasized this week that they had no intention of walking away from those problems any time soon.

The effect on sea life of the large amounts of oil that dissolved below the surface is still a mystery. Two preliminary government reports on that issue have found concentrations of toxic compounds in the deep sea to be low, but the reports left many questions, especially about an apparent decline in oxygen levels in the water.
And understanding the effects of the spill on the shorelines that were hit, including Louisiana’s coastal marshes, is expected to occupy scientists for years. Fishermen along the coast are deeply skeptical of any declarations of success, expressing concern about the long-term effects of the chemical dispersants used to combat the spill and of the submerged oil, particularly on shrimp and crab larvae that are the foundation of future fishing seasons.

After 86 days of oil gushing into the gulf, the leak was finally stopped on July 15, when BP managed to install a tight-fitting cap on the well a mile below the sea floor, then gradually closed a series of valves. Still, the well has not been permanently sealed. Until that step is completed in several weeks, the risk remains that the leak will resume.

Gulf bacteria eat oil
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.”

Scale of effort to change, but when?
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. He said the risk of shoreline oiling might continue for at least several more weeks.

Still, it is becoming clear that the Obama administration, in conjunction with BP, will soon have to make decisions about how quickly to begin scaling down the large-scale — and expensive — response effort. That is a touchy issue, and not just for environmental reasons.

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The response itself has become the principal livelihood for thousands of fishermen and other workers whose lives were upended by the oil spill. More than 1,400 fishing boats and other vessels have been hired to help deploy coastal barriers and perform other cleanup tasks. Those fishermen are unconvinced that the gradual disappearance of oil on the surface means they will be able to return to work soon.

“Surface is one thing; you know that’s going to dissipate and all,” said Mickey Johnson, who owns a shrimp boat in Bayou La Batre, Ala., pointing out that shrimpers trawl near the sea floor.

“Our whole big concern has always been the bottom,” Mr. Johnson said.