Since BP capped the renegade Macondo well at the center of the Gulf oil disaster 12 days ago, the oil slick has shrunk to about 10,000 square miles from 80,000 square miles in just a matter of weeks.
The reduction has amazed scientists who are tracking the spill and raised many questions about where all the oil has gone. An 800-vessel skimming fleet that weeks ago pulled in 25,000 barrels of oil a day could barely find 50 barrels a day late last week. That means much of the up to 3 million barrels suspected to be remaining in the Gulf has largely gone off the radar.
It's clear the Gulf is doing what Louisiana State University biologist Ed Overton calls "Mother Nature's work" in breaking down the patchy oil. Oil-eating bacterial microbes are working at a fast pace. But scientists are still unsure of the longer term environmental impact. Many fishermen are convinced that much of the oil is suspended in the water column or has drifted to the bottom, where it's impacting oyster beds, crab herds, and spawning fish schools.
"Superficially, everything is going to look fine and within a couple of years we'll be back to normal shrimp catches, but we will have certain species on which this will have had a catastrophic effect and that will be a long time in recovery," says Thomas Shirley, a marine biologist at Texas A&M University in Corpus Christi.
The Gulf oil spill is not a containable event like the 1989 Exxon Valdez disaster, which dropped thick sludge in Prince Edward Sound, Alaska. Days after the Deepwater Horizon accident in the Gulf, the government sent skimmers hunting for visible slicks. Boats managed to sop up about 1 million barrels of the estimated 4 million barrels from the ruptured Macondo well, showing how widely the oil has spread.
As oil now becomes harder to find, BP is considering whether to shift the Vessels of Opportunity program, a BP initiative that's employed hundreds of out-of-work fishing crews, from oil collection to oil detection missions in order to keep them working.
"What we have is an aggregation of hundreds of thousands of patches of oil and the challenge is to find out where they are at right now because they are widely dispersed," said retired Coast Guard Adm. Thad Allen, who is overseeing the recovery effort for the federal government. "Maybe patches is a misnomer on my part," he added. "What we're seeing are mats, patties, small concentrations, very hard to detect, but they're out there."
At least part of the disappearing act has to do with fast-feeding and fast-breeding bacterial microbes that are thriving on a huge new food supply, and egged on by 88-degree water and a blistering sun that's heating surface air temperatures to nearly 100 degrees.
"To bacteria, this oil spill is filet mignon, not chopped beef," says Mr. Overton. "And after this spill stopped, you have a large biomass of bacteria now looking around for food, and it's my belief that they're gobbling up anything that looks like oil."
About 600 miles of the Gulf Coast have been oiled, and a southerly wind in the coming days could mean more oil coming ashore. However, government officials say their focus is now shifting to figuring out how fast and to what extent the spill is degrading by itself.
"The sheer volume of oil that's out there has to mean there will be some very significant impacts," says Jane Lubchenco, administrator of the National Oceanic and Atmospheric Administration (NOAA). "We've already seen some of those impacts play out in ways that are more obvious, more visual, because they're at the surface, but what we have yet to determine is the full impact that the will have not just on the shorelines or on the wildlife, but beneath the surface."
While the baking-hot temperatures break down the volatility of the spill, toxicity is still a concern. While hundreds of animal autopsies have been inconclusive as to the cause of death, oil and its byproducts are extremely difficult to locate in animal tissue since they break down very quickly, says Mr. Shirley.
For many on the coast, the disappearing oil isn't a mystery at all.
Many suspect it has settled either on the bottom or in mid-layers of the Gulf. (There's little evidence the oil has drifted to the bottom, says Ms. Lubchenco at NOAA.) Gulf Coast residents remain concerned about the long-term health of the Gulf fishery, even as NOAA recently reopened some fishing grounds that were closed because of the spill. Locals' concern is that out-of-sight oil will be out-of-mind oil for BP and the Coast Guard.
Three dozen NOAA vessels are currently working to track the oil spill and major research cruises are being put together to study the effects of the BP oil spill in the Gulf. The biggest question for many scientists is the Gulf's environmental resilience: Whether the oil spill in some parts of the Gulf brought some local ecologies to a tipping point where they simply can't recover.
"We'll continue to stumble upon caches of this oil," says Shirley. "Some of it will be in deep water, some of it will be in a few hundreds of feet as this oil combines with other organic compounds and sediment. We'll find other pools of oil that are exposed, some of which will be in mousse or asphalt, some of it in the form of liquid oil covered over with sand or sediment."
Experts look to the 1979 Ixtoc blowout off Mexico's Yucatan Peninsula for clues to how oil disperses and degrades in the Gulf. While fisheries in the nearby area completely rebounded two years after the spill, a return expedition this year by Texas A&M researcher Wes Tunnell, who studied the Ixtoc spill, showed some species, especially oysters, never fully recovered.
Within minutes of arriving on site, Mr. Tunnell found asphalt and tar blocks from the Ixtoc spill wedged into coral in the same places they had been 20 years ago. When researchers broke open the blocks, they got fresh whiffs of petroleum hydrocarbons, Shirley says. "It's tough stuff, that's why we put it on roads."