HOUSTON (Reuters) - With most of a once-massive Gulf of Mexico oil slick no longer a threat, environmental experts say the Gulf coast may have dodged the worst nightmare of a massive catastrophe.
The slick from BP Plc.'s blown-out Macondo well has shrunk to the point that cleanup vessels are having trouble finding skimmable oil, and tiny bacteria that thrive in the Gulf's warm waters may have neutralized much of the danger.
"The vast majority of the oil" -- about 75 percent of the 4.9 million barrels of oil spewed by the well -- has evaporated or otherwise been contained," according to a U.S. government study released on Wednesday.
"The Gulf Coast has been extremely fortunate in terms of the nature of the winds and the currents," said Larry Mayer, an oceanographer at the University of New Hampshire in Durham. "It could have been much, much worse."
About a third of the Gulf coast's shoreline -- about 640 miles -- has been oiled, but only a fraction of that -- perhaps 40 miles -- falls into the "heavily oiled" category. Shoreline impacts, though formidable, are less than half the 1989 Exxon Valdez spill, which affected about 1,500 miles.
But with about a quarter of the spilled oil still unaccounted for, comparable to about five Exxon Valdez spills, it could be years before scientists can definitively pronounce that the region has dodged the bullet.
"The effects of this spill will likely linger for decades," said Jane Lubchenco," head of the National Oceanic and Atmospheric Administration. "The total amount of oil was immense."
Scientists know little about an underwater oil plume that could create undersea "dead zones," and the impact of oil-based toxins could take years to filter through the thousands of species that live in the Gulf.
"We're seeing a pendulum swing from what had being portrayed as oil Armageddon to what is now looking like oil nothing," said John Lopez, coastal program director for the Lake Pontchartrain Basin Foundation in Metairie, Louisiana. "The truth has been in between."
About 99 percent of an oil spill's environmental impacts are obvious within weeks, but it's the other one percent that is worrisome, said Edward Overton, professor emeritus at Louisiana State University in Baton Rouge.
"It probably is a bit too early to declare victory," Overton said. "Whatever damage is going to be done has been done. We just don't know what that is yet."
The good news: Louisiana's coastal marshes, the dense grasslands that absorb damage from incoming storms and filter pollutants, have seen little impacts from the spill, according to Irv Mendelssohn, a wetland ecology expert.
"Considering the volume of oil that has been spilled, only a small proportion of that has reached the coast," said Ed Owens with Polaris Applied Sciences Inc, who is under contract for BP as a shoreline response technical adviser.
But lurking beneath the water's surface is a plume of oil -- perhaps millions of gallons -- which scientists are struggling to track and understand.
U.S. scientists have confirmed the presence of low concentrations of oil moving in ocean currents over 1,000 meters (3,280 feet) deep, but have offered few insights into its ecological effects.
"That's totally unknown territory to many scientists - what happens at those kind of depths," Mayer said. "The potential problem there is that this microbial degradation of deep oil uses up oxygen."
The oxygen-depleted "dead zone" phenomenon is well-documented in the Gulf of Mexico during summer months, when farm chemicals carried by the Mississippi and Atchafalaya rivers stimulate algae growth that chokes off ocean life.
Now scientists wonder if the oil spill could create a similar problem.
"We've made a gigantic bio-reactor out on the Gulf," Overton said. "You had massive transfer of petroleum hydrocarbons to bacteria biomass."
As for the thousands of species that live in the Gulf Coast's fragile grasslands and estuaries, it could be years for the spill's impacts to come to light.
Will oil-based toxins have an impact on reproductive systems of birds and fish? What effect did the oil slick have on the countless fish eggs and larvae that are deposited on the ocean floor, and in the marshes?
"We can see the birds killed, we can see oil on the shoreline. But what we can't see is what damage has been done to juvenile and larval species near shore, as well as offshore," Overton said.
The impact on commercially significant species like the blue crab and shrimp could be hard to track, because their lives are so interlinked with other species, Lopez said.
"Those are the kind of species that could have a cascading impact though the ecosystem," Lopez said. "We need at least three years to feel comfortable understanding these impacts."
"What you've done is killed or wiped out a generation of fish," Overton said. "We may not see the impact of that for 10 years."
For the birds that nest in the Gulf Coast's marshes and swamps, the effects of the oil could be delayed for years, until this year's breed of birds reaches breeding age.
For example, Brown Pelicans, Louisiana's state bird, do not mate until 3-4 years after they hatch.
"Did they survive but have their reproductive systems compromised?" asked Melanie Driscoll, director of bird conservation for the National Audubon Society's Louisiana program. "We won't know some of these impacts for years. It's just too early to tell."
(Editing by Alan Elsner)