A year after the Gulf oil spill unleashed nearly five million barrels of oil into the Gulf of Mexico, scientists are still trying to figure out how much oil is left on the little-studied seafloor.
There's no doubt oil has concentrated on the shallow seafloor, close to the Gulf coastlines—and will remain there for some time, experts say.
"There is oil on some of the shelf areas that mixes in with the sand and tar balls continue to wash up," said Larry McKinney, director of the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University in Corpus Christi.
"So the long-term chronic effect of that is something we have to be concerned about."
But getting a handle on the status of the remote, deep seafloor is proving far more difficult, with research expeditions turning up different results.
Seafloor Mostly Oil-Free?
There are those—including Edward Overton, professor emeritus of environmental sciences at Louisiana State University—who believe most of the oil was diluted and dispersed in the water column, where it remains in tiny, suspended particles.
"I'm going to go out on a limb here," Overton said. "Contrary to what some think, I don't believe that there is a lot of oil on the bottom of the Gulf."
Overton stressed that the Gulf oil spill released a very light oil that lacks many of the dense asphaltenes, resins, and other components that make heavier types of petroleum more prone to sinking. Heavy oil is also less desirable to the petroleum industry, because it doesn't flow easily and is harder to extract and refine.
Because the oil gushed into the Gulf 1 mile (1.6 kilometers) below the surface, a number of factors—including high pressures at depth—broke the crude into smaller and smaller particles. "Tiny droplets of this oil, in the water column, have basically neutral buoyancy," he said.
"In a tidal area you see oil sink when the waves break and the globs hit the bottom and pick up grains of sand and shell fragments. That's how you get sunken oil in the tidal areas. But there aren't major mechanisms for massive downdrafts in that part of the [deep] water column.
"So I think it was dispersed throughout the Gulf in the water column, and I think it's still in the water column [in tiny, mostly undetectable concentrations]."
Terry Hazen, a microbial ecologist at California's Lawrence Berkeley National Laboratory, was part of an expedition that looked long and hard for oil in sediment samples—but found little in some 1,440 cores collected at 120 different sites throughout the Gulf.
"Only six percent of them were contaminated to any level, and those were probably associated with the drilling mud" from May's "top kill" maneuver, a failed effort to stop the spill by pumping heavy mud into the well.
That's because most of the contaminated samples were found within 0.6 mile (1 kilometer) of the wellhead and all were found within 1.7 miles (2.7 kilometers).
"According to sediment cores from the deep that we have seen the data on, there is little evidence so far that any large amount of oil is on the deep seafloor," Hazen said.
Lots of Oil Covering Deep Seafloor?
Samantha Joye, a marine scientist at the University of Georgia, said her research paints a different picture. Joye photographed thick oil on parts of the deep seafloor during three submersible dives near the site of the Macondo wellhead in December.
She saw a panorama of oily corals and dead brittle stars, but failed to find worms, sea cucumbers, and other species that would typically be seen in such spots. (See pictures of colorful sea creatures.)
Oil seeps naturally into the Gulf each year, but Joye has studied many such areas before and said these sites don't look at all the same now.
"The bottom line is that there is a lot of oil on the seafloor," she said. "BP itself has fingerprinted some of that oil to Macondo. They found the same thing we did at the same sites."
"We have samples from before the sedimentation of oil began in May, and this layer wasn't there at that time," she said.
"When we went back in September, we revisited several sites and at all of them found this layer that wasn't there in May. And this is not a normal-looking layer. It's a very bizarre-looking layer.
"It's hard for me to come up with an explanation, independent of the oil spill, that could explain why you have a similar layer over such a large area."
Deep-sea Corals Mostly Unscathed by Gulf Spill?
"The good news about what we've seen is that we've done dives all over the Gulf of Mexico since the spill, and most places where we dove in the deep Gulf, things looked pretty good," said Fisher, of Penn State University.
"In most places you couldn't see that there were any obvious signs of damage."
Fisher cautioned, however, that harm to the coral, such as reproductive disorders or cancer, might take years to appear.
He also noted that sites he's seen close to the wellhead itself presented a far bleaker picture—masses of corals covered with a suspicious dark substance that scientists believe is tied to the spill.
"In November we found a reef site that had been very severely impacted, and we've been back to the site in December and again just a few weeks ago," Fisher said.
"We found that when we sucked this brown material off the corals, the tissues underneath were dead. Wherever the corals were covered with this stuff it killed them."
Parts of those same colonies remained uncovered and alive, however—which means Fisher and colleagues are anxiously waiting to see what will happen next.
"Unfortunately with corals, things happen slowly," he said. "These corals are hundreds of years old and it takes a long time for them to grow at all. The best case is that colonies could continue growing from the parts that are still alive, or the colonies could completely die. And I would not want to venture a gamble on how it's going to play out."
As one might expect, Fisher explained, it looks like corals closer to the well site fared far worse than distant ones exposed to more diluted oil. But his team faces an interesting challenge when trying to map the extent of coral damage—no one knows exactly where all the Gulf's deepwater coral sites are. (See coral pictures.)
"Like most of the deep sea, this area is largely unexplored," Fisher said. "So now we're just trying to find more coral sites so we can see how far the acute damage did go and in what direction. I know there are more rocky, possible coral sites within 15 miles [24 kilometers] of Macondo, and I think we're going to find them in the next few months."
Imagining the Elephant
It may seem that scientists are studying two entirely different events or areas, noted Harte Research Institute's McKinney.
But their results simply show that the larger picture of oil-spill impacts on an enormous ecosystem like the Gulf seafloor will take a long time to emerge.
McKinney likened the Gulf oil spill scientific process to the old story about a group of blind men asked to describe an elephant, in which each person's perspective is shaped by the area he happens to grab.
"One grabbed the tail and said the elephant is like a rope, another touched a leg and said it was like a tree trunk," he said.
"It wasn't until they all got together that they recognized the whole picture. Some places where there seem to be contradictions here, I think it's because we can't see the whole elephant yet. And I think that picture will become more clear with time."