A new study confirming the existence of a massive plume of oil trapped deep underwater in the Gulf of Mexico defies notions that bacteria, while they are degrading the oil, will make as quick work of petroleum lingering in the water's cold depths as they have on the surface.
In a widely hailed study published today in the journal Science, researchers from the Woods Hole Oceanographic Institution found there was no "appreciable" surge in bacteria growth and oil consumption around a plume they traced near the well that exploded earlier this summer. The first published peer-reviewed study of the subsurface oil found a plume stretching over an area nearly the size of Manhattan.
Based on measurements taken in late June, the study confirms the existence of an invisible, finely diffused mile-wide swath of oil-infused water that snaked more than 20 miles southwest from the crippled BP PLC well, some 3,000 feet beneath the Gulf's surface. The plume may have stretched even farther, but the scientists had to cut short their late-June research journey in the face of hurricane threats before they found the end of the plume.
Along the plume, researchers did not find dissolved oxygen depleting at levels they anticipated, indicating that while bacteria growth was stimulated and likely degrading the oil, there was not a huge swell in microbial life. Bacterial growth can deplete undersea oxygen levels, creating "dead zones" that are harmful to other marine creatures.
"The overall result is that we did not see a marked oxygen draw-down in the plume location at the well," said Benjamin Van Mooy, a scientist who contributed to the report.
In measuring oxygen levels along the plume, researchers found oxygen decreasing "considerably faster" than one would expect at those depths but "considerably slower" than expected at the surface, Van Mooy said. It means it could take a long time to get to oxygen levels low enough to harm marine life but that it might also be a while before the plume disappears.
Rather than painting a damning picture of subsurface oil that will barely be touched by bacteria, however, the report does little to invalidate the overall expectation that minute bugs are degrading, with vigor, oil and methane leaked from BP's well, the scientists said.
"There could be patches of higher microbial activity or lower microbial activity" elsewhere, said Van Mooy. "The ocean is a mixed-up, heterogeneous place, and microbes reflect that."
Many scientists have long assumed that Gulf bacteria, weaned on a diet of natural oil seeps, have been eagerly chewing through the various components of oil and methane throughout the water column. Indeed, preliminary media reports hyped that this feeding frenzy could lead to "dead zones" underwater, the surge in bacteria sapping regions of all available oxygen. If the rates of degradation they observed in June continued, it would be a "long while," if ever, before oxygen levels surged, Van Mooy said.
But it is now clear that some early readings of deepwater oxygen were based on flawed components that may have given misleadingly low measurements in the presence of oil. Given such inconsistency, scientists turned to chemical analyses that revealed, for at least one specific plume earlier this summer, fewer signs of microbial degradation than expected.
But some other scientists questioned the report's conclusion of little "appreciable" bacteria surge. There is some dissonance between what the study's scientists are stating and their data, said David Valentine, a microbiologist at the University of California, Santa Barbara. Valentine led his own expedition into the Gulf earlier this summer and is currently readying his direct measurements of bacterial activity for publication.
The Woods Hole researchers may have simply set their expectations too high, Valentine said. The oxygen levels detailed in the report, especially at their upper boundaries, still leave a large space for bacterial life to be growing and feeding on oil, Valentine said.
"There's still room for significant biodegradation to be occurring there," he said.
Indeed, the group's own rates would see all of the oxygen removed from the water in less than a year -- not that such losses will occur, he added.
But the Woods Hole researchers themselves said there could be greater microbial activity elsewhere. Christopher Reddy, one of the lead scientists on the report, likened the oil-spattered Gulf to a "buffet" for "opportunistic" microbes, which may prefer some compounds in the spread more than others.
"Look, any self-respecting microbe will want to eat oil; it's like butter," Reddy said. "But microbes are like teenagers. They do what they want, when they want to."
At best, the study is a snapshot in time, revealing the migrations of oil that had escaped from the BP well and diluted into deepwater over a period of five days. The study does not speak to current conditions in the Gulf, and researchers are uncertain how the environment has shifted since BP finally plugged its leak a month ago. The findings are more like one piece of a jigsaw puzzle than one section of a house of cards, according to Reddy.
Oxygen, in particular, is proving to be a finicky proxy. For example, the Woods Hole researchers used Valentine's own studies, which showed strong anomalies. Most likely, oxygen-rich water is mixing with these plumes at variable times, complicating what can be inferred about the bacterial response, he said.
"These signals are going to vary over time," he said.
To track the 3,000-foot-deep plume, the Woods Hole researchers used a robot submarine that zigzagged through the Gulf and recorded information about the water's biological and chemical composition. They also gathered water samples and made deepwater measurements using instruments cabled to a ship. To measure oxygen, the group employed electrodes and a backup method known as "Winkler titration," a labor-intensive method that allows oxygen concentrates to be measured precisely.
In some cases, the group found lower-than-normal oxygen levels with the electrode, but they attributed this to flawed readings from oil-fouled instruments, since they were unable to reproduce the readings using the Winkler method.
But other scientists were still not convinced.
"I found it very hard to believe these findings," said Ed Overton, an environmental scientist at Louisiana State University. "Even in the cool abyss, there seems like there should be more degradation than they seemed to detect."
Too few direct measurements have been published on microbial activity, forcing researchers to rely on indirect gauges. "[And] the damn oxygen sensors seem to be affected by the dispersed oil," Overton said. "It would really be nice to know what the direct bacterial counts are."
Previous government reports have confirmed the presence of subsurface oil, with lab analysis finding concentrations of 1 to 2 parts per million (ppm) near the wellhead, and light-based sensors detecting preliminary levels of up to 7 ppm in subsurface regions mirroring the layer studied by the Woods Hole expedition.
The Woods Hole report definitively ties the plume to the oil spill, by analyzing specific components of oil that are not found in natural oil seeps. The researchers found that "even if all of the natural [oil] seeps in the Gulf of Mexico were flowing into the plume," they would support less than half of the monoaromatics found in the plume.
"These findings confirm that a mechanism exists for direct hydrocarbon transfer into deep marine ecosystems," they wrote, though the larger implications for marine life remain outside the paper's study.
Due to their study's narrow focus on measuring monoaromatic hydrocarbons, it is impossible for the Woods Hole scientists to say how much total oil is in the plume. However, concentrations of monoaromatic hydrocarbons in the plume reached up to 50 micrograms a liter some 10 miles from the site, and elevated methane levels were detected more than 20 miles from the spill's center.