Last week the debate about the fate of oil in the gulf took, according to major media reports, an  optimistic  turn . Now Representative Ed Markey (D–MA) is raising questions  about federal oversight of dispersant use in the gulf, and a Senate committee is holding a hearing on the issue  on Wednesday.
But while scientists acknowledge that dispersants can have negative effects, they are generally more worried about the oil than the dispersants, as the oil is far more toxic and more than 100 times more of it has been released. At a briefing last week in Washington, D.C., scientists were cautiously supportive of the government's gutsy decision on 15 May to allow BP to squirt tens of thousands of gallons of dispersants a day a mile deep. More than a million gallons of Corexit have been released on the sea floor since, with another 800,000 gallons sprayed by plane on the surface—in what amounts to a major, unprecedented experiment. Overall, the researchers said the move saved vast areas of coastal ecosystems and greatly reduced the amount of oil that would need to be collected or burned. Dispersants break the oil into tiny molecules that present tens of thousands of times more surface area than normal crude to microbes to be eaten; the molecules also rise to the surface much more slowly than raw crude. Some 50 scientists at a workshop at Louisiana State University predicted these benefits from the dispersants in May , signaling their support for the decision the government had made; some dissenters  led by Sylvia Earle have said it was too soon to declare it a success.
But scientists at last week's congressional briefing said say some important risks to the undersea environment remain:
- Oil drops could wreak havoc on tuna eggs and larvae. Atlantic bluefin tuna are now spawning in their warm gulf waters. How might the oil drops affect their food supply? Biologist Robert Diaz of the College of William and Mary in Williamsburg, Virginia, said scientists don't know. "Is the dispersed oil better or worse for the tuna—that's hard to judge," Diaz said.
- By virtue of their size, small oil drops could be deadly. Dispersing the oil means breaking it into smaller drops, which can do unexpected things—like get wedged into the layers of armor of baby crabs, a Tulane University scientist found, as ScienceInsider reported  earlier this month. Although the oil has yet to be detected in the bodies of the larvae, researchers don't know what effect it may have on the crabs and whether natural molting of their shells could rid them of the pollutant. Had the decision been made not to disperse the oil under water, the oil would have largely remained on the surface where "perhaps the larvae wouldn't see it," said Diaz.
By making the oil drops tiny—biologist Kenneth Lee of the Bedford Institute of Oceanography in Nova Scotia, Canada, estimated the dispersed oil drops are several hundred micrometers in size—they are far smaller than shrimp eggs or larvae, and that provides a new way for the pollutant to get into their cells. "Hopefully, [the oil] is diluted enough" that it's not affecting them much, said Diaz, although he has no data either way.
- Vast undersea plumes may have gone undetected. As ScienceInsider reported  last week, dozens of federal and academic scientists aboard seven research vessels in the gulf believe the undersea oil is very dilute and has remained, for the most part, within 50 km of the wellhead. Lee described how scientists use daily data from this monitoring to plan where to send ships the following day, lowering the chance that large oil plumes have remained undetected after weeks of searching.
But could giant plumes have been missed in the vast gulf anyway? "That's possible. We just don't know," said oceanographer Nancy Kinner of the University of New Hampshire in Durham. Undersea gliders operating at depth have given additional data to scientists on currents, but they lack strong detection devices for finding oil. "It's just not something you can rig to these gliders in 30 days," said Kinner, who laid out a series of new research priorities for spill scientists related to finding such plumes. (ScienceInsider will cover that next week.)
- Are dispersants playing chemical chaperones for poisons? Dispersants form minute oil droplets by coating the oily molecules, making them into small drops surrounded by dispersant molecules. Chemist Bob Gagosian of the Consortium for Ocean Leadership in Washington, D.C., told Science Insider yesterday that some scientists worry that this could allow the most toxic elements of crude oil—polycyclic aromatic hydrocarbons (PAHs)—to enter the bodies of ocean microbes, with unknown effects.
- Unk-unks. Dangers scientists don't know about so they can't gauge them—so-called unknown-unknowns—might lurk far below the surface. "Our knowledge declines with depth," said Diaz. For example, he said, researchers have scant details about how ecosystems that rely on deep-sea sediments work. Plus, pointed out Lee, natural oil seeps spew oil into the ocean all the time, which is, to some extent, naturally dispersed into smaller drops. Scientists have little ability thus far to differentiate between these sources and oil droplets formed by Corexit—or, for that matter, teasing out an anthropogenic signal.