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As this article goes to press, the scope of the worst oil spill in U.S. history remains a moving target. The explosion and collapse of the BP-owned Deepwater Horizon oil rig on 20 April 2010 uncorked an underwater geyser that for 85 consecutive days shot an estimated daily load of 1.47–2.52 million gallons1—and possibly more—into the Gulf of Mexico. Caused by igniting gases leaking from API Well No. 60-817-44169, located 42 miles off the Louisiana coast and 5,000 feet underwater, the explosion killed 11 workers. At press time more than 600 miles of coastline was fouled by the oil, and about one-third of the Gulf’s fishing grounds were closed.2 BP engineers stanched the flow on July 15 with a mechanical cap, but oil and methane seeps have since appeared near the wellhead, raising new questions about the integrity of the well and, indeed, of the seafloor.2 No one knows what will happen next.  The Deepwater Horizon spill has generated heart-wrenching scenes of dying birds, oil-fouled marshes and barrier islands, and traumatized coastal residents. But a key image from this story isn’t even visible: mysterious plumes of dispersed oil droplets flowing deep underwater. To some degree, these plumes arose from intense physical pressures at the mile-deep wellhead, which broke the oil into droplets that never reached the surface. But spill response workers also used chemical dispersants—mixtures of solvents, surfactants, and other proprietary additives—to achieve a similar effect. Sprayed from the air and applied directly at the gushing wellhead, dispersants changed the oil’s physical and chemical properties, splitting it into tiny droplets that measure roughly 10 microns in diameter (naturally dispersed oil droplets are about 10 times larger).3 Dispersed oil droplets get pulled (or “entrained”) into the water column, where they undergo a range of removal processes, mainly metabolism by marine bacteria.  The decision to use dispersants—which have been commercially available for oil spill response since the mid-1960s—always involves environmental tradeoffs, says Mahlon Kennicutt, a professor of chemical oceanography at Texas A&M University. Whereas undispersed oil floats on water, smothering birds and marine mammals and fouling coastal resources, dispersed oil is transported throughout the water column, where it’s more available to marine life. “Dispersants don’t make the oil go away,” Kennicutt emphasizes.  “Zooplankton mistake oil droplets for food,” adds Carys Mitchelmore, an associate professor at the University of Maryland’s Chesapeake Biological Laboratory. That’s a dangerous scenario because zooplankton are crucial to the marine food web. Kill them off, Mitchelmore says, and the consequences spiral upward.

BETWEEN THE DEVIL AND THE DEEP BLUE SEA: DISPERSANTS IN THE GULF OF MEXICO