The chemicals BP is now relying on to break up the steady flow of leaking oil from deep below the Gulf of Mexico could create a new set of environmental problems. 
 
 Even if the materials, called dispersants, are effective, BP has already bought up more than a third of the world’s supply. If the leak from 5,000 feet beneath the surface continues for weeks, or months, that stockpile could run out. 
 
 On Thursday BP began using the chemical compounds to dissolve the crude oil, both on the surface and deep below, deploying an estimated 100,000 gallons. Dispersing the oil is considered one of the best ways to protect birds and keep the slick from making landfall. But the dispersants contain harmful toxins of their own and can concentrate leftover oil toxins in the water, where they can kill fish and migrate great distances. 
 
 The exact makeup of the dispersants is kept secret under competitive trade laws, but a worker safety sheet for one product, called Corexit, says it includes 2-butoxyethanol, a compound associated with headaches, vomiting and reproductive problems at high doses. 
 
 “There is a chemical toxicity to the dispersant compound that in many ways is worse than oil,” said Richard Charter, a foremost expert on marine biology and oil spills who is a senior policy advisor for Marine Programs for Defenders of Wildlife and is chairman of the Gulf of the Farallones National Marine Sanctuary Advisory Council. “It’s a trade-off – you’re damned if you do, damned if you don’t -- of trying to minimize the damage coming to shore, but in so doing you may be more seriously damaging the ecosystem offshore.” 
 
 BP did not respond to requests for comment for this article. 
 
 Dispersants are mixtures of solvents, surfactants and other additives that break up the surface tension of an oil slick and make oil more soluble in water, according to a paper published by the National Academy of Sciences. They are spread over or in the water in very low concentration – a single gallon may cover several acres. 
 
 Once they are dispersed, the tiny droplets of oil are more likely to sink or remain suspended in deep water rather than floating to the surface and collecting in a continuous slick. Dispersed oil can spread quickly in three directions instead of two and is more easily dissipated by waves and turbulence that break it up further and help many of its most toxic hydrocarbons evaporate. 
 
 But the dispersed oil can also collect on the seabed, where it becomes food for microscopic organisms at the bottom of the food chain and eventually winds up in shellfish and other organisms. The evaporation process can also concentrate the toxic compounds left behind, particularly oil-derived compounds called polycyclic aromatic hydrocarbons, or PAHs. 
 
 According to a 2005 National Academy of Sciences report, the dispersants and the oil they leave behind can kill fish eggs. A study of oil dispersal in Coos Bay, Ore. found that PAH accumulated in mussels, the Academy’s paper noted. Another study examining fish health after the Exxon Valdez spill in Alaska in 1989 found that PAHs affected the developing hearts of Pacific herring and pink salmon embryos. The research suggests the dispersal of the oil that’s leaking in the Gulf could affect the seafood industry there. 
 
 “One of the most difficult decisions that oil spill responders and natural resource managers face during a spill is evaluating the trade-offs associated with dispersant use,” said the Academy report, titled Oil Spill Dispersants, Efficacy and Effects. “There is insufficient understanding of the fate of dispersed oil in aquatic ecosystems.” 
 
 A version of Corexit was widely used after the 1989 Exxon Valdez spill and, according to a literature review performed by the group the Alaska Community Action on Toxics, was later linked with health impacts in people including respiratory, nervous system, liver, kidney and blood disorders. But the Academy report makes clear that the dispersants used today are less toxic than those used a decade ago. 
 
 “There is a certain amount of toxicity,” said Robin Rorick, director of marine and security operations at the American Petroleum Institute. “We view dispersant use as a tool in a toolbox. It’s a function of conducting a net environmental benefit analysis and determining the best bang for your buck.” 
 
 Charter, the marine expert, cautioned the dispersants should be carefully considered for the right reasons. 
 
 “Right now there is a headlong rush to get this oil out of sight out of mind,” Charter said. “You can throw every resource we have at this spill. You can call out the Marine Corps and the National Guard. This is so big that it is unlikely that any amount of response is going to make much of a dent in the impacts. It’s going to be mostly watching it happen.” 
 
 Ryan Knutson contributed to this report ||||| The surface area of a catastrophic Gulf of Mexico oil spill quickly tripled in size amid growing fears among experts that the slick could become vastly more devastating than it seemed just two days ago. 
 
 A least tern checks her two eggs on the beach in Gulfport, Miss., Saturday, May 1, 2010. Environmentalists are concerned that the approaching oil slick will destroy this generation of the bird that nests... (Associated Press) 
 
 Leonard Lawton of Pass Christian, Miss., fishes next to an oil retaining boom in Bay St. Louis, Miss., Saturday, May 1, 2010. Environmentalists are concerned about the potential disaster the approaching... (Associated Press) 
 
 John Phillips waits to find out if he will be able to place oil booms along the Louisiana coast in Venice, La., Saturday, May 1, 2010. Choppy seas are frustrating efforts to prepare for a growing oil... (Associated Press) 
 
 This image provided by the U.S. Coast Guard shows a containment boom staged at the Breton National Wildlife Refuge, Thursday, April 29, 2010. The spill _ a slick more than 130 miles long and 70 miles... (Associated Press) 
 
 Eric Duvernay, of Pass Christian, Miss., fishes next to an oil retaining boom in Bay St. Louis, Miss., Saturday, May 1, 2010. Environmentalists are concerned about the potential disaster the approaching... (Associated Press) 
 
 Emmit Brooks waits to find out if he will be able to place oil booms along the Louisiana coast in Venice, La., Saturday, May 1, 2010. Choppy seas are frustrating efforts to prepare for a growing oil spill... (Associated Press) 
 
 A fishing boat passes the oil skimming ship Mississippi Responder, which is anchored on the Mississippi River near Venice, La., Saturday, May 1, 2010. The ship has been assisting in the containment of... (Associated Press) 
 
 UPDATES map with latest information; map locates the progression of the oil leak in the Gulf of Mexico since April (Associated Press) 
 
 Frustrated fishermen eager to help contain the spill from a ruptured underwater well had to keep their boats idle Saturday as another day of rough seas kept crews away from the slick. 
 
 President Barack Obama planned a Sunday trip to the Gulf Coast to see the damage himself. 
 
 Documents also emerged showing BP downplayed the possibility of a catastrophic accident at the offshore rig that exploded. 
 
 How far the spill will reach is unknown, but the sheen already has reached into precious shoreline habitat and remains unstopped, raising fears that the ruptured well could be pouring more oil into the gulf than estimated. 
 
 The Coast Guard has estimated that about 200,000 gallons (757,060 liters) of oil are spewing out each day _ which would mean 1.6 million gallons (6.06 million liters) of oil have spilled since the April 20 explosion that killed 11 workers. The environmental mess could eclipse the Exxon Valdez disaster, when an oil tanker spilled 11 million gallons (42 million liters) off Alaska's shores in 1989. 
 
 The slick nearly tripled in just a day or so, growing from a spill the size of Rhode Island to something closer to the size of Puerto Rico, according to images collected from mostly European satellites and analyzed by the University of Miami. 
 
 On Thursday, the size of the slick was about 1,150 square miles (nearly 3,000 square kilometers), but by Friday's end it was in the range of 3,850 square miles (9,900 square kilometers), said Hans Graber, executive director of the university's Center for Southeastern Tropical Advanced Remote Sensing. That suggests the oil has started spilling from the well more quickly, Graber said. 
 
 "The spill and the spreading is getting so much faster and expanding much quicker than they estimated," Graber told The Associated Press on Saturday. 
 
 Louisiana State University professor Ed Overton, who heads a federal chemical hazard assessment team for oil spills, cautioned that the satellite imagery could be deceiving. 
 
 He said satellites can't measure the thickness of the sheen and makes it difficult to judge how much oil is on the water. 
 
 Another issue is that the oil slicks are not one giant uniform spill the size of an island. Instead, they are "little globs of oil in an area of big water," Overton said. 
 
 One expert also cautioned that if the spill continues growing unchecked, sea currents could suck the sheen down past the Florida Keys and then up the Eastern Seaboard. 
 
 The Florida Keys are home to the only living coral barrier reef in North America, and the third largest coral barrier reef in the world. About 84 percent of the nation's coral reefs are located in Florida, where hundreds of marine species live, breed and spawn. 
 
 "If it gets into the Keys, that would be devastating," said Duke University biologist Larry Crowder. 
 
 Ian R. MacDonald, an oceanography professor at Florida State University, said his examination of Coast Guard charts and satellite images indicated that up to 9 million gallons (34 million liters) had already spilled by April 28. 
 
 However, officials with the Coast Guard brushed off such fears and said the estimates were imprecise. 
 
 BP suggested in a 2009 exploration plan and environmental impact analysis for the well that an accident leading to a giant crude oil spill _ and serious damage to beaches, fish and mammals _ was unlikely, or virtually impossible. 
 
 The plan for the Deepwater Horizon well, filed with the federal Minerals Management Service, said repeatedly that it was "unlikely that an accidental surface or subsurface oil spill would occur from the proposed activities." 
 
 The company conceded a spill would impact beaches, wildlife refuges and wilderness areas, but argued that "due to the distance to shore (48 miles, or 77 kilometers) and the response capabilities that would be implemented, no significant adverse impacts are expected." 
 
 The spill threatens hundreds of species of wildlife, including birds, dolphins, and the fish, shrimp, oysters and crabs that make the Gulf Coast one of the nation's most abundant sources of seafood. 
 
 Although the cause of the explosion was under investigation, many of the more than two dozen lawsuits filed in the wake of the explosion claim it was caused when workers for oil services contractor Halliburton Inc. improperly capped the well _ a process known as cementing. Halliburton denied it. 
 
 The Coast Guard said Saturday it had shut down two offshore platforms and evacuated one of them near the spill as a safety precaution. 
 
 BP has tried applying chemicals underwater to break up the oil before it reaches the surface. That has never before been attempted at such depths. 
 
 BP and federal authorities said the dispersant was released overnight at the site of the leak, nearly 5,000 feet (1,524 meters) underwater, and they were evaluating the effort Saturday. 
 
 ___ 
 
 Associated Press writers Seth Borenstein, Melissa Nelson, Michael Kunzelman, Chris Kahn, Allen G. Breed, Vicki Smith, Janet McConnaughey, Alan Sayre, Tamara Lush and Brian Skoloff contributed to this report. ||||| R. Camilli, Woods Hole Oceanographic Institution/Science Still images taken June 1 by a remote-operated camera descending through the Gulf of Mexico just southwest of the well site showed what scientists called a "highly turbid" layer of emulsified oil between 1,065 meters and 1,300 meters deep. 
 
 Oil from the Deepwater Horizon spill formed an underwater plume of hydrocarbons the size of Manhattan, scientists said Thursday, raising fears of a lingering cloud of trace chemicals in the Gulf with an unknown long-term impact. 
 
 The new findings from the Woods Hole Oceanographic Institution add to evidence from other research groups this week that the offshore spill—the largest in history—is confounding scientists' assumptions about how the Gulf waters are interacting with the mass of oil. 
 
 Stefanie Ilgenfritz has details of a just-released-survey indicating the oil that spilled from the Deepwater Horizon produced drifting plume of hydrocarbons the size of Manhattan. 
 
 The new data, based on measurements taken in June when oil was still gushing from BP PLC's Macondo well, also challenge government estimates that the vast majority of the 4.9 million barrels of spilled oil is already gone from the Gulf or being rapidly broken down by bacteria, several marine experts said. The Woods Hole scientists reported their preliminary findings Thursday online in the journal Science. 
 
 Instead, some of that oil may persist deep under water—at levels thousands of times higher than those caused by the natural oil seeps that dot the Gulf sea floor—where it can elude conventional detection and cleanup efforts, scientists said. 
 
 Up to 79% of the spilled oil may still be in the Gulf, researchers at the Georgia Sea Grant program and the University of Georgia said earlier this week. Oil is already settling into the sea floor in a prime spawning ground for fish called DeSoto Canyon east of the damaged well, according to University of South Florida scientists. 
 
 "I think the imprint of the BP release, the discharge, will be detectable in the Gulf of Mexico for the rest of my life," oceanographer Ian MacDonald from Florida State University told a congressional hearing on the spill Thursday. 
 
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 The Woods Hole researchers saw little evidence in June, two months after a wellhead explosion triggered the spill, that oil-eating microbes had reduced the cloud of chemicals. "It looks like the oil is degrading relatively slowly," said Woods Hole chemist Ben Van Mooy. 
 
 Contrary to previous predictions by other researchers, however, the Woods Hole group found no evidence of "dead zones," in which bacteria feasting on oil can use up so much oxygen in the process that no fish or marine life can survive. They speculated that earlier oxygen readings might have been wrong because measuring devices can give artificially low readings when coated by oil. 
 
 Renewed concerns over leftover oil in the Gulf of Mexico could deal a further blow to the region's battered fishing industry, which is eager to return to normal after big stretches of state and federal waters were reopened in recent weeks. The five Gulf Coast states had $10.54 billion in combined seafood sales and employed more than 200,000 people in 2008, according to government data. 
 
 But demand for Gulf seafood has shrunk amid images of oil-slicked waters. That is despite heightened monitoring by the U.S. Food and Drug Administration, which says seafood from the parts of the Gulf open to fishing pose no health risks. 
 
 So far, the chemicals in the deep plume aren't concentrated in these depths at levels high enough to be directly toxic to most marine life, several ocean experts who study the Gulf said. 
 
 No one knows yet how long oil plumes will last or what their long-term impact will be. But experts are concerned that if the trace chemicals linger long enough, they could damage fish eggs and larvae, as well as the plankton on which many fish feed. 
 
 Researchers suggest the plume may last for a year or more, but it isn't yet known whether it has dissipated since the Woods Hole researchers measured it, at the end of June. 
 
 "These hydrocarbons may well show up somewhere else, running undetected below the surface," said Richard Camilli from the Massachusetts-based Woods Hole group, who was chief scientist on the June research. 
 
 Enlarge Image Close C. McIntyre, Woods Hole Oceanographic Institution The scientists made 57,000 measurements, mainly using sensors aboard a remote-controlled robot deployed from the ship. 
 
 For 10 days in June, Dr. Camilli and his colleagues aboard a U.S. National Science Foundation research vessel explored the plume—22 miles long and more than a mile wide—as it snaked along 3,000 feet below the surface of the Gulf. 
 
 Writing in the journal Science, the scientists confirmed that oil from the well had been caught below the surface of the Gulf in pools of microscopic oil drops and petroleum-based trace chemicals, which were degrading more slowly than many had expected. The plume resembled a mist of trace chemicals largely invisible to the eye, rather than a river of oil. 
 
 In normal circumstances, crude oil floats to the surface, where it can be skimmed, burned off or evaporated. Floating on the waves, it can be churned into smaller drops readily digested by bacteria. 
 
 But oil from the ruptured well, broken down by sprays of chemical dispersants and held at depth by water pressure, has formed microscopic droplets not buoyant enough to break through the transition layer that separates warm surface currents from the cold bottom water, several experts said. 
 
 In their analysis, the Woods Hole researchers said they found high concentrations of benzene, toluene, xylene and other so-called BTEX petroleum compounds that could be traced to the leaking well. They calculated that the plume contained between 5% and 6% of the signature BTEX petroleum hydrocarbons released during the spill. 
 
 "This is the first number that anybody has been able to put on how much of the stuff from the well is ending up in the plume," said University of Georgia oceanographer Samantha Joye, who is studying the Gulf spill but wasn't involved in the project. 
 
 At the National Oceanic and Atmospheric Administration, a senior agency official said the new data would become part of the federal damage assessment used to help determine any penalties and fines that may be levied against BP and its contractors for damages from the spill. 
 
 BP officials referred requests for comment on the new research to NOAA and the federal joint command overseeing the spill cleanup. 
 
 Since the well was capped on July 15, there has been virtually no dispersant use—only 200 gallons total applied on July 19, according to the Environmental Protection Agency. 
 
 The question of how much oil is left in the Gulf, and where it has gone, is a contentious political issue in Washington. 
 
 On Aug. 4, White House environmental policy czar Carol Browner and Jane Lubchenco, administrator of the NOAA, released a study that indicated that nearly 75% of the 4.9 million barrels spilled from BP's well in the Gulf had been dispersed, evaporated, or collected in cleanup operations. 
 
 That report has since drawn criticism. Rep. Ed Markey (D., Mass.), chairman of a House Energy and Commerce subcommittee, used Thursday's hearing to criticize the Obama administration for "giving many people a false sense of confidence," and chided the administration for releasing the report. 
 
 "We remain confident in our estimates," Ms. Lubchenco of NOAA said Thursday. 
 
 —Mike Esterl, Siobhan Hughes and Stephen Power contributed to this article. 
 
 Write to Robert Lee Hotz at sciencejournal@wsj.com
– BP has begun using huge amounts of chemical dispersants to contain the oil slick that has tripled in only a day and is now the size of Puerto Rico. (AP has more on the growing size here.) And while those chemicals—being used above and below the surface—will help reduce the amount of oil that makes landfall, they create a whole new set of environmental concerns in the water, reports ProPublica. The makeup of the dispersants is secret under trade laws, but their toxins can kill fish and collect on the seabed to reenter the food chain. "There is a chemical toxicity to the dispersant compound that in many ways is worse than oil,” says an expert on marine biology. “It’s a trade off—you’re damned if you do damned if you don’t—of trying to minimize the damage coming to shore, but in so doing you may be more seriously damaging the ecosystem offshore."