The “missing” oil from the Deepwater Horizon disaster has been found: It’s on the ocean floor dozens of miles off the Louisiana coast, according to a new study.

That’s pretty well out of the way of fisheries—and the food supply. For now.

But scientists will be working for years to figure out how that contamination affects the Gulf of Mexico's marine life and food webs.

More than 200 million gallons of Louisiana sweet light crude gushed from BP’s damaged wellhead into the Gulf during the 2010 crisis. Less than 20 percent of that was removed from the water, leaving 134 to 176 million gallons (depending on whom you ask) of oil unaccounted for.

Using core samples from the sea bottom, marine chemist Jeff Chanton of Florida State University and his colleagues have determined that about six to 10 million gallons of that oil settled and mixed with ocean bottom sediments about 60 miles southeast of the Mississippi River delta. The findings were published in the journal Environmental Science & Technology.

Even though this means the oil likely won’t be washing up in coastal marshes or coating seabirds, it’s not a good outcome.

“Scientists tend to view anything that takes oil out of the water column as good,” Chanton said. “But in the water column there’s more oxygen, and on the seafloor, the potential for less.”

That’s important because contact with oxygen can speed up oil’s breakdown in the environment. However, oil buried in oxygen-poor seafloor sediments could endure for years.

Worms and fish that live in the sediments are continually exposed and re-exposed to the toxic compound and carry it into the food web when they’re eaten by other marine animals.

“Tilefish, which dig substantial burrows into the sediment, are particularly heavily polluted,” said Steve Murawski, a fisheries biologist at the University of South Florida who has studied fish diseases in the wake of Deepwater Horizon. “And they can be prey for other fish.”

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Fish, which like humans are vertebrates, metabolize the oil in their livers, gall bladders, and bile. So, unlike heavy metals or other persistent organic pollutants, oil does not increase or bio-intensify in their body tissues as it travels up the marine food chain.

But the Deepwater Horizon gusher contaminated the environment during spawning season for many fish species. That exposure killed about 12 percent of the year’s young bluefin tuna and 12 to 15 percent of red snapper, along with many juvenile Spanish mackerel and speckled trout, Murawski said, and may have damaged the DNA of survivors.

These, in turn, could be passing on genetic mutations to their offspring that over generations will prevent their populations from recovering to pre-2010 numbers.

“We are seeing some growth changes consistent with [Deepwater Horizon’s effects] in red snapper and some other fish,” Murawski said. His lab is examining the DNA of Gulf fish, including red snapper, tilefish, and king snake eel.

It’s complex work that’s been made harder by an undersupply of documentation for pre-2010 pollutant levels in the Gulf, he said.

That will complicate efforts to figure out which environmental health problems to credit to the BP oil spill.

“Given that we’re pulling 500 million barrels [of oil] a year out of the Gulf, we should have some basic environmental baselines,” Murawski said. “There are 4,000 oil structures out there. We should have a background signature for all of them, but we don’t.”