In these stagnant areas, oil-munching bacteria don't receive the nutrient-rich water flow they need to thrive, said Temple University's Boufadel.
"The assumption that oil is going to disappear is surprising—we have to put an effort into understanding oil spills and how they interact with the environment much better," Boufadel said.
Boufadel's research on the leftover oil was funded by the Exxon Valdez Oil Spill Trustee Council, a state and federal partnership to restore the damaged ecosystem.
Shangri-La of the North
Twenty years of oil exposure has injured this stretch of Alaska's coast, which WWF's Williams calls the Shangri-la of the north.
In its first toxic sweep, the oil spill killed about 250,000 seabirds, 4,000 sea otters, 250 bald eagles, and more than 20 orca whales, according to WWF.
Today, one of the orca pods that lost family members has not recovered.
Sea otters and harlequin ducks continue to die by digging into the sand for food and releasing buried oil.
At the bottom of the food chain, pink salmon eggs and small invertebrates such as mussels and clams are not yet back to their original population levels.
And local fishers, who lost more than U.S. $286 million after the herring fishery collapsed in 1989, are still waiting for the fishery to rebound.
Stake in the Heart
Solutions exist to clean up the oil, Temple University's Boufadel said: For instance, increasing water flow to the most isolated patches could help the oil-hungry microbes do their job.
But the focus now should be on how to prevent spills in the future, said Takahashi-Kelso, now executive vice president at the Ocean Conservancy.
"Twenty years out, the real value of what we've learned is what decisions we make and how wise we are in managing risk," he said.
For instance, the disaster inspired the 1990 Oil Pollution Act, which, among other measures, requires that by 2020 all oil tankers like the Exxon Valdez have double hulls—a layer of protection that may prevent oil spills.
But the U.S. government's recent plan to sell offshore oil-drilling leases is risky, especially when the region is already threatened by climate change, WWF's Williams said. (See how climate change is changing the Arctic.)
For instance, oil drilling could hurt Bristol Bay, "the little engine" of the Bering Sea that produces up to half of the United States' wild seafood, Williams said.
"To add new carbon dioxide emissions by developing petroleum resources," she said, "is driving another stake in the heart of America's Arctic."
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