The 1989 Exxon Valdez disaster is a bad gift that keeps on giving. A new study finds that oil from the spill still lingers throughout the tidal zone of the region in southeastern Alaska first affected by the spill--an area frequented by marine birds and mammals. The findings could help explain why sea otters and ducks in the initially hardest hit area of Alaska's Prince William Sound have recovered more slowly than other nearby populations.
The Knight Island region of Prince William Sound was one of the first areas to be inundated with oil when the Exxon Valdez began to leak 11 million gallons into the water. In the immediate aftermath, thousands of sea otters, seals, birds and other marine species died. Valdez oil continued to be found in the area over the next decade, but most researchers believed it was confined mainly to the high tidal regions, where the shore is only covered with water at high tide. This comforted scientists concerned about sea otters and marine ducks, which spend much of their time lower on the shore, foraging for food by digging pits in the wet sand. In 2001, however, a report conducted by environmental chemist Jeff Short of the National Marine Fisheries Service in Alaska had some disturbing news: The middle tidal shore seemed to be harboring a substantial amount of oil below the surface.
Short and colleagues confirmed the results by surveying 32 randomly selected shorelines along the northern Knight Island region in 2003. This time, they looked for oil throughout the tidal zone, checking for it both on the shore's surface and in pits dug half a meter deep. The researchers found oil in 59 of 662 quadrants assessed, with subsurface oil distributed over the lower, middle, and higher tidal zones. Finding substantial amounts of oil in the lower tidal zone is concerning, says Short, because that's where organisms spend time foraging. Estimating that an average sea otter digs three pits a day, it would come across oil about once every 2 months, the team reports online today in Environmental Science and Technology. These run-ins with oil are the most likely explanation for why sea otters and sea ducks haven't been reproducing successfully in the area. Says Short, "There are no other stressors out there that would come anywhere close to rivaling this one."
"This work is going to be useful for other oil spill sites," says marine chemist Chris Reddy of the Woods Hole Oceanographic Institution in Massachusetts, who believes knowing how oil lingers and deposits along shores will help future clean-up efforts. And marine ecologist Charles Peterson of the University of North Carolina in Chapel Hill notes that recent studies have shown elevated levels of detoxification enzymes in ducks and otters in the Knight Island area. The new study clears up this mystery, he says.