It's now been more than three and a half years since the Deepwater Horizon oil drilling rig leased to BP exploded, causing over 200 million gallons of crude oil to spill into the Gulf of Mexico, the largest offshore oil spill in U.S. history.
In terms of the national news cycle, that duration might seem like a lifetime. In terms of an ecosystem as enormous and complex as the Gulf, it's more like a blink of an eye.
"Oil doesn't go away for a very long time," says Dana Wetzel, a biochemist at Mote Marine Laboratory in Florida who's been sampling water, sediments and the tissues of animals living in the Gulf for evidence of persisting oil. "The assumption had been that in a higher temperature environment, bacteria are going to degrade things much more rapidly, and it'll degrade quicker." But in previous research, she's found that even in warm environments, oil residue persists much longer than experts previously thought—in the waters of Tampa Bay, for instance, she found oil a full eight years after a spill.
If you simply dunked a bucket into Gulf waters and tested for petroleum, she notes, you might not find any. But as part of an ongoing project, Mote researchers are employing innovative sampling mechanisms that use pieces of dialysis tubing, which trap oil residue much like a marine organism's tissue does as it filters water. Deployed in metal containers, the pieces tubing gradually filter water over time, collecting any contaminants present.
This oil can persist through a few different mechanisms. After coating sediments, the viscous substance can stick to them for years. There's also evidence that some oil was trapped in the sunken Deepwater Horizon rig itself and continues to slowly bubble upward, accounting for the visible sheens of oil occasionally seen on the water's surface.
Wetzel's team is also interested in seeing how this oil impacts the area's biological health. To do this, they're collecting tissue samples from a variety of fish and other marine animals over time. "We've been going out in ships to the location of the blowout every year and taking transects, so we can get both a temporal picture of what's happening over time and a spatial picture of how the oil is attenuating the farther you go away from the spill," she says. In liver, gonad and spleen tissue, they're tracking DNA damage; in bile, they're testing for metabolites that result from oil absorption; and in blood, they're examining the animals' reproductive health and immune system response.
With these metrics, they can calculate correlations between exposure to specific amounts of contaminants and various health impacts. Once potentially concerning trend the scientists are looking for is the presence of vitellogenin (a protein found in fish who are producing eggs) in male fish—potential evidence of endocrine disruption.
But, even if found, it'd be impossible to say for sure that these health measures are the result of the Deepwater Horizon spill. "We can't assume these particular contaminants are the only stressors the animal is experiencing," Wetzel says. Instead of correlation, they'd really like to go a step further and prove causation.
To do that, they're beginning to conduct controlled exposure studies, in which microorganisms, corals or fish in captivity are exposed to particular concentrations of oil and other contaminants over time. Earlier this year, they published some of the first research in this area, showing that two common Gulf species of coral are significantly less like to survive when exposed to either oil or the dispersant used to break up the the spill at environmentally-relevant concentrations. Next, they plan to conduct similar tests on full-sized adult fish.
This sort of environmental sampling, marine health analysis and controlled experimentation will be essential in helping scientists understand how the Deepwater Horizon spill continues to affect the Gulf ecosystem. But the sad truth is that—given the frequency of oil spills paired with efforts to expand offshore drilling to new areas, like the North Slope of Alaska—it could also be crucial in providing a picture of how future oil spills will impact diverse environments and organisms.