The Oil Spill's Effects on Deep-Sea Ecology


The oil spill in the Gulf of Mexico falls into a distinct category from any other oil catastrophe; it's the first blowout in history to release oil in such deep waters, nearly a mile below the surface.

As a result, scientists say, the impacts of this spill are likely to go far beyond the oiled birds and dead sea turtles, spoiled beaches and wetlands that we think of when we think "oil spill." A substantial piece of the total impact is likely occurring under the sea, invisible (for now at least) but no less ominous than the more traditional shoreline effects. Far below the sea, the spill threatens organisms of all kinds and, indirectly, the ecosystem at large, though the extent of the danger is still obscured.

Oil on the surface of the ocean is a known quantity, says Ed Overton, an oil-spill expert at the Louisiana State University who is analyzing water, sediment, and other samples for NOAA's scientific-support team. "It's going to cause very substantial and noticeable damage—but it won't take very long to find the marsh loss and coastal erosion and impact on fisheries," he says. The effects of oil in the water column and at the sea floor, on the other hand, remains a mystery.

The first scientific mission to assess deepwater impacts of the Deepwater Horizon blowout, conducted from the research ship Pelican and funded by NOAA, discovered massive plumes of dispersed oil up to 30 miles long by seven miles wide and hundreds of feet thick. Though the data collected by the Pelican was criticized by NOAA as being too preliminary to draw conclusions from, scientists say the finding is not surprising and is in line with the results of previous studies.

One such study, a 2003 report by the National Research Council, considered what the effects of a deepwater well blowout might be and predicted that such an event, particularly of a reservoir rich in gas (as the Deepwater Horizon reservoir appears to be) would generate diffuse underwater plumes of microaerosolized oil much like what the Pelican scientists found.

A few years earlier, the U.S. Minerals Management Service (MMS) organized a study in 2000 in which scientists released oil into deep seas off the coast of Norway, but could only account for a small amount of it on the surface—suggesting that much of it remained in the water column. (While the conditions of this study aren't identical to the conditions of the current spill, Overton says the general findings could be expected to apply).

Conventional wisdom suggests that oil is lighter than water and therefore floats, but that's not entirely the case when a complex mixture of crude oil and natural gas is gushing from a well a mile below the surface, at high temperature and pressure, as is happening right now in the gulf. In this case, the gas can effervesce out of the oil, aerosolizing it into tiny droplets, much the way a fine mist emerges from the top of an aerosol can. Some of these droplets may be neutrally buoyant, meaning they move to a point in the water column where they neither rise nor sink, possibly resulting in underwater "plumes" like the ones reported. Adding subsea dispersants, which similarly break up the oil and are intended to prevent it from reaching the surface, may exacerbate this and could have toxic effects themselves.

A major impact of subsea oil plumes is that they lead to a bloom in oil-chomping microbes. These bugs eat the oil, but use oxygen in the process—meaning that oxygen levels in the water can drop rapidly and threaten the organisms living there. Samantha Joye, one of the principal investigators for the Pelican mission, says her team found that water within the plumes was 30 percent less oxygenated than normal. That's not enough of a drop to suffocate organisms—but she worries that it could get there relatively soon.

There is plentiful life in the deep sea that's in danger: fish, deep-sea corals, gelatinous zooplankton like jellyfish, and benthic-dwelling sharks, not to mention the diverse communites of shrimp, crabs, worms, and other critters that live near natural methane seeps. "It's like a lush jungle down there," Joye says. Even if oil exposure doesn't kill these organisms, it could have chronic, long-term effects, like impaired growth or reproduction.

Over time, any impact on the deep-sea communities is likely to have more broad effects, since the whole ocean is connected by various biological processes. "All the different zones of life are interactive in one way or another," says Lisa Levin, a marine ecologist at the Scripps Institution of Oceanography.

And any oil in the deep-sea environment could persist for a long time. The majority of oil on the surface evaporates, washes up on shore, or is degraded by natural weathering and oil-eating microbes. In the deep sea, on the other hand, it's dark and still, meaning no weathering and no evaporation. Microbial degradation is pretty much the only mitigating process—but it's slow. As a result, there's some possibility that deep-sea oil could get churned up by storms and have a limited shoreline impact sometime in the future, Joye says.

It could take years to find out the extent of the oil's subsea impact, but the scientists interviewed for this article stressed the importance of beginning the search immediately, even before the gushing well is capped. "If you don't look you won't find," says Rick Steiner, a marine biologist who worked on the Exxon Valdez spill back in 1989. "Hats off to the Pelican for doing what they could out there, but they might have sampled 1 percent or less of the total volume of the impact." Many other questions about the plumes remain, Joye says, including what's happening inside them, how are they moving, whether they're growing or shrinking, and if there are more of them.

It's also essential to get an accurate measure of the amount of oil being released, as this would allow scientists to deduce how much oil could be hiding below the surface based on the size of the oil slick and estimates of other factors like evaporation.

On these points, the scientific community has been increasingly critical of the official response to the spill, alleging that both the government and BP have resisted entreaties to either investigate the spill's magnitude and subsea impacts themselves or to allow independent scientists to do so.

"These deepwater effects are not going to mess the beaches up, and they're not going to have an immediate impact on the shrimp fishery, but they could have long-standing impacts," Joye says. These hidden impacts—and the way they are handled—could one day be considered the Deepwater Horizon's legacy.