The oil gusher on the Gulf seabed may be stopped, but much of the spilled oil still lurks in a plume of oil and dissolved methane gas 3,200-4,300 feet below the surface.
New research predicts that this plume will likely create a low-oxygen "dead zone" inhospitable to life in these deep waters, as microbes consume the oil and gas entrained in the plume.
The cold temperatures in the plume will slow the growth of the microbes compared to microbes acting at the surface. Because of this slower growth, the team predicts that it will be sometime in the fall before the oxygen levels hit their minimum.
But weak currents at that depth mean that the low oxygen levels may persist for a long time, with little mixing to bring in oxygenated waters: the team estimates it will be a couple of years before the oxygen levels return to normal.
The findings, now in press at the journal Geophysical Research Letters, predict that the zone will be similar in size to the well-known seasonal dead zone at the mouth of the Mississippi River caused by nutrient runoff upriver, though the new zone will remain within about 62 miles (100 kilometers) of the spill site.
"The area of the seasonal dead zone on the shelf is much larger, but it's much thinner," said study author Robert Hallberg of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, NJ. "This one is a smaller area, but it's thick. It's still a small area compared to the Gulf of Mexico."
Many researchers have been speculating that the deepwater plume would create a dead zone as microbes ate the undersea oil and gas, consuming oxygen in the process. The new work combines the best estimates for how much oil was released with detailed models of ocean currents and information about typical microbial oil degradation rates to show that the conditions in the Gulf should, indeed, produce one.
Unlike the seasonal dead zone, which occurs in shallow waters, the oil-caused zone will be deep enough that fisheries shouldn't be affected. "I'm not expecting that this is going to have really dire consequences for people," Hallberg said. "There may be consequences for deep ecosystems."
"It's almost a separate ocean down there," agreed Monty Graham of the Dauphin Island Sea Lab in Alabama, who observed oxygen depletion in shallower waters following the spill. "That doesn't mean it's not important for diversity. We don't know the impact on the deep-diving marine mammals that might be going down searching for food."
While this study predicts oxygen depletion near the spill site, plume measurements made in June and published in Science last week showed very little oxygen consumption by microbial oil degradation.
"On the face of it, it might appear to be in contradiction to what we are saying," Hallberg said. "but if you take into account the temperature, the oxygen depletion should be peaking in the fall. Those observations from June may have been taken too soon and too close to the source so that the oxygen drawdown may not have been realized yet."
Another possibility, Graham said, is that the oil degradation rate estimates that the team used in their simulations may not match those of the actual microbial community in the plume. "It could be a time lag difference that the community of microbes hasn't established itself and therefore it's not operating at its maximum efficiency."
The study also provided information about the spill's toxicity by estimating the concentrations of various oil components in the deepwater plume over time.
"We weren't finding widespread concentrations of the oil that would be acutely toxic at the level that kills organisms over a couple of days," Hallberg said.
"I am less worried about the hypoxia than I am about the potential chronic effects of these hydrocarbons on the organisms and larvae," said Nancy Kinner, head of the Center for Spills in Environment at the University of New Hampshire in Durham.
"What we really don't know is if you have a low dose — a low concentration in the water for a long exposure — what's the chronic impact going to be?"
Hallberg agreed. "We know that compounds like toluene and benzene are regulated as known human carcinogens. You could imagine that there could be something similar for marine organisms, or something that affects development, or something that doesn't actually kill them. That's something where I think there's going to be a lot of research in the coming year."