The NOAA-Princeton team’s computer analyses also suggest why deep-sea plumes can hang around for months or longer. Currents at great depths, two-thirds of a mile below the surface, move far more slowly than those near shorelines or the surface. So don’t expect deep hydrocarbon plumes to swoosh rapidly out into the Atlantic, Hallberg says. They’re more likely to slosh back and forth with the tides and in response to local eddies. Indeed, his group’s modeling data suggest “they will stay very much confined — within, say, about 100 kilometers of the spill site.”
He predicts that if the Woods Hole team resurveyed the plume site three to nine months from now, it would likely still find much of the oil there. By then, microbes may be dining on the hydrocarbons in earnest, locally drawing down oxygen levels. In the deep ocean, Hallberg notes, oxygen isn’t replenished quickly, so any losses tend to accumulate over time. "According to our simulations, these [very low oxygen] areas will be peaking in October," he says, potentially making some portions of the northern Gulf inhospitable to sea life.
The Woods Hole team wouldn’t speculate about how much of BP’s oil and methane has ended up in the plume they measured, or how many similar plumes might be snaking along the Gulf’s seafloor. But an August 17 report by the University of Georgia in Athens and Georgia Sea Grant attempts just that. The Georgia analysis estimates that between 70 and 79 percent of the BP oil is still in the water.
A panel of experts convened by the Georgia team calculated what share of subsurface BP oil has likely degraded and now estimates it could be just “8 percent of the total oil released into the water.”
Oil that has resisted dispersion and evaporation likely will “remain potentially harmful for decades,” MacDonald said at the congressional hearing, adding: “I expect the hydrocarbon imprint of the BP discharge will be detectable in the marine environment for the rest of my life.”