Poisonous hydrocarbons spiked near seafloor

By Janet Raloff

Web edition: November 1, 2010

In the early weeks after the damaged BP well began gushing huge quantities of oil and gas, attention focused on surface slicks and tar balls. Meanwhile, some 1,100 to 1,500 meters below, a toxic brew was developing within plumes emanating from the wellhead. While finned fish and marine mammals probably steered well clear of the spewing hydrocarbons, planktonic young — larval critters and algae that ride the currents — would have been proverbial sitting ducks.

Indeed, they were probably hammered, a new study suggests.

Researchers at five Gulf Coast universities mapped deep-sea concentrations of polycyclic aromatic hydrocarbons shortly after the spill began. They focused on the compounds “because we knew PAHs were going to be critical to understanding the ecological impacts of this event.,” explains Steven Lohrenz of the University of Southern Mississippi at the Stennis Space Center. After all, he notes, “These are some of the most toxic components in oil.”

His group tallied concentrations of naphthalenes, fluorenes, phenanthrenes and anthracenes, dibenzothiophenes, fluoranthenes and chyrsenes. By three weeks into the accident, total-PAH concentrations had spiked to about 190 parts per billion in the near-seafloor plume they were tracking — one that extended out at least 13 kilometers from the wellhead.

These researchers have just reported their findings online, ahead of print, in Geophysical Research Letters.

Explains Lohrenz, “190 [ppb] is considered pretty toxic” and would be expected to pose a risk to small organisms, especially larvae that “are rather passive in their movement.” In fact, the biological oceanographer notes, these larvae “might even be entrained by the same physics and currents that move the oil.” So if they were in the path of the plume, he says, “they may have been just out of luck.”

PAHs that proved especially high in the plume — particularly the smaller, two-ringed molecules like napthalenes — tend to be broken down pretty rapidly by bacteria in the water. “Like on the order of days,” he says. And that seems to have occurred in the Gulf, he observes, as later tracking of formerly oil-rich subsea plumes “showed that the signals [of these PAHs] had really dissipated.”

Although the university scientists had known the BP oil would contain the PAHs, “we weren’t sure, quite honestly, that we would see anything substantial,” Lohrenz concedes. In fact, his team uncovered a PAH bonanza — both in terms of the pollutants’ concentrations and their persistence over time and space.