No ‘dead zone’ from BP oil

Deep sea mixing kept microbes from creating suffocating parcels of water in the Gulf

As aquatic microbes dine, they consume oxygen. When too many congregate at some temporary smorgasbord of goodies, they can use up so much oxygen that a so-called dead zone develops — water with too little oxygen to sustain fish, mammals or shellfish. On Sept. 7, federal scientists reported that despite the massive release of oil from the damaged BP well in the Gulf of Mexico, no such dead zone developed.

And that was as of August 9, three-and-a-half weeks after the well was capped. By that point, subsea plumes of oil were already disappearing, presumably due in large measure to their consumption by oil-eating bacteria.

The new data came from continuous monitoring, beginning on May 8, from nine vessels plying the Gulf. At a briefing for reporters, Steve Murawski, the chief science advisor for the National Oceanic and Atmospheric Administration’s Fisheries Service, noted concern had developed about the potential for oxygen depletion at great depth — 1 to 1.3 kilometers. That’s where diffuse plumes of oil had been wandering. So the largely federal National Incident Command arranged to have oxygen measured regularly, beginning around 0.8 kilometers from the wellhead and extending out another 95 kilometers or so.

Aquatic critters need about 1 milliliter of oxygen per liter of water. The long-term average value in the Gulf at the depths studied has been about 4.8 ml/l, according to Murawski. The minimum value seen after the spill — and it reflected a single sample out of more than 400 collected — was 2.6 ml/l. The second lowest value: 3 ml/l. The average of the depressed values measured was still a fairly robust 3.8 ml/l.

The reason values didn’t go lower, Murawski said, was that neighboring waters shared some of their oxygen. Computer modeling indicated that without this mixing, dead zones would indeed have developed.

There has been considerable confusion about the status of the Gulf’s roving deep water plumes of oil: Have they been eaten up, moved away or just become diluted? I asked Murawski what his data indicate.

While predicating his answer on the fact that the data set ended in early August, he said that at least as of then plumes still existed. Indeed, he said. “we’re tracking the dissolution” of those current-driven clouds of oil. As of a month ago, measured levels of the deep oily pollution already were quite low: at parts per billion — if not parts per trillion — concentrations. And, he added, concentrations tended to fall off with distance from the wellhead.

Janet Raloff

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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