WASHINGTON — The amount of oxygen in the Gulf of Mexico near oil plumes created by the BP well blowout is 20 percent lower than normal, but not so low as to create dead zones where little life exists, federal scientists reported Tuesday.
The drop in oxygen levels is caused by microbes that use oxygen as they consume oil that's drifting 3,300 feet or more below the Gulf's surface, the scientists reported.
Dissolved oxygen levels would have to drop by an additional 70 percent to create dead zones, and that's unlikely, said Steve Murawski, the National Oceanic and Atmospheric Administration's chief scientist for fisheries and the head of the group of federal scientists who analyzed data and produced the report.
Dead zones are less of a threat as the oil degrades and spreads out, and as the oxygen-rich waters mix in from surrounding areas, he said.
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Still, the impact of the oil plume at such deep levels isn't fully understood, Murawski said.
"While it's a relief not to see deep dead zones as a result of the oil droplets that remain below the surface, there's lots of work to be done to understand their long-term impacts on the ecosystem," he said.
Scientists are continuing to track the plume, and NOAA has started what Murawski called a "ramped-up effort" to account for the oil and chemical dispersants.
The agency has been criticized for painting too rosy a picture with a report it issued during the summer that said as much as three-quarters of the oil that escaped into the Gulf between April 20 and July 15 had dissipated or been captured.
The latest report was based on samples taken in 419 locations at depths of 3,300 to 4,300 feet, from the wellhead to 60 miles out, where federal and independent scientists found the plumes. The samples were taken by researchers on four NOAA vessels and five academic vessels from May 8 to Aug. 9.
None of the dissolved oxygen readings approached the level of a dead zone, the scientists said.
"Dead zones are commonly observed in the near-shore waters of the western and northern Gulf of Mexico in the summer, but not normally in this deepwater layer and that's one of the reasons we're concerned about it," he said.
One of the world's largest dead zones, off Texas and Louisiana, is caused by pollution discharged from the Mississippi and Atchafalaya rivers.
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