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Gulf oxygen levels are lower, but not deadly, in wake of spill, researchers report

Published: Wednesday, September 08, 2010, 6:00 AM

The biodegradation of oil in plumes within 60 miles of the failed BP Macondo oil well have caused levels of dissolved oxygen in deep water of the Gulf of Mexico to drop by as much as 20 percent, but no oxygen-void dead zones have been created, a panel of scientists concluded in a report released Tuesday.

gulf_oil_microbes_science.JPGView full sizeMicrobes degrade oil, indicated by the circle of dashes, in the deepwater plume from the BP oil spill in the Gulf of Mexico, as documented in a study by Berkeley Lab researchers.

"To date, the decrease in oxygen has not been significant enough to cause hypoxia at depth -- that is, a dead zone -- nor is it likely to, going forward," said Steve Murawski, chief science adviser for the National Oceanic and Atmospheric Administration and lead scientist for the oil spill Unified Command's Joint Analysis Group, which authored the report.

The measurements were made in a layer of water between 3,300 feet and 4,300 feet deep, where scientists have found plumes of tiny oil and dispersant droplets, and the byproducts of their breakdown by microbes.

The lowest levels measured were about 3.7 parts per million. A dead zone is an area of water considered to be so low in oxygen that oxygen-dependent organisms will die. That level -- called hypoxia -- is generally set at 2 parts per million or less of oxygen dissolved in water.

Such levels are routinely found in much shallower water off of the shores of Louisiana and Texas each spring and summer, a result of algal blooms caused by fertilizer runoff.

"In most cases, most forms of life cannot exist in these low-oxygenated waters," Murawski said. "That's why there's considerable concern about this, because at those depths, you've got a biological community that doesn't grow very fast and doesn't reproduce at a very high rate."

The new report is the third released by the Joint Analysis Group. It was based on data collected between the first week of May and August 9 at 419 sampling sites by nine research vessels: the NOAA ships Gordon Gunter, Henry Bigelow, Nancy Foster and Thomas Jefferson and the research vessels Brooks McCall, Ferrel, Jack Fitz, Ocean Veritas and Walton Smith.

The measurements indicate that microbes are feasting on and biodegrading the oil, a process that uses up oxygen in the water. But the lost oxygen is being partially replenished by the mixing of water containing more oxygen from areas around and beneath the plumes, Murawski said.

He said computer modeling that assumed no mixing would occur indicated that the speed with which the molecules were degrading the oil could have caused the formation of dead zones.

"In the absence of being refreshed from high-oxygen water from the surrounding area, this region would have been depleted in a few weeks," he said.

Murawski said that while the results are good news, they do not represent a clean bill of health for Gulf waters.

"Because we're seeing the oxygen sag we're seeing (the drop in oxygen content), that means there's some level of degradation," he said.

"It's contingent on us to learn everything we can about this," Murawski said. "What are the potential ecological ramifications of this -- even at low levels -- to marine life that occupies these depths?"

He said the long-term effects of the remaining oil, dispersant and the chemicals remaining after they've been broken down by microbes will be monitored as part of the Unified Command's "natural resource damage assessment" process, which will end with recommendations on how to mitigate any damage.

"We continue to monitor the region," Murawski said. "We're tracking the dissolution of the plume. It's both being dispersed laterally by the ocean currents and it's also being degraded by microbes" and is now seen in even smaller particle sizes, "down to the parts-per-billion to parts-per-trillion level."

The most dangerous chemicals within the plumes would be compounds called polyaromatic hydrocarbons, which are poisonous, he said.

The study results seem to indicate at least limited success in the controversial decision to use dispersants at the wellhead, Murawski said. Several scientists and a number of environmental groups criticized the use of the chemicals, both because of their potential to create more toxic compounds when mixing with oil, and because some felt more oil could have been recovered from the surface without their use.

"The whole theory of using dispersants was that you would make the particles small enough that they could be readily consumed by bacteria, and that apparently is happening," he said.

But he said only a portion of the droplets were created by the dispersants. The explosive ejection of oil and natural gas at high speed from the wellhead also caused the oil to disperse in tiny particles that were neutrally buoyant, meaning their mass and weight keeps them from sinking or rising, and thus would continue to float in plumes in the Gulf's deeper water.

While the research was limited to a 60-mile area around the well, the majority of data was collected within 12 miles of the well because that's where oil was found at the time of several of the cruises.

Mark Schleifstein can be reached at mschleifstein@timespicayune.com or 504.826.3327.

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