Gulf oil spill: Oxygen dropped near oil plumes

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Oxygen levels fell significantly in deep-sea areas of the Gulf of Mexico contaminated by oil plumes from the BP spill.

But although researchers found a 20% decline in dissolved oxygen, the drop was not steep enough to create biological ‘dead zones’ that some scientists feared might form in the wake of the BP disaster.

Moreover, the oxygen levels seem to have stabilized.

‘We are not seeing a continued downward trend over time,” said Steve Murawski, chief scientist for fisheries in the National Oceanic and Atmospheric Administration, which released the findings Tuesday. “None of the dissolved oxygen readings have approached the levels associated with a dead zone, and as the oil continues to diffuse and degrade, hypoxia becomes less of a threat.”

Researchers believe the oxygen drop is a result of a rise in oil-munching microbes that feasted on the subsurface plumes and consumed oxygen in the process.

The oxygen drop was detected within 60 miles of BP’s blown-out wellhead at water depths of 3,300 to 4,300 feet -- a water layer that is normally relatively rich in oxygen. The natural mixing of water from surrounding areas with the depleted oxygen layer is helping counter the decline, the report suggests.
The findings were based on data collected at 419 locations sampled by nine research ships between early May and early August.

Before the well was capped in mid-July, it released more than 200 million gallons of oil, most of it into gulf waters nearly a mile deep. Much of the oil rose to the surface, but cloud-like plumes of tiny oil droplets also drifted at depths of several thousand feet.

In August, scientists from the Woods Hole Oceanographic Institution published a paper in which they documented a plume at least 22 miles long and 1.2 miles wide that was detected during a June research cruise.

The Woods Hole crew also did not find any dead zones -- where oxygen levels are so low they can’t support marine life -- associated with the plume. But the team said it was possible the oil layer could persist for some time.

-- Bettina Boxall