No signs of ‘dead zones’ near gulf oil spill, scientists say

Oxygen levels fell significantly in deep-sea areas of the Gulf of Mexico contaminated by the BP oil spill, researchers said Tuesday, but not enough to create biological “dead zones” that cannot harbor marine life.

Scientists found a 20% decline in dissolved oxygen, not enough to create the lifeless zones that some biologists had feared might form around the oil plumes left by the disaster. Moreover, the water’s oxygen levels appear to have stabilized.

“We are not seeing a continued downward trend over time,” said Steve Murawski, chief scientist for fisheries at the National Oceanic and Atmospheric Administration, which released the findings. “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 lower oxygen levels were detected within 60 miles of BP’s blown-out wellhead at water depths of 3,300 to 4,300 feet — a layer that is normally relatively rich in oxygen. The natural mixing of water from surrounding areas with the depleted 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 BP well was capped in mid-July, it released more than 200 million gallons of oil, mostly 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 in Massachusetts 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 associated with the plume. But the team said it was possible the oil layer could persist for some time.

bettina.boxall@latimes.com