* Oxygen sags 20 pct in Gulf of Mexico near BP spill
* Drop in oxygen did not create “dead zone” around well
* Explosive blowout at Macondo wellhead may have helped
By Deborah Zabarenko, Environment Correspondent
WASHINGTON, Sept 7 (Reuters) - Hungry microbes feasting on spilled BP oil caused a drop in oxygen levels in the Gulf of Mexico, but did not create a marine “dead zone” near the wellhead, U.S. scientists reported on Tuesday.
The amount of oxygen decreased by 20 percent from the long-term average in areas where oil from the broken BP Macondo wellhead was detected by government and independent observers, scientists from the National Oceanic and Atmospheric Administration told reporters.
“All the scientists working in the Gulf have been carefully watching dissolved oxygen levels because excess carbon in the system might lead to a dead zone,” said NOAA’s Steve Murawski. “While we saw a decrease in oxygen, we are not seeing a continued downward trend over time.”
Summer dead zones are common in shallower areas of the Gulf of Mexico, caused by run-off from farm chemicals flowing down the Mississippi River.
Dead zones have such low oxygen levels that most marine life — including commercial important fish and shellfish — cannot survive, and scientists feared the BP spill would create such a zone in deep water around the Macondo wellhead after the April 20 blowout at the Deepwater Horizon rig.
That did not happen, Murawski said, and at this point is unlikely. He said oxygen levels had hit a “sweet spot,” with microbes consuming enough of the dispersed oil to cause what he called a sag in oxygen, but not enough to cause a low-oxygen dead zone.
Part of the spilled BP oil was broken up into tiny particles by chemical dispersants, which made it more palatable to petroleum-eating microbes, Murawski said.
“Has it hit a sweet spot? Yes. Was it by design? Partly.”
The NOAA findings are in line with research presented by scientists from the Woods Hole Oceanographic Institution and Lawrence Berkeley National Laboratory.
Both of these institutions published articles in August noting the initial presence of an underwater plume of oil in the Gulf of Mexico and the disappearance of the oil when deep-water microbes efficiently ate it without giving off much oxygen.
The ruptured BP well had been secured with no threat of spewing crude. A cap on top of the failed blowout preventer at the Macondo well sealed in all oil flow since July 15, and on Friday, the company replaced the failed equipment with a new giant stack of valves and pipes.
Once the new blowout preventer is tested, BP can resume drilling a relief well that will bore into the Macondo well, pump in mud and cement to plug it for good.
Editing by Stacey Joyce