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.”