OGJ Senior Staff Writer
HOUSTON, Sept. 7 -- Dissolved oxygen levels in the deepwater Gulf of Mexico dropped by about 20% from their long-term average in the area of the oil spill from the Macondo well, but federal and independent scientists found no evidence of any deepwater dead zones.
A dead zone is an area of very low dissolved oxygen that cannot support most life. Dissolved oxygen levels were measured within 60 miles of the wellhead in 3,300-4,300 ft of water. Some researchers had suggested application of chemical dispersants at the wellhead could cause low dissolved oxygen levels.
Scientists attributed the lower dissolved oxygen levels to microbes using oxygen to consume the oil from the oil spill. Agencies involved were the National Oceanic and Atmospheric Administration, the US Environmental Protection Agency, and the Office of Science and Technology Policy.
Dead zones are common in the near-shore waters of the western and northern gulf during June through August. Dead zones, also known as hypoxic areas, are defined in marine waters as areas in which dissolved oxygen concentrations are below 2 mg/l. The lowest dissolved oxygen measured was 3.5 mg/l, a NOAA news release said.
“While we saw a decrease in oxygen, we are not seeing a continued downward trend over time,” said Steve Murawski, NOAA’s chief scientist for fisheries. “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.”
He said the mixing of the Macondo-area water with other gulf water helped prevent the creation of hypoxia areas.
The report was based upon 419 locations sampled on multiple expeditions by nine ships during a 3-month period. The Joint Anallysis Group report does not specifically address the question of the rate of biodegradation of oil, which cannot be determined looking only at dissolved oxygen data, Murawski said.
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