Washington, D.C. – Lower oxygen levels have been detected in areas of the Gulf of Mexico affected by the BP oil spill, but are not low enough to become “dead zones” incapable of supporting marine life, according to the U.S. Environmental Protection Agency (EPA). The federal agency released the report in conjunction with the National Oceanic and Atmospheric Administration and the Office of Science and Technology Policy.
Dissolved oxygen levels have dropped by about 20 per cent from their long-term average in areas where scientists previously reported the presence of subsurface oil. Researchers from agencies involved in the report attribute the lower levels to microbes that are using oxygen to consume the oil from the spill.
The dissolved oxygen levels, measured within 60 miles (96 km) of the well head, have stabilized and are not considered low enough to become dead zones. Dead zones are commonly observed in the near-shore waters of the western and northern Gulf of Mexico in summer, but not normally in the deep-water layer where the lowered oxygen areas in the study occurred. Dead zones are defined in marine waters as areas in which dissolved oxygen concentrations are below two milligrams per litre.
“All of 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 Steve Murawski, Chief Scientist for Fisheries and head of the joint analysis group. “While we saw a decrease in oxygen, we are not seeing a continued downward trend over time. 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.”
While the report does not specifically address the rate of biodegradation of oil, it references a recently-published peer-reviewed study that found that the half-lives of some components of the oil were in the range of 1.2 to 6.1 days. This suggests that the light components of the oil are being rapidly degraded by microbes.