Gulf oxygen levels are lower, but not deadly, in wake of spill, researchers report

By Mark Schleifstein, The Times-Picayune
Wednesday, September 08, 2010, 6:00 AM The biodegradation of oil in plumes within 60 miles of the failed BP Macondo oil well have caused levels of dissolved oxygen in deep water of the Gulf of Mexico to drop by as much as 20 percent, but no oxygen-void dead zones have been created, a panel of scientists concluded in a report released Tuesday. “To date, the decrease in oxygen has not been significant enough to cause hypoxia at depth — that is, a dead zone — nor is it likely to, going forward,” said Steve Murawski, chief science adviser for the National Oceanic and Atmospheric Administration and lead scientist for the oil spill Unified Command’s Joint Analysis Group, which authored the report. The measurements were made in a layer of water between 3,300 feet and 4,300 feet deep, where scientists have found plumes of tiny oil and dispersant droplets, and the byproducts of their breakdown by microbes. The lowest levels measured were about 3.7 parts per million. A dead zone is an area of water considered to be so low in oxygen that oxygen-dependent organisms will die. That level — called hypoxia — is generally set at 2 parts per million or less of oxygen dissolved in water. Such levels are routinely found in much shallower water off of the shores of Louisiana and Texas each spring and summer, a result of algal blooms caused by fertilizer runoff. “In most cases, most forms of life cannot exist in these low-oxygenated waters,” Murawski said. “That’s why there’s considerable concern about this, because at those depths, you’ve got a biological community that doesn’t grow very fast and doesn’t reproduce at a very high rate.” The new report is the third released by the Joint Analysis Group. It was based on data collected between the first week of May and August 9 at 419 sampling sites by nine research vessels: the NOAA ships Gordon Gunter, Henry Bigelow, Nancy Foster, and Thomas Jefferson, and the research vessels Brooks McCall, Ferrel, Jack Fitz, Ocean Veritas, and Walton Smith. The measurements indicate that microbes are feasting on and biodegrading the oil, a process that uses up oxygen in the water. But the lost oxygen is being partially replenished by the mixing of water containing more oxygen from areas around and beneath the plumes, Murawski said. …