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Home > Current News > 2004

News: 2004

Average Size "Dead Zone," In Anything But An Average Year
Posted: 7/26/04

Image: Map of Area of Bottom Hypoxia

The coast wide extent of the Louisiana "dead zone" mapped this week is slightly larger than average at 15,040 km2 (or 5,800 square miles), according to officials at the Louisiana Universities Marine Consortium. Through it, scientists have been studying the dead zone annually for many years. The “dead zone” is a low oxygen area in the Gulf of Mexico along the Louisiana coast. The long-term average since mapping began in 1985 is 13,000 km2 (or 5,000 square miles). The river flow and the offshore conditions prior to the mapping cruise this year, however, were abnormal, reminiscent of the Great Mississippi River Flood of 1993. The river in 2004 peaked in discharge several times in January, February, March, again in May, then persisted in a prolonged, above average flow in June and into July.

This year's low oxygen area extended from the Mississippi River delta almost to the Texas coast. The low oxygen bottom waters were very close to shore during this summer's mapping, as a result of north winds and onshore currents that pushed the zone towards the beach. Water depths affected were as shallow 12 feet and as deep as 100 feet, but mostly within the 70-foot contour. The close proximity of the low oxygen close to shore precluded the presence of shrimp trawlers from that area.

Freshwater from the Mississippi and Atchafalaya rivers was distributed far across the Louisiana coast, accompanied by massive algal blooms. These massive algal blooms are not toxic, but noxious in that they create scummy water at the surface and the organic matter that sinks to the Gulf bottom leads to the depletion of oxygen there. The algal blooms were not the same as the noxious ones in May along the southeastern Louisiana coast that created problems for fishers.

The scientific word for the commonly named “dead zone” is “hypoxia” or low oxygen. It was coined by fishers to describe the failure to capture fish, shrimp, and crabs with bottom-dragging trawls when the oxygen falls below the critical level of 2 parts per million (ppm) in bottom waters. Higher in the water column, however, there is sufficient oxygen to support sizeable numbers of fish, and they often seek refuge there from the low oxygen.

The seasonal formation and persistence of hypoxia are influenced by the discharges of the Mississippi and Atchafalaya rivers. The fresh water forms a fresher layer above the saltier Gulf waters. Nutrients in the water stimulate the growth of microscopic plants, phytoplankton. These are either transferred up the food web, which supports valuable commercial fisheries, or end up as organic debris on the sea floor. The decomposition of the organic debris depletes oxygen in the lower waters until the conditions no longer sustain the life of most marine animals there.

This year's mapping of the dead zone is the 20th anniversary of the systematic survey of the low oxygen that began in 1985 under the direction of Dr. Don Boesch, then director of the Louisiana Universities Marine Consortium (LUMCON), with initial funding from the National Oceanic and Atmospheric Administration (NOAA). Since then, LUMCON and LSU collaborators under the direction of Dr. Nancy Rabalais have maintained the mapping of the low oxygen each summer with funding primarily from NOAA. The scientific party that mapped this year's hypoxic zone was from LUMCON, Louisiana State University, Texas A&M University Galveston, and the University of Scranton and was funded by NOAA’s National Ocean Service, Coastal Ocean Program. The mapping was conducted from July 21-25, 2004, from aboard the research vessel, Pelican. For further information contact Nancy Rabalais, LUMCON, 985-851-2836, nrabalais@lumcon.edu.

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