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Dead Zone Mapping Expedition Findings Released

Dead Zone Mapping Expedition Findings Released

July 30, 2013


The 2013 area of low oxygen, commonly known as the ‘Dead Zone,’ measured 15,120 square kilometers (= 5,800 square miles) in this summer’s mapping expedition. Based on the May nitrogen load from the Mississippi River, the area was predicted to be 18,900 to 22,200 square kilometers (7,300 to 8,600 square miles), depending on the model. While not one of the larger areas mapped since systematic surveys started in 1985 by LUMCON and LSU researchers, the size of this year’s zone of oxygen-depleted bottom-water is above the long-term average and above the average size of the last five years.

Hypoxia forms as a result of the nutrient-overloaded waters of the Mississippi River stimulating the excess growth of phytoplankton. Not all of the phytoplankton is consumed by higher levels of the food web, and it sinks to the seabed where bacteria decompose the remains and deplete the oxygen. The low oxygen forms in the lower half of a stratified water column (warmer, fresher water overlying cooler, saltier water), which keeps the plentiful oxygen in the surface waters from reaching into the lower layer and replenishing the oxygen depleted by the microbial activity.

The size, while large, was a result of mixed conditions on the southeastern part of the study area, and winds from the west pushing the hypoxic water mass towards the east and thus reducing the bottom area footprint. At stations where hypoxia was found, the values were extremely low and close to zero.

For further information, contact:
Dr. Nancy Rabalais, nrabalais@lumcon.edu, or Dr. Gene Turner, euturne@lsu.edu
Visit the Gulf Hypoxia web site at http://www.gulfhypoxia.net for maps, figures, additional graphics and more information concerning this summer’s research cruise, and previous cruises.