The "Dead Zone" is an area in the Gulf of Mexico where seasonal oxygen levels drop too low to support most life in bottom and near-bottom waters. It is caused when phytoplankton growth, stimulated by nutrients such as nitrogen and phosphorus from the Mississippi and Atchafalaya rivers, settles and decays in the bottom waters. The decomposition of these algae consumes oxygen faster than it can be replenished from the surface, leading to decreased levels of dissolved oxygen.
Hypoxia began to develop early in 2008 with above average Mississippi River discharge in February and near maximum flooding in April. The recent June floods in the mid West just aggravated an already worsening situation offshore. "Low oxygen conditions were present off Terrebonne and Barataria Bays since March, and continued to increase through the spring and summer," reported Nancy Rabalais of Louisiana Universities Marine Consortium.
This year’s area of low oxygen was predicted by Dr. R. Eugene Turner of Louisiana State University to be the largest to date (8,800 square miles) based on the flux of nitrate-nitrogen from the Mississippi River in the May preceding the July mapping cruise. Another forecast by Dr. Donald Scavia of the University of Michigan was to fall between 8,300 and 8,700 square miles. Both forecasts are driven by the high nitrate loads from the Mississippi and Atchafalaya rivers. The nitrogen loading to the Gulf of Mexico in May of this year was 37% higher than 2007 and the highest since measurements began in 1970. "The intensive farming of more land, including crops used for biofuels, has definitely contributed to this high nitrogen loading rate," reported Turner.
Tropical storms and hurricanes have the
potential to disrupt hypoxia and aerate the bottom waters, which is
exactly what Hurricane Dolly did as it crossed the Gulf of Mexico from
Yucatan to Brownsville, Texas on July 21-23. "Hurricane Dolly’s winds
and waves caused regeneration of parts of the Dead Zone, especially along
its western and shoreward edges," said Dr. Rabalais. "If it were not
for Hurricane Dolly, the size of the Dead Zone would have been
substantially larger." Still, "an amazingly large area of hypoxia
persisted" despite the mixing from the hurricane. A similar large area
was mapped during groundfish surveys by the National Marine Fisheries
Service during June 11 – July 16, 2008.
The 12-member research
team of scientists and graduate students from the Louisiana
Universities Marine Consortium (LUMCON), Louisiana State University
(LSU) and the University of Iowa mapped the ever-present ‘Dead Zone’
along with conducting scientific experiments and collecting data for
several research and modeling programs. Funding for the research
program is provided by the NOAA Center for Sponsored Coastal Ocean
Research, Coastal Ocean Program, Grant No. NA06NOS4780197 to Louisiana
Universities Marine Consortium and Louisiana State University.
The area of low oxygen expands over 20,720 square kilometers or 8,000 square miles of Gulf of Mexico seabed, similar to last year’s size, but slightly smaller than the prediction of a record size of 8,800 square miles. The 2008 size ranks 2nd in size, along with that of 2001, for the area of hypoxia since mapping began in 1985.
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