Survey Cruise Records Second-Largest "Dead Zone" in Gulf of Mexico Since Measurements Began in 1985
July 28, 2008
Map showing dead zone.
High resolution (credit: NOAA )
NOAA-supported scientists from the Louisiana Universities Marine Consortium found the size of this year’s Gulf of Mexico dead zone to be 7,988 square miles, slightly smaller than the predicted record size of 8,800 square miles and similar to the area measured in 2007. Scientists think Hurricane Dolly’s wind and waves may have added oxygen to the zone to reduce its size.
The research cruise, led by LUMCON’s Nancy Rabalais, PhD., found this year’s dead zone is the second largest on record since measurements began in 1985 and is larger than the land area of the state of Massachusetts. The average size of the dead zone over the past five years has been 6,600 square miles, much larger than the interagency Gulf of Mexico/Mississippi River Watershed Nutrient Task force goal of reducing the Zone to 2,000 square miles.
“The continuing presence of a large dead zone highlights the need to implement ways to reduce the amount of nutrients coming from the Mississippi River watershed which have contributed the dead zone growth in recent years,” said said Rob Magnien, director of NOAA's Center for Sponsored Coastal Ocean Research. “Reducing nutrient pollution to protect coastal resources is one of the greatest ecosystem management challenges that we face nation-wide.”
Average size of dead zone since 1985.
High resolution (credit: NOAA )
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. This low oxygen, or hypoxic, area is primarily caused by high nutrient levels, which stimulate an overgrowth of algae that sinks and decomposes. The decomposition process in turn depletes dissolved oxygen in the water. The dead zone is of particular concern because it threatens valuable commercial and recreational Gulf fisheries.
Earlier this summer, a NOAA-sponsored forecast model developed by Dr. R. Eugene Turner of Louisiana State University, predicted a record-breaking dead zone area of 8,800 square miles. A similar NOAA-sponsored forecast by Donald Scavia, PhD. of the University of Michigan, predicted a dead zone size of approximately 8,300 to 8,700 square miles. Both forecasts were driven primarily by the high nitrate loads and high freshwater flows from the Mississippi and Atchafalaya rivers in spring 2008.
Rabalais and Turner conclude that the difference between predicted and observed areas was due to weather conditions associated with Hurricane Dolly that partially disrupted the dead zone that earlier cruises had already shown to extend well into waters off of Texas.
"Hurricane Dolly's winds and waves caused reaeration of parts of the dead zone, especially along its western and shoreward edges, just before measurements were taken." said Rabalais. "Had the cruise been a few days earlier the measured extent would have been substantially larger."
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