Gulf of Mexico ‘dead zone’ larger than average, but no record

By Mark Schleifstein, The Times-Picayune
1 August 2011

The annual low-oxygen "dead zone" along the coasts of Louisiana and Texas covers 6,765 square miles, larger than average and bigger than the state of Connecticut, but is below the size predicted by scientists as a result of the record-breaking Mississippi River floodwaters entering the Gulf of Mexico this spring and summer, said Nancy Rabalais, chief scientist for the just-completed, annual Louisiana Universities Marine Consortium monitoring cruise.

Map showing the 2011 ‘dead zone’ in the Gulf of Mexico off the Louisiana and Texas coast, which is larger than the state of Connecticut.

"The major disruptor of the size wasTropical Storm Don that followed the Research Vessel Pelican across the Gulf of Mexico towards Texas and whipped up the winds and waves," Rabalais said in a news release issued overnight.

The record level measured since the annual cruises began in 1985 was just over 8,500 square miles in 2002, another year when flooding occurred along the Mississippi.

In June, scientists estimated this year’s dead zone would be between 8,500 square miles and 9,400 square miles, based on the record levels of floodwaters entering the Gulf from the Mississippi. 

Weather conditions caused by Tropical Storm Don may actually have resulted in an underestimate of the size of the low-oxygen area, the news release said, as measuring oxygen in the water level closest to bottom "on a ship rolling in 5 to 6 ft swells presented additiional sampling issues that interfered with precise measurements at some stations."

Other possible reasons for the lower-than-expected size of the hypoxia area were that more southerly and southwesterly winds in the weeks before the cruise may have pushed the low-oxygen water towards the east, and the Mississippi River floodwater flow slowed dramatically during July, the news release said.

The cruise sampled oxygen levels in a 25-mile to 60-mile wide area along the Gulf Coast from just west of the Mississippi River to just east of Galveston Bay between July 24 and Saturday.

The National Oceanic and Atmospheric Administration, sponsor of the cruise, will hold a news conference about its findings at noon today.

Low oxygen levels, below 2 parts per million of oxygen in water, is defined as hypoxia by scientists. Fish tend to avoid such areas, and the lack of oxygen can kill organisms that live on or in the sediment of the Gulf’s bottom.

The low oxygen was more severe and extensive at two lines of "stations" — points where the ship made measurements — off Terrebonne Bay and Atchafalaya Bay, the news release said. Those stations will be restudied in August. 

The team of scientists led by Rabalais, who is director of LUMCON, also mapped a second, extensive area of low oxygen to the east of the Mississippi River, stretching from Mobile Bay to the Chandeleur Islands, on July 20 and 21.


A second ‘dead zone’ was discovered in the Gulf of Mexico, east of the Mississippi River.

"It was similar in severity to the area just west of the Mississippi River," the news release said.

The low-oxygen water can occupy much of the water column, from the Gulf’s bottom to its surface, or just the lowest level of water.

It is formed when blooms of phytoplankton grow, die and sink to the bottom, where they decompose, using up oxygen in the water there.

The phytoplankton blooms are driven by nutrients — nitrogen and phosphorus — that pour into the Gulf from the Mississippi and Atchafalaya rivers, along with fresh water. Most of the nutrients are the result of fertilizer runoff from farmland in the Midwest, although some comes from sewage treatment plants and other sources in the huge watershed of the Mississippi.

The less-dense fresh water generally stays on the surface, creating a layer above the Gulf’s salt water that reduces the ability of oxygen at the surface reaching the bottom until the salt and fresh water are mixed by winds or storms, as in the case this year with Tropical Storm Don.

The LUMCON mid-summer cruises date back to 1985. Their findings have resulted in the formation of the federal-state Mississippi River/Gulf of Mexico Nutrient Task Force Hypoxia Action Plan, which has a goal of reducing the size of the low oxygen area to 5,000 kilometers, or 1,930 square miiles.