‘Average’ dead zone predicted for Gulf in 2015, scientists say

By Mark Schleifstein, NOLA.com | The Times-Picayune
June 17, 2015

how dead zone forms

 This year’s low oxygen "dead zone" in the Gulf of Mexico along the Louisiana and Texas coast lines is likely to be "average" in size, but still as large as the state of Connecticut, according to a study by university and federal scientists, including researchers at Louisiana State University.

The dead zone is a name used to describe very low oxygen conditions that are created when nutrients from fertilizers on farmland and other uses, mostly in the Mid-West, flow down the Mississippi and Atchafalaya rivers into the Gulf each spring.

  This chart shows the size of dead zones since 1985 in square kilometers, in blue, and the predicted size of the 2015 dead zone, in red.   

The freshwater and nutrients form a layer above saltier Gulf water, and feeds huge blooms of algae, which eventually die and sink to the Gulf floor. There, they decompose and are eaten by bacteria, using up oxygen.

The low oxygen can reach levels below 2 parts per million, a condition called hypoxia that kills most creatures that live in and on bottom sediments, unless they’re able to swim away.

The scientists used four computer models to estimate the size, based on measurements by U.S. Geological Survey-operated stream gauges and nitrate sensors, which measured nitrogen and phosphorous compounds in water in the watersheds of the Mississippi and Atchafalaya rivers between October 2014 and May 2015.

The average size predicted by the four models was 5,483 square miles, still well above the goal of the federal-state Mississippi River/Gulf of Mexico Watershed Nutrient Task Force to reduce the low-oxygen zone to a five-year average of only 1,950 square miles by 2015. The average size since 1995 of the low-oxygen zone has been 5,911 square miles.

"The size of the annual Gulf of Mexico dead zone varies from year to year in response to changing weather patterns, primarily in the Corn Belt," said University of Michigan aquatic ecologist Don Scavia, a member of NOAA-funded teams that produce annual forecasts for the Gulf of Mexico, the Chesapeake Bay and Lake Erie.

"But the bottom line is that we will never reach the action plan’s goal of 1,950 square miles until more serious actions are taken to reduce the loss of Midwest fertilizers into the Mississippi River system, regardless of the weather," Scavia said in a news release announcing the study results.

The 2014 dead zone was 5,052 square miles.


The individual model predictions ranged from 4,344 square miles to 5,985 square miles. The annual forecast is sponsored by the National Oceanic and Atmospheric Administration and includes scientists from the U.S. Geological Survey, LSU, the Louisiana Universities Marine Consortium, University of Michigan, Virginia Institute of Marine Sciences/College of William and Mary, Texas A&M University and North Carolina State University.

Between July 28 and August 4, a research cruise headed by scientists with the Louisiana Universities Marine Consortium will take measurements of oxygen in the Gulf to estimate the actual size of this year’s low-oxygen zone.

The USGS estimated that 104,000 metric tons of nitrate and 19,300 metric tons of phosphorus flowed down the two rivers into the Gulf in May, which is about 21 percent below the long-term average for nitrogen and 16 percent above the long-term average for phosphorus, based on readings between 1980 and 2014.