Dead Zone

This year’s severe drought, centered in the Midwest, cut runoff and nutrient loading into the Mississippi and the Gulf of Mexico, resulting in the smallest dead zone in the Gulf in several years.

Texas A&M Oceanography professor Steve DiMarco cruised more than 1,200 miles in the Gulf in mid-August to measure this year’s oxygen-starved area, finding that only about 1,500 square miles affected by hypoxia, mostly near the Mississippi River delta off the coast of Louisiana.

“We had to really hunt to find any hypoxia at all and Texas had none,” DiMarco said. “In all, we found about 1,580 square miles of hypoxia compared to about 3,400 square miles in August 2011. What has happened is that the drought has caused very little fresh-water runoff and nutrient load into the Gulf, and that means a smaller region for marine life to be impacted.”

Hypoxia is when oxygen levels in seawater drop to dangerously low levels, defined as concentrations less than 2 milligrams per liter, and persistent hypoxia can potentially result in fish kills and harm marine life, thereby creating a “dead zone” of life in that particular area.

A trend toward larger dead zones in recent years spurred concern for commercial and sport fisheries, as marine life either dies or moves away from the oxygen-starved waters.

This was the first time in several years that the size of the dead zone stayed below the target of 1,900 miles set by the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force.

The average size of the dead zone between 2005 and 2010 was 6,653 square miles, and the 2011 dead zone, fueled by a record runoff, was larger yet. The task force has been working to reduce nutrient loading.

“Despite fluctuations in the size of the dead zone due to short-term weather events, the fact remains that we have a large and persistent area of severely degraded habitat that is caused by excessive nutrient pollution draining into the Gulf,” said Robert Magnien, Ph.D., director of NOAA’s Center for Sponsored Coastal Ocean Research.

DiMarco said the size of the dead zone off coastal Louisiana has been routinely monitored for about 25 years. Previous research has also shown that nitrogen levels in the Gulf related to human activities have tripled over the past 50 years.

During the past five years, the dead zone has averaged about 5,700 square miles and has reached as high as 9,400 square miles.

The Mississippi is the largest river in the United States, draining 40 percent of the land area of the country, including vast areas of chemically fertilized farmlands. The river accounts for almost 90 percent of the freshwater runoff into the Gulf of Mexico.

“These findings confirm what we found in a trip to the Gulf back in June, and also what other researchers in Louisiana have discovered, so there is general agreement that the dead zone this year is a very, very small one.

“The changes we see year to year are extreme. For example, last year, record flooding of the Mississippi River and westerly winds in the Gulf led to a much larger hypoxic area, particularly earlier in the summer. "