THE annual voyage led by Louisiana Universities Marine Consortium executive director Dr. Nancy Rabalais to measure the Gulf hypoxia "dead zone" took place in the last two weeks.
The official measurement, 5,840 square miles — equivalent to the size of Connecticut — was smaller than originally predicted.
Prior to the July 21-28 survey cruise, National Oceanic & Atmospheric Administration (NOAA)-supported scientists predicted this year's dead zone to be the 10th largest. Models developed by Dr. Donald Scavia at the University of Michigan and Dr. R. Eugene Turner at Louisiana State University estimated the size to be between 7,286 and 8,561 square miles.
"A near-record area was expected because of wet spring conditions in the Mississippi (River) watershed and the resultant high river flows, which deliver large amounts of nutrients, but nature's wind-mixing events and winds forcing the mass of low-oxygen water towards the east resulted in a slightly above-average bottom footprint," Rabalais said.
In contrast, the size of the dead zone last year was the fourth smallest as a result of the drought in the Mississippi River region.
Hypoxia, or low oxygen, can be caused by numerous factors, such as excess nutrient pollution. Weather factors, including wind speed, wind direction, precipitation and temperature, also affect the size of dead zones.
Nutrient runoff in the watershed stimulates algae growth. When the algae die and sink, bottom-dwelling bacteria decompose the organic matter, consuming oxygen. As a result, a low-oxygen or no-oxygen region forms, commonly known as the dead zone because the dangerously low oxygen levels are unable to support bottom-dwelling marine life. Fish will avoid dead zone areas and adapt by relocating farther out into open water.
As a non-point source, nutrient runoff from agricultural fields is often named as a culprit when scientists discuss the cause of the dead zone. Nutrients can also come from residential lawns, urban stormwater, discharge from sewage treatment plants and the combustion of fossil fuels.
The fact remains that nutrients are transported to the Gulf of Mexico through tiles, drainage ditches, creeks, streams and rivers. The Mississippi River, the largest river in the U.S., drains 40% of U.S. lands and provides 90% of the freshwater runoff into the Gulf of Mexico.
Surface and subsurface drainage is a necessary part of growing crops in the Mississippi River Basin. The roots of most crops cannot tolerate excessively wet conditions for more than a couple of days. Many areas in the Midwest would not consistently produce a crop if artificial drainage systems were not installed.
The Gulf dead zone has been routinely measured since 1985. The dead zone normally peaks in July or August and starts to break up in the fall.
There are 400 hypoxic zones in the world, and the Gulf of Mexico hypoxic zone is the largest.
Dr. Paul Montagna, professor and chair of the Harte Research Institute for Gulf of Mexico Studies (HRI), has studied dead zones for more than 20 years.
"The Gulf of Mexico may be far from the corn fields of the Midwest, but it's those fields that are causing a big problem for the Gulf Coast water this year. NOAA predicts a record-size dead zone in the Gulf this summer, stretching from south Texas all the way to Alabama," HRI said in a July 9 press release.
"The hypoxia zones are not dangerous to fish but cannot support bottom-dwelling life such as clams, crabs and shrimp," Montagna said. "Because fish avoid these areas, commercial shrimp boats and recreational fishermen must go farther out, to open water, to make their catch."
According to Dr. Larry King, executive director of HRI, ethanol is the singular cause of this year's dead zone.
"Corn prices are high right now, so farmers are planting more of it. While you can grow many crops without fertilizer, corn requires it. The U.S. Department of Agriculture estimates that as much as 40% of last year's corn crop was used to make ethanol," McKinney explained.
Still, McKinney noted that USDA's 40% estimate could not be documented. According to the National Corn Growers Assn., in 2012, the largest user of corn was livestock feed, taking 39.5%, followed by ethanol at 30.8%.