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Geography in the News: The Gulf of Mexico’s Dead Zone

By Neal Lineback and Mandy Lineback Gritzner, Geography in the NewsTM

The Gulf’s Growing Dead Zone

With rising demand over the past decade for the corn-based fuel additive ethanol, American farmers have grown more corn than at any time since World War II. Unfortunately, the nitrogen fertilizer being applied to cornfields is contributing to a growing “dead zone” in the Gulf of Mexico, a serious blow to the fisheries of the northern Gulf.

The size of the Gulf dead zone this summer is predicted by NOAA to be a 7,286 square mile (15,125 sq. km) area of the Gulf paralleling the Louisiana shoreline. This is an area larger than the state of Connecticut and has historically been prime fishing grounds off Louisiana’s shore. This dead zone typically is so depleted of oxygen that shellfish, crabs and shrimp suffocate, leaving it virtually devoid of life.

gitn_923_Gulf Dead Zone
Map by Geography in the News and Maps.com
Boundaries and names shown do not necessarily reflect the map policy of the National Geographic Society.

America’s corn farmers apply nitrogen-based fertilizer to their fields annually. When that nitrogen runs off fields in the Corn Belt states (primarily Iowa, Indiana, Illinois and Ohio, but other states as well), surface water carries it to the Mississippi River and into the Gulf of Mexico.

Nitrogen is a wonderful plant nutrient, resulting in quick greening and rapid growth when applied to crops or pasture. Corn responds particularly well to high amounts of nitrogen. However, large amounts of this nutrient also stimulate rapid algae and phytoplankton growth in both fresh and salt water. As these aquatic blooms die, they sink and decompose. This decaying material creates carbon dioxide, which robs the deeper water of its oxygen.

The weak westward circulation and stratification of the water during the summer in the northern Gulf of Mexico prevents the deepest water from becoming re-oxygenated. The scientific term for the severely depleted levels of oxygen in water bodies is hypoxia.

When hypoxia occurs, oxygen levels fall below two parts per million, a level at which most marine life cannot survive. Shellfish and other slow-moving animals die in place, while finfish may be able to flee to more oxygen-rich waters. The Gulf’s dead zone is now one of the largest hypoxic zones of water in the world and growing in size.

With the average concentration of nitrate-nitrogen in the main stem of the Mississippi River more than doubling since 1950, commercial fertilizer is considered the prime cause of the Gulf’s dead zone. The use of nitrogen fertilizer in the Mississippi Basin in 2008 was estimated to be about 7.7 million tons (7 million metric tons) per year and accounted for more than one-half of the annual nitrogen input to the river. Other sources are animal manure, municipal and domestic waste, and atmospheric deposition from power plants and vehicles.

The Gulf of Mexico’s dead zone typically begins in the spring and persists into the summer. Its size and location vary each year depending on currents, weather and other factors, but it is generally near and just west of the mouth of the Mississippi River.

Scientists working on the dead zone issue in the Gulf of Mexico had hoped to reduce nitrogen runoff by encouraging farmers to apply less fertilizer and establish buffers along waterways. But the demand for ethanol drove up the price for corn. The increased demand enticed farmers in the Corn Belt to plant more than 93 million acres (37.6 million hectares) of corn in 2007, an estimated 10 per cent increase over 2006 and the most acreage planted in corn since 1944. And the acreages of both corn and soybeans continue to increase. The Environmental Protection Agency estimated that as recently as 2005 up to 210 million pounds (95,254 metric tons) of nitrogen fertilizer entered the Gulf of Mexico.

The Gulf of Mexico is but one of approximately 150 dead zones around the world. Irregular coastlines where water currents are weak are conducive to the development of dead zones, particularly if entering streams carry heavy loads of soluble nutrients. Almost all the dead zones of the world are exclusively located in the Northern Hemisphere. Circumstantial evidence supports the conclusion that the higher levels of economic development north of the equator play a distinct role in the geographic pattern.

Dead zones are not irreversible. The Black Sea dead zone, previously the largest dead zone in the world, largely disappeared between 1991 and 2001. Fertilizers became too costly following the collapse of the Soviet Union and subsequent economic problems in Eastern and Central Europe. Fishing has once again become a major economic activity in the region.

In the United States, market pressure to grow corn is currently very high. The United States will likely need a comprehensive multi-state or federal approach to solve the problem of the Gulf’s dead zone.

And that is Geography in the News.

Sources: GITN #923, “The Gulf’s Dead Zone,” Maps.com, Feb. 8, 2008; GITN #493, “Fishing Empty Waters,” Maps.com, Nov. 2, 1999; Associated Press, “Midwest corn boom threatening Gulf of Mexico,” Winston-Salem Journal, Dec. 18, 2007; and http://www.sierraclub.org/cleanwater/waterquality/deadzone.asp

Co-authors are Neal Lineback, Appalachian State University Professor Emeritus of Geography, and Geographer Mandy Lineback Gritzner. University News Director Jane Nicholson serves as technical editor. Geography in the NewsTM  is solely owned and operated by Neal Lineback for the purpose of providing geographic education to readers worldwide.