The Why Files The Why Files --

Bringing life to the dead zone POSTED 3 JULY 2008

Dead zone: Heading for a record year
As American farmers scramble to meet a surging global demand for food, the oxygen-deprived "dead zone" in the Gulf of Mexico seems destined to set a record this summer, largely due to expanded use of fertilizer in the Mississippi River watershed.

In other words, after decades of effort and even some government spending, the hypoxia (low oxygen) problem is getting worse. Much worse.

long the craggy coast of Louisiana, a large swath, extending miles, of dark blue, yellow and orange denotes area of dead zoneThe 2008 dead zone is likely to be even bigger than the 2007 version. Dead zone Action plan.

The zone’s average size (8,200 square kilometers between 1985 and 1992) has reached about 14,000 square kilometers since 2003 (see #1 in the bibliography).

This year, Louisiana experts predict the dead zone will explode to 26,000 square kilometers, an area larger than Massachusetts.

Half-dozen grey, diamond-shaped fish, their black eyes staring into the camera The low-oxygen zone is bad news for the Gulf fishing industry, the second largest in the country, because fish and other animals die if they cannot leave an oxygen-depleted scene. This is a gnarly job for shellfish, yet the Gulf is the nation's biggest single source of shrimp and oysters. Setting the normal ecology of such a large area topsy-turvy could also trigger other unwanted effects, like blooms of toxic algae or species extinctions.

Grey snappers, like all other fish in the Gulf of Mexico, prefer water with abundant oxygen and food. The Dead Zone provides neither. Photo: NOAA

Too fertile for its own good
Gulf waters near New Orleans -- and in hundreds of other locations along the marine coast -- die each summer from too much of a good thing: The nutrient-rich Mississippi River water feeds an explosion of algae, and when bacteria decompose the algae, they exhaust the water's supply of dissolved oxygen. Water with less than two milligrams of dissolved oxygen per liter of water is considered a dead zone.

Often, the hypoxic zone is near the bottom, because organic matter falls as it decomposes, and because of "thermal stratification:" if surface waters are warmer than deep waters, the water has no tendency to mix through convection.

Because the dead zone is mainly caused by excess levels of the plant nutrients nitrogen and phosphorus, its primary cause is agriculture, and particularly corn, which typically gets a big dose of nitrogen fertilizer. Each year, the Mississippi River delivers to the Gulf about 1.6 million metric tons of nitrogen, much of it in the form of nitrate, (-NO3), a mobile compound that is phenomenally good at fertilizing plants -- on land and in water.

Young, green stalks of corn grow towards the sun amidst shallow, lapping watersFood prices and weather have combined to create a giant dead zone this summer. Amid intense concern about the global food supply, crop prices have soared, and this year’s 86-million-acre corn crop is the second-largest planting since 1949.

Carried on the same spring rains that have brought floods to the Midwest, a large portion of the vast tonnage of nitrogen applied to Midwestern crops this year has already washed into the Mississippi River.

Submerged corn in Illinois, one of the Midwestern states hard hit by floods this spring. Photo by L. Brian Stauffer, University of Illinois

Noxious nitrate
As a result, the Mississippi River is awash in nitrate, according to R. Eugene Turner, a Louisiana State University expert on the dead zone, who said this June that the level of nitrogen in the Gulf of Mexico in May was "37 percent higher than last year and the highest since measurements of nitrogen loading began in 1970. The intensive farming of more land, including crops used for biofuels, has definitely contributed to this high nitrogen loading rate.” (Dead-zone researchers consider nitrogen somewhat more important than phosphorus, but both elements are vital plant fertilizers.)

For two reasons, the soaring prices of corn and other crops have exacerbated the dead-zone problem. First, corn farmers give their crop a heavy dose of nitrogen fertilizer (soybeans, another huge crop, takes its nitrogen from the air). And as crop prices soar, farmland is being withdrawn from the federal conservation reserve program, decreasing the acreage of fallow land that can naturally attenuate phosphate and nitrogen pollution.

Map of US in olive green, river and tributaries branching out, extending like branches over most of center of countryThe Mississippi River drains 41 percent of the lower 48 states and has plenty of opportunities to pick up nutrients that can harm the Gulf ecosystem. Each year, the Big Muddy brings about 1.6 million metric tons of nitrogen to the Gulf, including about 1 million tons of nitrate. Map: Army Corps of Engineers

Biofuels also play a role. The U.S. government has set a goal of brewing 36 billion gallons of renewable fuels by 2022, and the only feasible feedstock now is corn. (In 2007, the United States produced 6.5 billion gallons of ethanol from corn.) According to a projection from Chris Kucharik, of the University of Wisconsin-Madison, and Simon Donner of the University of British Columbia, corn planted to produce 15 billion gallons ethanol would raise the nitrogen load in the Mississippi River by 10 to 19 percent.

Agriculture is already responsible for more than 70 percent of the nitrogen and phosphorus in the Mississippi, according to recent study (see #2 in the bibliography). Although corn (phosphorus and nitrogen) and soybeans (phosphorus) crops are major offenders, animal manure was the largest single source of phosphorus.

Short of halting agriculture, what can be done to tame the dead zone?


Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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