The Mississippi River Basin forms a large funnel, channeling nitrogen and other nutrients into the Gulf of Mexico, where a growing dead zone wrecks havoc on the marine ecosystems.

The river basin includes parts of 31 different states, draining over 41 percent of the continental United States. With a watershed this large, over 18 million people depend upon the Mississippi for their water supply.

Of these 31 states, nine are responsible for more than 75 percent of the nitrogen and phosphorous entering the gulf. These states – Illinois, Iowa, Indiana, Missouri, Arkansas, Kentucky, Tennessee, Ohio and Mississippi – are roughly one third of the total basin area. According to some studies, Iowa leads the pack, delivering about 25 percent of the nitrogen to the gulf.

A report released by the U.S. Geological Survey in 2008 shows slightly different figures. It places Illinois as the top culprit, with Iowa coming in second.

StatePercent of Total Nitrogen Flux
Source: U.S. Geological Survey

The data on the subsurface draining used by farmers, called tile drains, is limited, but Iowa and Illinois are recognized as heavily tiled. A report by the World Resources Institute estimates that 48 percent of Illinois’ cropland is tiled versus about 32 percent of Iowa’s cropland. Indiana, another high-ranking nitrogen contributor, has about 42 percent of its cropland tiled. According to the Geological Survey report, corn and soybean crops are responsible for 52 percent of the nitrogen that arrives in the gulf. Illinois and Iowa are the two largest corn-producing states, so the high ranking makes sense.

Agricultural workers point to the urban and suburban use of fertilizer as another contributor to nutrient runoff and studies do show urban dwellers over-apply fertilizer. However, the relative area of cities and towns is small in comparison to the expansive agricultural land and, according to the Geological Survey report, urban and population-related sources represent only 9 percent of nitrogen delivered to the gulf.

Because so many different states drain into the Mississippi, the effort to reduce the amount of nutrients flowing into the gulf becomes a very complex project. The goal of the Mississippi River/Gulf of Mexico Task Force is to reduce the hypoxic zone to 5,000 square miles; that would require a nitrogen reduction of about 45 percent. However, most environmentalists admit that the goal is out of reach.

Unless the states are able to reduce their nutrient pollution, scientists worry that the gulf will eventually reach a “tipping point” and species will be unable to recover from the summer hypoxia. Nancy Rabalais, executive director for the Louisiana Universities Marine Consortium, points to global examples of hypoxia as warnings for what could happen in the gulf.

In the Baltic Sea, the Black Sea and the Kattegat, species were lost due to hypoxia. According to a report in Water Science and Technology, the Kattegat and Black sea are recovering, but there is no recovery in the Baltic Sea.

System Area Affected (km2)Response and RecoveryFisheries Response
Louisiana Shelf15,000Mortality, Annual RecoveryStressed, but still highly productive.
Kattegat2,000Mass Mortality, Slow RecoveryCollapse of Norway Lobster, Sweden-Denmark reduction of demersal fish.
Black Sea20,000Mass Mortality, Annual RecoveryLoss of demersal fisheries.
Baltic Sea 100,000Eliminated, No recoveryLoss of demersal fisheries.
Source: C.W. Randal. “Potential Societal and Economic Impacts of Wastewater Nutrient Removal and Recycling.” Water Science and Technology.

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