Gulf of Mexico Hypoxia:
Do Iowa Farmers cause the "Dead Zone"?

by: Christiana Lang

In the 70's, there was a zone of oxygen depletion found in the Gulf of Mexico. Since then that zone has been defined as a zone of hypoxia. In recent years, much research has been done on this zone to try and find the main villain in reducing the oxygen concentration where the Mississippi River drains into the Gulf of Mexico. There have also been numerous ideas about ways nutrient fluxes can be reduced.

The area where the Mississippi River drains into the Gulf of Mexico is becoming depleted of oxygen. There are two main natural causes of this depletion. The first is water column stratification. As fresh water comes into the Gulf from the Mississippi River, the fresh water floats on top of the more dense salty water of the Gulf. This phenomenon is enhanced in the spring when run off and river flow are high. As summer approaches, mixing is inhibited when the thermal stratification is set up by warm weather. Since this mixing is inhibited, a pycnocline, the near-surface freshwater/saltwater boundary, forms and gives the necessary conditions for hypoxia. The supply rate of oxygen to the bottom is cut off (Council of Agriculture Science and Technology Task Force).

The second natural cause of hypoxia occurs when decomposition of organic matter uses up the available oxygen. This leads to the rate of consumption exceeding the rate of resupply. As these situations occur oxygen concentrations become too low to sustain animal life.

Figure 1. Louisiana Coast - Frequency of occurrence of hypoxia during mid-summer of 1985-1997, The map is 275 miles wide (Council of Agriculture Science and Technology Task Force).

The Council for Agricultural Science and Technology points out that the Mississippi River Basin covers 41% of the contiguous United States and accounts for 36% of the runoff. That area represents 55% of the agricultural land. Since this area thrives on agriculture, there is a lot of concern on what impacts nutrient load regulations will bring, therefore raising concern for water quality issues in the Gulf of Mexico.

Figure 2. Mississippi River Basin and the major tributary rivers (Council of Agriculture Science and Technology Task Force).

Agriculture has been implicated in 60% of the assessments of river water quality degradation in the United States (Task Force Report). In this area, scientists have estimated that agriculture contributes two-to-three pounds of nitrate per acre to the nitrate concentration of the Mississippi River (Council of Agriculture Science and Technology Task Force). If every farm acre is contributing two-to-three pounds of nitrate per acre, this number turns from very small to extremely large. I believe it is a problem Iowa's farmers need to be concerned about.

In the Gulf of Mexico, it has been found that the majority, 51%, of nitrate flux comes from the Upper Mississippi River, which only supplies 22% of the river's water discharge. The Upper Mississippi River is comprised of portions of Minnesota, Wisconsin, Iowa, Missouri, and Illinois. These percentages show agriculture in the Upper Mississippi River to be the main villain in harming the Gulf of Mexico, and causing the hypoxic zone. In comparison, the Illinois River contributes 12% of the nitrate flux and four percent of the discharge. This nitrate flux in the Mississippi River has increased about threefold between 1954 and the early 80's. Seventy percent of the nitrogen that enters the Gulf is in the Mississippi River before the Ohio River connects with it at Cairo.

Figure 3. Calculated Nitrogen Sources for Mississippi River Basin (Council of Agriculture Science and Technology Task Force).

Ninety percent of the nitrogen delivered to the Gulf by the Mississippi River originates from non-point sources consisting predominantly of nitrogen in agricultural runoff and atmospheric deposition. Atmospheric deposition is derived from nitrous oxides from fossil fuel combustion. Only about 1 percent comes from point sources like municipal treatment plants and industries (Alexander et al). If these non-point sources, like agricultural runoff were reduced by 50% it is predicted that the hypoxic zone will increase the average dissolved oxygen by 150%.

These nutrients that end up in the Mississippi are transported four different ways including air, surface runoff water, sediment and subsurface drainage water. The nutrients in the air are released through volatilization or through wind erosion from bare soils. Dissolve nutrients are transported either in solution or as adsorbed particles. These transport methods occur most frequently in intense precipitation and flood years. During these intense spring rains sediment carries nutrients into streams, increasing the nutrient load. Finally, the dissolved nutrients transported in subsurface drainage water are dominated by nitrate. The most important factor for this method is the timing of precipitation or irrigation and the amount of nitrate in the soil at that time.

Management decisions can aid in reducing the amounts of nutrients escaping to the rivers. Cropping, tillage, and weather are the main factors influencing surface runoff. Also having vegetative buffer or filter strips and wetlands reduces the transport of sediment and nutrients carried over land. Using controlled drainage, where subsurface drainage is restricted during certain periods of the year, creates wet, anaerobic environments upstream of the restriction which can result in beneficial denitrification (Council of Agriculture Science and Technology Task Force).

Many management techniques are now available to reduce the nutrient flux. Within agriculture we need to use reduce tillage and maintain soil conservation, which reduces surface runoff. Manure management plans used to calculate the timing, amounts, and acres are needed for proper application to reduce the hypoxic zone.

With the new technology available like GPS and GIS farmers are allowed to apply the optimum nutrients for a crop. This will not only help reduce the hypoxic zone, but it will reduce production costs. Using variable rate technology and applying their fertilizer in the spring should reduce the overall application of nutrients on farms, therefore reducing the cost of fertilizer for the farmer. This method will put the needed nutrients in areas where the farm may need more or less than the constant rate. In combination with a spring application, the nutrients will be more readily available to the crop when demands are highest. There will be less of a chance for leaching and denitrification helping to reduce the amount of nutrients that flow into our streams and eventually end up in the Gulf of Mexico.

Putting in cover crops, stream buffers, creating wetlands, and having conservation programs allow farmers to reduce the nutrient losses through runoff and infiltration to ground water. The government has even stepped in by passing the Food Security and 1990 Farm Bill. To meet the goals of Conservation Compliance, 1.7 million conservation compliance plans were developed on 143 million acres of the NationM-Us most highly erodible cropland. This is about one third of all cropland in the U.S. with a major portion in the Mississippi River Basin (Burt and Klaus).

Figure 4. Distribution of erosion rates before FSA, erosion rates with respect to soil loss tolerance values preliminary data as of February 9, 1995 (Burt and Klaus).

Figure 5. Summary 1994 Status Review Results- Distribution of erosion rates after full implementation with respect to the soil loss tolerance values of preliminary data as of February 9, 1995 (Burt and Klaus).

Nitrogen can stay in ground water for months or even years before entering surface water. By choosing the best management practices for each individual farm, the nutrient flux can be reduced. The key is to find a way that is beneficial for both the farm and the environment.

The government has also taken notice of this problem. They have passed the Food Security Act and the 1990 Farm Bill, which will allow more steps to be taken in the future to reduce nutrient loads in the rivers. The "Dead Zone" attention has created a need for programs that make it financially feasible for farmers to take their land out of production and put it into wetlands, or conservation lands. All of these steps have helped in reducing the nutrient flux in the rivers. A decrease in the agricultural nutrient flux of the Corn Belt can be practically accomplished with current technology and is a necessity to reduce the hypoxia in the Gulf of Mexico.

Hypoxia is a zone where oxygen levels are too low to support animal life. The leading cause of the growth of this zone is through non-point sources, like agriculture. There are ways to reduce the nutrient flux and the hypoxic zone. If farmers practice good management of nutrient levels and become land stewards the nutrient levels in the Mississippi and Gulf of Mexico will continue to fall. The most important thing to realize is that farming in a small town in the Midwest can alter life of people and animals in the Gulf of Mexico.


Alexander,R.B, Smith R.C. and Schwarz,G.E. "The Regional Transport of Point and Nonpoint-Sources Nitrogen to the Gulf of Mexico." February 2000.

Alt, Kluas, and Burt, J.P. "What is being done in the basin to control nutrient loads from agricultural sources?". February 2000.

Boesch, Donald F. "Gulf of Mexico Hypoxia: Where are we on the learning curve?"February 2000.

Task Force Report. 1999. Gulf of Mexic Hypoxia: Land and Sea Interactions. Council for Agricultural Science and Technology, June 1999. Ames, IA.