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Gulf of Mexico Hypoxia

The Iowa Nutrient Reduction Strategy was developed in response to persistent hypoxic conditions in the Gulf of Mexico. Hypoxia occurs in water bodies when the dissolved oxygen in water decreases to a level that can no longer sustain aquatic life. Hypoxia can occur naturally, but the global prevalence of hypoxia is expanding because of human activity. In the Gulf of Mexico near the mouth of the Mississippi River, conditions are considered hypoxic when oxygen levels are lower than 2 milligrams per liter (i.e. 2 parts per million). Gulf hypoxia is driven by excessive levels of nutrients—nitrogen and phosphorus—transported by the Mississippi River. There are four general ecological stages that lead to hypoxia in the Gulf of Mexico:

  • Freshwater discharge and nutrient loading from the Mississippi River
  • Nutrient-enhanced primary production of microorganisms
  • Decomposition of excessive biomass by bacteria on the ocean floor
  • Depletion of oxygen due to the decomposition process and stratification

In the last half of the 20th century, the nutrient levels from the Mississippi River increased and are attributed to soil erosion, dissolved nitrates in row crop production, and effluent from wastewater treatment processes, among other agricultural, municipal, and industrial activity in the Mississippi/Atchafalaya River Basin.

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