Relation of Nitrogen and Nitrates to Hypoxia

Currently, the United State’s Gulf of Mexico experiences an annual, seasonal “dead zone” as a result of hypoxia. Hypoxia is a low level of dissolved oxygen (<2mg/L) in an area of water. Hypoxia is typically temporary and seasonal, but the low oxygen levels can be devastating to aquatic organisms. Hypoxia occurs in many oceanic waters worldwide, but there is a growing area of concern in our Gulf of Mexico coast. Hypoxia is largely caused by nitrogen fertilizer application for agriculture, with heavy concentrations coming from the Midwestern US. Nitrogen mobilizes as nitrate, and is transported via surface water runoff. The runoff enters tile-drain supported ditches, enters streams and joins the Mississippi river, eventually reaching the gulf. Nitrate mobilization is a problem for human health, and ag. runoff is also often attributed to the contamination of surface and well-water sources in rural areas. High levels of nitrates restrict the ability of red blood cells to carry oxygen. This can harm humans, and is the major cause of infant methemoglobinemia, or “blue baby syndrome”. The safe drinking level standard is 10mg/L.

Nitrogen and nitrates relate to Hypoxia via the process of eutrophication. Since Nitrogen is a limiting nutrient in most waters, the added input of nitrate causes massive growth in algae. The algae rapidly consume all available N, and once the nutrient is limited again, the alga dies en masse. As the alga decomposes, oxygen is depleted in the water. This lowers dangerously lowers the level of dissolved oxygen in the water, which harms living organisms in the area. Small organisms and organisms that are immobile or unable to escape low-oxygen areas are particularly vulnerable. Hypoxia and resulting “dead zones” are harmful to local fishing and shrimping industries and algal blooms hurt the tourism industry. Hypoxia has lead to a decrease of about 25% in the brown shrimp habitat, forcing shrimping operations further offshore. As the hypoxia issue continues to grow, negative human effects will only increase. Since nitrate runoff from ag. has been proven to be the dominant source of hypoxia, policies could be enacted to effectively deal with “point-source” pollution. This makes enacting environmental policy more easily adapted, possibly included in past policy such as the Clean Water Act.

According to the USDA Economic Research Service, the most cost-effective solution would be to simply decrease the amount of nitrate entering the gulf by 1.2 million metric tons. According to calculations, this would not result in significant crop yields but would decrease the amount of hypoxia.