Nitrate loads entering the Gulf of Mexico have not changed despite reductions at local scales

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Reducing delivery of nitrate to the Gulf of Mexico is critical to decreasing the size of the “dead zone”—an area of hypoxia, or low dissolved oxygen—in Louisiana coastal waters. A new USGS study reports that larger, more widespread decreases in nitrate loading to the Gulf are needed to achieve the target levels of a 25% reduction by 2025.

Hypoxia kills or drives away aquatic life and the economic and environmental impacts of hypoxia can be severe. To reduce the size of the hypoxic zone in the Gulf of Mexico, the interagency Mississippi River/Gulf of Mexico Hypoxia Task Force has set target levels for nitrate loading.

The USGS study quantified trends in nitrate loading from 2002 to 2012 at more than 100 river monitoring sites throughout the Mississippi River basin. Although many smaller watersheds successfully reduced nitrate loading, the reductions generally were small and were offset by increased loading in other parts of the basin.

Even at sites where nitrate concentrations decreased overall, large nitrate loads occurred during high-flow (wet) years, and this effect was greatest in the Upper Mississippi River basin. Efforts to reduce extremely large loads in high-flow years might be most effective if targeted in the upper basin. High average nitrate loads and large fluctuations in loading from one year to the next continue to negatively impact water quality, ecosystem function, and fisheries in the Gulf of Mexico.

 

Citation: Network controls on mean and variance of nitrate loads from the Mississippi River to the Gulf of Mexico. Crawford, J.T., Stets, E.G., and Sprague, L.A. 2019, Journal of Environmental Quality (48) 1789-1799. https://doi.org/10.2134/jeq2018.12.0435

 

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