USGS Estimates Nutrients to Gulf Highest in 30 Years

Spring nutrient delivery from the Mississippi-Atchafalaya River Basin to the northern Gulf of Mexico is estimated to be among the highest in the last three decades, according to a July 11 U.S. Geological Survey press release.

Nutrient delivery, particularly during the months of April through June, has been identified as one of the primary factors controlling the size of the hypoxic zone that forms during the summer in the northern Gulf of Mexico. The Gulf hypoxic zone is an area where oxygen levels drop too low to support most life in bottom and near-bottom waters. Estimated nutrient contributions from October 2007 through June 2008 are available at http://toxics.usgs.gov/hypoxia/mississippi/oct_jun/index.html.

The large nutrient contributions are primarily due to near record-breaking streamflows this spring (April through June) in the Mississippi River Basin. Streamflows were about 50 percent higher this year compared to the long-term spring average flows since about 1980. Nutrient contributions for a given spring vary depending on the amount of flow in the Mississippi-Atchafalaya River Basin, as well as average stream water nutrient concentrations.

Contributions of phosphorus to the Gulf of Mexico are estimated to be the highest since the 1980s. Total phosphorus (83,000 tons) and dissolved orthophosphate (26,000 tons) are about 60 and 85 percent higher than the long-term spring average for the nearly 30-year period.

Similarly, nitrogen contributions to the Gulf of Mexico from April through June are estimated to be about 35 to 40 percent higher than the long-term spring average since the early 1980s. Contributions for total nitrogen and dissolved nitrate during the three months are estimated to be about 817,000 and 578,000 tons, respectively.

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