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Streamflow and Nutrient Delivery to the Gulf of Mexico for October 2007 to June 2008 (Preliminary)

Preliminary estimates of monthly streamflow and nutrient fluxes from the Mississippi and Atchafalaya Rivers to the Gulf of Mexico are provided in July each year for the previous nine months (October through June). The delivery of fresh-water runoff and nutrients by the Mississippi-Atchafalaya River Basin are two primary controls on the size of the hypoxic zone that develops in the northern Gulf of Mexico each summer. The hypoxic zone is an area where oxygen levels drop too low (dissolved oxygen concentrations of less than 2 milligrams per liter) to support most life in bottom and near-bottom waters. The low oxygen conditions cause fish to leave the area and can kill bottom-dwelling organisms that cannot leave.

Nutrient data herein are used by scientists to annually estimate the size of the mid-summer hypoxic zone in the northern Gulf of Mexico. Dr. R. Eugene Turner (Louisiana State University) estimates the size of the hypoxic zone using the May dissolved nitrite plus nitrate fluxes delivered to the Gulf (Turner and others, 2006; Turner and others, 2008). Dr. Don Scavia (University of Michigan) estimates the size of the hypoxic zone using the sum of May and June total nitrogen fluxes delivered to the Gulf (Scavia and others, 2003; Scavia and others, 2004). The Gulf hypoxic zone is later measured by the Louisiana Universities Marine Consortium each summer in July when it is anticipated to be at its greatest extent.

• Streamflow and Nutrient Flux to the Gulf of Mexico October 2007 to June 2008 (Excel^© Spreadsheet)
• Discussion of 2008 Preliminary Spring (April, May, June) Nutrient Fluxes
• USGS News Release: Spring Nutrient Delivery to the Gulf Estimated Among Highest in Three Decades

Although streamflow and nutrient flux information is usually calculated on a water year (October to September) basis, this information is provided in preliminary form in July so that the spring nutrient fluxes can be utilized in a timely manner for modeling of the areal extent of the hypoxic zone in the Gulf of Mexico. Note that the provisional data have been checked for errors, but some minor changes can be expected for stream-discharge data as rating curves for converting streamwater stage to stream discharge are finalized only after the completion of that water year. Final estimates, using the full 5-year calibration dataset, are provided after the water year is completed and streamflow and water-quality data are finalized.

The monitoring network, dataset preparation steps, and nutrient flux estimation methods used are the same as used by Aulenbach and others (2007). Links to specific information on data and methodology in the report are available here:

• Monitoring Network Used to Estimate Total Delivery of Flow and Nutrients to the Gulf
• Sources and Preparation of Data Used for Nutrient Flux Estimation
• Methods Used to Estimate Nutrient Fluxes

Note that flux estimates on a monthly time-step can be quite inaccurate and should be used with caution. They are provided with the intent to allow the user to sum up nutrient fluxes on either a seasonal basis or an annual basis other than water year.

References

Aulenbach, B.T., Buxton, H.T., Battaglin, W.T., and Coupe R.H., 2007, Streamflow and nutrient fluxes of the Mississippi-Atchafalaya River Basin and subbasins for the period of record through 2005: U.S. Geological Survey Open-File Report 2007-1080

Scavia, Donald, Rabalais, N.N., Turner, R.E., Justić, Dubravko, and Wiseman, W.J., Jr., 2003, Predicting the response of Gulf of Mexico hypoxia to variations in Mississippi River nitrogen load: Limnology and Oceanography, v. 48, no. 3, p. 951–956.

Scavia, Donald, Justić, Dubravko, and Bierman, V.J., Jr., 2004, Reducing hypoxia in the Gulf of Mexico—Advice from three models: Estuaries, v. 27, no. 3, p. 419-425.

Turner, R.E., Rabalais, N.N., and Justić, Dubravko, 2006, Predicting summer hypoxia in the northern Gulf of Mexico—Riverine N, P, and Si loading: Marine Pollution Bulletin, v. 52, no. 2, p. 139-148.

Turner, R.E., Rabalais, N.N., and Justić, Dubravko, 2008, Gulf of Mexico hypoxia—Alternate states and a legacy: Environmental Science and Technology, v. 42, no. 7, p. 2323-2327.

For users that do not have Excel^© installed on their computer system, a free viewer is available for Windows^© systems (Excel Viewer 2003) that allows users to open, view, and print Excel^© spreadsheets. Third party viewers are also available for other operating systems (Mac OSX, UNIX, ...). The use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government or the U.S. Geological Survey.