Hypoxia in the Gulf of Mexico
Preliminary estimates of monthly streamflow and nutrient fluxes from the Mississippi and Atchafalaya Rivers to the Gulf of Mexico are provided in June each year for the previous eight months (October through May). 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 May 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. More information is available on research and management of the hypoxic zone in the northern Gulf of Mexico from the National Oceanic and Atmospheric Administration's (NOAA) National Centers for Coastal Ocean Science's (NCCOS) Center for Sponsored Coastal Ocean Research (CSCOR).
Although streamflow and nutrient flux information is usually calculated on a water year (October to September) basis, preliminary monthly nutrient fluxes from October to May are provided so that the spring nutrient fluxes can be utilized to predict the areal extent of the hypoxic zone in the Gulf of Mexico. In previous years, preliminary nutrient fluxes were estimated through June, and were provided in July. Researchers have reported that the May nutrient fluxes are more critical than June nutrient fluxes in determining the extent of the hypoxic zone for that summer, due to the time lag between the delivery of nutrients to the Gulf and the peak development of the hypoxic zone along with the 3-month residence time of surface waters along the coast during the summer (Turner and others, 2006). Thus, the USGS is releasing preliminary estimates of the nutrient flux in mid-June to better address the needs of researchers predicting the size of the hypoxic zone.
Note that the provisional data have been checked for errors, but some minor changes can be expected. Stream discharges are finalized only after the completion of that water year. Nutrient flux estimates are calculated using a model with an abbreviated water-quality calibration dataset that only goes through May of the current water year. Estimates of the uncertainty associated with the monthly nutrient fluxes, which can vary significantly, are also provided. Final estimates, using the full 5-year water-quality calibration dataset, are provided after the water year is completed and streamflow and water-quality data are finalized. Approved streamflow and nutrient delivery to the Gulf of Mexico for 1979 to 2011 are available.
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:
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.
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.
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