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      Nitrate in the Mississippi River and Its Tributaries, 1980 to 2008: Are We Making Progress?

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          Changes in nitrate concentration and flux between 1980 and 2008 at eight sites in the Mississippi River basin were determined using a new statistical method that accommodates evolving nitrate behavior over time and produces flow-normalized estimates of nitrate concentration and flux that are independent of random variations in streamflow. The results show that little consistent progress has been made in reducing riverine nitrate since 1980, and that flow-normalized concentration and flux are increasing in some areas. Flow-normalized nitrate concentration and flux increased between 9 and 76% at four sites on the Mississippi River and a tributary site on the Missouri River, but changed very little at tributary sites on the Ohio, Iowa, and Illinois Rivers. Increases in flow-normalized concentration and flux at the Mississippi River at Clinton and Missouri River at Hermann were more than three times larger than at any other site. The increases at these two sites contributed much of the 9% increase in flow-normalized nitrate flux leaving the Mississippi River basin. At most sites, concentrations increased more at low and moderate streamflows than at high streamflows, suggesting that increasing groundwater concentrations are having an effect on river concentrations.

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          Most cited references 7

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          Differences in phosphorus and nitrogen delivery to the Gulf of Mexico from the Mississippi River Basin.

          Seasonal hypoxia in the northern Gulf of Mexico has been linked to increased nitrogen fluxes from the Mississippi and Atchafalaya River Basins, though recent evidence shows that phosphorus also influences productivity in the Gulf. We developed a spatially explicit and structurally detailed SPARROW water-quality model that reveals important differences in the sources and transport processes that control nitrogen (N) and phosphorus (P) delivery to the Gulf. Our model simulations indicate that agricultural sources in the watersheds contribute more than 70% of the delivered N and P. However, corn and soybean cultivation is the largest contributor of N (52%), followed by atmospheric deposition sources (16%); whereas P originates primarily from animal manure on pasture and rangelands (37%), followed by corn and soybeans (25%), other crops (18%), and urban sources (12%). The fraction of in-stream P and N load delivered to the Gulf increases with stream size, but reservoir trapping of P causes large local- and regional-scale differences in delivery. Our results indicate the diversity of management approaches required to achieve efficient control of nutrient loads to the Gulf. These include recognition of important differences in the agricultural sources of N and P, the role of atmospheric N, attention to P sources downstream from reservoirs, and better control of both N and P in close proximity to large rivers.
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            Nitrate flux in the Mississippi River.

            Increased delivery of biologically available nitrogen to estuaries and coastal oceans in recent decades has been linked to eutrophication and seasonal hypoxia in the northern Gulf of Mexico and elsewhere. We have developed a model that accounts for 95% of annual variation in delivery of nitrate to the Gulf of Mexico by the Mississippi River in 1960-98. Retrospective analysis indicates that this nitrate flux could have been reduced by 33% if the use of nitrogen-containing fertilizer in the Mississippi River basin had been cut by 12%.
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              Predicting summer hypoxia in the northern Gulf of Mexico: riverine N, P, and Si loading.

              We conducted a statistical analysis to discern the relative strengths of the loading of various forms of nitrogen, phosphorus, dissolved silicate and their molar ratios on the variance in the size of the summertime low oxygen zone found off the Mississippi River, northern Gulf of Mexico. A stable statistical model that included Year and riverine nitrate+nitrite loading for the 2 months prior to the measurement of hypoxic zone size described 82% of its variation in size from 1978 to 2004. The usefulness of the term Year is consistent with the documented increase in carbon stored in sediments after the 1970s, which is when the hypoxic zone is predicted to have become a regular feature on the shelf and to have expanded westward. The increased carbon storage is anticipated to cause a sedimentary respiratory demand influencing the size of the zone, and whose temporal influence is cumulative and transcends the annual variations in nitrogen loading. The variable Year is negatively correlated with the TN:TP ratio in a way that suggests N, not P, has become more important as a factor limiting phytoplankton growth in the last 20 years. Nitrogen, in particular nitrate+nitrite, and not phosphorus, dissolved silicate, or their molar ratios, appears to be the major driving factor influencing the size of the hypoxic zone on this shelf. This conclusion is consistent with cross-system analyses that conclude that the TN:TP ratio in the Mississippi River, currently fluctuating around 20:1, is indicative of nitrogen, not phosphorus, limitation of phytoplankton growth. Nutrient management that places stronger emphasis on reducing nitrogen loading as compared to phosphorus loading, is justified.

                Author and article information

                Environ Sci Technol
                Environmental Science & Technology
                American Chemical Society
                09 August 2011
                01 September 2011
                : 45
                : 17
                : 7209-7216
                []U.S. Geological Survey , Denver Federal Center, Box 25046 MS 415, Denver, Colorado 80225, United States
                []U.S. Geological Survey , 432 National Center, Reston, Virginia 20192, United States
                [§ ]U.S. Geological Survey , 3039 Amwiler Road, Suite 130, Atlanta, Georgia 30360, United States
                Author notes
                [* ]Phone: 303-236-6921; fax: 303-236-4912; e-mail: lsprague@ 123456usgs.gov .
                Copyright © 2011 U.S. Government

                This is an open-access article distributed under the ACS AuthorChoice Terms & Conditions. Any use of this article, must conform to the terms of that license which are available at http://pubs.acs.org.

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