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Discussion of 2011 Preliminary Spring (April and May) Nutrient Fluxes

Nutrient delivery from the Mississippi-Atchafalaya River Basin (MARB) to the Gulf of Mexico has been identified as one of the primary factors controlling the size of the hypoxic zone that forms in the northern Gulf of Mexico every summer in recent years. Each year since 1985, the Louisiana Universities Marine Consortium has measured the size of the hypoxic zone in July, when the zone is anticipated to be near its greatest extent (in recent years, additional measurements have been made at other times). Models of the relations between the size of the hypoxic zone and nutrient delivery to the Gulf of Mexico indicate that nutrient delivery during the spring has a stronger relation to hypoxic zone size than annual nutrient flux or nutrient flux for other time periods during the year (Scavia and others, 2003, and Scavia and others, 2004).

Graphics of runoff and preliminary nutrient fluxes (dissolved nitrite plus nitrate, total nitrogen, total phosphorus, dissolved orthophosphate, and dissolved silica) for Spring (April and May) 2011 are presented in figures 1 through 6. (Figures 2 through 6 are modified from figures 1 through 5 of Aulenbach and others (2007). The figures provide a comparison of conditions in Spring 2011 to the period of record. Note that 2011 nutrient flux estimates are preliminary as they are based on provisional data, which are subject to change. More information on the preliminary data (including confidence intervals) and access to the data used on this page are available. The historic approved data (including confidence intervals) used on this page are also available.

Figure 1. April and May runoff from the Mississippi and Atchafalaya Rivers to the Gulf of Mexico for 1979 through 2011. Maximum, minimum, and average runoff are determined for the period 1979 through 2009.	Figure 2. Estimated April and May dissolved nitrite plus nitrate flux as N to the Gulf of Mexico for 1979 through 2011. Maximum, minimum, and average fluxes are determined for the period 1979 through 2009. *Note that 2011 fluxes are preliminary as they are based on provisional data.
Figure 3. Estimated April and May total nitrogen flux as N to the Gulf of Mexico for 1980 through 2011. Maximum, minimum, and average fluxes are determined for the period 1980 through 2009. *Note that 2011 fluxes are preliminary as they are based on provisional data.	Figure 4. Estimated April and May total phosphorus flux as P to the Gulf of Mexico for 1979 through 2011. Maximum, minimum, and average fluxes are determined for the period 1979 through 2009. *Note that 2011 fluxes are preliminary as they are based on provisional data.
Figure 5. Estimated April and May dissolved orthophosphate flux as P to the Gulf of Mexico for 1982 through 2011. Maximum, minimum, and average fluxes are determined for the period 1982 through 2009. *Note that 2011 fluxes are preliminary as they are based on provisional data.	Figure 6. Estimated April and May dissolved silica flux as SiO2 to the Gulf of Mexico for 1980 through 2011. Maximum, minimum, and average fluxes are determined for the period 1980 through 2009. *Note that 2011 fluxes are preliminary as they are based on provisional data.
Figure 7. Spring (April - May) net mean streamflow for the five large subbasins that make up the Mississippi-Atchafalaya River Basin. *Note that some 2011 flows are preliminary as they contain some provisional data. Also note that Lower Mississippi mean streamflows for 2011 includes the streamflows from the Arkansas and Red Rivers as streamflows for the Arkansas and Red Rivers were not yet available.	Figure 8. Box plots showing the distribution of average spring (April and May) dissolved nitrite plus nitrate concentrations, for the years 1979 to 2008, for four of the five large subbasins that comprise the Mississippi-Atchafalaya River Basin. (The Lower Mississippi River subbasin was excluded due to the large errors in estimating the average concentrations.)
Figure 9. Box plots showing the distribution of average spring (April and May) total phosphorous concentrations, for the years 1979 to 2008, for four of the five large subbasins that comprise the Mississippi-Atchafalaya River Basin. (The Lower Mississippi River subbasin was excluded due to the large errors in estimating the average concentrations.)

Runoff from the Mississippi and Atchafalaya Rivers in Spring 2011 is well above the average for the period of record 1979 to 2010 (46 percent, 3^rd highest runoff; figure 1; period of record defined by concurrent availability of sufficient water-quality samples for flux estimation for both rivers), with April runoff being slightly above average for April (6.9 percent) and runoff being extremely high for May (85 percent above average). The extreme flooding in the Lower MARB in May exceeded the highest May runoff for the prior 32-year period of record.

The above average spring flows resulted in elevated Spring nutrient fluxes that ranged from 18 to 63 percent above average, depending on the nutrient. Dissolved nitrite plus nitrate flux for Spring 2011 was about 325,000 metric tons as N, about 22 percent above the average for the period of record (1979 - 2010; figure 2). This is the 8^th highest spring dissolved nitrite plus nitrate flux over the 33-year period of record (1979-2011), and is significantly higher than last year's flux of 272,000 metric tons, which was the 14^th highest flux. Total nitrogen flux for Spring 2011 was about 456,000 metric tons, about 18 percent above average (figure 3). Phosphorus fluxes for Spring 2011 were 44,400 metric tons as P for total phosphorus and 13,000 metric tons as P for dissolved orthophosphate, about 32 and 50 percent above average, respectively (figures 4 and 5). Dissolved silica flux in Spring 2011 was about 1,650,000 metric tons as SiO₂, about 63 percent above average (figure 6).

Nutrient fluxes for a given spring vary depending on the amount of flow in the Mississippi-Atchafalaya River Basin, as well as the source of flow within the Basin. Preliminary streamflow estimates indicate that about 23 percent of the Spring 2011 runoff occurred in the Upper Mississippi subbasin, about 50 percent occurred in the Ohio/Tennessee River subbasin, about 10 percent occurred in the Missouri subbasin, and the remaining 16 percent of runoff occurred from the combined Lower Mississippi and Arkansas/Red subbasins (figure 7). Spring 2011 runoff was about 61 percent above average for the Upper Mississippi subbasin, about 100 percent above average for the Ohio/Tennessee subbasin, about 27 percent above average for the Missouri subbasin, and about 24 percent below average for the combined net contributions of the Lower Mississippi and Arkansas/Red subbasins. Figures 8 and 9 show the distributions of average spring concentrations of dissolved nitrite plus nitrate and total phosphorus for four of the large subbasins in the Mississippi-Atchafalaya River Basin. The differences in concentrations among the subbasins help to explain how variations in the source of water can yield different nutrient fluxes for the MARB for similar flows.

Extreme flooding occurred in the Lower MARB during May 2011. To minimize downstream flooding, the U.S. Army Corps of Engineers diverted portions of the lower Mississippi River to the Atchafalaya River via the Morganza Spillway and Floodway starting on 5/14/11 and to Lake Pontchartrain via the Bonnet Carre’ Spillway starting on 5/9/11. These flow diversions continued into June 2011. Due to these diversions, supplemental flow and flux estimates for May 2011 are included here to better interpret nutrient transport in the Lower MARB and delivery to the GOM. Estimates for nutrient fluxes for the entire MARB are calculated at the traditional stations on the Mississippi River (Tarbert Landing, Miss. and St. Francisville, La.) and the Atchafalaya River (Simmesport, La. and Melville, La.) and alternatively at the two downstream outlets of the Atchafalaya River, downstream of the Morganza Floodway, at the Southern Atchafalaya River at Morgan City, La. and Wax Lake Outlet at Calumet, La. stations. Estimated nutrient fluxes for the lower Mississippi River downstream of the Bonnet Carre’ Spillway and for the entire MARB excluding the flow/fluxes diverted to Lake Pontchartrain via the Bonnet Carre’ Spillway are included as these flux estimates may better represent the delivery of nutrients to the GOM.

In May 2011, about 9.6 percent of the total MARB flow and 8.6 percent of the dissolved nitrite plus nitrate flux (15,500 of the total 180,000 metric tons as N) was diverted into Lake Pontchartrain. A delay and attenuation of nutrient transport due to lake-water storage and in-lake processes is not assessed in this analysis.

Note that flows/fluxes measured and estimated at stations are independent of each other; so the sum of upstream flows/fluxes may not be equal to downstream flows/fluxes. Discrepancies can occur due to errors in streamflow measurements and flux estimates, errors in streamflow due to over-bank flooding, off-channel storage of ponded floodwaters, additional sources of nutrients, and in-stream processing of nutrients.

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.