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Discussion of 2009 Preliminary Spring (April and May) Nutrient Fluxes

Nutrient delivery from the Mississippi-Atchafalaya River Basin 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) 2009 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 2009 to the period of record. Note that 2009 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 2009. Maximum, minimum, and average runoff are determined for the period 1979 through 2008.	Figure 2. Estimated April and May dissolved nitrite plus nitrate flux as N to the Gulf of Mexico for 1979 through 2009. Maximum, minimum, and average fluxes are determined for the period 1979 through 2008. *Note that 2009 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 2009. Maximum, minimum, and average fluxes are determined for the period 1980 through 2008. *Note that 2009 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 2009. Maximum, minimum, and average fluxes are determined for the period 1979 through 2008. *Note that 2009 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 2009. Maximum, minimum, and average fluxes are determined for the period 1982 through 2008. *Note that 2009 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 2009. Maximum, minimum, and average fluxes are determined for the period 1980 through 2008. *Note that 2009 fluxes are preliminary as they are based on provisional data.
Figure 7. Spring (April and May) mean streamflow for the five large subbasins that make up the Mississippi-Atchafalaya River Basin. *Note that some 2009 flows are preliminary as they contain some provisional data. Also note that Lower Mississippi mean streamflows for 2009 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 2009 was about 17 percent above the average for the period of record 1979 to 2008 (figure 1), with April runoff being slightly below average for April (-3.6 percent) and May runoff being quite high (about 39 percent above the May average). The above average spring flows was one of the factors resulting in above average Spring nutrient fluxes. Dissolved nitrite plus nitrate flux for Spring 2009 was about 295,000 metric tons, about 11 percent above the average for the period of record (1979 - 2008; figure 2). This is the 10th highest spring dissolved nitrite plus nitrate flux over the 31-year period of record (1979-2009), but much lower than last year's flux of 382,000 metric tons as N, which was the 4th highest flux. Total nitrogen flux for Spring 2009 was about 409,000 metric tons as N, about 5.5 percent above average (figure 3). Phosphorus fluxes for Spring 2009 were 37,800 metric tons as P for total phosphorus and 12,100 metric tons as P for dissolved orthophosphate, about 13 and 42 percent above average, respectively (figures 4 and 5). Dissolved silica flux in Spring 2009 was about 1,120,000 metric tons as SiO₂, about 11 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, and other factors. The source of flow affects the nutrient flux because average spring streamwater nutrient concentrations can vary significantly by subbasin. Preliminary streamflow estimates indicate that runoff in Spring 2009 was 42 percent above average for the Upper Mississippi subbasin, 28 percent above average for the Ohio/Tennessee subbasin, 23 percent above average for the Missouri subbasin, and 14 percent below average for the combined Lower Mississippi and Arkansas/Red subbasins (figure 7). 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 helps to explain how variations in the source of water can yield different nutrient loads for the Basin for similar Basin flows.

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.