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Discussion of 2014 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) 2014 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 2014 to the period of record. Note that 2014 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 .

Graph of April and May runoff from the Mississippi and Atchafalaya Rivers to the Gulf of Mexico for 1979 through 2009.
Figure 1. April and May runoff from the Mississippi and Atchafalaya Rivers to the Gulf of Mexico for 1979 through 2014. Maximum, minimum, and average runoff are determined for the period 1979 through 2013.
Graph of estimated April and May dissolved nitrite plus nitrate flux as N to the Gulf of Mexico for 1979 through 2009.
Figure 2. Estimated April and May dissolved nitrite plus nitrate flux as N to the Gulf of Mexico for 1979 through 2014. Maximum, minimum, and average fluxes are determined for the period 1979 through 2013. *Note that 2014 fluxes are preliminary as they are based on provisional data.
Graph of estimated April and May total nitrogen flux as N to the Gulf of Mexico for 1980 through 2009.
Figure 3. Estimated April and May total nitrogen flux as N to the Gulf of Mexico for 1980 through 2014. Maximum, minimum, and average fluxes are determined for the period 1980 through 2013. *Note that 2014 fluxes are preliminary as they are based on provisional data.
Graph of estimated April and May total phosphorus flux as P to the Gulf of Mexico for 1979 through 2009.
Figure 4. Estimated April and May total phosphorus flux as P to the Gulf of Mexico for 1979 through 2014. Maximum, minimum, and average fluxes are determined for the period 1979 through 2013. *Note that 2014 fluxes are preliminary as they are based on provisional data.
Graph of estimated April and May dissolved orthophosphate flux as P to the Gulf of Mexico for 1982 through 2009.
Figure 5. Estimated April and May dissolved orthophosphate flux as P to the Gulf of Mexico for 1982 through 2014. Maximum, minimum, and average fluxes are determined for the period 1982 through 2013. *Note that 2014 fluxes are preliminary as they are based on provisional data.
Graph of estimated April and May dissolved silica flux as SiO2 to the Gulf of Mexico for 1980 through 2009.
Figure 6. Estimated April and May dissolved silica flux as SiO2 to the Gulf of Mexico for 1980 through 2014. Maximum, minimum, and average fluxes are determined for the period 1980 through 2013. *Note that 2014 fluxes are preliminary as they are based on provisional data.
Graph of Spring (April - June) mean streamflow for the five large subbasins that make up the Mississippi-Atchafalaya River Basin.
Figure 7. Spring (April - May) net mean streamflow for the five large subbasins that make up the Mississippi-Atchafalaya River Basin. *Note that some 2014 flows are preliminary as they contain some provisional data. Also note that Lower Mississippi mean streamflows for 2014 includes the streamflows from the Arkansas and Red Rivers as streamflows for the Arkansas and Red Rivers were not yet available.
Box plots showing the distribution of average spring (April and May) dissolved nitrite plus nitrate concentrations
Figure 8. Box plots showing the distribution of average spring (April and May) dissolved nitrite plus nitrate concentrations, for the years 1979 to 2013, 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.)
Box plots showing the distribution of average spring (April and May) total phosphorous concentrations
Figure 9. Box plots showing the distribution of average spring (April and May) total phosphorous concentrations, for the years 1979 to 2013, 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.)
Click on graphs for larger versions.

Runoff from the Mississippi and Atchafalaya Rivers in the Spring of 2014 is below the average for the period of record from 1979 to 2013 (about 15 percent below average, 16th lowest spring runoff in 36 years; figure 1; period of record defined by concurrent availability of sufficient water-quality samples for flux estimation for both rivers). April (about -11 percent) and May (about -18 percent) runoff were both below average.  Spring 2014 runoff is about 17 percent below last year's spring runoff.

Below average April and May streamflow resulted in below average nitrate (25 percent below average from 1979-2013) and TN fluxes (24 percent below average). Dissolved nitrite plus nitrate flux for Spring 2014 was about 199,000 metric tons as N, which is ranked as the 9th lowest spring dissolved nitrite plus nitrate flux over the 36-year period of record (1979-2014).  Spring 2014 nitrate loads were about 33 percent lower than last year's spring flux of 300,000 metric tons (when streamflows were about average), which is the 12th highest spring flux. Total nitrogen flux for the Spring of 2014 was about 294,000 metric tons, which was the 9th lowest recorded from 1979-2014 (figure 3). May dissolved nitrite plus nitrate and total nitrogen fluxes, which are used to estimate the size of the hypoxic zone that forms in the northern Gulf of Mexico each summer, are about 25 and 23 percent below average, respectively. Phosphorus fluxes for the Spring of 2014 were 35,700 metric tons as P for total phosphorus and 10,600 metric tons as P for dissolved orthophosphate, 6 and 20 percent above average respectively (figures 4 and 5). The dissolved silica flux in the Spring of 2014 was about 853,000 metric tons as SiO2; about 17 percent below average (figure 6).

Nutrient fluxes for a given spring vary depending on the amount of flow in the MARB, as well as the source of flow within the Basin. Preliminary streamflow estimates indicate that about 26 percent of the Spring 2014 runoff occurred in the Upper Mississippi subbasin, about 42 percent occurred in the Ohio/Tennessee River subbasin, about 7 percent occurred in the Missouri subbasin, and the remaining 25 percent of runoff occurred from the combined Lower Mississippi and Arkansas/Red subbasins (figure 7). While overall MARB Spring 2014 runoff is about 15 percent below average, the Upper Mississippi subbasin is about 2 percent above average, the Ohio/Tennessee subbasin is about 2 percent below average, the Missouri subbasin is about 50 percent below average, and the combined net contributions of the Lower Mississippi and Arkansas/Red subbasins are about 28 percent below average. 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 MARB. 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.

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, doi:10.4319/lo.2003.48.3.0951.

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, doi:10.1007/BF02803534

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