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Discussion of 2017 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. Most years 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) 2017 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 2017 to the period of record. Note that 2017 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 2017. Maximum, minimum, and average runoff are determined for the period 1979 through 2016.	Figure 2. Estimated April and May dissolved nitrite plus nitrate flux as N to the Gulf of Mexico for 1979 through 2017. Maximum, minimum, and average fluxes are determined for the period 1979 through 2016. *Note that 2017 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 2017. Maximum, minimum, and average fluxes are determined for the period 1980 through 2016. *Note that 2017 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 2017. Maximum, minimum, and average fluxes are determined for the period 1979 through 2016. *Note that 2017 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 2017. Maximum, minimum, and average fluxes are determined for the period 1982 through 2016. *Note that 2017 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 2017. Maximum, minimum, and average fluxes are determined for the period 1980 through 2016. *Note that 2017 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 2017 flows are preliminary as they contain provisional data. Also note that Lower Mississippi mean streamflows for 2017 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 2016, 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 2016, 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 the Spring of 2017 is larger than average for the period of record from 1979 to 2017 (about 8 percent above average, 15th highest spring runoff in 38 years; figure 1; period of record defined by concurrent availability of sufficient water-quality samples for flux estimation for both rivers). April runoff was about 16 percent below average, while May runoff was about 31 percent above average. Spring 2016 runoff is about 9 percent above last year's spring runoff.

Above average April and May streamflow resulted in above average nitrate (13 percent above average from 1979-2016) and TN fluxes (5 percent above average). Dissolved nitrite plus nitrate flux for Spring 2017 was about 299,000 metric tons as N, which is ranked as the 13th highest spring dissolved nitrite plus nitrate flux over the 39-year period of record (1979-2017). Spring 2017 nitrate loads were about 19 percent higher than last year's spring flux of 252,000 metric tons, which is the 22nd highest spring flux.

Total nitrogen flux for the Spring of 2017 was about 399,000 metric tons, which was the 17th highest recorded from 1980-2017 (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 17 percent above average, respectively. Phosphorus fluxes for the Spring of 2017 were 38,400 metric tons as P for total phosphorus and 11,800 metric tons as P for dissolved orthophosphate, 12 and 32 percent above average respectively (figures 4 and 5). May total phosphorus (22,600 metric tons) flux in 2017 was about 33 percent above the average from 1979-2016, and was the 7th highest May flux over that period. May dissolved orthophosphate (7,170 metric tons) flux was about 48 percent above the average from 1982-2016, and ranked as the 4th highest May flux over that period. The dissolved silica flux in the Spring of 2017 was about 1,290,000 metric tons as SiO2; about 26 percent above 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 28 percent of the Spring 2017 runoff occurred in the Upper Mississippi subbasin, about 43 percent occurred in the Ohio/Tennessee River subbasin, about 16 percent occurred in the Missouri subbasin, and the remaining 13 percent of runoff occurred from the combined Lower Mississippi and Arkansas/Red subbasins (figure 7). While overall MARB Spring 2017 runoff is about 8 percent above average, the Upper Mississippi subbasin is about 42 percent above average, the Ohio/Tennessee subbasin is about 27 percent above average, the Missouri subbasin is about 53 percent above average, and the combined net contributions of the Lower Mississippi and Arkansas/Red subbasins are about 55 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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