USGS - science for a changing world

Hypoxia in the Gulf of Mexico

Home
USGS Info on MRB Nutrients
Gulf of Mexico Hypoxic Zone
Hypoxia Task Force
Other Agency Info
USGS Publications

Discussion of 2013 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) 2013 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 2013 to the period of record. Note that 2013 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 2013. Maximum, minimum, and average runoff are determined for the period 1979 through 2012.
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 2013. Maximum, minimum, and average fluxes are determined for the period 1979 through 2012. *Note that 2013 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 2013. Maximum, minimum, and average fluxes are determined for the period 1980 through 2012. *Note that 2013 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 2013. Maximum, minimum, and average fluxes are determined for the period 1979 through 2012. *Note that 2013 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 2013. Maximum, minimum, and average fluxes are determined for the period 1982 through 2012. *Note that 2013 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 2013. Maximum, minimum, and average fluxes are determined for the period 1980 through 2012. *Note that 2013 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 2013 flows are preliminary as they contain some provisional data. Also note that Lower Mississippi mean streamflows for 2013 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 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.)
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 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.)
Click on graphs for larger versions.

Runoff from the Mississippi and Atchafalaya Rivers in Spring 2013 is very close to the average for the period of record 1979 to 2012 (about 4 percent above average, 15th highest spring runoff in 35 years; figure 1; period of record defined by concurrent availability of sufficient water-quality samples for flux estimation for both rivers). April runoff is below average (about -16 percent) while May runoff is well average (about 23 percent). Spring 2013 runoff is about 66 percent above last year’s well below average spring runoff.

Below average April streamflow resulted in below to near average April nutrient fluxes that ranged from 23 percent below average to 7 percent above average, depending on the nutrient. Above average May streamflow resulted in above average May nutrient fluxes that ranged from 16 to 59 percent above average. Dissolved nitrite plus nitrate flux for Spring 2013 was about 265,000 metric tons as N, about 3 percent below the average for the period of record (1979 - 2012; figure 2). This is the 18th highest spring dissolved nitrite plus nitrate flux over the 35-year period of record (1979-2013), and is about 77 percent higher than last year's spring flux of 145,000 metric tons (when streamflows were much lower), which is the 2nd lowest spring flux. Total nitrogen flux for Spring 2013 was about 380,000 metric tons as N, about 1 percent below average (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 16 and 19 percent above average, respectively. Phosphorus fluxes for Spring 2013 were 43,800 metric tons as P for total phosphorus and 11,500 metric tons as P for dissolved orthophosphate, both about 31 percent above average (figures 4 and 5). Dissolved silica flux in Spring 2013 was about 987,000 metric tons as SiO2, about 3 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 32 percent of the Spring 2013 runoff occurred in the Upper Mississippi subbasin, about 39 percent occurred in the Ohio/Tennessee River subbasin, about 11 percent occurred in the Missouri subbasin, and the remaining 19 percent of runoff occurred from the combined Lower Mississippi and Arkansas/Red subbasins (figure 7). While overall MARB Spring 2013 runoff is slightly above average, the Upper Mississippi subbasin is about 55 percent above average, the Ohio/Tennessee subbasin is about 6 percent above average, the Missouri subbasin is about 6 percent below average, and the combined net contributions of the Lower Mississippi and Arkansas/Red subbasins are about 36 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.

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.

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://toxics.usgs.gov/hypoxia/mississippi/oct_jun/graphics.html
Page Contact Information: Webmaster
Page Last Modified: Friday, 14-Jun-2013 15:22:09 EDT