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 2015 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) 2015 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 2015 to the period of record. Note that 2015 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 2015. Maximum, minimum, and average runoff are determined for the period 1979 through 2014.
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 2015. Maximum, minimum, and average fluxes are determined for the period 1979 through 2014. *Note that 2015 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 2015. Maximum, minimum, and average fluxes are determined for the period 1980 through 2014. *Note that 2015 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 2015. Maximum, minimum, and average fluxes are determined for the period 1979 through 2014. *Note that 2015 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 2015. Maximum, minimum, and average fluxes are determined for the period 1982 through 2014. *Note that 2015 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 2015. Maximum, minimum, and average fluxes are determined for the period 1980 through 2014. *Note that 2015 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 2015 flows are preliminary as they contain some provisional data. Also note that Lower Mississippi mean streamflows for 2015 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 2014, 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 2014, 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 2015 is above the average for the period of record from 1979 to 2015 (about 10 percent above average, 13th highest spring runoff in 37 years; figure 1; period of record defined by concurrent availability of sufficient water-quality samples for flux estimation for both rivers). April (about 20 percent) and May (about 2 percent) runoff were both above average. Spring 2014 runoff is about 29 percent above last year's spring runoff.

Above average April and May streamflow did not result in above average nitrate (21 percent below average from 1979-2014) and TN fluxes (12 percent below average). Dissolved nitrite plus nitrate flux for Spring 2015 was about 210,000 metric tons as N, which is ranked as the 10th lowest spring dissolved nitrite plus nitrate flux over the 37-year period of record (1979-2015). Spring 2015 nitrate loads were about 0.3 percent higher than last year's spring flux of 209,000 metric tons (when streamflows were below average), which is the 9th lowest spring flux.

Total nitrogen flux for the Spring of 2014 was about 338,000 metric tons, which was the 14th lowest recorded from 1980-2015 (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 22 and 14 percent below average, respectively. Phosphorus fluxes for the Spring of 2014 were 39,800 metric tons as P for total phosphorus and 10,300 metric tons as P for dissolved orthophosphate, 18 and 16 percent above average respectively (figures 4 and 5). The dissolved silica flux in the Spring of 2014 was about 1,070,000 metric tons as SiO2; about 5 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 12 percent of the Spring 2015 runoff occurred in the Upper Mississippi subbasin, about 35 percent occurred in the Ohio/Tennessee River subbasin, about 8 percent occurred in the Missouri subbasin, and the remaining 45 percent of runoff occurred from the combined Lower Mississippi and Arkansas/Red subbasins (figure 7). While overall MARB Spring 2014 runoff is about 10 percent above average, the Upper Mississippi subbasin is about 36 percent below average, the Ohio/Tennessee subbasin is about 3 percent above average, the Missouri subbasin is about 19 percent below average, and the combined net contributions of the Lower Mississippi and Arkansas/Red subbasins are about 66 percent above 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

USGS Home Water Climate Change Science Systems Ecosystems Energy and Minerals Environmental Health Hazards

Accessibility FOIA Privacy Policies and Notices

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:
Page Last Modified: Monday, 08-Jun-2015 11:51:48 EDT