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Report Site Map > Discussion of Streamflow and Nutrient Delivery to the Gulf of Mexico

USGS Open-File Report 2007-1080 - Streamflow and Nutrient Fluxes of the Mississippi-Atchafalaya River Basin and Subbasins for the Period of Record Through 2005

Discussion of Streamflow and Nutrient Delivery to the Gulf of Mexico

The average annual streamflow delivered to the Gulf of Mexico during the period 1981 to 2005 was 21,700 cubic meters per second (m3/s). The average annual flux of total nitrogen and total phosphorus during that period was 1,470,000 and 140,000 metric tons as nitrogen (N) and phosphorus (P), respectively. Annual average streamflow ranged from 13,000 m3/s in the year 2000, to 29,500 m3/s in 1993. The "Great Flood of 1993" occurred in the American Midwest in 1993. The annual average flux of total nitrogen ranged from 810,000 metric tons as N in 2000, to 2,210,000 metric tons as N in 1983. The annual average flux of total phorphorus ranged from 80,700 metric tons as N in 1985, to 180,000 metric tons as N in 1987.

A strong relation between annual nutrient flux and streamflow for most nutrient species is apparent in the graphics of annual streamflow and nutrient flux. This is attributed largely to the fact that streamflow is the delivery mechanism for nutrient transport to the Gulf. For example, for the Mississippi-Atchafalaya River Basin for the period from 1979 through 2005, water year 2000 had the lowest annual average streamflow and the lowest annual dissolved nitrite plus nitrate flux; while water year 1993, a year when a large flood occurred on the Mississippi River, had the highest annual average streamflow and the highest annual dissolved nitrite plus nitrate flux (see figure 1 on the annual graphics page).

Similarly, a strong relation is apparent between monthly nutrient flux and streamflow; see the graphics of monthly nutrient flux and streamflow. As a measure of the strength of the relation between monthly nutrient flux and monthly streamflow, a model R2 was calculated (expressed as a percentage) from a simple linear regression model between these two variables. The model R2 represents the percentage of the variation in the monthly nutrient flux that can be explained by the linear relation with monthly average streamflow. The R2 for simple linear regressions of monthly nutrient flux and streamflow are high for dissolved nitrite plus nitrate (83 percent), total Kjeldahl nitrogen (68 percent), total nitrogen (85 percent), total phosphorus (75 percent), and dissolved silica (92 percent) indicating the significance of the role of streamflow on the amount of nutrient flux for these water-quality constituents. Dissolved orthophosphate had a moderate relation with streamflow (R2 = 47 percent), and dissolved ammonia had a poor relation with streamflow (R2 = 25 percent) indicating that streamflow had a lesser role in the amount of flux for these constituents.

Nutrient delivery from the Mississippi-Atchafalaya River Basin to the Gulf of Mexico has been identified as one of the 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). For example, the two years with the lowest spring dissolved nitrite plus nitrate and total nitrogen fluxes, 1988 and 2000 (figures 1 and 2) correspond to the only two years when the area of the hypoxic zone in the northern Gulf of Mexico was less than 5,000 km2. Graphics of the spring (April, May, June) nutrient flux for the period of record for other constituents (total phosphorus, dissolved orthophosphate, and dissolved silica) are presented in figures 3, 4, and 5, respectively.

Figure 1. Estimated April, May, and June dissolved nitrite plus nitrate flux as N to the Gulf of Mexico for 1979 through 2005.
Figure 1. Estimated April, May, and June dissolved nitrite plus nitrate flux as N to the Gulf of Mexico for 1979 through 2005.
Figure 2. Estimated April, May, and June total nitrogen flux as N to the Gulf of Mexico for 1980 through 2005.
Figure 2. Estimated April, May, and June total nitrogen flux as N to the Gulf of Mexico for 1980 through 2005.
Figure 3. Estimated April, May, and June total phosphorus flux as P to the Gulf of Mexico for 1979 through 2005.
Figure 3. Estimated April, May, and June total phosphorus flux as P to the Gulf of Mexico for 1979 through 2005.
Figure 4. Estimated April, May, and June dissolved orthophosphate flux as P to the Gulf of Mexico for 1982 through 2005.
Figure 4. Estimated April, May, and June dissolved orthophosphate flux as P to the Gulf of Mexico for 1982 through 2005.
Figure 5. Estimated April, May, and June dissolved silica flux as SiO2 to the Gulf of Mexico for 1980 through 2005.
Figure 5. Estimated April, May, and June dissolved silica flux as SiO2 to the Gulf of Mexico for 1980 through 2005.
Click on graphs for larger versions.

Spring runoff varies from 23 to 42 percent of annual runoff during the period 1979 to 2005 (figure 6). Spring nitrogen flux as a percent of annual flux is almost always higher than spring runoff as a percent of annual runoff. Spring dissolved nitrite plus nitrate flux ranges from 30 to 50 percent of the annual flux (figure 6) and spring total nitrogen flux ranges from 27 to 47 percent of the annual flux (figure 7). These results indicate that a disproportionately high amount of nitrogen flux occurs in the spring because of higher spring nitrogen concentration and higher spring streamflow. For the other nutrients, the spring flux as a percent of annual flux is similar to spring runoff as a percent of annual runoff. Spring total phosphorus varies from 20 to 43 percent of the annual flux (figure 8); spring orthophosphate varies from 18 to 40 percent of the annual flux (figure 9); and spring dissolved silica varies from 20 to 42 percent of the annual flux (figure 10). These results indicate that the amount of spring flux for these constituents is largely controlled by just one variable, higher spring streamflow.

Figure 6. Estimated April, May, and June dissolved nitrite plus nitrate flux and spring runoff (April to June) to the Gulf of Mexico for 1979 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 6. Estimated April, May, and June dissolved nitrite plus nitrate flux and spring runoff (April to June) to the Gulf of Mexico for 1979 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 7. Estimated April, May, and June total nitrogen flux and spring runoff (April to June) to the Gulf of Mexico for 1980 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 7. Estimated April, May, and June total nitrogen flux and spring runoff (April to June) to the Gulf of Mexico for 1980 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 8. Estimated April, May, and June total phosphorus flux and spring runoff (April to June) to the Gulf of Mexico for 1979 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 8. Estimated April, May, and June total phosphorus flux and spring runoff (April to June) to the Gulf of Mexico for 1979 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 9. Estimated April, May, and June orthophosphate flux and spring runoff (April to June) to the Gulf of Mexico for 1982 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 9. Estimated April, May, and June orthophosphate flux and spring runoff (April to June) to the Gulf of Mexico for 1982 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 10. Estimated April, May, and June dissolved silica flux and spring runoff (April to June) to the Gulf of Mexico for 1980 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Figure 10. Estimated April, May, and June dissolved silica flux and spring runoff (April to June) to the Gulf of Mexico for 1980 through 2005. Monthly fluxes and spring runoff are expressed as the percentage of the annual flux or runoff for that water year.
Click on graphs for larger versions.

References

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, 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.

Report Site Map > Discussion of Streamflow and Nutrient Delivery to the Gulf of Mexico

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