Streamflow and Nutrient Delivery from the Mississippi River Basin to the Gulf of Mexico

The mouth of Mississippi River Basin where it flows into the northern Gulf of Mexico

U.S. Geological Survey (USGS) scientists have published a new analysis of streamflow and nutrient (nitrogen, phosphorus, and silica) delivery from the Mississippi River Basin to the northern Gulf of Mexico. Scientists have linked the delivery of nutrients and streamflow from the Basin to the formation and extent of a "hypoxic zone" — a zone of waters with low dissolved oxygen that forms each summer in the northern Gulf along the Louisiana-Texas coast. The resulting lack of oxygen can cause stress or death in bottom-dwelling organisms that cannot escape to more oxygen-rich areas of the Gulf.

The Mississippi River Basin drains about 3 million square kilometers or about one third of the land area of the United States. It discharges to the Gulf of Mexico in Louisiana via the main stem of the Mississippi River and the Atchafalaya River.

The report also provides information on streamflow and nutrient delivery for 30 subbasins. The subbasins vary in size from 16,200 square kilometers to 1,847,000 square kilometers and have varied nutrient yields based on differing hydrology, land use, and climate. The information is presented for the available period of record for each subbasin, which for some dates back to the early 1960's.

Annual total nitrogen flux and streamflow for total Mississippi-Atchafalaya River Basin

Annual total phosphorus flux and streamflow for total Mississippi-Atchafalaya River Basin

Total Nitrogen flux and total phosphorus flux (in metric tons as nitrogen and phosphorus) delivered to the Gulf of Mexico by the Mississippi River Basin during the period 1981 to 2005. The average annual streamflow to the Gulf of Mexico, for the period 1981-2005 was 21,700 cubic meters per second. The average annual flux of total nitrogen and total phosphorus was 1,470,000 and 140,000 metric tons as nitrogen (N) and phosphorus (P), respectively. For the period, annual average streamflow ranged from 13,000 (2000) to 29,500 (1993) cubic meters per second. Annual average flux of total nitrogen ranged from 810,000 (2000) to 2,210,000 (1983) metric tons as N. Annual average flux of total phosphorus ranged from 80,700 (1985) to 180,000 (1987) metric tons as P. Among other factors, nutrient flux was dependent upon streamflow, the major mechanism for downstream transport and delivery to the Gulf of Mexico. For the period 1981-2005, the Upper Mississippi and Ohio/Tennessee subbasins contributed 39 and 34 percent of total nitrogen and 27 and 31 percent of total phosphorus, while comprising only 15.7 and 16.7 percent of the land area, respectively.
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Scientists will use this information to investigate causal linkages between the delivery of nutrients and streamflow to the northern Gulf and the magnitude and duration of the "hypoxic zone". Managers, including the Mississippi River/Gulf of Mexico Watershed Nutrients Task Force, will use this information to guide actions to mitigate problems associated with excess nutrients in local receiving waters, as well as the Gulf of Mexico. Currently, the Task Force is conducting a science assessment of the causes of hypoxia in the northern Gulf of Mexico.

The report also presents information for the five major subbasins that comprise the entire Mississippi River Basin. This information will be used by subbasin teams established by the Task Force to address nutrient runoff at the level of major subbasins of the Mississippi River Basin.

The Report

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