Spring 2009 Nutrient Delivery to the Gulf of Mexico Above 30-Year Average


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Spring 2009 Nutrient Delivery to the Gulf of Mexico Above 30-Year Average

By Brent Aulenbach and Jennifer LaVista
August 2009

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Nutrient delivery to the northern Gulf of Mexico during spring 2009 was among the highest measured by the U.S. Geological Survey (USGS) in 30 years.

An abundance of nutrients, which are essential for plant growth, is not necessarily a good thing. Excessive nutrients can decrease the amount of oxygen in the water, a phenomenon known as hypoxia. This oxygen depletion can result in an area called a hypoxic zone, or "dead zone," where organisms living on or near the bottom experience stress or even death.

Hypoxia, along with overfishing, habitat loss, and toxic contamination, can adversely affect the Gulf of Mexico coastal region, an important fishing ground that provides the Nation with about 1.2 billion pounds of fresh seafood every year.

Above: Dissolved oxygen in bottom waters, measured from June 8 through July 17, 2009, during the annual summer Gulf of Mexico Southeast Area Monitoring and Assessment Program (SEAMAP) cruise in the northern Gulf of Mexico. Orange and red colors indicate lower dissolved oxygen concentrations. Data were received from the NOAA ship Oregon II twice weekly and assimilated into a geospatially enabled database; more information and maps from previous years are available on NOAA's Gulf of Mexico Hypoxia Watch Web site. [larger version]

The amount of nutrients transported from the Mississippi River Basin to the Gulf during the spring is a major factor controlling the size of the hypoxic zone. The northern Gulf of Mexico hypoxic zone is the second largest in the world (the largest is in the Baltic Sea) and threatens the economic and ecological health of one of the Nation's largest and most productive fisheries.

Nutrients can come from many sources, such as fertilizers applied to agricultural fields, golf courses, and suburban lawns; atmospheric contributions; erosion of soils; and discharge from sewage-treatment plants.

In early June each year, the USGS releases estimates of nutrients delivered to the Gulf of Mexico from the Mississippi and Atchafalaya Rivers (http://toxics.usgs.gov/hypoxia/mississippi/oct_jun/). These estimates are used by the National Oceanic and Atmospheric Administration (NOAA), the Louisiana Universities Marine Consortium, and other researchers to predict the areal extent of the hypoxic zone.

Predictions of the size of the 2009 hypoxic zone, released June 18 (see NOAA press release), reflect USGS estimates of about 295,000 metric tons of nitrogen (in the form of nitrate) delivered in April and May 2009 to the northern Gulf. In 2008, the hypoxic zone exceeded 20,000 km², an area similar in size to the State of New Jersey. Spring delivery of nitrogen in 2009 was about 23 percent lower than that measured in 2008, but still about 11 percent above the average from 1979 to 2009.

The amount of nutrients delivered to the Gulf each spring depends in large part on precipitation and the resulting amounts of nutrient runoff and streamflow in the Mississippi-Atchafalaya River Basin. Streamflows in spring 2009 were about 17 percent above the past 30-year average. Streamflows in spring 2008 were even higher, contributing to that year's higher nutrient levels (see graphs at Discussion of 2009 Preliminary Spring (April and May) Nutrient Fluxes).

State and Federal partners serving on the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force are trying to reduce nutrients transported to the Gulf in order to reduce the 5-year-running-average areal extent of the Gulf of Mexico hypoxic zone to less than 5,000 km² by the year 2015. (The 5-year running average is the average of areal extents measured in 5 consecutive years, used to smooth the effect of an anomalously large or small hypoxic zone in any given year.) Tracking nutrient levels every year is important to determine if partners are on target for that goal.

The USGS has monitored streamflow and water quality in the Mississippi River Basin for decades; to access more information, visit Streamflow and Nutrient Flux of the Mississippi-Atchafalaya River Basin and Subbasins Through Water Year 2008.

For more than 125 years, the USGS has served as the Nation's water-monitoring agency, collecting data on streamflow and water quality in selected streams and rivers across the United States. Access data from more than 7,400 streamgages, many of which provide real-time data in 15-min increments, at the USGS WaterWatch site.

For an even greater variety of USGS data, including data on groundwater as well as surface water, visit the National Water Information System's Web Interface, which provides access to water-resources data collected at approximately 1.5 million sites in all 50 States, the District of Columbia, and Puerto Rico.

Delivery of Sediment-Associated Contaminants to the Gulf of Mexico
June 2003