Spring Nutrient Delivery to the Gulf Estimated Among Highest in Three Decades

Spring Nutrient Delivery to the Gulf Estimated Among Highest in Three Decades

Spring nutrient delivery from the Mississippi-Atchafalaya River Basin to the northern Gulf of Mexico is estimated to be among the highest in the last three decades.  

Nutrient delivery, particularly during the months of April through June, has been identified as one of the primary factors controlling the size of the hypoxic zone that forms during the summer in the northern Gulf of Mexico. The Gulf hypoxic zone is an area where oxygen levels drop too low to support most life in bottom and near-bottom waters. Estimated nutrient contributions from October 2007 through June 2008 are available at http://toxics.usgs.gov/hypoxia/mississippi/oct_jun/index.html.

The large nutrient contributions are primarily due to near record-breaking streamflows this spring (April through June) in the Mississippi River Basin. Streamflows were about 50 percent higher this year compared to the long-term spring average flows since about 1980. Nutrient contributions for a given spring vary depending on the amount of flow in the Mississippi-Atchafalaya River Basin, as well as average stream water nutrient concentrations.    

Contributions of phosphorus to the Gulf of Mexico are estimated to be the highest for the USGS record since the 1980s. Total phosphorus (83,000 tons) and dissolved orthophosphate (26,000 tons) are about 60 and 85 percent higher than the long-term spring average for the nearly 30-year period.

Similarly, nitrogen contributions to the Gulf of Mexico from April through June are estimated to be about 35 to 40 percent higher than the long-term spring average since the early 1980s. Contributions for total nitrogen and dissolved nitrate during the three months are estimated to be about 817,000 and 578,000 tons, respectively.

USGS releases preliminary estimates of monthly nutrient fluxes from the Mississippi and Atchafalaya Rivers to the Gulf of Mexico in early July each year for the previous nine months (October through June). These estimates are used by the National Oceanic and Atmospheric Administration, Louisiana Universities Marine Consortium, U.S. Environmental Protection Agency, and other researchers for modeling the areal extent of the hypoxic zone.

Final USGS nutrient fluxes are based on streamflows and nutrient information for the full water year (through September 2008), and will be available in 2009 after data are finalized. Nutrient fluxes for previous years are available at http://toxics.usgs.gov/hypoxia/mississippi/nutrient_flux_yield_est.html.

USGS has monitored streamflow and water quality in the Mississippi River Basin for decades. More information on the monitoring network used to estimate total delivery of flow and nutrients to the Gulf; dataset preparation steps; and nutrient flux estimation methods are available in an online report at http://toxics.usgs.gov/pubs/of-2007-1080/index.html.

For more than 125 years, the USGS has monitored flow and (or) quality in selected streams and rivers across the U.S. Access data from more than 7,400 streamgages, many of which provide real-time data in 15 minute increments at http://water.usgs.gov/waterwatch/. For an even larger variety of USGS data, such as for ground water and water quality, access the National Water Information System Web Interface (NWISWeb), which contains over 1.5 million sites, and averages over 25 million hits per month (at http://waterdata.usgs.gov/nwis/).

In addition, USGS uses innovative geo-spatial modeling (referred to as SPARROW) to help understand relations among sources of nutrients, watershed characteristics, and resulting transport of nitrogen and phosphorus throughout the Mississippi-Atchafalaya River Basin. http://water.usgs.gov/nawqa/sparrow/gulf_findings/.