Gulf of Mexico dead zone could reach 8,000 square miles

Mississippi River plume meets Gulf of Mexico at Southwest Pass. Photo courtesy of the Louisiana Universities Marine Consortium

This year’s “Dead Zone” in the Gulf of Mexico could be one of the largest on record. The Gulf dead zone is formed each spring and summer off the Louisiana and Texas coast when oxygen levels drop too low to support life in bottom and near-bottom waters.

This latest forecast, headed by University of Michigan aquatic ecologist Donald Scavia and colleagues, sees a Gulf dead zone of between 7,450 and 8,456 square miles or an area about the size of New Jersey.

This puts the years 2009, 2008, and 2001 as a virtual tie for second place on the list of the largest Gulf dead zones and continues a decades-long trend that threatens the health of the fishery industry.

It would also mean that the five largest Gulf dead zones on record have occurred since 2001. The biggest of these oxygen-starved, or hypoxic, regions developed in 2002 and measured 8,484 square miles.

The forecast for a large 2009 dead zone is based on above-normal flows in the Mississippi and Atchafalaya rivers this spring. In April and May, the flows in the two rivers were 11% above average.

The flows contain large amounts of nutrient nitrogen and phosphorus, mostly farmland run-off from fertilization and livestock waste. These nutrients cause algae blooms, which die and sink. Bacteria decompose the organic matter and use up the oxygen in the water, making it uninhabitable for other marine life.

Additional flooding of the Mississippi since May could also result in a dead zone that exceeds the upper limit of the current forecast. Conversely, northeast of the Gulf, the Chesapeake Bay could find its dead zone shrinking between 0.7 to 1.8 cubic miles.

A regional dry spell that lasted from January to April has decreased the flow and the amount of nitrogen expected to enter the Bay via the Susquehanna River. If this happens, this will be the lowest level since 2001 and the third lowest on record.

Despite this, scientists caution that continued high flows in June, beyond the period used for forecasts, could make the actual size of the dead zone higher at the end of the forecast range.

"While it's encouraging to see that this year's Chesapeake Bay forecast calls for a significant drop in the extent of the dead zone, we must keep in mind that the anticipated reduction is due mainly to decreased precipitation and water runoff into the Bay," said Scavia.

"The predicted 2009 dead-zone decline does not result from cutbacks in the use of nitrogen, which remains one of the key drivers of hypoxia in the Bay,” added Scavia.

The Gulf hypoxia research team is supported by the U.S. National Oceanic and Atmospheric Administration's Center for Sponsored Coastal Ocean Research and includes scientists from Louisiana State University and the Louisiana Universities Marine Consortium.

The official size of the 2009 hypoxic zone will be announced following a NOAA-supported monitoring survey led by the Louisiana Universities Marine Consortium on July 18-26. In addition, NOAA's Southeast Area Monitoring and Assessment Program's (SEAMAP) is currently providing near real-time data on the hypoxic zone during a five-week summer fish survey in the northern Gulf of Mexico.


-  Katrice R. Jalbuena

Sources:

¹ http://www.ns.umich.edu/htdocs/releases/story.php?id=7203
² http://www.gulfhypoxia.net/