Mississippi Floods to Raise the Dead Zone in the Gulf

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A record-setting dead zone is predicted to occur in the Gulf of Mexico and expected to kill bottom-dwelling fish and other marine life over a significant portion of the seafloor this summer following the rise in nutrient runoff from the Mississippi floods, according to marine scientists supported by the National Oceanic and Atmospheric Association.

This year’s forecast estimates that the size of the low-oxygen or hypoxic region in the Gulf will reach up to 9,421 square miles, the size of New Jersey and Delaware combined. City-sized portions of this region could see oxygen levels in the water column dropping to zero.

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“While there is some uncertainty regarding the size, position and timing of this year’s hypoxic zone in the Gulf, the forecast models are in overall agreement that hypoxia will be larger than we have typically seen in recent years,” said Jane Lubchenco, undersecretary of commerce for oceans and atmosphere and NOAA administrator, in a statement.

Oceanographers from the Louisiana Universities Marine Consortium, Louisiana State University, and the University of Michigan use nutrient inputs compiled from the U.S. Geological Survey’s extensive stream gauge network along the Mississippi River to forecast the marine biogeochemical reaction to the uploads of nitrogen and phosphorus into the Gulf of Mexico. In May 2011, the Mississippi watershed’s nitrogen transport into the Gulf was 35 percent higher than the average for that month over the last 32 years.

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Nancy Rabalais, director of Louisiana Universities Marine Consortium will lead a two-week survey of Gulf waters later this summer to monitor the actual size and variation of this year’s dead zone. Monitoring the dead zone in the Gulf of Mexico began in 1985 and the largest event came in 2002 when low-oxygen levels affected 8,400 square miles of coastal waters.

Farm fertilizers washed downstream are one of primary sources of excess nitrogen and phosphorus into the Gulf of Mexico. The added nutrients allow for sudden marine algal blooms that color the sea green with chlorophyll.

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But the blooms of phytoplankton die almost as quickly as they form, and sink to the seafloor where bacteria consume the snowfall of organic matter as well as oxygen from the water column. Schools of fish can get caught in the oxygen-depleted waters and choke to death as they swim. Lobsters, crabs, and other marine animals that dwell on the bottom are the most susceptible.

Areas that are shallow and warm provide for a thriving seafloor bacterial community and an increased risk for greater oxygen depletion. In deeper, cooler waters, the metabolism of the bacteria is slowed and they consume less oxygen.

Storms and coastal winds can mix the water column and reduce the severity of a dead zone, as was the case in 2009. That year, the dockside value of commercial fisheries in the Gulf of Mexico was $629 million, NOAA reported. Despite the storms, nearly three million recreational fishers took 22 million fishing trips to the Gulf, contributing more than $1 billion to the Gulf economy.

IMAGE: A relatively cloud-free Sea-viewing Wide Field-of-view Sensor (SeaWiFS) image of the Gulf of Mexico captured on November 7, 2004.

2002 and 2008 Maps: Showing Dissolved Oxygen contours. Credit: Gulf of Mexico Hypoxia Watch.

Chart: Annual measurements of the size of the Gulf of Mexico's hypoxic zones with the 2011 forecast. Dark gray represents the range of forecasts. Credit: Nancy Rabalais/Louisiana Universities Marine Consortium, National Oceanic and Atmospheric Administration.