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2012 Gulf of Mexico 'Dead Zone' Size

Published on: 07/27/2012
  • 2012 Gulf Hypoxia in Brief
  • Mid-summer forecast:1,197 to 6,213 square miles
  • June survey result: 295 square miles
  • Mid-summer survey result: 2,889 square miles

Mid-Summer Survey Results
Results of an annual mid-summer hypoxic zone survey by the Louisiana Universities Marine Consortium found the fourth smallest dead zone on record in the northern Gulf of Mexico. The LUMCON-led team found hypoxia limited to a thin band near the mouth of the Mississippi River followed by a large non-hypoxic area to the west before finding several stations with hypoxia off of Galveston Bay. The patchy pattern of hypoxia observed in 2012 was unlike any other year documented, including those during drought years. The worsening drought in much of the United States, particularly in the Midwest, resulted in near record low levels of water traveling down the Mississippi into the Gulf.

Long-term measured size of the Gulf of Mexico hypoxic zone
This lack of fresh water entering the Gulf had two effects. First, if substantially reduced the amount of nutrients delivered to Gulf and limited the amount of plankton that could bloom and lead to hypoxia. The LUMCON team found a significant amount of clear 'blue' water during the survey, particularly further offshore, which indicates a relative lack of phytoplankton and suspended sediments.

Second, the lack of fresh water entering the Gulf did not allow the physical conditions for hypoxia to form. Fresh, or low salinity, water from the Mississippi River has a lower density (e.g., lighter) than the salt water of the Gulf of Mexico. As this fresh water enters the Gulf, it rests atop the salt water and creates a phenomenon known as stratification, where very little mixing occurs between the salty bottom water and the fresh surface water. When plankton sinks to the bottom water and decays, the bacteria feeding on the dead plankton consume oxygen, and this low-oxygen water is trapped under the fresh layer.

The hypoxic zone that forms each spring and summer off the coast of Louisiana and Texas, threaten valuable commercial and recreational Gulf fisheries. In 2009, the dockside value of commercial fisheries in the Gulf was $629 million. Nearly three million recreational fishers further contributed about $10 billion to the Gulf economy, taking 22 million fishing trips.

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