Gulf of Mexico Dead Zone Smaller Than Predicted, But Deeper

Written by on August 5, 2009 in Marine Life, Other News
Gulf of Mexico Dead Zone - July 2009 - Credit: LUMCON

Gulf of Mexico Dead Zone – July 2009 – Credit: LUMCON

A group of scientists, supported by the National Oceanic and Atmospheric Administration (NOAA) and led by Nancy Rabalais, Ph.D. from the Louisiana Universities Marine Consortium (LUMCON), found the size of this year’s Gulf of Mexico dead zone to be smaller than forecasted, but deeper.

The Gulf of Mexico dead zone is the second largest hypoxic zone worldwide.  This years areal footprint of the low oxygen was smaller, the dead zone only measures 3,000 square miles, which she believes is due to unusual weather patterns that re-oxygenated the waters, among other factors.  However the thickness was greater, the area was very close to shore; the dead zone, which is usually limited to water just above the sea floor, was severe where it did occur, extending closer to the water surface than in most years.

Earlier this summer NOAA predicted a larger than normal dead zone area of between 7,450 – 8,456 square miles (read previous article).  The forecast was driven primarily by the high nitrate loads and high freshwater flows from the Mississippi and Atchafalaya rivers in spring 2009 as measured by the U.S. Geological Survey.

Rabalais said, “The winds and waves were high in the area to the west of the Atchafalaya River delta and likely mixed oxygen into these shallower waters prior to the cruise, thus reducing the area of the zone in that region. The variability we see within each summer highlights the continuing need for multiple surveys to measure the size of the dead zone in a more systematic fashion.”

The average size of the dead zone over the past five years, including this cruise, is now 6,000 square miles. The interagency Gulf of Mexico/Mississippi River Watershed Nutrient Task Force has a goal to reduce or make significant progress toward reducing this dead zone average to 2,000 square miles or less by 2015.

Robert Magnien, Ph.D., director of NOAA’s Center for Sponsored Coastal Ocean Research said, “The results of the 2009 cruise at first glance are hopeful, but the smaller than expected area of hypoxia appears to be related to short-term weather patterns before measurements were taken, not a reduction in the underlying cause, excessive nutrient runoff. The smaller area measured by this one cruise, therefore, does not represent a trend and in no way diminishes the need for a harder look at efforts to reduce nutrient runoff.”

Dead Zone - Sediment from the Mississippi River carries fertilizer to the Gulf of Mexico - Credit: NOAA

Dead Zone – Sediment from the Mississippi River carries fertilizer to the Gulf of Mexico – Credit: NOAA

The dead zone is fueled by nutrient runoff, principally from agricultural activity, which stimulates an overgrowth of algae that sinks, decomposes, and consumes most of the life-giving oxygen supply in the water. But dead zones are reversible. The Black Sea dead zone, previously the largest dead zone in the world, largely disappeared between 1991 and 2001 after fertilizers became too costly to use following the collapse of the Soviet Union and the demise of centrally planned economies in Eastern and Central Europe. From 1985 to 2000, the North Sea dead zone had nitrogen reduced by 37% when policy efforts by countries on the Rhine River reduced sewage and industrial emissions of nitrogen into the water. Other cleanups have taken place along the Hudson River and San Francisco Bay.

The models used to forecast the area of the dead zone, developed by R. Eugene Turner, Ph. D. of Louisiana State University and Donald Scavia, Ph. D. of the University of Michigan, are constructed for understanding the important underlying causes to inform long-term management decisions, but they do not include short-term variability due to weather patterns.

Related links:

Science Museum of Minnesota

LUMCON 2009 Gulf Hypoxia

LUMCON Hypoxia in the Northern Gulf of Mexico – supported by NOAA

NOAA’s National Ocean Service

Mississippi River/Gulf of Mexico Watershed Nutrient Task Force

Dead Zone Summer/Winter - Credit: NASA

Dead Zone Summer/Winter – Credit: NASA

Copyright ©  2009 by Marine Science Today, a publication of OceanLines LLC

About the Author

About the Author: Celia is Director of Business Operations for OceanLines LLC and is a frequent contributor to both OceanLines and Marine Science Today. She is a certified diver and her favorite topic is marine biology, especially stories about whales. .


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  1. Sweetwater Tom says:

    1) What is the largest hypoxic zone on earth? It used to be the Black Sea, according to the article.

    2) What does “deeper” mean? The hypoxic zone starts from the bottom up, it seems.

    3) Do scientists consider the hypoxic volume? Should they?


  2. Celia says:

    Dave Kidwell from NOAA responded the following to our inquiry:

    1) According to recent publications (see Diaz and Rosenberg, 2008 /Nature/), the Baltic remains the largest hypoxic zone on average.

    2) I’m not sure what you are referring to by “deeper”. Hypoxia does form on the bottom. This year, the hypoxic zone extended closer to the surface than in normal years.

    3) Yes, scientists consider the hypoxic volume, although area remains the primary measure to assess progress towards nutrient reduction goals. Efforts are underway to improve monitoring in the Gulf of Mexico that would allow for complimentary reporting of hypoxic volume in addition to area.

    In response to the other questions….Unfortunately, I am not an expert on Baltic Sea hypoxia, so would not be speaking from an informed position if I were to attempt to answer your question on land-use changes in Europe. In terms of the application of science to reduce hypoxic zones, there has been the direct use of this research to attempt to reduce their impacts. For example, in the Gulf of Mexico, science has identified nutrients as the primary cause of the hypoxic zone as well as identified agriculture as the primary source of these nutrients.

    Further, research indicated that to implement the 2001 Gulf Hypoxia Action Plan a 30% reduction in nitrogen would be required. Continued scientific investments have led to refinements in that approach. Thus, at the time of the release of the 2008 Gulf Hypoxia Action Plan it was recommended that a 45% reduction in nitrogen would be required as well as a 45% reduction in phosphorus. Ongoing research will likely continue to refine nutrient levels required to reduce the hypoxic zone as well as advance our understanding of the ecological and socioeconomic impacts of the hypoxic zone.

    For more information on the science and management of the hypoxic zone, I would recommend the following web sites:

  3. Sweetwater Tom says:

    My thanks to all for your efforts!