Issue 6

Hypoxia and the Dead Zone

Runoff and erosion from both the Upper and Lower Mississippi Alluvial Valleys contribute to the dead zone in the Gulf of Mexico. (photo by NASA)

The dead zone in the northern Gulf of Mexico is an area along the Louisiana- Texas coast where the deepest water contains less than 2 percent parts per million of dissolved oxygen, not enough for fish and other aquatic organisms to live. Fish leave the area; bottom-dwelling organisms that cannot move experience extreme stress, often dying.

This hypoxia—defined as the absence of oxygen available to living tissues—is caused mainly by the excess of nitrogen delivered to the Gulf of Mexico by the Mississippi River as it flows through both the Upper and Lower Mississippi Alluvial Valleys. Sources of excess nitrogen and other nutrients include: runoff from developed land, soil erosion, agricultural fertilizers, and atmospheric deposition. Nitrogen promotes the growth of algae. As the algae dies, it sinks to the bottom, using up available oxygen.

Since 1993, the average extent of the dead zone in the Gulf of Mexico has been around 6,200 square miles, twice the average size measured between 1985 and 1992. The hypoxic zone reached its maximum size so far in 2002, when it was measured at around 8,500 square miles—larger than the State of Massachusetts.

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