Hypoxia, or oxygen depletion, is an environmental phenomenon where the concentration of dissolved oxygen in the water column decreases to a level that can no longer support living aquatic organisms.

Hypoxia in the northern Gulf of Mexico is defined as a concentration of dissolved oxygen less than 2 mg/L (2 ppm). This figure is based on observational data that fish and shrimp species normally present on the sea floor are not captured in bottom-dragging trawls at oxygen levels < 2mg/L. In other oceans of the world, the upper limit for hypoxia may be as high as 3-5 mg/L.

Hypoxia occurs naturally in many of the world’s marine environments, such as fjords, deep basins, open ocean oxygen minimum zones, and oxygen minimum zones associated with western boundary upwelling systems. Hypoxic and anoxic (no oxygen) waters have existed throughout geologic time, but their occurrence in shallow coastal and estuarine areas appears to be increasing as a result of human activities (Diaz and Rosenberg, 1995). The largest hypoxic zone currently affecting the United States, and the second largest hypoxic zone worldwide, occurs in the northern Gulf of Mexico adjacent to the Mississippi River on the Louisiana/Texas continental shelf. The maximum areal extent of this hypoxic zone was measured at 22,000 km2 during the summer of 2002; this is approximately the same size as the state of Massachusetts.

The average size of the hypoxic zone in the northern Gulf of Mexico over the past five years (2004-2008) is about 17,000 km2, the size of Lake Ontario. For comparison, the entire surface area of the Chesapeake Bay and its major tributaries measures about 11,000 km2.