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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.

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.

What causes hypoxia? Major events leading to the formation of hypoxia in the Gulf of Mexico include: Freshwater discharge and nutrient loading of the Mississippi River Nutrient-enhanced primary production, or eutrophication Decomposition of biomass by bacteria on the ocean floor Depletion of oxygen due to stratification