Every spring the process begins and culminates in a vast region of oxygen-starved ocean bottom that stretches along the Louisiana and Texas coasts. Hypoxia is defined by a dissolved oxygen concentration in seawater of no more than 2 ppm. At oxygen concentrations below this level, fish trawlers are unable to find or capture any live shrimp or bottom-dwelling fish in their nets. The low oxygen levels drive away fish, while bottom-dwellers such as shrimp, crabs, snails, clams, starfish, and worms eventually suffocate. These features of an otherwise highly productive coastal ecosystem are cause for concern since fisheries are a vital renewable resource of the northern Gulf of Mexico.
Hypoxia results from a combination of natural and human-influenced factors.
Upon entering the Gulf of Mexico, freshwater from the Mississippi River drainage basin floats over the saltier, denser water of the Gulf, resulting in a stratification of the water column. This stratification intensifies in the summer and prevents any oxygen housed within the upper layers of the Gulf of Mexico from being introduced to the bottom. In addition, the Mississippi River discharge contains high levels of nutrients, such as nitrogen, phosphorus, and silica, some of which is natural but much of which is derived from the widespread application of fertilizers on farmlands that drain into the Mississippi River and ultimately into the Gulf of Mexico. These microscopic plants, in turn, support the rest of the marine food web. However, as these plants die and sink to the bottom, the natural decomposition of this dead plant material depletes the deeper waters of the Gulf of Mexico of what little oxygen it may contain.