Mississippi Basin nitrogen flux believed to cause Gulf hypoxia

An expanding hypoxic zone develops each spring and summer on the Louisiana-Texas shelf of the Gulf of Mexico, and nitrogen from the Mississippi River Basin has been implicated as one of the principal causes. Hypoxic conditions, which occur when dissolved oxygen concentrations are less than 2 mg/L, can cause stress or death in bottom-dwelling organisms that cannot leave the zone.

The mid-summer extent of the hypoxic zone has more than doubled since it was first systematically mapped in 1985 [Rabalais et al., 1999]. The largest hypoxic zone measured to date occurred in 1999, when it reached ∼20,000 km², about the size of the state of New Jersey [Rabalais, 1999].