Gulf of Mexico hypoxia: alternate states and a legacy.

A 20+ year data set of the size of the hypoxic zone off the Louisiana-Texas coast is analyzed to reveal insights about what causes variation in the size of the hypoxic zone in summer, the accumulation of carbon storage in sediments, and pelagic and sediment oxygen demand. The results of models support the conclusion that some of this variation can be explained by a higher sedimentary oxygen demand, which may be larger than water column respiration rates in summer. Proxies for organic loading to sediments reveal that carbon losses continue after accumulation, and results from other studies indicate that sediment oxygen demand is directly related to surface water phytoplankton production, which has increased because of higher nutrient loading from the Mississippi River watershed. The potential size of the hypoxic zone for a given nitrogen load has increased as a result and has doubled from 1980 to 2000. The development of widespread hypoxia after the early 1980s and its consequences could, therefore, be considered a shift to an alternate ecosystem state. The Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico goal of reducing the size of the hypoxic zone to an average of 5000 km2 by 2015 becomes more difficult to achieve for every year there is no significant reduction in nutrient loading. The decisions made to reduce the size of the hypoxic zone must incorporate these nonlinear responses and, we think, err on the side of caution in assuming that existing management efforts are sufficient to restore water quality on this shelf. The legacy of a higher sediment respiratory demand following eutrophication should apply to other coastal systems.