Hypoxia, or low dissolved oxygen, is defined as oxygen levels less than 2mg per l. The northern Gulf shelf is home to one of the largest seasonal hypoxia events in the world, primarily resulting from riverine-derived nutrients from the Mississippi-Atchafalaya River watershed described above, as well as salinity stratification of oceanic waters which inhibits re-aeration of bottom waters. Sufficient concentrations of dissolved oxygen in the subsurface depths of the ocean are critical to maintaining ecosystem function. Benthic organisms may die when exposed to extended hypoxic conditions, and mobile organisms may move out of the area, altering fishery harvest and bycatch rates. In the GoM, hypoxic events are most common during the summer months, and in the shelf waters off of the Louisiana coast but have been documented on the Texas shelf as well as further east.
Bottom water dissolved oxygen is measured regularly as part of the fishery-independent Southeast Area Monitoring and Assessment Program (SEAMAP) trawl and hydrographic survey which has been conducted bi-annually since 1987. Estimates of bottom dissolved oxygen concentrations show an overall decreasing trend over time, although in recent years concentrations have increased off the coast of Louisiana in both summer and fall. Hypoxia off the coast of Texas is typically less severe, although bottom oxygen concentrations have decreased more dramatically in recent years. The severity and spatial extent of the hypoxic zone varies greatly from year to year, due to a number of factors such as wind stress, freshwater discharge, hurricane activity, coastal circulation, and atmospheric warming (Bianchi et al. 2010). Interestingly, the recent increase in bottom dissolved oxygen off the coast of Louisiana coincides with a decrease in sea surface temperature in that region, whereas the decrease in dissolved oxygen off the coast of Texas coincides with a recent increase in sea surface temperature in that region. Increased stratification reduces vertical mixing and prevents oxygen from diffusing below the pycnocline (Scavia et al., 2003), and the observed recent trends in hypoxia may be partially explained by changes in temperatures.