Oxygen Depletion in the Gulf of Mexico Adjacent to the Mississippi River

The seasonal formation of a bottom water layer severely depleted in dissolved oxygen has become a perennial occurrence on the Louisiana continental shelf adjacent to the Mississippi River system. Dramatic changes have occurred in this coastal ecosystem in the last half of the 20th century as the loads of dissolved inorganic nitrogen tripled. There are increases in primary production, shifts in phytoplankton community composition, changes in trophic interactions, and worsening severity of hypoxia. The hypoxic conditions (dissolved oxygen less than 2 mg l-1) cover up to 22,000 km2 of the seabed in mid-summer. Dissolved oxygen concentrations seldom decrease to anoxia, but are often below 1 mg l-1 and down to 0.5 mg l-1. The continental shelf of the northwestern Gulf of Mexico is representative of systems in which nutrient flux to the coastal ocean has resulted in eutrophication and subsequently hypoxia. The Mississippi River influenced continental shelf is similar to systems, such as deep basins and fjords, with regard to biogeochemical processes of oxic versus suboxic conditions in the water column and sediments. However, the suboxic conditions for the Gulf of Mexico are less persistent in time and space due to the dynamic nature of the open continental shelf system. Also, anoxia at the seabed is not as common or long lasting