Hypoxia in the Gulf of Mexico

Seasonally severe and persistent hyporia, or low dissolved oxygen concentra tion, occurs on the inner- to mid-Louisiana continental shelf to the west o f the Mississippi River and Atchafalaya River deltas. The estimated areal e xtent of bottom dissolved oxygen concentration less than 2 mg L-1 during mi d-summer surveys of 1993-2000 reached as high as 16 000 to 20 000 km(2). Th e distribution for a similar mapping grid for 1985 to 1992 averaged 8000 to 9000 km(2). Hypoxia occurs below the pycnocline from as early as late Febr uary through early October, but is most widespread, persistent, and severe in June, July, and August. Spatial and temporal variability in the distribu tion of hypoxia exists and is, at least partially, related to the amplitude and phasing of the Mississippi and Atchafalaya discharges and their nutrie nt flux. Mississippi River nutrient concentrations and loadings to the adja cent continental shelf have changed dramatically this century, with an acce leration of these changes since the 1950s to 1960s. An analysis of diatoms, foraminiferans, and carbon ac accumulation in the sedimentary record provi des evidence of increased eutrophication and hypoxia in the Mississippi Riv er delta bight coincident with changes in nitrogen loading.