HYPOXIA HOT SPOTS IN THE MISSISSIPPI BIGHT

Foraminifer proxies of hypoxia indicate low oxygen conditions in several hot spots in the Mississippi Bight.  The foraminifer hypoxia proxy, the Ammonia to Elphidium ratio, was tabulated from core-top data published in 1954 and 1955 and compared to core tops collected in 2000-2001 during the Northern Gulf of Mexico Littoral Initiative (NGLI) and other samples of opportunity collected since 2000 Additionally, the oxygenation history of a site near the Balize delta was evaluated over the past one hundred years based on samples from a gravity core dated by ²¹⁰Pb.

The results from the 1950s core-top collections show persistent low to hypoxic conditions on the shelf at hot-spot locales seaward of the Mississippi-Alabama barrier islands and the eastern distributaries of the Balize delta. Specifically, the ratio of Ammonia to Elphidium exceeds 80% seaward of Horn Island Pass, Ship Island Pass, and Mobile Bay in the Mississippi-Alabama barrier island region, and seaward of Pass à Loutre and the southernmost outlet from Breton Sound in the Balize delta area. Consistent with these results are reports of present-day low to hypoxic oxygen concentrations in bottom waters at several of these sites associated with seasonally-high, average surface chlorophyll a and seasonal strengthening of a freshwater cap. Results from core tops collected since 2000 indicate that the hypoxic hot spots may be increasing in size. Results from the gravity core off Pass à Loutre indicate no clear change in conditions over the past century.