HYPOXIA HOT SPOTS IN THE MISSISSIPPI BIGHT

Foraminifer proxies of hypoxia indicate low-oxygen conditions and possible hypoxia in several hot spots in the Mississippi Bight. Two foraminifer proxies of hypoxia, the Ammonia-Elphidium (A/E) index and the Pseudononion-Epistominella-Buliminella (PEB) index, were tabulated from published census data from core tops collected in 1951-1956. Additionally, the PEB index was compiled from census data spanning the past 100 years in a ²¹⁰Pb-dated gravity core from the middle shelf northeast of the Balize delta. Biogeography of the indicator species dictates that the A/E index is most effective at depths <30 m, whereas the PEB index is effective at shelf depths >30 m.

The results from the 1950s core-top collections suggest recurrent, seasonal low oxygen to hypoxic conditions in bottom water at hot-spot locales seaward of the Mississippi-Alabama barrier islands and the eastern distributaries of the Balize delta. Specifically, the A/E index exceeds 90% seaward of Horn and Dauphin Islands of the Mississippi-Alabama barrier islands, and seaward of Pass à Loutre and Main Pass of the Balize delta. Consistent with these results are reports of present-day low to hypoxic oxygen concentrations in bottom waters at several of these locales associated with seasonally-high, average surface chlorophyll a and seasonal persistence of a freshwater cap. In contrast, the PEB indexes from 1950s core tops suggest oxygenated conditions >30 m in the Bight. The PEB index from the gravity core indicates good oxygenation throughout the past century in the middle Mississippi Bight, and suggests an eastward limit to the low oxygen conditions near the Balize delta.