FORAMINIFERAL ASSEMBLAGES AND OXYGEN: TIME SERIES OF SEASONAL LOW-OXYGEN EVENTS AT A SITE IN THE MISSISSIPPI BIGHT, USA

Assemblages of live, benthic foraminifera vary progressively with oxygen content of bottom water based on annual, summertime samples from a single locale compared to a well oxygenated site, all from about 20 m water depth in the Mississippi Bight. Short cores were taken from a site that was well-oxygenated in June 2005 and from a single site that ranged from low oxygenation to hypoxia in August 2006, July 2008, and June 2009. Oxygenation of bottom water at each site was determined by Winkler analyses. Two replicate cores were taken from each site, sliced at 1-cm intervals from 0-10 cm sediment depth, and stained with rose Bengal. Samples were sieved at 63 µm and split with a settling-type splitter, and stained specimens from entire splits suspended in tap water were counted in a gridded Petri dish until 100-400 specimens were tabulated. Stained specimens were picked for identification.

The oxygenated site (Winkler titration: 6.46 mg/L) had an average of 51 ± 11 species (Margalef's d = 9 ± 2) in each of its slices from 1-4 cm. The two replicates of the topmost sample (0-1 cm), however, averaged only 29 ± 6 species (Margalef's d = 5 ± 1). In comparison, the hypoxic/low-oxygen sites (Winkler titrations: 2006, 2.82 mg/L; 2008, 0.99 mg/L; 2009, 4.22 mg/L) were significantly lower in diversity, averaging 13 ± 3 species (Margalef’s d 2.1 ± 0.5). Like samples from the nearby Louisiana Bight, the hypoxic/low-oxygen sites had lost most miliolid, textularid, lagenid, and spirillid species and had high dominance among the remaining rotalids. Sites in order from hypoxic to low-oxygen were dominated by Bolivina lowmani plus Buliminella morgani, then Nonionella basiloba, and then Ammonia tepida. MDS analysis of samples based on all species supports the assemblage succession with oxygenation and further suggests population structuring with depth in the sediment. Unanswered by these snapshots in time are the effects of oxygenation history on population decline and recovery, the role of geochemical preconditioning of sediment over short to long timescales, and ultimate imprinting of an hypoxia signal into the death assemblage. These issues are vitally important for several reasons, including refinement of paleoenvironmental interpretation.