BATHYAL ASSEMBLAGES OF LIVE, BENTHIC FORAMINIFERA NEAR THE DEEPWATER HORIZON OIL SPILL, NORTHERN GULF OF MEXICO

The study’s main objective is to compare live benthic foraminiferal assemblages at contaminated and uncontaminated bathyal sites around the source of the Deepwater Horizon oil spill. This work is part of a larger project to determine the spill effects on benthic ecology. Samples were taken from un-oiled (A, E, F, and G), moderately oiled (C and D), and heavily oiled (B) sites, in which polycyclic aromatic hydrocarbons (PAHs) ranged from 29 to 7,553 ng/g. The topmost 0-1 cm interval from seven multicores, and two multicores sliced at 1-cm intervals to 10 cm from the least and most oiled sites, were stained with rose Bengal, washed through a sieve with 45 μm openings, and split with a settling-type splitter. Wet aliquots were fully picked until at least 300 stained (live) specimens were tallied, and specimens were identified at successive depth intervals until 95% of the living assemblage was accumulated (= depth of habitation). Five pseudo-replicates were analyzed at site G to determine the variability between samples. To date, over 150 species of foraminifera have been identified with 12 to 50% of specimens yet to be identified to species, though generic assignments have been made.

The surface densities of six sites range from 390 to 2,221 tests/10cm³ and weakly correlate with increasing [PAH]. The surface density for the heavily oiled site is 3 to 5 times greater than that of the un-oiled sites and 3 and 6 times greater than that of the moderately oiled sites. The relative species frequencies are equitably distributed, and all surface samples share >3% Bulimina aculeata, Epistominella levicula, and Rutherfordoides mexicanus.

The distributions of live specimens in the un-oiled and heavily oiled sites are distinctly different and mirrored by the macrofauna. The depth of habitation is 2 times greater at the un-oiled site, but the standing stock is 1.5 times greater at the heavily oiled site. The H’ diversity index is greater throughout the depth of habitation of the un-oiled site compared to the heavily oiled site. Tentatively, the trends in density, standing stock, and diversity appear consistent with the point source pollution model (Alve, 1995) as though from patches of oil on the seafloor (although the heavily oiled site is not yet ruled out as natural seep). The depth of habitation is also useful for recognition of pollution effects.