INTERTIDAL FORAMINIFERA OF THE MISSISSIPPI AND CHANDELEUR SOUNDS: EFFECT OF DEEPWATER HORIZON OIL SPILL OBSCURED BY EROSION

This work is part of a larger study to determine the impact of Deepwater Horizon oil on populations of intertidal marsh foraminifera. Short cores were collected from the marsh edge of two pristine sites in May 2010 and two oiled sites in August 2010 in the Mississippi and Chandeleur Sounds. The cores were sliced into 1-cm intervals to a depth of 10 cm, and prepared samples were stained with rose Bengal to mark live specimens. Additionally, concentrations of ⁷Be fallout from the atmosphere were determined from gamma counts.

Seasonal succession is evident at the four sites, with Arenoparrella mexicana dominating in May and Trochammina inflata and carbonate species dominating in August. Interestingly, the sites on the Mississippi Sound are at all times dominated by agglutinated species whereas those in the Chandeleur Sound contain significant numbers of carbonate species like Helenina anderseni and Ammonia parkinsoniana. The density of live specimens suggests a stark contrast between conditions in the Mississippi and the Chandeleur Sounds. Densities of live specimens at the Chandeleur sites are drastically less than those of the Mississippi Sound. Peak densities of 600 and 1000 tests/10cc are seen in August (oiled) and May (pristine), respectively, in cores from the Mississippi Sound. In contrast, peak densities of 50 and 200 tests/10cc are seen in May (pristine) and August (oiled), respectively, in the Chandeleur Sound. The amount of atmospheric ⁷Be with depth in cores was used to determine the maximum depth of sediment mixing over the last year. Although ⁷Be was found mixed to 0.75-1.75 cm in Mississippi Sound sites, neither site in Chandeleur Sound accumulated any ⁷Be at the surface, indicating possible removal of surface sediment. Removal of surface sediment is consistent with the low density of live foraminifera in the Chandeleur Sound. Erosion from Tropical Storm Bonnie’s storm surge (July 22-24) and possible remediation efforts at the contaminated sites may be responsible for the removal of surface sediment at the oiled Chandeleur site. However, high erosional rates under ambient conditions may ultimately be responsible for the erosion inferred at the less protected Chandeleur sites. The erosion of surface sediment from the western margin of the Chandeleur Sound has obscured possible effects due to oil contamination.