A Decline in Benthic Foraminifera following the Deepwater Horizon Event in the Northeastern Gulf of Mexico

Abstract

Sediment cores were collected from three sites (1000–1200 m water depth) in the northeastern Gulf of Mexico from December 2010 to June 2011 to assess changes in benthic foraminiferal density related to the Deepwater Horizon (DWH) event (April-July 2010, 1500 m water depth). Short-lived radioisotope geochronologies (210Pb, 234Th), organic geochemical assessments, and redox metal concentrations were determined to relate changes in sediment accumulation rate, contamination, and redox conditions with benthic foraminiferal density. Cores collected in December 2010 indicated a decline in density (80–93%). This decline was characterized by a decrease in benthic foraminiferal density and benthic foraminiferal accumulation rate (BFAR) in the surface 10 mm relative to the down-core mean in all benthic foraminifera, including the dominant genera (Bulimina spp., Uvigerina spp., and Cibicidoides spp.). Cores collected in February 2011 documented a site-specific response. There was evidence of a recovery in the benthic foraminiferal density and BFAR at the site closest to the wellhead (45 NM, NE). However, the site farther afield (60 NM, NE) recorded a continued decline in benthic foraminiferal density and BFAR down to near-zero values. This decline in benthic foraminiferal density occurred simultaneously with abrupt increases in sedimentary accumulation rates, polycyclic aromatic hydrocarbon (PAH) concentrations, and changes in redox conditions. Persistent reducing conditions (as many as 10 months after the event) in the surface of these core records were a possible cause of the decline. Another possible cause was the increase (2–3 times background) in PAH’s, which are known to cause benthic foraminifera mortality and inhibit reproduction. Records of benthic foraminiferal density coupled with short-lived radionuclide geochronology and organic geochemistry were effective in quantifying the benthic response and will continue to be a valuable tool in determining the long-term effects of the DWH event on a larger spatial scale.

How Did the Deepwater Horizon Oil Spill Affect Coastal and Continental Shelf Ecosystems of the Gulf of Mexico?

“…A substantial proportion of DWH oil sedimented to the seafloor (Schwing et al, 2014;Brooks et al, 2015;Romero et al, 2015;Valentine et al, 2015;Chanton et al, 2015;Daly et al, 2016). Concentrations in shelf sediments were most problematic in coastal areas, especially east of the Mississippi River, although few post-spill benthic samples were collected in this region.…”

“…With respect to contamination of GoM sediments, a number of published studies indicate that a substantial proportion of DWH oil was transported to the seafloor (Schwing et al, 2014;Valentine et al, 2014;Brooks et al, 2015;Chanton et al, 2015;Romero et al, 2015;Daly et al, 2016). Because so much DWH oil was located in the water column above the continental shelf and in coastal areas, it is likely that the sediments in these areas contain substantial quantities of sedimented oil as well, although at concentrations likely higher than the deep areas adjacent to the well.…”

Synthesis and Crosscutting Topics of the GoMRI Special Issue

“…A year later, one site closest to the well site showed evidence of recovery, while the other site continued to show a decline in benthic foraminifera (Schwing et al, 2015). Further assessments at these sites are in progress.…”

How Did the Deepwater Horizon Oil Spill Impact Deep-Sea Ecosystems?

“…Thus, the oil spill likely changed the nature of microbial communities in the deep sea. There was an 80%-93% decline in benthic foraminifera following the DWH event related to persistent reducing conditions and increased PAH concentrations (Schwing et al, 2015).…”