The need for comprehensive benthic baselines and monitoring programs prior to marine petroleum extraction: Lessons learned form the Deepwater Horizon Oil Spill

The Deepwater Horizon (DWH, 2010) released 3.19 million barrels of petroleum into the northeastern Gulf of Mexico. The delivery of DWH-derived hydrocarbons to the sediments was caused primarily by Marine Oil Snow, Sedimentation, and Flocculent Accumulation (MOSSFA). MOSSFA resulted in a 4-10-fold increase in sedimentation rate, 2-3 fold increases of oil-derived hydrocarbon concentrations and surface-derived biotic material (e.g. planktic foraminifera, diatoms), and a 2-3 order of magnitude increase in Corexit 9500A dispersant concentration. Estimates of the MOSSFA seafloor spatial extent ranged from 1,030 to 35,425 km², accounting for between 4-14% of the total petroleum released. Increased microbial respiration of organic carbon caused intensifying reducing conditions up to three years following DWH. The depositional event had detectable impacts for more than three years (higher sedimentation rates, absence of bioturbation), followed by a return to normal sedimentation from 2013 to 2015. Benthic foraminifera density (80-93%) and diversity (30-40%) declined in 2010, subsequently increased from 2011 to 2014 and reached a steady state by 2015. Due to a lack of baseline measurements, it is uncertain whether these assemblages have fully recovered to pre-DWH status. Benthic macrofaunal and meiofaunal diversities surrounding the wellhead were moderately to severely impacted, did not recover as of 2011, and indicated a disturbed environment through 2014. Comprehensive benthic baselines prior to future perturbations allow for the quantitative characterization of impact and recovery. They also provide a benchmark on which to develop monitoring programs and marine biotic indices that can establish longer-term trends in both natural and anthropogenic stressors, their controls, and reduce monitoring costs over the lifetime of a project. Through these analyses of benthic assemblages, we have gained a basic understanding of the characteristics of the benthic community and functional linkages, both spatially and between trophic levels. Benthic environmental baselines have proven to be critical following DWH and should be established to a common high standard in order to assess environmental impacts over a variety of scales prior to deep sea benthic resource exploration.