Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon

Significance The Deepwater Horizon event led to an unprecedented discharge of ∼4.1 million barrels of oil to the Gulf of Mexico. The deposition of ∼4–31% of this oil to the seafloor has been quantified previously on a bulk basis. In this work, we assess the extent of degradation over 4 y postspill for each of 125 petroleum hydrocarbons that contaminated the seafloor. As expected, chemically simpler compounds broke down more quickly than complex compounds, but degradation rates also depended on environmental context: Breakdown often was faster before seafloor deposition than after and for oil trapped in small droplets than for oil in large particles. These results provide a basis to predict the long-term fate of seafloor oil.