The Deepwater Horizon (DWH) oil spill originated at the base of the continental shelf in the northern Gulf of Mexico (GoM), but large quantities of the oil were transported to the shelf (≤200 m water depth) and into coastal waters (herein defined as ≤15 km from the coast). Water-column effects were generally limited to the period of the ongoing oil releases, although, due to an extensive oil sedimentation event (“dirty blizzard”), effects on the benthos have the potential to be chronic, especially in soft sediments. Impacts on phytoplankton, zooplankton, and ichthyoplankton were relatively short-lived, and the abundance and species composition of planktonic communities returned to pre-spill conditions within a year of the event. Mortalities of larval fish were generally less than 20% of Gulf-wide species populations owing to the extensive and extended spawning periods of most species. Impacts on the productivity of the region’s fisheries were also relatively short-lived and influenced by extensive fishery closures to protect seafood safety, although long-term effects may eventually alter the productivity of some stocks.

Benthic communities exhibited effects from the spill that ranged from negligible to significant. Hard-bottom communities, including natural and artificial reefs, suffered injuries that were severe and long lasting. Due to the patchy nature of oil deposition, high tolerance of toxins, and low bioavailability, effects on soft-sediment communities appear to be minimal except in areas, such as beaches, where oil settled in very high amounts. However, DWH oil may persist in coastal and continental shelf sediments for decades if it is sequestered by continuing sedimentation in the absence of events such as tropical storms that may resuspend contaminated bottom material. Nevertheless, vertebrates and shellfish foraging or living in the sediments may be continuously exposed to weathered DWH oil. Understanding the full impacts of the spill requires sustained monitoring in order to separate event-induced impacts from normal variability, and it also requires research that spans the natural range of variation in benthic and pelagic communities. Collection of routine contaminant baselines in GoM waters, sediments, and biota should be viewed as a high priority moving forward.

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