Ecotoxicological effects of chemically dispersed crude oil in the marine environment

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In April 2010, the drilling rig Deepwater Horizon (DWH) in the Gulf of Mexico exploded and sank. Over the course of three months, 780 million L crude oil leaked from the wellhead at 1500 m depth. In an effort to limit the negative spill effects, 7 million L dispersants were injected in the well head and applied at the surface. Chemical dispersants break up the oil slick into tiny droplets which disperse into the water column (Figure 1, top), thereby reducing the risk of oil slicks landing on coasts and oiling of birds and marine mammals. Although this will result in a temporary increase in toxicity in the water column, it is assumed that the period of exposure will be short by dilution and enhanced degradation of the dissolved contaminants. With the DWH spill, however, it was revealed that complexes of dispersed oil with sea snow, particulate matter and plankton had settled at the ocean floor (Figure 1, bottom). A thick toxic oily layer still is impairing the recovery of the benthic ecosystem years after the spill. The greatly enhanced formation of sea snow and resulting persistence was unexpected and suggested to be related to the extensive dispersant application.