The Deepwater Horizon explosion caused a wellhead on the ocean floor to rupture, leaking crude oil into the ocean. In the subsequent months, as efforts continued to disperse the spill or burn it off, toxic crude oil made its way into the Gulf of Mexico, spreading into the gulf stream and northward along the Atlantic coast. According to a report by the National Resources Defense Council on the first anniversary of the incident, only an estimated 8 per cent of the spill was successfully removed, with the remainder either sinking to the ocean floor or contaminating beaches and shoreline ecosystems.
The spill spread quickly throughout the Gulf of Mexico, as oil spewed to the surface and sank back to the bottom. As the spill dispersed hundreds of miles from the explosion site, it entered the gulf's loop current, a powerful ocean current along the coast of Florida. The loop current then drew the oil around the tip of the Florida coastline, where it entered the gulf stream, a fast-moving current stretching northward along the coast of the United States.
When oil from the Deepwater Horizon spill entered the gulf stream, Atlantic coastal ecosystems suffered severe damage not only to vulnerable coastline habitats, but also to the breeding conditions of variety of animals relying on these areas for egg-laying and hatching young. According to the National Wildlife Federation, the spill was responsible for the deaths of more than 8,000 marine creatures, including birds, sea turtles and dolphins. By the time the well was finally capped in July 2010, more than 4,200 miles of coastline had been affected.
Although a cleanup effort launched by British Petroleum and numerous state and federal agencies worked to clear the surface of floating oil, large amounts of leaked oil sank back to the ocean floor, making a total cleanup virtually impossible. In a retrospective on the spill, the environmental activist organization Earth Justice states that over a year after the Deepwater Horizon explosion, ocean water environments remained polluted, with consequences for ocean food webs and the survival of numerous marine organisms.