Ecological Insults and Injuries Revealed Four Years after Deepwater Horizon

On April 20, 2010, the drilling rig Deepwater Horizon blew out, caught fire, and eventually sank in Gulf waters a mile deep, taking the lives of 11 people. Over the next 86 days, in the largest ever accidental discharge of oil into marine waters, the out of control Macondo 252 well spewed at least 175 million liquid gallons of petroleum, plus an additional 180,000 metric tons of gaseous hydrocarbons, into the Gulf of Mexico. The cumulative oil slick from this discharge covered an area the size of New York State, extending from coastal marshes and bayous across the continental shelf far out into the deep-sea zone.

At the time of the spill, impacts to the Gulf’s fish, wildlife, and habitat from Deepwater Horizon could not be fully appreciated. Media interest and public attention in the story gradually waned. Today, however, pictures emerge in which harm to the Gulf is observed at the ecosystem, food web, and organism levels. Even the Gulf’s physical structure was altered by the spill.

Clear impacts of oil

Deep sea coral colonies exhibited multiple signs of stress, including tissue loss, skeletal (sclerite) enlargement, excess mucous, and smothering from a substance chemically linked to the Macondo well. Almost half of the corals studied exhibited impacts to more than half of the colony, and nearly a quarter of the corals showed impacts to more than 90 percent of the colony. Many corals died as a result of the spill. Due to their physiology, deep water corals are a sensitive indicator of exposure from petroleum hydrocarbons in the water column, even if the spilled oil does not actually settle onto the sea floor.

At the other end of the geographic continuum, oil changed the structure of the very coastline itself. Especially in Louisiana, Deepwater Horizon oil caused permanent losses in marsh area. As oil arrived along the coast, it concentrated primarily along the seaward edges of the marshes. The more oil that arrived, the more it damaged marsh vegetation. Plant death from this oil exposure more than doubled shoreline erosion, thereby permanently undercutting the marsh platform due to loss of plant root anchoring. In some locations, shoreline retreat increased by more than 125%, reaching 9 feet of shoreline loss per year at oil impacted sites. Given other stressors, including sea level rise from climate change, natural subsidence, and channelization of the Mississippi River, oil impacts have now made salt marshes along the Gulf even more vulnerable.

Green sea turtle

In surface waters, oil from Deepwater Horizon harmed the floating Sargassum plant community and the animals that rely upon this highly-distinct habitat. Sargassum forms a localized oasis of productivity and biodiversity in what would otherwise be a featureless ocean surface. Researchers have found that the oil-spill caused direct, sub lethal, and indirect injuries along three different pathways. First, Sargassum acted to accumulate and concentrate oil, exposing animals to greater levels of contaminants. Second, the dispersants used sank the Sargassum, causing direct habitat loss and possibly transporting both dispersant and oil even deeper into the water column. And lastly, oil-affected Sargassum depleted oxygen levels, potentially stressing animals that use this habitat for resting, cover, and food. Just how important is Sargassum in the Gulf of Mexico? As documented in one Gulf study, almost 90% of all sea turtles, including the endangered Kemp’s ridley (Lepidochelys kempii), were observed within 3 feet of Sargassum.

Arguably the most worrisome injuries from Deepwater Horizon were those that were less visible, but that affect the very fabric of life down at tissue and organ levels of the animals exposed. Steep health declines have been observed in common bottlenose dolphins of Barataria Bay, Louisiana. Dolphins in this heavily-impacted part of the spill zone showed evidence of adrenal toxicity consistent with oil exposure, they were five times more likely to have contracted severe lung disease, and they had poor prognosis for survival. Fish in the Gulf fared no better. Even at low concentrations of oil exposure, Gulf killifish (Fundulus grandis) exhibited changes to both their genetic composition and gill structures, the latter especially significant because gills are the site of oxygen uptake through respiration. Some of the same contaminants from Deepwater Horizon oil, especially the polycyclic aromatic hydrocarbons (PAHs), caused heart defects to form in larval fish, including the endangered, ESA-listed bluefin tuna. Such defects are likely to prove fatal over time.

Oil in the Gulf

Looking ahead, the focus on impacts from Deepwater Horizon will shift to the ecological processes, linkages, offsets and other synergies that arise from indirect and sub lethal injuries. For example, some species such as seaside sparrow Ammodramus maritimus may have experienced multiple, simultaneous stressors from the same spill. Seaside sparrows were less abundant and their reproductive performance was impaired at oiled sites after the spill. Researchers will scrutinize how this tiny bird was affected by direct habitat loss from the spill, insect declines following the spill and compromised immune function from exposure, all of which may be exacerbated by sedentary habits that prevent it from moving away from impacted sites.

Deepwater Horizon taught us that oil spills can be considerably bigger than we imagined, and difficult to control or contain when they do occur. While science may appear to just tally more insults to the injuries, assessing the full scope of harm from this unprecedented oil spill is a vital step to recovery. Otherwise, we will not know how to best deploy our resources to restore the Gulf of Mexico, and to protect the endangered sea turtles and marine fish that make this sea their home.

— J. Christopher Haney, Ph.D., Chief scientist