The spill is the "worst environmental disaster the US has faced", according to White House energy adviser Carol Browner. Indeed, the spill was by far the largest in US history, almost 20 times greater than the Exxon Valdez oil spill. Factors such as petroleum toxicity, oxygen depletion and the use of Corexit dispersant are expected to be the main causes of damage. Eight U.S. national parks were threatened and more than 400 species that live in the Gulf islands and marshlands were at risk, including the endangered Kemp's Ridley turtle, the Green Turtle, the Loggerhead Turtle, the Hawksbill Turtle, and the Leatherback Turtle. In the national refuges most at risk, about 34,000 birds were counted, including gulls, pelicans, roseate spoonbills, egrets, terns, and blue herons. A comprehensive 2009 inventory of offshore Gulf species counted 15,700. The area of the oil spill includes 8,332 species, including more than 1,200 fish, 200 birds, 1,400 molluscs, 1,500 crustaceans, 4 sea turtles, and 29 marine mammals. As of 2 November 2010, 6,814 dead animals had been collected, including 6,104 birds, 609 sea turtles, 100 dolphins and other mammals, and 1 other reptile. According to the U.S. Fish and Wildlife Service, cause of death had not been determined as of late June. According to NOAA, since 1 January 2011, 67 dead dolphins had been found in the area affected by the oil spill, with 35 of them premature or newborn calves.
In May 2010, Duke Universitymarine biologist Larry Crowder said threatenedloggerhead turtles on Carolina beaches could swim out into contaminated waters. Ninety percent of North Carolina's commercially valuable sea life spawn off the coast and could be contaminated if oil reaches the area. Douglas Rader, a scientist for the Environmental Defense Fund, said prey could be negatively affected as well. Steve Ross of UNC-Wilmington said coral reefs could be smothered. In early June Harry Roberts, a professor of Coastal Studies at Louisiana State University, stated that 4,000,000 barrels (640,000 m3) of oil would be enough to "wipe out marine life deep at sea near the leak and elsewhere in the Gulf" as well as "along hundreds of miles of coastline." Mak Saito, an Associate Scientist at Woods Hole Oceanographic Institution in Massachusetts indicated that such an amount of oil "may alter the chemistry of the sea, with unforeseeable results." Samantha Joye of the University of Georgia indicated that the oil could harm fish directly, and microbes used to consume the oil would also reduce oxygen levels in the water. According to Joye, the ecosystem could require years or even decades to recover, as previous spills have done. Oceanographer John Kessler estimated that the crude gushing from the well contained approximately 40% methane by weight, compared to about 5% found in typical oil deposits. Methane could potentially suffocate marine life and create dead zones where oxygen is depleted. Also oceanographer Dr. Ian MacDonald at Florida State University believes that the natural gas dissolving below the surface has the potential to reduce the Gulf oxygen levels and emit benzene and other toxic compounds.In early July, researchers discovered two new previously unidentified species of bottom-dwellingpancake batfish of the Halieutichthys genus, in the area affected by the oil spill. Damage to the ocean floor is yet unknown. In particular was the Louisiana pancake batfish, whose range is entirely contained within the area affected by the spill.
In late July 2010, Tulane University scientists found signs of an oil-and-dispersant mix under the shells of tiny blue crab larvae in the Gulf, indicating that the use of dispersants had broken the oil into droplets small enough to easily enter the food chain. Marine biologists from the University of Southern Mississippi's Gulf Coast Research Laboratory found "orange blobs" under the shells of crab larvae "in almost all" of the larvae they collected from over 300 miles (480 km) of coastline stretching from Grand Isle, Louisiana, to Pensacola, Florida.
In September 2010, Oregon State University researchers announced the oil spill waters contain carcinogens. The team had found sharply heightened levels of chemicals in the waters off the coast of Louisiana in August, the last sampling date, even after BP successfully capped its well in mid-July. Near Grand Isle, Louisiana, the team discovered that polycyclic aromatic hydrocarbons or PAHs, which are often linked to oil spills and include carcinogens and chemicals that pose various risks to human health, remained at levels 40 times higher than before the oil spill. Researchers said the compounds may enter the food chain through organisms like plankton or fish. The PAH chemicals are most concentrated in the area near the Louisiana Coast, but levels have also jumped 2 to 3 fold in other spill-affected areas off Alabama, Mississippi and Florida. As of August, PAH levels remained near those discovered while the oil spill was still flowing heavily. Kim Anderson, an OSU professor of environmental and molecular toxicology, said that based on the findings of other researchers, she suspects that the abundant use of dispersants by BP increased the bioavailability of the PAHs in this case. "There was a huge increase of PAHs that are bio-available to the organisms – and that means they can essentially be uptaken by organisms throughout the food chain." Anderson added that exactly how many of these toxic compounds ended up in the food chain was beyond her area of research.
On 22 October 2010, it was reported that miles-long strings of weathered oil had been sighted moving toward marshes on the Mississippi River delta. Hundreds of thousands of migrating ducks and geese spend the winter in this delta.
Researchers reported in early November 2010 that toxic chemicals at levels high enough to kill sea animals extended deep underwater soon after the BP oil spill. Terry Wade of Texas A&M University, Steven Lohrenz of the University of Southern Mississippi and Stennis Space Center found evidence of the chemicals as deep as 3,300 feet (1,000 m) and as far away as 8 miles (13 km) in May, and say the spread likely worsened as more oil spilled. The chemicals (PAHs), they said, can kill animals right away in high enough concentrations and can cause cancer over time. "From the time that these observations were made, there was an extensive release of additional oil and dispersants at the site. Therefore, the effects on the deep sea ecosystem may be considerably more severe than supported by the observations reported here," the researchers wrote in the journal Geophysical Research Letters. They added that PAHs include a group of compounds, and different types were at different depths, and said "It is possible they dissipate quickly, but no one has yet showed this".
In November 2010, federally funded scientists found damage to deep sea coral several miles from BP's Macondo well. While tests are needed to verify that the coral died from the well, expedition leader Charles Fisher, a biologist with Penn State University, said, "There is an abundance of circumstantial data that suggests that what happened is related to the recent oil spill." According to the Associated Press, this discovery indicated that the spill's ecological consequences may be greater than what officials have said. Previous federal teams have stated that they found no damage on the ocean floor. "We have never seen anything like this," Fisher added. "The visual data for recent and ongoing death are crystal clear and consistent over at least 30 colonies; the site is close to the Deepwater Horizon; the research site is at the right depth and direction to have been impacted by a deep-water plume, based on NOAA models and empirical data; and the impact was detected only a few months after the spill was contained."
A Coast Guard report released on 17 December 2010, said that little oil remained on the sea floor except within a mile and a half of the well. The report said that since 3 August, only 1% of water and sediment samples had pollution above EPA-recommended limits. Charlie Henry of NOAA warned even small amounts of oil could cause "latent, long-term chronic effects". And Ian R. MacDonald of Florida State University said even where the government claimed to find little oil, "We went to the same place and saw a lot of oil. In our samples, we found abundant dead animals."
In February 2011, the first birthing season for dolphins since the spill, the director of the Institute for Marine Mammal Studies in Gulfport reported that dead baby dolphins were washing up along the Mississippi and Alabama shorelines at about 10 times the normal number for the first two months of the year. "For some reason, they’ve started aborting or they were dead before they were born; the average is one or two a month. This year we have 17 and February isn’t even over yet.” It is not yet certain if the deaths are related to the oil spill.
From mid-January to late March 2011, scientists counted almost 200 dead dolphins in the Gulf, with another 90 in 2010. After investigating the deaths, NOAA put a gag order on the results, saying that the research is part of a criminal investigation of the oil spill. Numerous independent scientists said they have been "personally rebuked by federal officials for speaking out of turn to the media about efforts to determine the cause" of the deaths. A study published in the journal Conservation Letters showed the actual number of mammal deaths due to the spill may be as much as 50 times higher than the number of recovered carcasses. "The Deepwater oil spill was the largest in US history, however, the recorded impact on wildlife was relatively low, leading to suggestions that the environmental damage of the disaster was actually modest. This is because reports have implied that the number of carcasses recovered... equals the number of animals killed by the spill." said Rob Williams from the University of British Columbia.
In April 2011, one year from the onset of the spill, scientists confirmed that they had discovered oil on dead dolphins found along the Gulf Coast. Fifteen of the 406 dolphins that had washed ashore in the last 14 months had oil on their bodies; the oil found on eight of them was linked to the April 2010 BP oil spill. A NOAA spokesperson stated,"It is significant that even a year after the oil spill we are finding oil on the dolphins, the latest just two weeks ago." A study performed by NOAA in the summer of 2011 showed dolphins that came in contact with the petroleum were "seriously ill" with drastically low weight, low blood sugar and for some, cancer of the liver and lungs.
According to a March 2012 study, oil from the Macondo well entered the ocean's food chain through zooplankton. Dr. Michael Roman of the University of Maryland Center for Environmental Science stated "traces of oil in the zooplankton prove that they had contact with the oil and the likelihood that oil compounds may be working their way up the food chain".
In March 2012, definitive link was found between the death of a Gulf coral community the size of half a football field and the BP oil spill.
A NOAA study in Spring 2012, along with two other studies reported at the same time, suggest that the long-term environmental effects of the spill may have been "far more profound than previously thought". The joint study by NOAA and BP found "many of the 32 dolphins studied were underweight, anemic and suffering from lung and liver disease, while nearly half had low levels of a hormone that helps the mammals deal with stress as well as regulating their metabolism and immune systems". Other researchers discovered dead and dying corals "coated in brown gunk". Deep sea corals are usually unaffected by oil spills, but scientists surmised that the depth of the oil and cold temperatures were to blame for the unprecedented affects. Another researcher found that some types of spiders and other insect were far less numerous than before the spill.
A 2012 study of the sands of the contamimated beaches and marshes showed that the variety of organisms, one of the lowest links in the food chain, had dropped dramatically since the spill. The remaining species are believed to be those that favor polluted conditions and that consume hydrocarbons. This could result in long-lasting effects to the ecosystem. The lead author said, "We went from this very diverse community with an abundance of different organisms to this really (impoverished) community that was really dominated by a couple of fungal species". The authors also expressed concerns that trace minerals and metals such as mercury and arsenic deposited by the oil may cause harm to both wildlife and humans. 
"Disturbing numbers" of mutated fish are appearing in the Gulf. Scientists and fishermen are pointing to the BP oil spill, the dispersants and chemicals used in its cleanup as the cause of these deformities which include shrimp born without eyes, fish with lesions, fish with oozing sores and, according to a local fisher-woman, "We are also finding eyeless crabs, crabs with their shells soft instead of hard, full grown crabs that are one-fifth their normal size, clawless crabs, and crabs with shells that don't have their usual spikes ... they look like they've been burned off by chemicals". The dispersants are known to be mutagenic. In Barataria Bay, LA, one of the most heavily oiled areas, 50 percent of shrimp were found lacking eyes and eye sockets. Another lifelong fisher-woman reported seeing "fish without covers over their gills and others with large pink masses hanging off their eyes and gills".
Prior to the spill, only 1/10 of 1 percent of Gulf fish had lesions or sores. Today, many locations showed between 20 and 50 percent of fish with lesions, according to the University of South Florida.
Dr. Jim Cowan of NOAA believes polycyclic aromatic hydrocarbons (PAHs), released from BP's submerged oil, are likely the cause of mutations he is finding since the spill. "There's no other thing we can use to explain this phenomenon. We've never seen anything like this before."