On 20 April 2010, the drilling rig Deepwater Horizon exploded after a surge of natural gas blasted through its concrete core, spilling 795 million litres (210 million gallons) of crude oil into the Gulf of Mexico, off the coast of Louisiana. Two days later, the rig capsized and sank into a valley in the continental shelf.

The ignition killed 11 people aboard and injured 17. The number of other casualties – including marine mammals, sea turtles, birds, fish and invertebrates – were countless, and several species are still experiencing new health consequences.

Oil and natural gas uncontrollably burst into one of the planet’s most productive ecosystems for nearly three months. At its largest, the oil spill covered over 15,000 sq miles (39,000 sq km) of the ocean,according to an environmental damage assessment conducted by the National Oceanic and Atmospheric Administration (NOAA).

When the world first learned about the BP Deepwater Horizon oil spill, Win McNamee, chief photographer for Getty Images, was stuck thigh-deep in pluff mud, an umber quicksand-like miasma, on East Grand Terre Island, Louisiana. McNamee was starting another 14-hour day, working to illustrate the depth and breadth of the crisis.

With petroleum malodour lodged in his nose and the early morning sun on his back, McNamee, cemented in the swamp for 30 minutes, was snapping photos of the spill’s aftermath. But he was not the unluckiest party on the island; that was the wildlife, thrashing 10ft (3m) away from a future-Pulitzer Prize winner and his camera. 

Before him, a brown pelican was baptised in the Gulf’s backwash of petroleum and Corexit, a toxic chemical meant to break up the oil.

“It’s difficult to describe the feelings of helplessness that a person feels when they see wildlife caught up in the middle of an environmental disaster,” McNamee says.

With its Brobdingnagian bill and lithe neck, the pelican is not the obvious choice for a galvanising symbol. But there’s more here than meets the eye.

The brown pelican is treasured as Louisiana’s state bird, not only because it is native to the state, but also because of different symbolic, historical and religious connotations. Under French and Spanish colonialism, the settlers of Louisiana were officially Roman Catholic, and in historical Catholic tradition, the pelican is associated with self-sacrifice. During times of famine, the mother pelican pecked at her body to feed her kin, according to legend.

The bird had been removed from the endangered species list only a few months before the disaster, in November 2009. Yet here it was, once again a symbol of sacrifice in “the worst environmental disaster in US history,” as US President Barack Obama described the incident in a prime-time address. 

Now, when Americans think of the BP Deepwater Horizon oil spill, they likely envision an oil-soaked pelican.

McNamee describes the now-iconic images as “tragic combination of beautiful light and a horrific subject”. But personally, McNamee felt an “overwhelming sense of sadness”.

“You have to push that feeling to the side to do your job,” McNamee says. “Frankly, the justification for staying focused on the job is that you will, hopefully, have a larger impact in bringing understanding, assistance and fundraising to ameliorate the situation.”

The northern Gulf Coast is a hotspot for avian diversity and abundance, and that’s particularly so in southeastern Louisiana, where the oil spill hit. Some 2.1 billion land birds migrate across the Gulf of Mexico every year. The region supports more than half of the global population of seaside sparrows or marshes. Coastal Louisiana supports about a third of all the brown pelicans in the eastern United States, says Erik Johnson, director of conservation science for Audubon Delta, a three-state regional office of the non-profit National Audubon Society.

The oil spill ravaged this community. Somewhere between 100,000 and a million birds were killed outright, says Johnson. Audubon Delta estimates 10% of all the brown pelicans in the northern Gulf of Mexico died because of the oil spill. 

Oil-soaked feathers have a particularly devastating effect on birds, because feathers help them maintain a healthy body temperature. Birds cannot thermoregulate, meaning, they cannot keep their own body temperature steady when the outside temperature changes, explains Johnson. When oil cakes their feathers, birds can freeze to death, even in lukewarm waters. In the middle of summer, birds will internally bake, without the cooling measures of their feathers. The direct effects of the spill also included choking and gagging to death.

The chemical byproducts of oil got into the tissue of birds: in their blood, livers and feathers. This oil intrusion even had cascading effects on how birds’ DNA functioned. Researchers saw lower nesting success among birds due to the spill.

“I mean, it was a disaster,” says Johnson, who was in the heart of the crisis as he was completing the final year of his doctorate program in Wildlife, Fish and Wildlands Science and Management at Louisiana State University. “A human-caused disaster.”

McNamee’s photo series, distributed from Getty Images, was widely published by global news outlets. The close-ups by him and other photographers, of pelicans but also hermit crabs and laughing gulls. Marine mammals probably inhaled vapours, and ingested and absorbed oil, says Cynthia Smith, CEO of the US National Marine Mammal Foundation. She was a lead clinical veterinarian for NOAA’s marine mammal injury assessment following the spill.

McNamee’s photo told the story of animal species, many already endangered, who experienced increased mortality, developmental defects and reproductive declines due to exposure to the oil.

News crews would also capture the Gulf Coast’s aerial ooze, but nothing stuck with audiences like the close-up shots. “We are social creatures. We connected with this intimate portrait,” says Johnson.

Viewers’ empathetic responses to a lone brown pelican heightened public pressure on the Obama administration and BP to stop the leak, he says.

“[McNamee’s] photos don’t capture the scale of the issue, but they tell the tragic story,” says Johnson. They helped consumers, news outlets, researchers and policymakers were able to understand and identify with the suffering of the environment, says Johnson.

“You can’t protect what you don’t know,” adds Johnson.

US federal authorities began their on-site response the night of the spill, according to official statements. However, the government’s damage assessment process gathered findings to be used for litigation, not public awareness, says Johnson. This niche is where ordinary citizens stepped in.

“We organised a community science project to monitor birds in real-time and be able to communicate those results to the public,” he says. “Within a couple of weeks, we had people on the ground, monitoring birds on the coast, faster than the state and federal government.”

Meanwhile, the world watched in dismay. The US accepted assistance from other nations, including Mexico, Norway and Canada. 

“You look at these pictures, you get angry,” Johnson adds. “That is a trigger for people to want change: social change, environmental change, policy change.” The mass education prompted a new cohort of conservationists, he says, “which will make the Gulf Coast more resilient in the long term”.

The environment has rebounded, helped along by the investment of over $5bn (£4bn) in restoration, part of the global settlement that the Department of Environmental Protection reached with the Department of Justice in 2015. The settlement funding allowed researchers to not only address the ecological damage from the spill but also to protect the Gulf ecosystem from inevitable intense weather and rising sea levels.

“Today, I’m happy to say that we haven’t seen a complete collapse of the ecosystem and of bird populations,” Johnson adds.

Now in coastal Louisiana, the water management community provides enough jobs to rival the oil and gas industry – a sharp contrast from a state that was once defined by a reliance on fossil fuels.

Both during and after the spill, the investment and research allowed over 1,000 graduate students to receive degrees in aspects of Gulf Coast ecology, Johnson says.

“There’s this whole new generation of scientists and conservation professionals out there, who learned from this monumental event,” says Johnson. “That’s a really, really important outcome.”

Environmental accountability extended to the US government, which made a new agency, the Bureau of Safety and Environmental Enforcement, to investigate offshore drilling issues and execute commonsense safety rules following Deepwater Horizon. Before the spill, the agency that approved leases to oil companies also oversaw regulatory enforcement, an oversight that some might argue has had deadly implications. 

“I hope this photo reminds all generations of people that they can absolutely make a difference,” says Smith. But she also warns that there is more work to be done.”I think most people agree that the Deepwater Horizon oil spill was a devastating environmental disaster, but I don’t know if people realise that it is still impacting wild animals and our coastal communities.”

Her job as a clinical veterinarian was to determine if dolphins and whales living in the Gulf of Mexico at the time of the oil spill were harmed. Prior to the oil spill, most scientists believed that dolphins and whales would avoid oil, Smith says. Obviously, this belief proved false. Over 1,000 animals from at least 10 species of dolphins and whales were visually documented swimming through oil, with no evidence of avoidance. Since then, Smith has been documenting the spill’s longer-term impact on wildlife.

For her, the pelican photo is a motivation for her work and an emblem of the public’s changing mindset. 

“As someone who grew up on the Gulf Coast, I hope this photo inspires everyone to consider wild animals when making decisions about the environment,” Smith says. “The animals need our protection and our compassion.”