After the rig exploded, killing 11 people, the well pumped at least 3.1 millions of barrels of oil into the Gulf of Mexico for 87 days.
Oil giant BP has disputed the implication that the spill is to blame for the dolphins' demise.
Conclusive vs consistent
But the scientists have a different interpretation of their own data, and it implicates the historic spill.
The dolphins' deaths are consistent with raised exposure to petroleum, they said in a study released this week by the National Oceanic and Atmospheric Administration
and a group of independent scientists.
Though oil that spewed out of the uncapped wellhead may not have poisoned the dolphins outright, swimming through it probably broke down the health of many of them, NOAA said
, leading them to get sick and die.
It proposes that the exposure is a cause of the sickness and a contributor to their deaths.
A government official familiar with BP's counter assessment accused the oil company of having "cherry-picked" the scientific data
for its purposes. BP denies that.
In the photo, the sign over the silvery carcass reads "Tursiops truncatus." That's the scientific name for the bottlenose dolphin, the most common dolphin species off of Gulf shores. Seeing one washed up dead on a beach may not be common, but it's also no anomaly.
But the sign in the photograph also includes a locator, Fourchon Beach. It's on Barataria Bay, which took a big hit from Deepwater Horizon, the costliest man-made disaster in U.S. history.
And during and after the spill, dolphins washed up there dead in particularly high numbers, the study said.
The scientists working for and with NOAA focused on dolphins that died there and on nearby beaches in Mississippi and Alabama during and after the spill in an "unusual mortality event," or UME, when dead dolphins turned up in peak numbers.
They discovered some marked health damage among dolphin carcasses -- many had extremely rare lesions in their adrenal glands and their lungs. The two ailments were likely related to each other, the scientists say, and likely caused by the spill.
"The timing, location, and nature of the lesions support that oil compounds from the Deepwater Horizon oil spill caused these lesions and contributed to the high numbers of dolphin deaths within this oil spill's footprint," NOAA said.
The weakening of the adrenal glands probably came first, the scientists said. And in many cases they probably led to the lung ailments.
One-third of the entire group of dolphin cadavers had the same kinds of damage to the adrenal gland. Half of the Barataria Bay dolphins had it. The scientist had never come across it before.
"To our knowledge, adrenal cortical atrophy as found in this study has not been previously described in free-ranging (dolphins, porpoises and whales), including bottlenose dolphins previously studied in the northern Gulf of Mexico," they said.
Damaged glands produce less hormones, which throws off the dolphins' metabolism and blood pressure, and can make them more susceptible to coldlike diseases.
"Animals with adrenal insufficiency are less able to cope with additional stressors in their everyday lives," said Dr. Stephanie Venn-Watson, the study's lead author, "and when those stressors occur, they are more likely to die."
Death by infection
Many of the dolphin carcasses studied had forms of pneumonia that either killed them or helped kill them, the scientists said. They were markedly more prevalent than would normally be expected in randomly found dolphin carcasses.
Altogether, the dolphins they inspected were four times more likely to die from infections than normal. Dolphin carcasses from the spill zone also had a high prevalence of problems in their lymph systems, which aid in fighting infection.
Researchers also tested living dolphins in the spill zone in 2011, and found that many of them had the same health problems.
"Barataria Bay dolphins had a high prevalence of moderate to severe lung disease and blood value changes indicative of hypoadrenocorticism," the study said. The latter is a reduction in adrenal gland hormones.
Nearly half were expected not to live very long, the scientists said, who suspected oil pollution from the spill as the root cause.
Previous studies on dolphins have also implicated the Deepwater Horizon spill in their diminished health, the study said. "These studies have increasingly pointed to the presence of petroleum hydrocarbons as being the most significant cause of the illnesses and deaths plaguing the Gulf's dolphin population," said scientist Teri Rowles.
This new study underlines and bolsters their findings.