Researchers point to the Deepwater Horizon oil spill as the likely cause for an increase in the number of dead dolphins discovered along the northern Gulf of Mexico since 2010, a report released Wednesday says.

“We feel this study is a critical link in a chain of studies,” said Kathleen Colegrove, lead veterinary pathologist for the report in the online journal PLOS ONE.

Previous studies on the increased death of dolphins in the northern Gulf of Mexico, along with this new one, point to a connection to oil exposure at the time of the spill.

“Dolphins found within the Deepwater Horizon footprint show life-threatening adrenal and lung functions,” Stephanie Venn-Watson, lead author of the report with the National Marine Mammal Foundation, said during a press conference Wednesday.

BP officials have consistently disputed that there is any direct link or proof that the dolphin deaths are connected to the 2010 oil spill.

“The data we have seen thus far, including the new study from NOAA, do not show that oil from the Deepwater Horizon accident caused an increase in dolphin mortality,” BP said in a Wednesday statement. “This new paper fails to show that the illnesses observed in some dolphins were caused by exposure to Macondo oil.

“In fact, numerous studies conducted over the last several decades have shown that respiratory illness — one of the conditions cited — is among the most common causes of death for bottlenose dolphins, including a study where half of the dolphins examined had pneumonia.”

Study researchers, however, said they ruled out the usual suspects in the cause of bacterial pneumonia leaving oil exposure as the cause in these cases.

The study released Wednesday is the most recent in a string of studies into the increased bottlenose dolphin deaths in the northern Gulf of Mexico. Researchers looked at lung and adrenal gland tissues from 46 dead dolphins found stranded in Louisiana, Mississippi and Alabama — 22 of which were from Barataria. These dead dolphins were found between June 2010 and December 2012.

Researchers compared these dolphins from oiled areas to 106 dead dolphins from 1996 to 2012, from outside of the northern Gulf of Mexico or before the Deepwater Horizon oil spill.

Those from the northern Gulf of Mexico were more likely to have a bacterial pneumonia — 22 percent compared to 2 percent — and impaired adrenal glands — 33 percent compared to 7 percent.

In the heavily oiled Barataria Bay in Louisiana, about half of the dead dolphins had adrenal gland problems.

Almost 20 percent of the dolphins examined in the Deepwater Horizon impact area had lung lesions compared to just 2 percent of the dolphins outside of the area.

“These dolphins had some of the most severe lung lesions I have seen,” Colegrove said. She added, “We found dolphins dying after the oil spill had lesions that were not present before the oil spill.”

The report says, “During the (Deepwater Horizon) oil spill and response period, numerous dolphins, including dolphins in Barataria Bay, were observed swimming through visibly oiled waters and feeding in areas of surface, subsurface and sediment oiling.”

Dolphins breathe at the surface where oil fumes would be heaviest. They take deep breaths and hold their breaths for an extended period of time, Venn-Watson said. Many of the dolphins were found to have bacterial pneumonia which is one of the most common complications of chemical inhalation.

“The lung is damaged and bacteria can set up in the lungs,” Colegrove said.

Other problems were found in the adrenal glands which produce hormones to let the animal deal with stresses like cold temperatures or infections.

Scientists also looked for bacteria and viruses that have caused dolphin deaths in the past to see if they were the cause, but what they found was the lung and adrenal gland problems.

The Deepwater Horizon oil rig exploded in April 2010, leading to the largest marine oil spill in United States history. Although the increase in dolphin deaths in the northern Gulf of Mexico started in February 2010, NOAA has stated that it appears the pre-spill deaths were because of cold weather and short-term exposure to fresh water localized in Lake Pontchartrain and western Mississippi waters.

BP, in its five-year post spill report, disagrees that the dolphin deaths and the oil spill can be linked.

“An examination of this and other information reveal there is no evidence to conclude that the Deepwater Horizon accident had an adverse impact on bottlenose dolphin populations in the Gulf of Mexico,” BP claims. The report calls an “unusual mortality event” as something that happens in the Gulf of Mexico and around the world.

“The increase in reported mortality observations after the spill may reflect an increase in the number of observers in the Gulf and an increased awareness among Gulf residents,” the report says.

NOAA is one of the federal members of a group of state and federal trustees currently researching ecological damage caused by the 2010 spill. Under the federal Natural Resource Damage Assessment process, these trustees must quantify the amount of damage and then put a dollar value on what it would take to bring species or ecosystems back to pre-spill conditions. The study released Wednesday is part of that assessment work and involved multiple federal and state agencies as well as university researchers.

Although a few whales have been reported dead in northern Gulf of Mexico, the unusual mortality event in the northern Gulf of Mexico has mostly involved bottlenose dolphins.

Researchers said Wednesday that work continues to determine if the increased dolphin deaths seen in the last few years is a concern for the overall population of dolphins in the oil-impacted area.

“It is a goal for us to understand what the long-term impacts could be,” said Teri Rowles, head of NOAA’s Marine Mammal Health and Stranding Response Program.

Follow Amy Wold on Twitter, @awold10.