An oil rig explosion off the coast of Louisiana killed eleven workers on 20 April 2010. The world then watched helplessly as BP's oil gushed out into the Gulf of Mexico for 87 days, killing untold millions of marine animals.
Dolphins that survived one of the worst environmental disasters ever, still appear to be suffering the effects over a decade later.
Comparing populations of common bottlenose dolphins (Tursiops truncatus) of Barataria Bay in Louisiana that live within the area of oil exposure to populations from Sarasota Bay in Florida, which haven't been exposed to serious pollution, researchers have found there are still troubling health differences between them.
Previous studies have already revealed the shorter-term impacts of the oil spill on the Barataria Bay dolphins, including abnormal adrenal function, lung disease, impaired reproduction, immune system problems and decreased survival. Dolphins living within the spill area only gave birth to living calves 19 percent of the time.
Oiled dolphin swimming in Northern Barataria Bay. (Louisiana Department of Wildlife and Fisheries)
Analysing tissue samples from 34 Barataria Bay dolphins, University of Connecticut veterinary scientist Sylvain De Guise and colleagues found immune system impairments in 2018 similar to those found in the population in 2011. They then performed laboratory tests on dolphin cells and mice to confirm if oil exposure can trigger these conditions.
"The parallel between findings in dolphins exposed following the Deepwater Horizon spill and laboratory mice experimentally exposed to oil was impressive and really helped build the weight of evidence between oil exposure and specific effects on the immune system," explained De Guise.
Both the mice exposed to oil and the dolphins had increased T lymphocyte (white blood cell) proliferation and more cells that suppress the immune system – T regulatory cells. These cells usually prevent autoimmune diseases.
While immunological changes have also been seen in dolphins exposed to other stresses, like algal bloom toxins, the pattern of changes was different from those seen in the Barataria Bay dolphins.
Studies in rodents have previously linked such immune system changes to increased susceptibility to disease. De Guise's team showed these immune differences could also be passed down from rodents who had been exposed to oil pollution to their young. And as the changes were not just present in the older dolphins, the team is concerned these impairments are being passed down through the generations of dolphins.
There is, however, another potential explanation for this though.
"It is possible that there is continued exposure to the Deepwater Horizon oil that may not have been completely removed from the Barataria Bay ecosystem," the team wrote in their paper.
Samples of the Barataria Bay marsh sediments showed oil concentrations were still 10 times higher than before the spill, eight years later. Further research will be needed to determine exactly what is happening.
Researchers are concerned the long term effects of oil spills may not be limited to dolphins. We still know little about the health outcome from other long-lived species like turtles. Immune effects have also been reported in humans who have worked to clean up oil spills, suggesting there's a common response to oil exposure across mammals.
"The long-term effects and potential for multigenerational effects raise significant concerns for the recovery of dolphin populations following the spill," De Guise said.
This research was published in Environmental Toxicology and Chemistry.