Bottlenose dolphins swimming off the Florida Everglades, beloved for zipping alongside lonely boaters in the remote bays and rivers chiseled out of the vast marshes, have the highest levels of mercury ever documented in the mammals, researchers have found.
But why that is may not be so easy to unravel.
The findings surfaced in a study in the journal Environmental Pollution that looked at pesticides and toxins in South Florida dolphins. Dolphins are considered a sentinel species, providing valuable insight into ecosystems and public health. Last year, for example, in a study that looked at dolphins in the Indian River Lagoon, researchers discovered that elevated mercury levels in dolphins accurately reflected high amounts in the nearby human population.
But in the Everglades, nothing is ever simple.
The high amounts of mercury likely come from the miles of mangrove that line the coast and form countless islands, said Florida International University marine ecologist and co-author Jeremy Kiszka. Mercury in the Everglades has long been linked to smoke stacks and fertilizer used in farming and led to declines in birds — as well as repeated health warnings over eating fish. But aggressive cleanup efforts have helped reduce levels.
Still, mangroves are remarkably efficient at producing and trapping mercury, and filtering it into the water, so determining where the toxic mercury comes from, and how long it’s been there, remains poorly understood, researchers say.
“I would love to answer this absolutely critical question,” Kiszka said. “I understand it’s frustrating. We don’t know where it’s coming from. OK, potentially the mangroves, but there could be other sources.”
That also means the dolphins inhabiting Florida Bay, Whitewater Bay, Joe River and other areas in Everglades National Park studied by the team have likely had high levels for a long time with no ill effects, he said.
“I doubt there is any impact on the population. However, without any data, it’s not reasonable to say,” he said. “This is a baseline study.”
While boaters to the remote rivers and bays are familiar with the dolphins, little is actually known about them. Kiszka said FIU scientists have been studying the dolphins since 2010 but have focused largely on their distribution and foraging habits, not their physiology. So for this study, the team wanted to see what kind of pollution they could find in the dolphins, as well as pods in the Florida Keys, examining tissue samples taken from scores of dolphins in 2008 and 2013.
Not surprisingly, they found higher levels of pesticides in dolphins near the Keys, which were more likely to be exposed to urban runoff. However, the levels were lower than expected, providing some good news, Kiszka said.
Because mangroves trap and transport mercury — other studies have found high levels in the Amazon and other mangrove forests — scientists expected to find elevated levels. But not in such high amounts, he said.
“I couldn’t believe those levels because that’s the highest ever recorded,” he said. “It raises a lot of other questions.”
FIU scientists now hope to expand the study to include other animals — alligators and bull sharks — that could give them a better understanding of what mercury is doing in the marine environment.
“Expanding to other species in the Everglades will definitely help to understand the pathways,” he said. “We’re trying to put together the many different pieces to understand what makes those animals or ecosystems more susceptible to high mercury. There’s a lot of work to be done.”
Follow Jenny Staletovich on Twitter @jenstaletovich.