Elevated mercury levels discovered in dolphins

A recent study published in the journal Toxics has identified elevated mercury levels in dolphins in the U.S. Southeast, with the highest concentrations found in those from Florida’s St. Joseph and Choctawhatchee Bays.

According to the experts, dolphins serve as a “sentinel species” due to their high position in the food chain, long lifespans, and physiological similarities to humans. Their diet, which includes fish like spot, croaker, and weakfish, overlaps with that of humans, indicating shared exposure risks to mercury pollution.

Dolphin mercury levels and human health risks

Although the study does not make direct conclusions about mercury levels or health risks for residents of Florida and Georgia, it references previous research indicating a correlation between high mercury levels in dolphins in Florida’s Indian River Lagoon and the human population in the same area.

“As a sentinel species, the bottlenose dolphin data presented here can direct future studies to evaluate mercury exposure to human residents in the Southeast as well as other potentially affected areas in the United States,” the authors explained.

The U.S. Environmental Protection Agency (EPA) recognizes fish as part of a healthy diet but cautions that groups such as pregnant individuals, children, the elderly, and those with weakened immune systems are more susceptible to the adverse health effects of mercury. Moreover, some people are at higher risk due to their high fish consumption.

Mercury toxicity in dolphins 

According to the Minamata Convention on Mercury, in marine mammals like dolphins, mercury toxicity can lead to severe health issues, including reproductive failure, behavioral changes, and even death.

While some mercury occurs naturally, most mercury pollution results from burning fossil fuels and industrial activities like mining, cement production, and chemical manufacturing. In aquatic environments, bacteria convert mercury into methylmercury, which accumulates up the food chain from small fish to top predators like dolphins.

Focus of the research 

The scientists analyzed 175 skin samples from bottlenose dolphins collected between 2005 and 2019 from various estuaries in Florida and Georgia, including St. Joseph, Choctawhatchee, and Biscayne Bays in Florida, and the Skidaway and Turtle/Brunswick river estuaries and Sapelo Island in Georgia. 

They measured mercury levels in dolphin skin, which correlate with methylmercury levels in their other tissues and organs.

Colleen Bryan, a research biologist and co-author of the study, noted that the National Institute of Standards and Technology (NIST) has been involved with dolphin health assessment and biopsy studies since 2002.

“We’ve helped standardize testing protocols and collection and storage methods, so any measurements taken are highly accurate and comparable across studies.”

Results of the study

The highest mercury levels recorded were in St. Joseph Bay, where dolphins averaged 14,193 nanograms of mercury per gram (ng/g) in their skin. 

Lead author Mackenzie Griffin, now a biologist at NOAA, suggested that industrial activities might partly explain these levels, and noted that the bay’s limited water exchange with other waterways exacerbates the problem.

The experts also found that dolphins in the Charleston, South Carolina area had the lowest average mercury levels at 509 ng/g, while those in the Florida Coastal Everglades had high levels at 10,916 ng/g. 

According to Bryan, Charleston Harbor benefits from tidal flows that help flush out mercury, whereas in the Everglades, decomposing mangrove leaves release mercury into the water, which is then converted to methylmercury by bacteria.

“Our research adds to other studies that have consistently shown elevated levels of mercury in dolphins in the Southeast. We hope it will lead to a better understanding of what’s happening in our oceans,” Bryan concluded.

Mercury in the ocean

Mercury in the ocean primarily originates from natural sources like volcanic eruptions and human activities such as coal burning, mining, and industrial processes. Once released into the atmosphere, mercury can travel long distances before settling into bodies of water. 

In the ocean, mercury undergoes a series of transformations, one of the most concerning being its conversion to methylmercury by certain microorganisms.

Methylmercury is a potent neurotoxin that bioaccumulates in marine food webs, meaning that its concentration increases as it moves up the food chain. As a result, top predators like large fish and marine mammals often have high levels of mercury, posing risks to both wildlife and humans who consume seafood. 

Efforts to monitor and reduce mercury levels in the ocean include international agreements like the Minamata Convention, aimed at controlling and reducing mercury emissions. 

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