The Minamata Convention on Mercury, which entered into force in August 2017, is a global effort to curb the use and release of mercury. However, a new study suggests that cutting mercury emissions alone may not be enough to combat mercury accumulation in commonly eaten fish (Nature 2019, DOI: 10.1038/s41586-019-1468-9).
Mercury is predominantly emitted from mining operations, coal combustion, and cement production. Aquatic species then accumulate mercury through the food web. More than 80% of exposure to neurotoxic methylmercury in the US is due to seafood consumption.
Despite the health risks associated with methylmercury, scientists do not have a good understanding of how the compound accumulates in marine predators such as Atlantic Bluefin tuna. “We can’t just grow them in a lab,” says Harvard University biogeochemist Amina Schartup, who led the new work.
Instead, the researchers built a mechanistic computational model, incorporating information on bioenergetics and predator-prey interactions. The model reproduces the food web of the Gulf of Maine, including more than 20 species—from phytoplankton to commercially important predators and shellfish.
By modifying external parameters, the team could monitor how the entire food web responded to changing environmental conditions. The study focused on changes to three environmental parameters: a 20% decrease in mercury emissions, a 1 °C ocean temperature increase, and a shift in diet due to overfishing of herring.
As the ocean warms, predatory species become more active, requiring more food to sustain themselves. This increase in appetite also leads to an increase in mercury intake, the model shows—even for a relatively small temperature change.
However, not all fish responded in the same way to the environmental perturbations. The model predicts that methylmercury content will decrease slightly in Atlantic cod, but increase by over 70% in spiny dogfish. The authors speculate that these differences could help explain why observed methylmercury concentrations have not been uniformly decreasing in fish in the Gulf of Maine, despite reductions of mercury inputs into the ecosystem.
Michael Bank, a senior scientist at the Institute of Marine Research, describes the model as “comprehensive and sophisticated,” but notes that it’s limited in its geographical scope and, in some cases, based on small sample sizes. He also points out that some of the predicted results contradict the long-term observational data within Norwegian marine ecosystems.
He says he hopes that the work done on this model will spark efforts to “develop a more robust modeling community” around bioaccumulation of mercury in fish, noting that governments worldwide have datasets that could be used to run the model for other areas of the ocean.
The bottom line, both Schartup and Bank say, is that attempts to limit mercury emissions must be accompanied by reductions in greenhouse gas emissions. “Mercury accumulating in fish is not happening in a vacuum,” Schartup says. “It’s happening in the environment we live in.”
This story was updated on Aug. 8, 2019, to correct the date for the Minamata Convention on Mercury. It went into force in August 2017, not September 2017.