Researchers at the University of Hawaii at Manoa and the University of Michigan have discovered that mercury accumulation in fish tends to take place at deeper depths. Mercury is transported through the atmosphere before falling on the ocean and becoming a part of the marine food web. Fish tend to take on more mercury at depth because of the photochemical reactions that break up organic mercury in the surface waters.

“A few years ago we published work that showed that predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna,” said co-author Brian Popp, professor of geology and geophysics at the University of Hawaii at Manoa. “We knew this was true, but we didn’t know why.”

“We knew that organic and inorganic mercury dissolved in seawater has a nutrient-like profile, with lower concentrations at the surface and higher concentrations at depth,” added Anela Choy, a PhD candidate in oceanography at the University of Hawaii at Manoa. “We saw it in the water, and we saw it in the fish. But we couldn’t explain the gradient we saw, nor did we know exactly where and how the bioavailable organic mercury was entering the marine food web.”

Bacteria in the oceans transform atmospheric mercury into the organic monomethylmercury form that can accumulate in fish. Big predatory fish have high levels of methylmercury due to the fact that they consume a lot of smaller, mercury-containing fish. Previously, researchers discovered that the depths at which a species eats is almost as significant as its position in the food chain in identifying how much methylmercury it has acquired.

The researchers discovered that chemical reactions induced by sunlight annihilate as much as 80 percent of monomethylmercury in the upper depths of the central North Pacific Ocean. They also came to the conclusion that a lot of monomethylmercury must be produced and enter marine food webs in oxygen-deficient, deeper waters.

“The crystal-clear waters surrounding Hawaii and the unique information that we had about the depths at which our local fish feed allowed us to clearly identify both the photochemical degradation of monomethylmercury at surface levels and the microbial production of monomethylmercury from inorganic mercury in deeper waters,” Popp said.

The discovery that mercury is being transformed to its toxic, bioavailable form at depth is significant due to the fact that researchers anticipate that mercury levels at medium depths in the North Pacific will increase in coming decades.

According to the researchers, learning more about mercury concentrations in fish has promising advantages for all fish-eating societies, but is extremely important in Hawaii where fish consumption is among the highest levels in the country.

The primary pathway for human exposure to methylmercury is the consumption of big predatory marine fish like swordfish and tuna. Impacts of methylmercury on humans can include harm to the central nervous system, the heart and the immune system.

The new results have substantial meanings for determining how mercury levels in marine fish might react to future alterations in mercury accumulation in the ocean.

“In the next few decades there will be changes in mercury concentrations in the Pacific Ocean, and those changes are likely to be different for surface waters than for deep waters,” Popp noted. “Understanding the competing processes that produce and destroy monomethlylmercury at different depths in the ocean is critical to tracing its bioaccumulation in fishes and the potential consequences for human food supply.”

According to the U.S. Environmental Protection Agency, mercury exists in various forms, and individuals are exposed to the industrial toxin in numerous ways. Generally, people in the U.S. are exposed to mercury by consuming fish containing methylmercury. Other exposures may be due to handling or damaging products containing mercury.

The findings are described in greater detail in the journal Nature Geoscience.