Mercury accumulation in the ocean fish we eat tends to take place at deeper depths, scientists found, in part because of photochemical reactions that break down organic mercury in well-lit surface waters. More of this accessible organic mercury is also being generated in deeper 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 Brian Popp, professor of geology and geophysics at UH Manoa, and a co-author of a new paper scheduled for online publication August 25 in the scientific journal Nature Geoscience. "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," said Anela Choy, a PhD candidate in oceanography at UH 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 change atmospheric mercury into the organic monomethylmercury form that can accumulate in animal tissue. Large predatory fish contain high levels of methylmercury in part because they eat lots of smaller, mercury-containing fish. In 2009, researchers at UH Manoa determined that the depths at which a species feeds is nearly as important as its position in the food chain in determining how much methylmercury it contains.
For the new research, the UH Manoa team worked closely with colleagues at the University of Michigan who used a highly sophisticated mass spectrometer to measure the stable isotopic compositions of mercury in nine species of marine fish that feed at different depths, including six predator fish and three prey fish.
Their analysis showed that chemical reactions driven by sunlight destroy up to 80 percent of monomethylmercury in the well-lit upper depths of the central North Pacific Ocean near Hawai'i. The scientists also determined that a significant amount of monomethylmercury must be formed and enter marine food webs in oxygen-poor, deeper waters.
The Michigan researchers had previously recorded mercury isotope measurements on fish in the Gulf of Mexico that suggested that up to 50 percent of monomethylmercury was destroyed by photochemical reactions before it was taken up by yellowfin and blackfin tuna living offshore.
In Hawaii, the conditions were different—and better—for this type of analysis. "The crystal-clear waters surrounding Hawai'i 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 finding that mercury is being converted to its toxic, bioavailable form at depth is important in part because scientists expect mercury levels at intermediate depths in the North Pacific to rise in coming decades.
"The implication is that predictions for increased mercury in deeper water will result in higher levels in fish," said Joel Blum of the University of Michigan, the lead author on the new paper and a professor in the department of earth and environmental sciences. "If we're going to effectively reduce the mercury concentrations in open-ocean fish, we're going to have to reduce global emissions of mercury, including emissions from places like China and India."
Research that helps us to better understand mercury concentrations in fish has potential benefits for all fish-consuming societies, but is particularly relevant here in Hawai'i where marine fish consumption is among the highest levels in the United States.
For example, the University of Hawai'i Pacific Biomedical Research Center recently found that women in Hawai'i are three times more likely to have elevated umbilical cord blood mercury levels compared with the national average. For this study group, there was a positive correlation between the cord blood mercury concentration and fish consumption, based on dietary surveys.
The main pathway for human exposure to methylmercury is the consumption of large predatory marine fish such as swordfish and tuna. Effects of methylmercury on humans can include damage to the central nervous system, the heart and the immune system. The developing brains of fetuses and young children are especially vulnerable.
"The results of our research allow us to determine which marine fish are likely to have lower mercury concentrations, and why mercury concentrations are higher in some fish compared to others," said Jeffrey Drazen, associate professor of oceanography at UH Manoa. "This research is exciting because it allows us to gather new insight about both the biogeochemistry of mercury and the ecology of animals living in Earth's largest habitat—the open ocean."
The new research findings also have important implications for estimating how mercury levels in marine fish might respond to future changes in mercury deposition to the ocean, especially in light of proposed international treaties on industrial mercury emissions.
"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 said. "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."
Blum led the effort to very precisely measure the ratios of the stable isotopes of mercury, relying on techniques his lab has developed to take advantage of a natural phenomenon called isotopic fractionation. Popp and Drazen led the Hawai'i group that sampled fish at various depths, measured the total amount of mercury in their muscle tissues, and determined their position in the marine food web.