Ocean fish acquire more mercury at deeper depths
New research from the University of Michigan and the University of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology combines biogeochemistry and direct marine ecology observations to show how the global mercury cycle is colliding with ocean fish—and the human seafood supply—at different depths in the water.
Mercury—a common industrial toxin—is carried through the atmosphere before settling on the ocean and entering the marine food web. Mercury accumulation in the ocean fish that is eaten 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 Mānoa, and a co-author of a new paper scheduled for online publication August 25, 2013, 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 Mānoa. “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 Mānoa 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 Mānoa 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 Hawaiʻi. 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.
Research more productive in Hawaiʻi
In Hawaiʻi, the conditions were different—and better—for this type of analysis. “The crystal-clear waters surrounding Hawaiʻi 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. “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.”
Results have big impact locally
Research that helps everyone better understand mercury concentrations in fish has potential benefits for all fish-consuming societies, but is particularly relevant here in Hawaiʻi, where marine fish consumption is among the highest levels in the United States.
For example, the University of Hawaiʻi Pacific Biomedical Research Center recently found that women in Hawaiʻi 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 Mānoa. “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.”
Future consequences for human food supply
Read the UH Mānoa news release for more on this research.