Aug. 25, 2013 â€” Mercury -- a common industrial toxin
-- is carried through the atmosphere before settling on the ocean and entering
the marine food web.
new research from the University of Michigan and the University of Hawai'i at
Manoa School of Ocean and Earth Science and Technology (SOEST) 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.
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.
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
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."
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.
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.
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.
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
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
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.
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
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."
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
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.