The long-standing mystery of how mercury gets into open-ocean fish
has been solved, claim University of Michigan researchers and their
University of Hawaii colleagues. Their findings suggest that levels of
the toxin in Pacific Ocean fish will likely rise in coming decades.Using isotopic measurement techniques developed at U-M, the
researchers determined that up to 80 percent of the toxic form of
mercury, called methylmercury, found in the tissues of deep-feeding
North Pacific Ocean fish is produced deep in the ocean, most likely by
bacteria clinging to sinking bits of organic matter. The study also confirmed that the mercury found in Pacific fish near
Hawaii likely traveled through the air for thousands of miles before
being deposited on the ocean surface in rainfall, said U-M environmental
scientist Joel Blum. The North Pacific fisheries are downwind from
rapidly industrializing nations such as China and India that are
increasingly reliant on coal-burning power plants, a major source of
mercury pollution. "This study reinforces the links between mercury emitted from Asian
countries and the fish that we catch off Hawaii and consume in this
country," said Blum, the lead author of a paper scheduled for online
publication Aug. 25 in Nature Geoscience. "The implications are that 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," Blum said. "Cleaning up our own shorelines is not going to
be enough. This is a global atmospheric problem." 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. In December 2011, the Environmental Protection Agency released new
standards sharply limiting future emissions of mercury and other toxic
pollutants from coal- and oil-burning power plants in the United States.
Earlier this year, the United Nations Environment Programme negotiated
the Minamata Convention on Mercury, an international treaty aimed at
curbing future mercury emissions; it is still unclear what level of
mercury-emission reductions will result. It has been known for some time that large predatory marine fish contain
high levels of methylmercury in part because they eat lots of smaller,
mercury-containing fish. The toxin builds up in the tissues of the
top-of-the-food-chain predators through a process called
bioaccumulation. In 2009, researchers at the University of Hawaii determined that the
depth at which a species of fish feeds is nearly as important as its
position in the food chain in determining how much methylmercury it
contains. "We found 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, a professor of geology and geophysics at the
University of Hawaii at Manoa and co-author of both the 2009 paper and
the new Nature Geoscience paper. "We knew this was true, but we didn't know why." That observation was difficult to explain because researchers had
presumed that if methylmercury production occurs in the open ocean, it
most likely takes place in the biologically active surface layer,
carried out by microbes that convert inorganic mercury into the toxic
organic form through a process called methylation. But in the latest study, the Michigan and Hawaii researchers showed that
perhaps as much as 80 percent of the methylmercury found at depth in
the central North Pacific is produced below what is known as the surface
mixed layer, a region extending down to about 165 feet. They found that
methylation continues down to a depth of about 2,000 feet, most likely
the work of oxygen-shunning bacteria attached to sinking particles of
dead plant and animal matter containing inorganic mercury. That finding is important in part because scientists expect mercury
levels at intermediate depths (660 to 3,300 feet) in the North Pacific
to rise in coming decades; one estimate calls for a doubling by
mid-century. At the same time, oxygen-depleted regions called oxygen
minimum zones, which typically occur at depths greater than 1,300 feet,
are expanding in oceans worldwide, and human-caused climate change is
expected to accelerate that process. The work by Blum and his colleagues suggests that if these two trends
unfold as expected, conditions will favor increased production of
methylmercury by microbes known as anaerobic bacteria, which will
increase the threat to the North Pacific fisheries, the world's most
important source of seafood. "The implication is that predictions for increased mercury in deeper
water will result in higher levels in fish," said Blum, a professor in
the Department of Earth and Environmental Sciences. In their study, the researchers analyzed tissue samples from nine
species of marine fish that feed at different depths in a region near
Hawaii called the North Pacific Subtropical Gyre. The work combined
biogeochemistry with direct marine ecology observations. 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
led the Hawaii 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. Together, the researchers showed how and where methylation occurs in the
open ocean and explained the previously observed increases in the
mercury concentration of predatory fish with depth. They found that
while methylation occurs in well-lit near-surface waters, sunlight
destroys up to 80 percent of the methylmercury formed there, through a
process called photochemical degradation. "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 methylmercury at
surface levels and the microbial production of methylmercury from
inorganic mercury in deeper waters," said Popp, a University of Michigan
graduate. In addition, the isotopic composition of the mercury found in the fish
tissues was "a nearly perfect match" with the chemical signature of
mercury in the atmosphere known to travel long distances, far from
pollution sources such as coal-burning power plants, Blum said. That finding confirms an idea that was long-suspected but previously
unsupported by hard evidence: "These results strongly support the
hypothesis that long-range transport of mercury deposited to the ocean
surface is ultimately what's ending up in these fish," Blum said. The nine species of fish used in the study, listed from shallowest- to
deepest-feeding, are flying fish, mahi-mahi, yellowfin tuna, skipjack
tuna, moonfish (opah), bigeye tuna, swordfish, and two species of
lantern fish. Source:Nature Geoscience
No comments:
Post a Comment