The protected old-growth forest in the Amazon of southeastern
Peru appears pristine: Ancient trees with massive trunks grow alongside young,
slender ones, forming a canopy so thick it sometimes feels to scientists like
evening during the day.
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But a new analysis of what’s inside the forest’s leaves and
birds’ feathers tells a different story: The same canopy that supports some of
the richest biodiversity on the planet is also sucking up alarming levels of
toxic mercury, according to a study published Friday.
The mercury is released into the air by miners searching for
gold along nearby riverbanks. They use mercury to separate the precious metal
from surrounding sediment and then burn it off. Carried in the air, particles
catch on leaves like dust and are washed onto the forest floor by rain. Other
particles are sucked into the leaves’ tissue. From there, mercury appears to
have transferred up the food web to songbirds, which showed levels of mercury
2-12 times as high as those in comparable areas farther from mining activity.
“The patterns were so much more stark and so much more
devastating than we expected to find,” said Jacqueline Gerson, a biogeochemist
at the University of California, Berkeley, who led the research as a doctoral
student at Duke University. The study was published in the journal Nature
Communications.
The findings, from the Madre de Dios region of Peru, provide new
evidence of how people are altering ecosystems around the world, as species
extinction rates accelerate, with little understanding of the consequences.
Scientists have long known that mercury, which is also released
into the air by burning coal, is a dangerous neurotoxin to humans and animals.
In aquatic ecosystems, it can easily convert into a very poisonous form called
methylmercury. As big fish eat smaller ones, the mercury sticks around,
accumulating up the food web. For this reason, doctors advise pregnant women
around the world to avoid eating large, predatory fish like shark, king
mackerel and swordfish.
In the Madre de Dios region, where illegal gold mining has
surged in recent years along with the price of gold on global markets, the
government declared a health emergency in 2016 after 40% of people tested in 97
villages had dangerously high levels of mercury in their systems.
Researchers have mostly focused on human exposure to mercury in
rivers, lakes and oceans. They have not been as worried about it on land, since
it is less likely to become methylmercury. But the sheer load of mercury going
into the forest, combined with rainy conditions and soil, are leading to
concerning levels of methylmercury there.
“It’s been assumed that people living in the Peruvian Amazon
have been getting all their methylmercury exposure from eating fish,” Gerson
said. “That may not be the case.”
The kind of gold mining that happens in the Madre de Dios region,
called artisanal and small-scale gold mining, occurs in about 70 countries,
often illegally or unofficially, and it is the largest source of mercury
pollution in the world. It also accounts for about 20% of global gold
production.
Julio Cusurichi Palacios, president of the Native Federation of
the Madre de Dios River and Tributaries, a group formed by Indigenous
communities in the region, said the government should combat illegal mining
with enforcement but also by strengthening alternative livelihoods for
Indigenous and other local people. They harvest fish, Brazil nuts, yucca and
corn, he said, but need help “improving their goods, selling their goods, so
they don’t fall into thinking, ‘I better go into mining, since my product
doesn’t have a market.’”
By capturing the mercury, the forests are helping to keep it out
of aquatic systems, said Emily Bernhardt, a professor of biogeochemistry at
Duke and co-author of the study.
“These are some of the most biodiverse forests on Earth,”
Bernhardt said. “We already knew they sequester tons of carbon in their biomass
and their soils, and we have now uncovered an additional, incredibly important
service.”
But the service is not without cost. Mercury poisoning can
affect birds’ ability to navigate and sing, and can cause them to lay fewer
eggs, she noted. It can also make their eggs less likely to hatch.
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