Although the Grand Canyon segment of the Colorado River
features one of the most remote ecosystems in the United States,
it is not immune to exposure from toxic chemicals such as
mercury according to newly published research in Environmental
Toxicology and Chemistry.
The study, led by the U.S. Geological Survey, found that
concentrations of mercury and selenium in Colorado River food
webs of the Grand Canyon National Park, regularly exceeded risk
thresholds for fish and wildlife. These risk thresholds indicate
the concentrations of toxins in food that could be harmful if
eaten by fish, wildlife and humans. These findings add to a
growing body of research demonstrating that remote ecosystems
are vulnerable to long-range transport and bioaccumulation of
contaminants.
“Managing exposure risks in the Grand Canyon will be a
challenge, because sources and transport mechanisms of mercury
and selenium extend far beyond Grand Canyon boundaries,” said
Dr. David Walters, USGS research ecologist and lead author of
the study.
David Uberuaga, superintendent of Grand Canyon National Park,
added, “studies like this continue to educate the public and
highlight the threats that face the park and its resources."
The study examined food webs at six sites along nearly 250
miles of the Colorado River downstream from Glen Canyon Dam
within Glen Canyon National Recreation Area and Grand Canyon
National Park in the summer of 2008. The researchers found that
mercury and selenium concentrations in minnows and invertebrates
exceeded dietary fish and wildlife toxicity thresholds.
Although the number of samples was relatively low, mercury
levels in rainbow trout, the most common species harvested by
anglers in the study area, were below the EPA threshold that
would trigger advisories for human consumption.
“The good news is that concentrations of mercury in rainbow
trout were very low in the popular Glen Canyon sport fishery,
and all of the large rainbow trout analyzed from the Grand
Canyon were also well below the risk thresholds for
humans,” said Dr. Ted Kennedy, USGS researcher and co-author of
the study.
“We also found some surprising patterns of mercury in rainbow
trout in the Grand Canyon. Biomagnification usually
leads to large fish having higher concentrations of mercury than
small fish. But we found the opposite pattern, where small,
3-inch rainbow trout in the Grand Canyon had higher
concentrations than the larger rainbow trout that anglers
target. This inverted pattern likely has something to do with
the novel food web structure that has developed in Grand
Canyon.”
Airborne transport and deposition -- with much of it coming
from outside the country -- is most commonly identified as the
mechanism for contaminant introduction to remote ecosystems, and
this is a potential pathway for mercury entering the Grand
Canyon food web. Also, long-range downstream transport from
upstream sources can deliver contaminants to river food webs.
This is the case for selenium in this study, where irrigation of
selenium-rich soils in the upper Colorado River basin
contributes much of the selenium that is present in the Colorado
River in Grand Canyon.
Exposure to high levels of selenium and mercury has been
linked to lower reproductive success, growth, and survival of
fish and wildlife. No human consumption advisories are currently
in place for fish harvested from the study area. However, to
assess potential risks to humans that may consume fish from
Grand Canyon or Glen Canyon National Recreation Area, additional
studies are planned.
Research partners in this study include the Cary Institute of
Ecosystem Studies, Montana State University, and Idaho State
University.
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Map of study area showing sample
location relative to Glen Canyon Dam on the Colorado
River, Grand Canyon (AZ, USA). |
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