U.S. Geological Survey scientists report the Grand Canyon’s food webs are contaminated with dangerously high levels of mercury and selenium. The source of the runoff pollution can be tracked down hundreds of miles upstream, coming from coal-burning electrical plants and other human sources. This shows that “remote ecosystems are vulnerable to long-range transport and subsequent bioaccumulation of contaminants,” the researchers write.

Horseshoe Bend displays the scenery and solitude of the Colorado River in the upper reaches of the Grand Canyon. Fishing on the river is peaking as temperatures are ideal for spring midge hatches. Image: Denver Post

Horseshoe Bend displays the scenery and solitude of the Colorado River in the upper reaches of the Grand Canyon. Fishing on the river is peaking as temperatures are ideal for spring midge hatches. Image: Denver Post

“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, lead author of the study.

The researchers analyzed samples taken from six distinct sites along the Colorado river. When minnows, invertebrates and fish were gauged for mercury and selenium levels, the researchers found these exceeded dietary limits. This not only endangers the health of wildlife but also humans who might eat the fish, for instance, and become contaminated.

Mercury pollution can make its way to oceans and waterways, contaminating fish and seafood, and accumulating in higher concentrations as it makes its way up the food chain. Across the United States, mercury pollution has contaminated 18 million acres of lakes, estuaries, and wetlands (43 percent of the total), and 1.4 million river miles. Once mercury in the body reaches dangerously high levels, it causes adverse effects to the nervous system. Selenium poisoning, on the other hand, has a much wider range of effects like hair and tooth loss or tumour growth.

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Surprisingly, the mercury levels in rainbow trout were below the EPA threshold. It’s surprising because according to the law of biomagnification, bigger fish should have a larger mercury concentration given they feed on fish which in turn feed on other smaller fish and so on. This may be due to the unique ecology of the Grand Canion.

“Insect food sources for fish are quite limited in the Colorado River below Glen Canyon Dam, most likely due to temperature and flow regimes of the regulated river,” said co-author of the study Dr. Ted Kennedy. “While smaller fish can satisfy their caloric needs by eating just insects, there aren’t enough of the invertebrates to make up the entire diet of larger fish, forcing them to feed on other less calorie-dense organic matter like algae.”

“We think [the mercury] is getting picked up by that algae in Lake Powell and exported into Grand Canyon,” Kennedy said.

Another interesting finding is that the sampled fish didn’t show any of the deformities normally associated with mercury contamination, even though the levels for the toxic levels should have caused this kind of effect. Remember, yesterday ZME Science published a post explaining how such deformities caused by toxic metals can be used to identify mass extinction events which happened millions of years ago. In this particular case, however, the complex interplay between the selenium and mercury canceled the deformity. “If both of these things are at high levels together, it can mitigate effects of having just one of them in a high concentration,” says Kennedy.

The number one source for these pollutants are coal-fired plants, even though there’s no such plant in the vicinity of the Grand Canyon. That’s because mercury gets airborne and stays airborne for months at a time. During this time, the mercury fired by a coal plant can wind up carried by currents thousands of miles away from its source. It’s practically a global phenomenon. The solution: install stacks that capture the mercury and other emissions and drastically reduce coal usage. In the meantime, be careful with fish sourced from the Colorado river.

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