The Grand Canyon reach of the Colorado River meanders through one of the most remote ecosystems in the United States. It would be easy to assume steep canyon walls and uninhabited shores resulted in pristine waters. But research by Cary Institute’s Dr. Emma Rosi-Marshall and colleagues has found that this isolated part of the Colorado harbors toxic levels of mercury and selenium.
As part of a food web study, Rosi-Marshall and collaborators from the U.S. Geological Survey, Montana State University, and Idaho State University surveyed aquatic life at six sites along a 250-mile stretch of river downstream of Glen Canyon Dam. Sampled invertebrates and minnows had mercury and selenium concentrations in excess of toxicity thresholds for fish, wildlife, and people. Mercury levels in fish were consistently higher than other U.S. rivers, even those draining more agricultural, industrial, and urban landscapes.
Animals exposed to excess mercury and selenium can suffer from impaired reproductive success, growth, and survival. The Grand Canyon reach of the Colorado River supports the largest population of endangered humpback chub, a fish whose diet relies on aquatic invertebrates. It is also an important foraging ground for a variety of fish-eating birds, such as the kingfisher.
Mercury in the Grand Canyon’s food web largely originates from air pollution. Coal-burning power plants are the largest human-caused source of mercury emissions, which can travel hundreds of miles before being deposited. Irrigation of selenium-rich soils in the upper Colorado River basin leads to an accumulation of this contaminant downstream.
The study adds to the growing body of evidence that seemingly remote ecosystems are vulnerable to long-range pollution generated by human activities.