Mercury concentrations in Lake Powell higher in side canyons

USGS scientists research where and why mercury is present following fish advisory.

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PAGE – Mercury concentrations within Lake Powell are highest around the side canyons, according to a new U.S. Geological Survey report.

This finding is part of a study to better understand why mercury concentrations in Striped bass are higher in the lower part of the lake.

USGS Scientist David Naftz says the results of this study may provide a basis for the identification and evaluation of remediation activities related to mercury.

“Findings indicate that more limited areas within Lake Powell, such as side canyons and river arms, could be targeted by resource managers for remediation strategies,” Naftz said, “if deemed necessary.”

Results show levels of methylmercury generally increased from the upper to the lower lake. Narrow side canyons in the lower half of the lake also showed higher levels of methylmercury. Mercury concentrations were found to be higher in tissue samples from Striped bass collected from Wahweap Bay, located at the lower end of the lake, than bass from Good Hope Bay at the upper end.

Results also indicated that fish tissue samples collected in the San Juan Arm contained higher mercury concentrations than fish tissue samples collected from the lower reservoir. The same factors that influence higher mercury concentrations in the lower lake likely influence higher mercury concentrations in the arms compared to the main stem. 

Conditions in the lower part of the reservoir were found to be more conducive for methylmercury production.

Findings show that inflowing water from the Colorado River at the upper end of the lake carries suspended sediment, which blocks light from penetrating very deep into the water.

This limits the production of microscopic algae, or phytoplankton in the upper portion of the lake.

As water moves farther downstream, the flow slows and suspended sediment settles to the bottom, allowing more light penetration. The report also suggests that increased light enhances phytoplankton production, resulting in oxygen consumption by microorganisms that feed on phytoplankton, which creates favorable low-oxygen conditions for methylmercury production.

Lake Powell serves as a critical Western U.S. reservoir and a source of power generation, attracting more than 3 million visitors a year. Many of those visitors, according to the USGS, are Striped bass anglers.

Findings from an earlier study resulted in 2012 fish consumption advisory for Striped bass for the lower portion of the lake. Striped bass in the lower half of the lake consistently exceeded the U.S. Environmental Protection Agency human consumption advisory limits for mercury.

USGS scientists, in a recent study, collected water, plankton, and sediment samples from the lake. Fish tissue samples were collected during November 2014 by the Utah Division of Wildlife Resources and were analyzed by the USGS Mercury Research Lab.

Data were compared to previous fish surveys to gain a better understanding of mercury dynamics within the lake.

Mark Marvin-DiPasquale, a USGS mercury scientist on the project, says mercury concentrations differ in certain parts of the lake.

“Our focus was to better understand the conditions that tend to promote higher or lower levels of toxic methylmercury in sediment, water, fish, and (in) plants,” Marvin-DiPasquala explained.

Mercury is a naturally-occurring metal that can pose a threat to humans, fish, and to wildlife if they are exposed to elevated levels of its most toxic form: methylmercury.

In aquatic systems, methylmercury is created from inorganic mercury by microorganisms under particular environmental conditions, such as low oxygen.

The movement of inorganic mercury from the atmosphere or land to the water does not always result in equivalent levels of methylmercury in fish and in wildlife unless environmental conditions are favorable for methylmercury production.

Critical work remains to be done in Lake Powell related to mercury trends during different times of the year, according to the USGS. Geographic areas and habitat types, as well as gaining a better understanding of the relative contribution of sediment versus water-column mercury methylation.