Long-Term Monitoring Reveals How Water and Biota in Remote Lakes Respond Differently to Changes in Atmospheric Deposition of Mercury

Science Center Objects

A comparison of regional mercury atmospheric deposition data with water, yellow perch, and dragonfly larvae samples from lakes in Voyageurs National Park indicates that decreases in mercury emissions resulted in mercury reduction in water from these remote lakes, but mercury declines in biota were significant in only one of three lakes, likely because resident biota integrate exposure over much longer time periods.

Scientists sampling fish from a canoe on Shoepack Lake in Voyageaurs National Park

Mark Sandheinrich and Sean Bailey of the University of Wisconsin - La Crosse sampling fish from a canoe on Shoepack Lake in Voyageurs National Park.

(Credit: Mark Brigham, USGS. Public domain.)

Mercury, a pervasive environmental contaminant, is a concern for human and wildlife health. Whereas mercury emissions have increased globally, mercury releases to the environment in the United States and Canada have decreased since 1990. However, questions remain about whether the decreases in atmospheric deposition have equated to decreases in bioaccumulation of mercury in fish from remote lakes and rivers where atmospheric deposition is the predominant source of mercury.

In response to these questions, U.S. Geological Survey and National Park Service scientists investigated and report on one of the longest known multimedia datasets for mercury. The dataset included aqueous total mercury, methylmercury, and sulfate from near-surface lake-water samples from four lakes in Voyageurs National Park in northern Minnesota. The dataset also included total mercury in aquatic biota from the same lakes, as well as atmospheric mercury deposition from two regional sites.

Dragonfly larvae in water in bottom of zip-lock plastic bag held at horizon level with lake

Sample of dragonfly larvae in water in bottom of zip-lock plastic bag, held at horizon level with lake. At Sand Point Lake in Voyageurs National Park.

(Credit: Roger Haro, University of Wisconsin - LaCrosse. Public domain.)

Mercury deposition at the regional sites decreased by an average of 22 percent from 1998 to 2018. Much of these decreases occurred prior to 2009, with relatively flat trends since 2009. In the four lakes, aqueous methylmercury concentrations declined by an average of 44 percent and aqueous total mercury declined by an average of 27 percent (2001–18). Near-surface aqueous methylmercury may be responding to a decline in atmospheric mercury deposition as well as a decline in sulfate deposition, which is an important driver of mercury methylation in the environment.

For the three lakes with long-term biomonitoring (2000–18), temporal patterns in yellow perch (Perca flavescens) and dragonfly larvae (Odonata, Anisoptera) total mercury concentrations declined significantly in only one lake. Two lakes had initial declines until about 2010, corresponding to the initial declines in atmospheric deposition. Results from this study indicate that regional-scale decreases in mercury and sulfate emissions have resulted in reduced mercury concentrations in local lake water. Conversely, biological tissues integrate exposure over much longer time periods, and owing to the large pool of mercury in these lake systems, reduction of mercury in biota may take longer.

This research was funded in part by the U.S. Geological Survey Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology), which is part of the U.S. Geological Survey Ecosystems Mission Area. Additional funding for the manuscript was provided by the National Park Service. Funding for some data collection and analysis was provided by several agencies and programs, including the U.S. Forest Service, U.S. Environmental Protection Agency, Minnesota Pollution Control Agency, University of Minnesota, University of Wisconsin at LaCrosse, and the Great Lakes Restoration Initiative.