What goes up must come down, and toxic mercury is no exception.
The metal is spewed into the sky worldwide by power plants, volcanoes and oceans. Mercury can travel half the globe and then fall in rain or snow back to Earth, where it's soaked up by soil, plants, trees and sediment.
And fish. And people who eat fish.
Once mercury has settled on a forest, it can be released again when, for example, a fire burns.
That's why Donna Olson was holding a Ziploc bag full of little perch on this little Boundary Waters Canoe Area Wilderness lake on a recent brilliant May morning.
Even here, among the most pristine waters in the nation, mercury is in the fish.
Olson was bagging the fish in the stern of a Kevlar canoe while Brent Flatten operated a battery-operated shocking device in the bow.
"There's one! Grab it!" Olson shouted as Flatten stretched to scoop it up, one of several dozen they bagged that day for later study in a lab.
Olson and Flatten are part of a U.S. Forest Service fisheries research team trying to determine the impact of forest fires on mercury levels in fish.
Crews are testing fish, water, soils and more.
The fish in Lum Lake and others across the Northland don't get mercury only from rain that falls into the lake. They also get an unknown amount from mercury that falls on nearby forests and eventually flushes into the lake.
That land-locked mercury can build up for years in forest trees and plants and then suddenly move into nearby waterways when it's released, including when forest fires burn through the area.
Fires don't generate new mercury. Instead, they rerelease it into the environment.
Scientists with the Forest Service and other agencies are, for the first time, trying to determine how much mercury is released by those fires, whether wild fires or intentional fires.
They're also finding interesting relationships between mercury on the land and mercury in the fish.
The Forest Service effort is joined by the University of Minnesota and the U.S. Geological Survey. The coalition won a $423,000 grant from the national fire sciences program to study fish from 10 Superior National Forest lakes before and after fires.
"We'll be able to see if there's a pulse of mercury in the fish after the fires," said Ken Gebhardt, aquatic specialist for the Superior National Forest.
PICKING PERCH
Perch are the perfect species to study because they are widespread across the region and their young are easy to capture in shallow water. Tiny fish are preferred so their mercury load, or exposure, can be traced to a single season.
Perch also are a common snack for walleyes, bass and pike that people eat -- a key element in the food chain that passes mercury to people.
The BWCAW is the perfect place to look because dozens of intentional fires have been and will be lit on thousands of acres. The study lakes were picked near those fires. Scientists won't have to wait for wildfires that may never come in order to gather their post-fire data.
The intentional fires are intended to reduce the amount of dead wood on the ground after the July 1999 windstorm that ravaged parts of the BWCAW.
So far, researchers primarily have gathered pre-fire data, and they hope more intentional fires are lit this summer so post-fire data flows in. They also hope for another grant to continue the study for several more years, until more post-fire data is available.
Known more for their work with trees, the research is part of the Forest Service's beefed-up fisheries program. While state agencies usually manage fish within national forests, the feds are taking an active role in protecting fish habitat and the forest's impacts on fish.
The project is so different from others that it recently won a national award among Forest Service fisheries projects.
TRENDS EMERGING
While data is preliminary, scientists already have made some observations after two summers in the field.
As expected, they found that mercury in fish is related to mercury levels in nearby soils. They also found that the higher the organic matter in the soil, the higher the mercury level.
"If mercury is high in the soil, it's high in the fish," Gebhardt said. "We were assuming from the start that there's a direct tie between the soil and the water and the fish. What's in the soil is in the fish."
Scientists also have used soil samples to prove that the mercury in the forest is coming from the sky, not from bedrock below. The mercury levels are highest near the surface and drop as researchers look deeper.
"There's no doubt this is airborne deposition. It's not coming from the rock," said Trent Wickman, air resources specialist for the Superior National Forest. "We didn't ask to get this mercury, but we have to deal with the mercury burden that's on the land now."
Scientists also have found a surprising relationship between lake size and mercury.
"The smaller the lake, the higher the mercury level" in the fish," Gebhardt said. "We don't exactly know why... but it's a pretty dramatic correlation so far."
In the end, the research may help determine if there's a short- or long-term mercury impact on fish from fires, if and where intentional fires are used in the future, or if some actions are needed to mitigate a fire's impact on fish.
"It won't be the factor, but it will be another factor to consider with fire management," Wickman said.Duluth News Tribune
