Do not eat: Researchers seek clues on mercury in St. Louis River estuary fish

Mercury can severely affect children's brain and neurological development.

UMD research assistants Amber White and Dan Fraser examine a sample of St. Louis River estuary water and sediment that will be tested for mercury levels. It's one of multiple research projects underway to determine why so much mercury in the river becomes toxic, building up in fish. John Myers /

If a little kid catches a big walleye on Lake Winnibigoshish, the state of Minnesota says she can safely eat that fish once a week.

If she catches the fish out of the upper St. Louis River, say around Brookston, the state suggests just one meal per month.

But if the same kid catches a big walleye on the St. Louis River estuary in Duluth, the state warns her not to eat it. Ever.

That’s because walleye in the estuary contain very high levels of toxic mercury — among the highest from any lake or river in Minnesota; only the Red River in northwestern Minnesota has higher mercury levels in fish than the St. Louis River estuary.


Amber White, UMD research assistant scrapes a sample of St. Louis River estuary sediment to be tested for mercury. Scientists from multiple universities and agencies are working to find out why so much mercury in the river methylates --- becomes toxic -- and builds up in fish, making them unsafe for some people to eat. John Myers /

That mercury can severely affect children's brain and neurological development.

So why are estuary fish so high in mercury — higher than fish in the same river upstream, higher even than Lake Superior just downstream? It’s the same river, with mostly the same water, and roughly the same levels of elemental mercury flowing through and falling from the sky.

Scientists from the University of Minnesota Duluth have joined those from state and federal agencies and several other universities to find the answer. They are busy this summer, and have been for several years now, trying to figure out why the estuary’s ecosystem converts more mercury into that toxic methylmercury form. And they want to know why fish in the river seem to acquire and store toxic mercury at a higher rate.

They are studying mud at the bottom of the estuary, water at the surface and below, and the creatures that live throughout the ecosystem — from tiny algae to mayfly larvae to perch and walleye.

“Most of the action is in the top layer, just a few centimeters, of the sediment,” said Amber White, a UMD research assistant, on a recent trip on the water to gather samples. “But we take it down further just to be sure.

”The goal is to find out the cause of mercury methylation and turn it off, or try to control it, by some sort of human action.

“We know there’s more mercury in the fish in the estuary than nearby waters. We still don’t know what the cause of that is,” said Nathan Johnson, a professor in UMD’s Department of Civil Engineering who has been studying mercury in the estuary for several years. “Is it high because of the characteristics of the waterway? Or because of some higher pollutant load coming in? That doesn’t seem to be the case.


”In his Sea Grant-funded project, Johnson is collaborating with scientists from the University of Wisconsin-Madison, University of Wisconsin-La Crosse and Gustavus Adolphus College in ongoing estuary mercury efforts. His graduate students and research assistants have been looking closely the sediment to see if it’s legacy mercury, deposited or left behind decades ago, that’s causing the problem.

Samples of St. Louis River estuary sediment. The top layer of sediment contains the most living organisms, and the most mercury. John Myers /

Water chemistry, hydrology or sulfate?

Much is at stake. According to a study by the Minnesota Department of Health released in 2012, one of every 10 babies born in the Lake Superior region of Minnesota has unsafe levels of toxic mercury in his or her bloodstream, likely because their mothers ate too much contaminated fish while pregnant.

But the estuary’s ecosystem is complex. And scientists say there may be no easy way to solve the problem.

“I don’t think we’re going to get a single, definitive answer,” Johnson said.

In essence, the science so far says it’s not that the estuary is receiving more mercury but that mercury in the estuary is reacting differently — it’s more available to the ecosystem, and more of it is in the toxic, methylated form.

“What we know is that watersheds vary dramatically in how much mercury they methylate. We don’t know a lot about how that process works in the estuary to get from inorganic mercury to the methylmercury stage,” said Bruce Monson of the Minnesota Pollution Control Agency. Joel Hoffman, research biologist with the Environmental Protection Agency in Duluth, looked at fish that spend their entire year in the estuary and compared them to fish that migrate into Lake Superior for part of the year, including walleyes and pike.


“The research is showing that the fish, as they leave the (estuary) and enter the lake, there’s less mercury in them. There's less mercury in the food web of the lake than the river,” Hoffman said.

Hoffman is studying the problem as part of the ongoing effort to fix historic pollution and habitat destruction problems in the Twin Ports so it can eventually be removed from the list of more than 40 contaminated “Areas of Concern” across the Great Lakes. He wants to know if cleaning up contaminated sediment in the estuary will help reduce the mercury problem.

The unusually high buildup of toxic mercury in estuary fish may be due to the chemical nature of the water, or how much water is rushed down the river system that then slows down when it hits the wider estuary. Or maybe it’s how much organic matter there is in the estuary. Or maybe it’s Lake Superior’s seiche effect, when lake water pushes back upriver into the estuary, that is part of the problem. Or all of the above.

“It’s very possible that this is part of a natural process and we don't have any way to control it,” Monson added.

Most of the mercury in the river got there from human causes, either being dumped in as effluent or falling in rain from air emissions from industrial sources — both far away and close to home.

But the process of methylating mercury is natural, Hoffman said. And it’s been known for years that the ecosystems that best methylate mercury are the ones with ample wetlands, bogs and carbon.

“There’s a lot of dissolved, organic carbon in the river. That's why it’s so brown. That’s a very good delivery system for mercury,” Hoffman said. The more tea-colored or brown the water is naturally, “generally, the more mercury is in the fish.

”But Hoffman said scientists want to find out if there’s more going on in the estuary. “Can we find out if there’s a human role to be played in the mercury cycle of this river?” he asked. “That’s really our goal."


Critics of Northeastern Minnesota taconite iron ore mining operations, which release sulfate in their effluent, suggest that high sulfate levels are spurring the methylation of mercury that accumulates in St. Louis River fish.

Bacteria in the water that methylate mercury thrive when sulfate levels are high. Water in the upper river may move too quickly for the bacteria to take hold, one theory holds. But mining critics say that, when the river slows in the estuary, that’s when the bacteria take off. And that's where mercury levels are highest in fish. Even if the sulfate levels are lower in the estuary, they say, the higher upstream sulfate levels are enough to kick-start the bacteria methylating mercury.

Scientists agree that sulfate is a factor in the process of methylating mercury into its toxic form. But researchers say the rivers in Northeastern Minnesota with the highest sulfate levels had lower levels of toxic mercury than those with lower sulfate levels.

“In general, they did not find a relationship between the high sulfate concentrations in the upper St. Louis River with mercury methylation or methylmercury concentrations,” Monson said. “But the question remains — is the sulfate from the upper St. louis River contributing to mercury methylation in the estuary? We still haven’t come up with a way to answer that question.

”UMD’s Johnson conducted laboratory experiments adding sulfate to estuary water; he said “the results were inconclusive. The sulfate additions (and subtractions) didn't obviously increase methylmercury when sulfate was added.

”But lab conditions are different from the estuary ecosystem, Johnson notes, and he says more work needs to be done in testing the relationship between sulfate and increased mercury methylation.

UMD research assistants Dan Fraser and Amber White gather a sample of water from the bottom of the St. Louis River estuary to be sampled for mercury. Scientists are studying the estuary's water chemistry and biology to find out why so much mercury in the system becomes toxic. John Myers /


Disturbing data, no answer soon

Walleyes all along the St. Louis River, indeed across most of northern Minnesota, have mercury levels above what’s considered safe for everyone to eat as often as they want. But estuary walleyes blow way past the standard.

In the most recent sample results available, estuary walleye held, on average, 0.624 parts per million mercury — 70 percent more than St. Louis River walleye above Cloquet. Over the past decade the mercury levels in estuary walleye have moved as high as 1.61 parts per million while upper river walleye remained below 0.47.Mercury levels in fish can vary widely over just a few miles, even without any local source of mercury pollution. Fish in the Island and Whiteface reservoirs north of Duluth have higher mercury levels than even the estuary. Yet walleyes in Fish Lake and Wild Rice Lake just a few miles away have far lower mercury levels.

Each water body appears to have a different capacity both to store mercury and then to make it available for fish at the top of the food chain and, indirectly, to people who eat fish, Monson said.

Minnesota is in the midst of a 20-year plan to cut mercury released from Minnesota smokestacks by 76 percent from 2005 levels, and already is near the goal. That should help fish in most of the state's lakes and rivers drop below the level of mercury whereby everyone can safely eat them at least once each week. Mercury from coal-fired power plants has gone from 1,800 pounds per year to below 500 pounds.

As Minnesota has pushed to curb mercury pollution, mercury levels in fish have been declining in many waterways for the past decade or so, according to analysis of pike and walleye across the state.

But in about 10 percent of the state's waterways, including the St. Louis River estuary, mercury levels in fish are so high that statewide efforts won’t be enough.

Mercury levels “have not dropped in the estuary, which is peculiar,” Monson said. “We’ve had some years recently where it’s been quite a bit higher.

”In 2013 several environmental groups and the Fond du Lac Band of Lake Superior Chippewa pushed for the PCA to join with the EPA to develop a Total Maximum Daily Load process for the St. Louis River, a move under the Clean Water Act that would strike at sources of pollutants that might be contributing to mercury in the fish. It was widely believed at the time that a TMDL plan would require huge investments by mining operations and, potentially, sewage treatment plants, to cut sulfate levels.


The PCA balked, saying the expensive measures might do little or nothing to make the fish safe to eat while potentially crippling the iron ore industry. Since then the PCA has moved ahead with research projects with no deadline yet to take any regulatory action toward solving the problem.

“The Legislature has given us the funding to go out and find some answers about the St. Louis River,” said Paul Hoff, supervisor of the water assessments section of the PCA. “At this point, we still don’t have the answers.”

“At some point, policymakers are going to have to make some tough decisions. No one questions that reducing mercury in these fish is a major priority,” Hoff said. “But, until then, it’s our job to provide the science that the policymakers can use to make those decisions. We’re still doing that.”

About toxic mercury

Mercury is a potent neurotoxin that can harm human health even in small amounts.

It occurs naturally in the earth's crust. It is released into the environment from volcanic activity, weathering of rocks and as a result of human activity — the largest source — such as burning coal in power plants, mining gold, incinerating garbage and processing taconite iron ore.

Most mercury found in Northland waters comes from the sky, falling in rain and snow, but other sources could be wastewater treatment plants or industrial effluent. The airborne mercury can come from industry close to home and from power plants as far away as China.

Once in the environment, especially in water, mercury can be transformed by bacteria into toxic methylmercury, an organic compound that bioaccumulates, meaning organisms — especially fish — contain higher levels of mercury than their surroundings. Methylmercury also biomagnifies, meaning levels get higher as it moves up the food chain. Large predator fish have much higher levels than minnows, which have higher levels than mayfly larvae, which have higher levels than zooplankton, and so on. It doesn’t break down and keeps building up.

Most people's exposure to toxic mercury comes from eating fish and shellfish, and both Minnesota and Wisconsin have advisories for people to limit how much fish they eat from many lakes and rivers.

Exposure to mercury, even small amounts, may cause serious health problems, especially for children and developing fetuses. Mercury may have toxic effects on the nervous, digestive and immune systems, and on lungs, kidneys, skin and eyes.

Even at low levels, mercury can harm the developing nervous system of a fetus and may harm adults' cardiovascular and immune systems, health experts warn. At high levels, mercury can trigger memory loss, slurred speech, hearing loss, lack of coordination, loss of sensation in fingers and toes, reproductive problems, coma and possibly death. In general, smaller fish have less mercury. Mercury is in all parts of the fish tissue; you can’t see it and you can’t trim it off.

Sources: World Health Organization, Environmental Protection Agency, Minnesota Department of Health.

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at
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