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Readers Want to Know…

Why is mercury more of a problem in the gamefish of Minnesota’s northeastern lakes than in other areas?

Map of Minnesota divided into various mercury regions.

The Minnesota Pollution Control Agency (MPCA) has divided Minnesota into two mercury regions— northeast and southwest— based on the way mercury moves from the landscape into a waterbody and the methylmercury content of fish. This division closely follows wetland distribution. (Source: MPCA)

Mercury is a useful liquid in your thermostat but it can become fiendishly toxic due to its uncanny ability to change forms, float around the globe, and accumulate to health-hazardous-levels of methylmercury in the higher reaches of aquatic food webs.

It seems to defy logic that walleye from the clear wilderness lakes of northeastern Minnesota have higher concentrations of methylmercury, a chemical that can damages nerves, than fish in other parts of the state. However, according to state and federal agency experts and researchers, that’s the case. Arrowhead Region walleye contain 0.6 parts per million (ppm) of methylmercury versus 0.4 ppm in a 16-inch walleye from warmer, murkier lakes elsewhere in the state.

When pondering possible reasons, it’s tempting to leap to the simple— but inaccurate— conclusion that more mercury descends on the northwoods. However, scientists and public health officials report that when it comes to methylmercury, the amount in a walleye fillet has more to do with complex relationships between chemical, physical, and biological factors than it does with the total amount of mercury loaded into a watershed.

“The lakes in northeastern Minnesota are more susceptible systems and vulnerable to pollutant contamination,” said Gary Glass, a senior research chemist retired from the Environmental Protection Agency’s Mid-Continent Ecology Division in Duluth. “The lakes in the northeast are naturally less productive and are more acidic due to extensive wetlands in the watersheds. These characteristics, along with the amount of exposed bedrock and other contributing factors, allow mercury to enter the food web at a faster pace.”

Mercury (or Hg in chemical terms, which is short for “hydrargyrum” meaning “water silver”) gains access to the food web when certain bacteria (called sulfate reducers) that usually live in the muddy sediments of rivers and lakes, change mercury from a relatively harmless form to the more harmful methylmercury (MeHg) during the normal course of their lives and metabolism. This process is called methylation and causes methylmercury to easily accumulate in the food web.

The production and abundance of MeHg in a lake reflects a variety of interrelated factors including:

  • Wetland abundance
  • Lake chemistry (temperature, dissolved organic matter, pH)
  • Watershed qualities (wetland abundance, soil type, ground cover)
  • Microbe activity
  • Mercury abundance and form (it exists in three oxidation states that respond differently to the environment)

Recently, in the journal Environmental Science and Technology, Minnesota researchers confirmed the significant influence that additional sulfate can have on the rate at which bacteria produce methylmercury in northern lakes. Like mercury, much of the sulfate reaching lakes is a byproduct of the way coal is burned and converted to power. Sulfates form from sulfur oxides that are washed from the sky as acid rain. They can also creep into the environment from sulfur-tainted municipal and industrial processes and wastes.

Methylmercury sticks to algae, which absorb the molecules. The algae are eaten by tiny aquatic grazers, such as the zooplankton Daphnia. The grazers are eaten by small fish, which in turn are eaten by larger fish. Methylmerecury binds with tissues and accumulates in animals as it continues up the foodchain.

The concentration of mercury in a fish depends on the species, age, and size of the fish, as well as the water in which it lives. The relatively high MeHg concentrations found in predator fish from colder northern lakes typifies the way heavy metals accumulate in older animals at the peak of the food chain. Fish grow more slowly in colder water and a slower-growing fish has more time to absorb heavy metals before being caught and eaten.

What You Can Do

The best way to enjoy fish without worrying about your or your family’s annual dose of mercury is to follow the Minnesota Department of Health annual fish consumption advisories. Eating smaller fish and species lower on the food web, like sunfish, crappies, or yellow perch, can also reduce exposure.

Current consumption advisories for Lake Superior’s bigger sports fish are based on both mercury and PCB (polychlorinated biphenyl) levels, said Pat McCann of the Minnesota Department of Health.

PCB exposure can be reduced by removing fatty tissue and skin from fillets. Sea Grant offers a handy free brochure called Cooking Your Catch, that illustrates where the fat is found and how to prepare fish.

Unlike PCBs, methylmercury binds tightly to proteins in fish tissue, including muscle. No method of cooking or cleaning can reduce the amount in a meal of fish. Direct human and wildlife exposure to methylmercury comes mostly through the consumption of fish.

Bar graph comparing mercury emissions in pounds per year.

A comparison of what processes emit mercury in Minnesota for 1990 and 2005. (Source: MPCA)

Mercury is the major pollutant in about two-thirds of Minnesota’s impaired waters. The State of Minnesota, local power companies, and sanitary districts have taken significant steps to reduce the mercury entering the environment. You can help too.

“We need to focus on reducing our individual energy consumption,” said Trent Wickman, air resource specialist with the Superior National Forest. “Using fluorescent light bulbs, turning down the thermostat, and unplugging appliances will minimize your environmental impact and your energy bill.”

By Sea Grant Staff
June 2006

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