Mercury still a danger in the Great Lakes

Nations work to regulate a chemical that knows no borders.

Mercury is responsible for fish contaminant advisories in all five of the Great Lakes. Michigan Department of Health and Human Services has issued fish consumption guidelines for all inland lakes and rivers. Brandon Schroeder | Michigan Sea Grant

Mercury is responsible for fish contaminant advisories in all five of the Great Lakes. Michigan Department of Health and Human Services has issued fish consumption guidelines for all inland lakes and rivers. Brandon Schroeder | Michigan Sea Grant

Mercury is one of the most persistent chemicals found in the Great Lakes. It gradually works its way up the food chain in a process called bioaccumulation. This means that mercury accumulates in greater quantities in predators at the top of the food chain such as large fish and humans. Imagine a lake trout that contains mercury built up from all of the smaller prey fish it has consumed over time or the fisherman that eats several of those lake trout every month. Exposure to sufficient levels of mercury over time can lead to significant health consequences including damage to the central nervous system, brain, heart, kidneys and immune system. Mercury is especially damaging to the still-developing brains of fetuses and children.

How does this harmful chemical end up in Great Lakes waters? There are some naturally occurring sources of mercury but most of the mercury in the Great Lakes comes from atmospheric deposition. Coal-fired power plants, waste incineration, metal smelting and mining release mercury into the air in its inorganic form. That mercury eventually settles back down onto the surface, some of it falling onto Great Lakes waters. This inorganic mercury is then converted through a series complex chemical processes into organic methylmercury. It is in this organic form that mercury causes health problems in humans, fish and wildlife. Bacteria living deep in lake sediments play a key role in this conversion process.

In 1997 the United States and Canadian governments came together to sign the Great Lakes Binational Toxics Strategy which called for the virtual elimination of Level 1 substances from the Great Lakes. A host of harmful chemicals fall under the category of Level 1 including PCBs, dioxins, DDT and mercury.  In 2010 the Great Lakes Regional Commission, a consortium of regional leaders and policy makers, authored the Great Lakes Mercury Emission Reduction Strategy with detailed regulatory and policy recommendations for state and federal action to reduce mercury emissions.

Mercury concentrations and trends vary across the Great Lakes. A 2011 study found mercury levels in sediment core samples taken from 91 inland lakes across the region were down 20 percent from 1980s levels but mercury concentrations remain three to four times higher than pre-industrial levels. All five of the Great Lakes still have fish consumption advisories in effect for mercury contamination and since the 1990s mercury levels in fish and wildlife in some areas of the Great Lakes have begun rising again. While atmospheric mercury emissions in the U.S. have declined in recent decades they’ve increased in other parts of the world and policy makers are increasingly concerned that these sources may be counteracting local regulatory efforts.  

A newly released report by the International Joint Commission (IJC) argues that comprehensive regional monitoring of mercury levels is critical to evaluate the impact of regulatory measures and inform future efforts to design international mercury reductions strategies. There is no comprehensive sustainably funded network for monitoring mercury deposition in the Great Lakes. The IJC recommends that the Canadian and U.S. governments make funding available to establish such a network and to investigate the sources of ongoing deposition. The IJC also recommends that the two countries advocate for international agreements to regulate global sources of bioaccumulative chemicals like mercury.

Michigan Sea Grant helps to foster economic growth and protect Michigan’s coastal, Great Lakes resources through education, research and outreach. A collaborative effort of the University of Michigan and Michigan State University, Michigan Sea Grant is part of the NOAA-National Sea Grant network of 33 university-based programs.

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