Mercury Contamination in Lakes

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By Guest Contributor Audrey Arnold, Environmental Scientist


Among the many technologies which bring humans convenience and comfort, many have the ability to inflict harm upon the environment through the production of wastes. One of the most prominent toxic by-products of human activity is mercury, a heavy metal known to have negative neurological and reproductive effects on fish, mammals, and birds.

What Is Mercury?

Mercury, Hg on the periodic table, is one of only two elements which are liquid at room temperature. It is used in thermometers, barometers, compact fluorescent lamps, and batteries, and is a known constituent in coal and other rocks. Production and improper disposal of mercury-containing products can lead to mercury pollution within the environment. However, the largest source of mercury pollution is dry or wet deposition from the atmosphere in the form of particulates or precipitation.

Where Does Atmospheric Mercury Come From?

While mercury occurs naturally in deposits and can be released into the atmosphere through volcanic eruptions, human activities have doubled or tripled the amount of mercury circulating through the atmosphere, with an increase of about 1.5% per year.

Coal power plants are the largest source of atmospheric mercury, producing about 33 tons annually. Other sources include industrial boilers, cement and metals processing plants, steel production, and waste incineration.

Atmospheric mercury also contaminates hydroelectric reservoirs indirectly: mercury accumulated in plant tissue is released into the water once new dams flood large swaths of forest.

Mercury’s Complex Pathway in Aquatic Ecosystems

Fish in crystal-clear, isolated lakes within the northern United States, Canada, and Scandinavia were first reported to have elevated concentrations of mercury in the 1980s.

More recently, contamination has been shown to have spread to streams, reservoirs, and wetlands within a larger geographic range, with a total of 33 states enacting fish-consumption advisories aimed towards residents and tourists. How does that happen?

Counterintuitively, while mercury concentrations in fish have increased the amount of mercury within the physical environment has remained relatively low – this is a direct result of the biochemistry of mercury and processes which allow it to take on its most toxic form, methylmercury. Consisting of one carbon atom, three hydrogens, and a mercury atom, methylmercury is the form of mercury present within living organisms. Its other major form, vaporous mercury, which occurs in the atmosphere and consists of a solitary mercury atom, is much less toxic as organisms are much more able to eliminate it from their bodies.

Mercury is believed to enter the food web after it undergoes a series of interactions with bacteria which transform it into methylmercury. Once in the food web, methylmercury bioaccumulates within an organism – meaning that the organism acquires mercury faster than it eliminates it. In addition, methylmercury biomagnifies; the amount of methylmercury increases with each level up the food chain.

This relationship means that top-level carnivores have higher concentrations of mercury within their tissues than do lower-level consumers.

A Source of Health Concern

In humans, serious health effects can come from even low mercury exposure, including neurological disorders. Mercury is a major health concern because we tend to eat fish which are mid- and top-level predators, such as tuna, swordfish, mackerel, and mahi mahi. Therefore, the most widely consumed seafood often contains higher amounts of mercury than lower-level consumers, such as herrings, anchovies, and sardines. And, although tuna contains less mercury than shark, marlin, mackerel, and other large fish, it is the most commonly eaten fish; this makes tuna the top contributor of mercury to the American diet. Moreover, since mercury accumulates within fish meat rather than skin and fat, it is impossible to eliminate through processing and cleaning.

The Natural Resource Defense Council has provided general guidelines for fish consumption here.

Over several decades, politicians have taken the health hazards of mercury into account when enacting environmental laws pertaining to industry. The years 2010-2013 marked a phase-in of mercury regulations for cement plants, power plants, mining operations, and industrial boilers under stipulations of the Clean Air Act. In addition, pollution control devices on smokestacks in industrial plants and incinerators can remove much of the mercury from emissions. A combination of these measures, along with general awareness of the health and environmental dangers of mercury, has led to a 65% decrease in the amount of mercury pollution over the past two decades. However, given the toxic effects of mercury, it is still important to remain vigilant about exposure to and release of this heavy metal.


Natural Resources Defense Council, Mercury Contamination in Fish.

United States Geological Survey, Mercury Contamination of Aquatic Ecosystems.