The mercury cycle
Once released into the environment, mercury can be converted to various forms and move easily between air, water, sediment, soil, plants, and animals for extended periods of time. Elemental mercury stays in the air for a long time and can be transported on wind currents, either as vapour or bound to particles. These emissions can circulate in the atmosphere for up to a year, travelling long distances to locations far from their original source before being deposited. Mercury emitted into the atmosphere can be deposited on land and surface waters, and can also enter water bodies as runoff from soil or through groundwater. Mercury can be re-emitted from land, fresh water and the oceans back into the atmosphere. These natural transformations and environmental pathways of mercury are very complex and are greatly affected by local conditions. Understanding the relationships between local conditions and mercury levels in the environment is key to predicting changes in the concentration of mercury and its ability to be absorbed by living things.
Methylmercury, bioaccumulation and biomagnification
In aquatic and other oxygen-poor environments, mercury can turn into a highly toxic compound called methylmercury. Over time, methylmercury can build up in the tissues of a living organism, a process known as bioaccumulation. Methylmercury also increases in concentration (biomagnifies) through the food chain as predators eat other organisms and absorb the contaminants that their food sources contained. As a result, top predators acquire greater body burdens of mercury than the fish and other animals they consume.
Mercury travels through the environment in complex ways and moves easily between the atmosphere, water, soil, and sediment in a process known as the mercury cycle. Mercury emissions result from natural sources such as forest fires and volcanoes or from human sources such as a factory. These emissions can travel long distances in the atmosphere, and be deposited far away from the initial source. Mercury is converted to various forms through complex atmospheric processes and is deposited to land or water. Mercury can be absorbed by vegetation. Mercury can also be deposited to water through runoff from the land and can be re-emitted from land and water back to the atmosphere. Mercury can also increase in the food chain through biomagnification, meaning that mercury concentrations increase in animals higher up the food chain as they consume mercury-contaminated prey. This process is illustrated by plankton being eaten by a fish, which is in turn eaten by a seal. Methylation and demethylation are also a part of the mercury cycle. Methylation is the process of forming methylmercury from inorganic mercury. Demethylation is the reverse process where methylmercury is transformed into an inorganic form of mercury. These reactions primarily occur in water and sediment and under certain environmental conditions.
For more scientific information on mercury, its effects and how it travels through the environment, see the Canadian Mercury Science Assessment.
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