Exposure to lower doses of mercury can also cause brain damage, decrease reproduction rates, growth, neurological development, and the ability to learn (Environment Canada 2004). .
Mercury found in water bodies and wetlands usually is converted to an organic compound called methylmercury (Environment Canada 2002). It is the most lethal of all forms of mercury because it is easily transported across cell membranes that usually form barrier against toxic substances (Environment Canada 2004). Less than one per cent of methylmercury taken up by an organism is excreted (Mason 1992). It directly reacts with cells since it has the ability to cross blood-brain and placental membranes (Environment Canada 2004). This neurotoxin is not very soluble in lipids. Consequently it binds strongly with sulphydryl groups in proteins and excretion from the body is slow causing bioaccumulation in tissues (Environment Canada 2002).
Mercury Cycle.
Mercury is cycled and recycled through a complex biogeochemical cycle (Environment Canada 2004). Simplified, mercury enters the environment through rock and soil erosion, and emissions from volcanic and human activity. The gaseous mercury then moves through the atmosphere and eventually is deposited on land and surface water. Areas in the world that don't have a direct source of mercury pollution may accumulate it in many ways. Soil may draw moisture out of the atmosphere and with it absorbs mercury, but mostly it comes from precipitation (Environment Canada 2003). Mercury that is deposited into water is almost always converted into methylmercury which bioaccumulates in the food chain. .
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Bioaccumulation.
Bioaccumulation of mercury increases at each tropic level as indicated in Figure 1. It can enter the body through the uptake of water by absorption of dissolve mercury or through the consumption of food (Environment Canada 2002). Top predator species have the highest amount of mercury because the elimination process is considerably slower then the in take process (Environment Canada 2003).