Releases of mercury to the environment are usually in the form of elemental or inorganic forms. Methylmercury is an organic form of mercury. Biological processes change the chemical form to methylmercury, which is the more toxic form found in fish. Methylmercury bioaccumulates through the food chain and, once in the body, can affect the fetal and adult nervous systems. In the atmosphere, mercury is transported by wind either as a vapor or as particles.
Mercury reaches waters either through direct deposition or as run-off from soil after rain.
In the water, biological processes can transform mercury into methylmercury a highly toxic form, which can accumulate in fish. When mercury is deposited into the water, microorganisms help convert it to methylmercury, a highly toxic form of mercury. Small organisms and plants take up the mercury as they feed. As animals higher up the food chain eat those plants and organisms, they, too, take in methylmercury. The process continues, with levels of mercury increasing, up the food chain.