Summary: Toxic methylmercury becomes a concern for human health as it bioaccumulates in fish. In fact, methylmercury is sometimes present in predatory fish at the top of the food chain at concentrations 1,000,000 times that of the background level. When these fish travel long distances, bioaccumulation then contributes to the global transport of mercury. As human populations consume fish, they become exposed to methylmercury, with the highest exposure correlating to the consumption of fish at the highest trophic levels.
The figure above depicts mercury cycling in a lake and its watershed. With regard to anthropogenic sources, this starts as emission and transport and ends with the bioaccumulation of methylmercury (here MeHg). These mercury emissions are transported across large distances in the form of elemental mercury [Hg (0)]. Elemental mercury is oxidized in the atmosphere to become a highly reactive gaseous form of mercury [Hg (II)]. Hg (II) is then deposited through precipitation (wet deposition) and by surface contact (dry deposition) into bodies of water, such as this lake. Anaerobic bacteria then convert some of this deposited Hg (II) to methylmercury which is bioaccumulated and then bioconcentrated. How much Hg (II) is converted to methylmercury depends on the convergence of various biotic and abiotic reactions such as photodegradation and redox (oxidation-reduction) reactions that result in some of the mercury being buried, some of it being released back to the atmosphere, and some of it being bioaccumulated.
A recent study by Harris et al. studied the response of lakes to changing inputs of atmospheric mercury. It found that mercury levels in fish respond directly and quickly to changing rates of mercurial atmospheric deposition. Furthermore, other studies have shown observed decreases in the amount of mercury in fish in areas where mercury deposition has recently declined. Mercury as a pollutant is widespread, and at on oceanic level, it will take much longer for inventories to respond to changes in atmospheric levels. However, evidence that lakes will respond to these changes support the idea that emissions reductions will be effective.
Note: This is particularly good evidence to support the idea that a global effort to reduce mercury emissions, through the Minamata Convention, will have some direct and quickly observable results.
Engstrom, Daniel R. 2007. “Fish respond when the mercury rises.” Proceedings of the National Academy of Sciences 104, no. 42: 16394-16395.