Mercury is a concern because it is a potent developmental neurotoxin that accumulates in aquatic food webs (Mason et al., 1995). One particular form of mercury, known as methyl-mercury, is preferentially bioaccumulated.
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| Bioaccumulation refers to the process by which a concentration of a substance, such as mercury, increases as it moves up the food chain. When mercury enters the water, it is transformed into a more toxic substance called methylmercury. Methylmercury is more absorbed easily by bacteria and small plants. These bacteria and plants are eaten by small fish, which in turn are eaten by larger fish (Sigel and Sigel, 1997). |
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| Biomagnification is a natural process whereby contaminants like mercury are found at progressively higher concentrations as they pass from prey to predator up the food chain. The biomagnification of mercury can lead to concentrations in fish that exceed the safe levels recommended for human consumption (Cleckner et al., 1998). |
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The bioavailability of a mercury source is defined as the relative ease with which Hg from the source is converted to methyl-mercury by methylating bacteria.In other contexts, the term “bioavailability” can also describe the relative ease with which different types of mercury are acquired by aquatic organisms. For example, although inorganic mercury and methyl-mercury can be equally bioavailable to phytoplankton, methyl-mercury is much more bioavailable to zooplankton that grazes on the phytoplankton (Mason et al., 1996).
Inorganic mercury coming from mine runoff, geothermal sources, air deposition, and other sources is converted into methylmercury in stream sediment and solids by bacteria and absorbed by small aquatic organisms, which are then consumed by small fish. The small fish are eaten by larger fish, which are consumed by humans. Once ingested by humans, mercury affects the human nervous system and may disrupt normal brain development in children. Losses of sensation in the hands, as well as hearing, speech, and vision problems are examples of the potential effect of mercury on adults (
Sacramento Regional County Sanitation District Mercury Web site)
Numerous fish-consumption advisories exist for the Sacramento region, including new guidelines for sport-fish in selected water bodies in the northern Sierra Nevada foothills (Nevada, Placer, and Yuba Counties) issued by the
California Office of Environmental Health Hazard Assessment. As a result of its toxicity, numerous environmental studies of mercury contamination have been undertaken by various Federal and State agencies throughout California; many of these studies have focused on the Sacramento River watershed.
