C. D. Snyder1, R. Webb2, J. Atkinson3 , S. Spitzer3
BACKGROUND AND JUSTIFICATION
Mercury (Hg) is a toxic element that naturally occurs in aquatic systems in very low concentrations. Past human use of the metal for industrial and agricultural purposes has resulted in serious contamination of many surface waters. Even in remote, relatively pristine areas where direct anthropogenic inputs are lacking, long-range atmospheric transport of Hg from fossil fuel combustion and other sources has led to increased concentrations in freshwater systems and biota (Downs et al. 1998, Fitzgerald et al. 1998). Concentrations of Hg sufficient to prompt fish consumption advisories (i.e., > 0.5 ug/g) have been reported for predatory fish from relatively remote areas with no on-site anthropogenic or geologic sources of Hg (e.g., Abernathy and Cumbie 1977, Bodaly et al. 1984).
The chemistry of Hg is complex and consequently its behavior is difficult to predict in nature. Total mercury concentrations in the environment have not been found to be effective predictors of bioaccumulation in fish. Depending on physical, chemical, and biological conditions at a site, Hg can remain largely tied up in sediments, released from sediments to the water column, be lost to the atmosphere, be transported with sediment particulate matter to other locations, or be taken up by aquatic biota where it may concentrate and become a threat to humans and other fish-eating animals (reviewed in Ullrich et al. 2001). Although the precise factors controlling the accumulation of Hg in aquatic biota are not fully understood, it is clear that fish and other aquatic species are much more efficient in accumulating methylmercury (MMHg) than the inorganic forms that predominate in the abiotic component of the environment (Mason et al. 1995). Thus, factors that influence the rate in which in inorganic Hg is transformed to MMHg also influence bioaccumulation as well.
Although the process of Hg methylation is complex, the results of numerous studies on contaminated lakes indicate that enhanced Hg methylation rates and bioaccumulation have been consistently linked to low pH, low salinity, and the presence of organic matter in low oxygen environments (reviewed in Ullrich et al. 2001). The relationship between water chemistry and Hg methylation has not been fully investigated in stream ecosystems.
Streams in SNP
vary considerably in terms of pH and other important water chemistry
parameters due to variation in dominant bedrock class underlying individual
catchments. All the streams in SNP are characterized by low salinity
or ionic strength. Streams underlain by siliciclastic bedrock have low
acid neutralizing capacities (ANC) and consequently very low pH. As
a result of their low pH, these streams may be the most vulnerable to
mercury contamination. Streams underlain by basaltic bedrock have higher
ANC and pH. Streams underlain by granitic bedrock have intermediate
ANC and pH. We thus expect that bedrock distribution in SNP may reflect
a gradient in watershed response to atmospheric deposition of mercury.
to variation in water chemistry, there is variation in landscape setting
that may be important. In particular, elevation has been shown to affect
Hg deposition in some areas though results have been contradictory.
For example, total Hg deposition has been shown to be greater in watersheds
at higher elevations due to cloud deposition (i.e., wet Hg deposition
associated with mist and fog) (Shanley et al. 2005). However, methylmercury
in sediments and biota have been shown to be higher in lower elevation
sites, presumably because the greater watershed areas support a larger
number of wetlands and other sites where Hg methylation occurs (Kamman
et al. 2005).
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