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Fish in Some Streams Accumulate Mercury

Looking downstream at the sampling site for investigating daily variations in mercury concentrations in the Madison River, Yellowstone National Park, Montana. A mobile water-quality laboratory and the gage house for a USGS streamflow station (Site ID: 06037500) are on the far bank. Water samples from the river were pumped through tubing into the mobile laboratory.
USGS scientist collecting samples of aquatic species from the Pike River, Wis., for later analysis of mercury contamination.
(Larger version)

Some stream ecosystems are much more sensitive to the trace amounts of mercury deposited in them from the atmosphere than others. This variation in the sensitivity of streams was discovered by a team of U.S. Geological Survey (USGS) scientists while they were studying the fate of atmospherically deposited mercury in eight watersheds. They reported their discovery in a recent series of articles published in Environmental Science and Technology. The team found that, for a given amount of atmospherically deposited mercury, fish in some watersheds will have high levels of mercury contamination while fish in other watersheds will not. The study's results show that total mercury and methylmercury concentrations in the streams were much more variable than the observed variation in the atmospheric deposition of mercury (nine orders of magnitude variation in concentration for streams and four for atmospheric deposition). The articles document that the characteristics of the watershed, particularly the abundance of wetlands and the amount of dissolved organic carbon in stream water, exert a strong influence on how atmospherically deposited mercury is transformed to methylmercury and subsequently bioaccumulated in stream food webs.

  • The eight streams in the study were located in Florida, Oregon, and Wisconsin. Streams in urban areas (Orlando, Florida; Portland, Oregon; and Milwaukee, Wisconsin), and streams in relatively undeveloped areas in these States were included in the study. The streams spanned a wide range of watershed characteristics that can impact the fate of mercury in streams. The major findings from the study include:
  • Watershed characteristics are a bigger factor in determining the transport, transformation, and bioaccumulation of methylmercury (the most toxic form of mercury) than the amount of mercury deposited in the watershed from the atmosphere. In addition to the abundance of wetlands noted above, additional key factors are how much dissolved organic carbon and suspended sediment are delivered to streams during rain events. Dissolved organic carbon and suspended sediment control how much mercury and methylmercury are delivered to streams. The scientists' data showed strong positive correlations between increasing dissolved organic carbon content of stream water and increasing methylmercury concentrations in fish.
  • Based on studies in lakes, scientists have known that bacteria in lake bottom sediments can produce methylmercury that ends up in lake water. A surprising result from this study is that the methylmercury generated in stream bottom sediments does not contribute significantly to observed methylmercury concentrations in stream water. Instead, methylmercury in stream water appears to be derived from production in the watershed.
  • Once methylmercury enters streams, it is available for uptake at the base of the food web. Concentrations of methylmercury in fish were strongly and positively correlated with concentrations of methylmercury in stream water. This correlation and other results indicated that methylmercury contamination in fish is most likely dominated by the amount of methylmercury available for uptake at the base of the food web, rather than by differences in where fish are located in food chains.

Over two decades of research on mercury contamination in the environment has greatly improved scientists' understanding of the fate of mercury in aquatic ecosystems. However, the vast majority of this research has been conducted on lakes, wetlands, and reservoirs, leaving streams relatively less well understood. These findings can help decision makers better anticipate concentrations of mercury and methylmercury in streams and understand how mercury makes its way into fish in streams in comparable environmental settings. The Toxic Substances Hydrology Program contributed to this large study conducted by the USGS National Water-Quality Assessment (NAWQA) Program.



Brigham, M.E., Wentz, D.A., Aiken, G.R., and Krabbenhoft, D.P., 2009, Mercury cycling in stream ecosystems--1. Water column chemistry and transport: Environmental Science and Technology, v. 43, no. 8, p. 2,720-2,725, doi:10.1021/es802694n. (Free download)

Chasar, L.C., Scudder, B.C., Stewart, A.R., Bell, A.H., and Aiken, G.R., 2009, Mercury cycling in stream ecosystems--3. Trophic dynamics and methylmercury bioaccumulation: Environmental Science and Technology, v. 43, no. 8, p. 2,733-2,739, doi:10.1021/es8027567. (Free download)

Marvin-DiPasquale, M., Lutz, M.A., Brigham, M.E., Krabbenhoft, D.P., Aiken, G.R., Orem, W.H., and Hall, B.D., 2009, Mercury cycling in stream ecosystems--2. Benthic methylmercury production and bed sediment-pore water partitioning: Environmental Science and Technology, v. 43, no. 8, p. 2,726-2,732, doi:10.1021/es802698v. (Free download)

Additional References on the Fate of Mercury in Watersheds

Gorski, P.R., Armstrong, D.E., Hurley, J.P., and Krabbenhoft, D.P., 2008, Influence of natural dissolved organic carbon on the bioavailability of mercury to a freshwater alga: Environmental Pollution, v. 154, no. 1, p. 116-123, doi:10.1016/j.envpol.2007.12.004.

Shanley, J.B., Mast, A.M., Campbell, D.H., Aiken, G.R., Krabbenhoft, D.P., Hunt, R.J., Walker, J.F., Schuster, P.F., Chalmers, A., Aulenbach, B.T., Peters, N.E., Marvin-DiPasquale, M., Clow, D.W., and Shafer, M.M., 2008, Comparison of total mercury and methylmercury cycling at five sites using the small watershed approach: Environmental Pollution, v. 154, no. 1, p. 143-154, doi:10.1016/j.envpol.2007.12.031.

Schuster, P., Shanley, J., Marvin-Dipasquale, M., Reddy, M., Aiken, G., Roth, D., Taylor, H., Krabbenhoft, D., and DeWild, J., 2008, Mercury and organic carbon dynamics during runoff episodes from a northeastern USA watershed: Water, Air, and Soil Pollution, v. 187, no. 1, p. 89-108, doi:10.1007/s11270-007-9500-3.

Nelson, S.J., Johnson, K.B., Weathers, K.C., Loftin, C.S., Fernandez, I.J., Kahl, J.S., and Krabbenhoft, D.P., 2008, A comparison of winter mercury accumulation at forested and no-canopy sites measured with different snow sampling techniques: Applied Geochemistry, v. 23, no. 3, p. 384-398, doi:10.1016/j.apgeochem.2007.12.009.

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Created on Wednesday, November 18, 2009