What we hope to answer with the USGS National Mercury Bioaccumulation Study?

1. Determine the effects of mercury loading from sources (such as coal-fired energy plants), mercury methylation efficiency (how efficiently certain bacteria in some ecosystems change other forms of mercury into methyl mercury), and food web interactions (algae, aquatic insects, and fish as food sources) on the bioaccumulation of mercury in predator fish.
2. Quantify the source and seasonality of mercury and methyl mercury as well as biogeochemical species and transformation rates related to the aquatic mercury cycle.

Background

The USGS National Water Quality Assessment program and Toxic Substances Hydrology program are conducting a national study of mercury in different rivers of the US. Mercury is a toxic chemical that can cause neurological damage in people. The most common way that people get mercury in their bodies is by eating fish that have been contaminated with the chemical. The source of mercury for most waters is airborne from coal-fired energy plants and this mercury falls on lakes, wetlands, and streams sometimes far from the source. Under just the right conditions, certain kinds of bacteria change this mercury into methyl mercury, the form most easily taken up by other organisms. Methyl mercury is the primary form in fish. Methyl mercury can increase in concentration from algae, stream invertebrates such as insects, to small fish, game fish, birds, and people. Many states have advisories about eating fish from their waters because of mercury concerns.

Mercury Issues in Wisconsin

• State agencies, tribes, environmental advocacy groups, and the public in Wisconsin have a relatively high level of concern regarding mercury.
• The primary source for mercury contamination in Wisconsin waters is airborne from coal combustion, and the Wisconsin Department of Natural Resources has estimated that 50% of the mercury deposited in the state comes from within Wisconsin.
• Nationwide, Wisconsin ranks second in the number of lakes and rivers with mercury advisories for fish consumption.
• The Western Lake Michigan drainage is 15% wetland overall and 40-60% wetland in the northern part of the study unit. In addition to a high percentage of wetland, many of these streams are characterized by additional factors considered to be important in creating methyl mercury.

Study Design

• In 2002, fish, water, and sediment were sampled in 14 NAWQA study units nationwide
• In 2003-4, more detailed sampling was done in three of these study units, including the Western Lake Michigan Drainages, Willamette River Basin in Oregon , and Georgia-Florida Coastal Plain. In each study unit, this sampling was done at 2-3 sites representing one urban and one to two rural/non-cultivated watersheds. Additional study units began similar mercury sampling in 2005.
• In 2004-5, limited sampling for long-term trends in mercury concentrations began in the Western Lake Michigan Drainages, Willamette River Basin in Oregon , and Georgia-Florida Coastal Plain study units.

Approach

• Stream sampling sites were selected based on the availability of target predator fish and mercury-source landscapes including urban and reference or non-cultivated.
• Total mercury (all forms) and methyl mercury in water were sampled monthly plus during selected storm events in 2003-04.
• Potential net methylation rates in sediment and mercury in pore water and sediment were measured seasonally.
• Total mercury and stable carbon and nitrogen isotopes were determined in predator fish to examine the effect of food sources; total and methyl mercury and stable isotopes also were determined in food chain organisms (forage fish, invertebrates, and algae).
• Data from the Mercury Deposition Network will be used to determine loading rates so that the amount of mercury entering the stream ecosystems can be assessed in relation to the amount of mercury accumulating in the water, sediment, and fish.

Table 1. Sites in Wisconsin sampled for the Mercury Topical, Mercury Synoptic and Mercury Trends Studies.

Study Sites in the Western Lake Michigan Drainages – 3 Streams

• Urban
• Oak Creek in South Milwaukee, WI: 42% urban land and 8% wetland
• Reference/Non-cultivated
• Evergreen River near Langlade, WI: 77% forested land and 9% wetland
• Pike River near Amberg , WI: 72% forested land and 18% wetland

Summary

The USGS mercury studies will provide a nationwide data set of total and methyl mercury in river ecosystems in a variety of geographic settings. This information will add considerably to basic data needs and an improved understanding of mercury chemistry in diverse landscapes. The detailed investigation will contribute toward a greater understanding of the many factors that control how methyl mercury is formed and its resulting bioaccumulation in aquatic organisms in streams and rivers.

For more information on national mercury studies, see Fact Sheet 016-03 - Mercury in Stream Ecosystems: New Studies Initiated by the U.S. Geological Survey.

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