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FIGURE 2-9 Increasing concentrations of mean methylmercury in U.S. streams with increased wetland density and mean dissolved organic carbon. Once deposited in wetlands, mercury is converted to methylmercury. Dissolved organic carbon binds strongly to mercury keeping mercury in the aquatic zone and available for uptake by organisms. SOURCE: USGS, 2009a.

lack of denitification (Nolan and Hitt, 2006). EPA uses this information to help prioritize monitoring and better assess its regulatory efforts.

During Cycle 2 NAWQA developed empirical models to probe hydrologic alteration nationwide as well as the connection between hydrologic alteration and the structure of macroinvertebrates and fish assemblages. NAWQA successfully modeled ecologically important flow metrics under a “natural” or “minimally disturbed” flow regime using geospatial data and a reference condition approach. This opened the possibility of quantification of hydrologic alteration across the United States (Figure 2-10). Using geospatial models and NAWQA data, Carlisle et al. (2011) demonstrated that diminished magnitude of flows was the best predictor of impairment of macroinvertebrate and fish assemblages nationally. NAWQA integrated macroinvertebrate data (collected by NAWQA and the EPA Wadeable Stream Assessment15) to expand the scope of a model assessment of biological condition in streams in the western United States (Carlisle and Hawkins, 2008). These studies are the foundational material for a USGS Circular summarizing findings on aquatic communities across the United States prepared by the Ecological National Synthesis Project, planned for 2012.

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15 See http://water.epa.gov/type/rsl/monitoring/streamsurvey/index.cfm.



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