1. National Water-Quality Assessment Program, US Geological Survey, 12201 Sunrise Valley Drive, MS 413, Reston, Virginia 20192 USA
2. Western Center for Monitoring and Assessment of Freshwater Ecosystems, Department of Aquatic, Watershed, and Earth Resources, and the Ecology Center, Utah State University, Logan, Utah 84322-5210 USA
3. Patrick Center for Environmental Research, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103 USA