1. U.S. Geological Survey, Water Resources Division/MS 480, 345 Middlefield Road, Menlo Park, California 94025, Department of Environmental Toxicology, University of California, Santa Cruz, California 95064, U.S. Geological Survey, Water Resources Division, 8505 Research Way, Middleton, Wisconsin 53562, and The Academy of Natural Sciences, Estuarine Research Center, 10545 Mackall Road, St. Leonard, Maryland 20685