Water level management and contaminant exposure to tree swallows nesting on the Upper Mississippi River † Thomas W. Custer & Paul M. Dummer & Christine M. Custer & Azusa U. Li & David Warburton & Mark J. Melancon & David J. Hoffman & Cole W. Matson & John W. Bickham Received: 28 June 2006 / Accepted: 30 November 2006 / Published online: 8 February 2007 # Springer Science + Business Media B.V. 2007 Abstract The U.S. Army Corps of Engineers con- ducted a water drawdown on Navigation Pool 8 of the Upper Mississippi River during the summers of 2001 and 2002 to increase aquatic vegetation production and thereby improve fish and wildlife habitat. Flood- ing of previously dried wetlands, however, may increase the rate of mercury methylation and make mercury more available to terrestrial vertebrates that feed in aquatic environments. Our objective was to determine if mercury, other elements, and organo- chlorine contaminants were more available to verte- brates following the 2001 drawdown. Tree swallow (Tachycineta bicolor) eggs and nestlings were col- lected at two sites on Pool 8 and a nearby Reference site in 2000 (pre-2001 drawdown), 2001 (pre-2001 drawdown) and 2002 (post-2001 drawdown) and tissues were analyzed for mercury, other elements, and organochlorine contaminants. Bioindicator meas- urements of genetic damage, oxidative stress, ethox- yresorufin-O-dealkylase activity, and the ratio of liver to nestling mass were also measured in nestlings at all sites and all years. Based on a multivariate analysis, the 2001 drawdown of Pool 8 did not influence element concentrations, organochlorine concentra- tions, or bioindicator response. Concentrations of inorganic and organochlorine contaminants in tree swallow eggs and nestlings were not at toxic levels. Hatching success did not differ among years and was comparable to the nationwide average. Keywords Organochlorines . Elements . Mercury . Tree swallows . Mississippi River Introduction The annual cycle of fluctuating water levels has been modified on the Upper Mississippi River, the reach upstream from St. Louis, Missouri, primarily by the system of dams installed by the U.S. Army Corps of Engineers in the 1930s to enhance commercial navigation. Over time, high water levels have con- Environ Monit Assess (2007) 133:335–345 DOI 10.1007/s10661-006-9588-8 T. W. Custer (*) : P. M. Dummer : C. M. Custer : A. U. Li US Geological Survey, Upper Midwest Environmental Sciences Center, 2630 Fanta Reed Road, La Crosse, WI 54603, USA e-mail: tcuster@usgs.gov D. Warburton US Fish and Wildlife Service, 4101 E. 80th Street, Bloomington, MN 55425, USA M. J. Melancon : D. J. Hoffman US Geological Survey, Patuxent Wildlife Research Center, 12011 Beech Forest Road, Laurel, MD 20708, USA C. W. Matson : J. W. Bickham Department of Wildlife and Fisheries, Texas A&M University, College Station, TX 77843, USA