Water level management and contaminant exposure to tree swallows nesting on the Upper Mississippi River†

Abstract

The U.S. Army Corps of Engineers conducted 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. Flooding 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 organochlorine contaminants were more available to vertebrates following the 2001 drawdown. Tree swallow (Tachycineta bicolor) eggs and nestlings were collected 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 measurements of genetic damage, oxidative stress, ethoxyresorufin-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 concentrations, 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.

Concentrations and spatial patterns of organic contaminants in tree swallow (Tachycineta bicolor) eggs at United States and binational Great Lakes Areas of Concern, 2010–2015

“…Polychlorinated biphenyls, which are one of the primary contaminant classes associated with impaired beneficial use at most of the AOCs, were present at or below background exposure levels in tree swallow eggs at 30% of AOCs (11 of 27 AOCs). The nationwide geometric mean background exposure for tree swallows (0.34 μg/g ) was similar to the concentrations at non‐AOCs in the present study (0.32 μg/g), indicating that the non‐AOC sites were adequate reference sites for PCB exposure in the present study. Approximately 78% of AOCs had mean exposure less than exposure at the most contaminated non‐AOC (Little Tail Point).…”

“…Tree swallows accumulate more dioxins and furans than higher–trophic level (piscivorous) avian species , so they are a better indicator of PCDD‐Fs within a system than are piscivorous waterbirds. Other organic contaminants tended to not differ between AOCs and non‐AOCs ( p,p ′‐DDE and mirex) or, if there was higher exposure at AOCs, exposure was near background for the United States and Canada or below reproductive effect thresholds (Σheptachlors, Σchlordanes, and dieldrin ). Midland, Michigan, had 100 times more benzene and 10 times more octachlorostyrene in tree swallow egg samples than the next most contaminated location.…”

Contaminants in the Upper Mississippi River: historic trends, responses to regulatory controls, and emerging concerns

“…4). Concentrations of total PCBs (0.44 lg/g wet weight) in eggs of tree swallows nesting on Pool 8 during 2000, 2001, and 2002 were much lower than concentrations associated with embryo toxicity (Custer et al, 2007).…”

“…Custer et al (1997), however, did not consider the thickness of these eggshells as a threat to the reproduction of great blue herons in the colonies sampled. Similarly, concentrations of p,p 0 -DDE (0.15 lg/g wet weight) in eggs of tree swallows (Tachycineta bicolor Vieillot) nesting on Pool 8 of the Upper Mississippi River (near La Crosse, Wisconsin) during 2000, 2001, and 2002 were much lower than concentrations associated with embryo toxicity (Custer et al, 2007).…”

“…Concentrations of DDE in piscivorous and insectivorous birds foraging in the Upper Mississippi River have also been quantified since the DDT ban (Custer et al, 1997(Custer et al, , 2007. In 1993, geometric mean concentrations of DDE in eggs of great blue herons ranged from 0.60 to 3.8 lg/g wet weight among 10 colonies sampled on the Upper Mississippi River (Custer et al, 1997).…”