An ecological risk assessment of fish-eating birds exposed to polychlorinated dibenzofurans and dibenzo-p-dioxins within the Tittabawassee River floodplain, MI, USA

Concentrations of polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzo-p-dioxins (PCDDs) in the Tittabawassee River (TR) and associated floodplains downstream of Midland, MI, USA are greater than at upstream locations and regional background concentrations. Sediments and floodplain soils in downstream study areas (SAs) contain total concentrations of the seventeen 2,3,7,8-substituted PCDD/DF congeners ranging from 1.0x10 2 to 5.4x104 ng/kg dry wt, respectively. In contrast, concentrations of SigmaPCDD/DFs in sediments and soils from upstream reference areas were 10- to 20-fold less. The majority of the contaminant mixture is composed of 2,3,7,8-tetrachlorodibenzofuran and 2,3,4,7,8-pentachlorodibenzofuran, which are likely present is the result of historical chemical production and associated waste management practices. Concerns about potential ecological impacts of the elevated concentrations of PCDD/DFs within the TR floodplain led to a site-specific multiple lines of evidence study was executed including dietary- and tissue-based exposures assessments and measurements of population health. Two fish-eating bird species that breed along the TR [great blue heron (Ardea herodias; GBH) and belted kingfisher (Ceryle alcyon; BKF)] were monitored both upstream and downstream of the putative source in order to elucidate the potential for contaminant driven adverse population-level effects. Additionally, measured exposures were compared to toxicity reference values (TRVs), and reproductive parameters were compared to literature values. During the 2005-2007 breeding seasons, a total of 37 BKF nest chambers were excavated for sample collection and monitored for reproductive effort and success. Three GBH breeding colonies located within the SA were monitored during the 2006 and 2007 breeding seasons and nests within each colony were accessed for sample collection. Concentrations of 2,3,7,8-tetrachlorodibenzo- p-dioxin equivalents (TE QWHO-Avian) in both eggs and nestlings of BKF from the SA were 5- to 21-fold greater than in those from upstream reference areas (RAs). Concentrations of TEQWHO-Avian blood plasma of adult GBH from the SA was 4- to 8-fold greater compared to those from the RA. Contaminant concentrations in GBH eggs and nestlings were similar among all studied breeding colonies. Predicted dietary exposures followed this same spatial trend in both species, being 150- to 190-fold greater along the TR compared to upstream RAs. Comparison of the predicted daily dietary dose to the TRV suggested there was the risk of adverse effects as a result of exposure to PCDD/DFs. This is in contrast to the conclusions drawn from both the tissue-based exposure and effects assessments and site-specific measures of individual and population health. This inconsistency is likely the result of the dietary exposure and effects assessments being more conservative, based on the greater number of assumptions that must be made and the greater uncertainty associated with the dosing methodology from which the TRV was derived. Therefore, the overall conclusion of the research presented herein is that the populations of BKF and GBH breeding along the TR are not at risk despite elevated concentrations of PCDD/DFs in the diet and tissues.