Great horned owl (Bubo virginianus) dietary exposure to PCDD/DF in the Tittabawassee River floodplain in Midland, Michigan, USA

Abstract

Soils and sediments in the floodplain of the Tittabawassee River downstream of Midland, Michigan, USA contain elevated concentrations of polychlorinated dibenzofurans (PCDF) and polychlorinated dibenzo-p-dioxins (PCDD). As a long-lived, resident top predator, the great horned owl (Bubo virginianus; GHO) has the potential to be exposed to bioaccumulative compounds such as PCDD/DF. Site-specific components of the GHO diet were collected along 115 km of the Tittabawassee, Pine, Chippewa, and Saginaw Rivers during 2005 and 2006. The site-specific GHO biomass-based diet was dominated by cottontail rabbits (Sylvilagus floridanus) and muskrats (Ondatra zibethicus). Incidental soil ingestion and cottontail rabbits were the primary contributors of PCDD/DF to the GHO diet. The great horned owl daily dietary exposure estimates were greater in the study area (SA) (3.3 to 5.0 ng 2,3,7,8-TCDD equivalents (TEQ(WHO-avian))/kg body wt/d) than the reference area (RA) (0.07 ng TEQ(WHO-Avian)/kg body wt/d). Hazard quotients (HQs) based on central tendency estimates of the average daily dose and no-observable-adverse effect level (NOAEL) for the screech owl and uncertainty factors were <1.0 for both the RA and the SA. Hazard quotients based on upper end estimates of the average daily dose and NOAEL were <1.0 in the RA and up to 3.4 in the SA.

Ecological risk assessment of great horned owls (Bubo virginianus) exposed to PCDD/DF in the Tittabawassee River floodplain in Midland, Michigan, USA

“…The lesser concentrations of PCDD/DF measured in blood plasma of nestling GHO and addled eggs are consistent with the results of a site-specific dietary study that predicted GHO in the Tittabawassee River floodplain study area have a potential average daily dose of 3.3 ng TEQ WHO-Avian /kg bw/d [19]. That study described the GHO diet and concentrations of PCDD/DF in prey items, such as rabbits, muskrats, and meadow voles, which make up 85% of the GHO biomass-based diet from the Tittabawassee River floodplain.…”

“…In addition, profiles of congener concentrations in GHO from both the reference and study areas are consistent with the congener patterns in the sitespecific diet (Fig. 2) [19]. This result is consistent with GHOs being exposed to PCDD/DF through prey from within the floodplain.…”

“…Profiles of relative concentrations of congeners in GHO blood plasma and addled eggs were consistent with congener profiles for other birds and mammals in the floodplains of the reference and study areas [19,35]. In addition, profiles of congener concentrations in GHO from both the reference and study areas are consistent with the congener patterns in the sitespecific diet (Fig.…”

“…This result is consistent with GHOs being exposed to PCDD/DF through prey from within the floodplain. While the predominant congeners in soil and sediment were 2,3,7,8-TCDF and 2,3,7,8-TCDD [19], wildlife in the floodplain preferentially retained 2,3,4,7,8-PeCDF but did not retain 2,3,7,8-TCDF at concentrations proportional to those in soils. This result is consistent with metabolism and rapid clearance of 2,3,7,8-TCDF that has been observed in mallards [36] and gulls [37].…”

“…The third line of evidence, dietary exposure to PCDD/DFs, is reported elsewhere [19]. Examining three lines of evidence provided greater confidence in the assessment of the potential for PCDD/DF in the floodplain soils and sediments to adversely affect resident GHO [20].…”