Polychlorinated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents in eggs of red-breasted mergansers near Green Bay, Wisconsin, USA, in 1977–78 and 1990

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The hypothesis that the concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEqs) contributed by non-ortho- and mono-ortho-substituted (coplanar) PCB congeners have changed relative to total concentrations of PCBs over time was tested by analyzing eggs of red-breasted mergansers collected from the same locality in 1977–78 and in 1990. Twelve eggs from each time period were analyzed for coplanar PCBs, using a porous graphitic carbon HPLC column and GCECD. TEqs were calculated from coplanar PCB concentrations and bioassay-derived toxic equivalency factors. Median total PCB concentrations decreased from 25 to 13 μg/g, fresh weight (fw), and TEqs decreased from 320 pg/g to 160 pg/g, fw. The relative potency of the PCB mixtures, expressed as the ratio of TEqs contributed by the coplanar PCB congeners to total PCBs, did not change, nor did the ratio of concentrations of coplanar congeners to total PCBs. Thus, the relative potency of the mixture of PCBs in the eggs has not changed even as the concentrations declined significantly. The greater relative potency of TEqs relative to that which would be expected from the Aroclor® mixtures released into the environment is due to selective bioaccumulation, biomagnification, and metabolism, rather than as a result of differential weathering as a function of time.