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The extent of bioaccumulation of the syn- and anti-isomers of Dechlorane Plus (DP) is assessed in archived food web samples from Lake Winnipeg and Lake Ontario. Concentrations of the isomers were determined using purified analytical solutions of individual isomers as opposed to the technical mixture. The syn-isomer was consistently detected in all samples from both lakes; the anti-isomer was detected in all Lake Ontario samples, but only 45% of the samples from Lake Winnipeg. The pattern of bioaccumulation was different for the isomers in Lake Winnipeg. The anti-isomer was dominant in higher trophic level (TL) organisms like walleye [arithmetic mean ±1 × standard error: 730 ± 120 pg/g, lipid weight (lw)] and gold eye (760 ± 170 pg/g, lw) while the syn-isomer dominated the lower TL organisms like zooplankton (550 ± 40 pg/g, lw) and mussels (430 ± 140 pg/g, lw). In Lake Ontario, the extent of bioaccumulation of the isomers and concentrations was greatest in the lower TL benthic organism, Diporeia (syn, 1307 ± 554; and anti, 3108 ± 898 pg/g lw) and also high in zooplankton (syn, 719; and anti, 1332 pg/g lw). This suggests that the isomers are bioavailable in sediment and that, despite their molecular size, diffusion from the water column into zooplankton can occur. Differences in the mean fractional abundance of the anti-isomer (mean fanti = mean concentration of the anti-isomer divided by sum of mean syn- and anti-concentrations) were pronounced in sediments between lakes (Lake Winnipeg mean fanti = 0.610, Lake Ontario mean fanti = 0.860) and the extent of enrichment (anti-) and depletion (syn-) of the isomers were more marked in Lake Winnipeg biota. There were also differences in the biomagnification potentials, as measured by the trophic magnification factor (TMF), between the isomers in the Lake Winnipeg food web; no statistically significant TMFs for either isomer were found for the Lake Ontario food web. A TMF of 2.5 (r² = 0.12, p = 0.04) for the anti isomer and <1 (r² = 0.17, p = 0.01) for the syn isomer suggests that differences in bioaccumulation in the Lake Winnipeg food web are not driven solely by configuration of the pendent chlorocyclopentene moieties. [ABSTRACT FROM AUTHOR]