Potential impacts of Dreissena on polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) levels in sediment were investigated by comparing PCDD/PCDF levels in colonized sediment with levels in sediment in the same area that was not colonized but that was exposed to similar environmental conditions. Levels of PCDDs/PCDFs were also determined in Dreissena tissues. The particle size distribution of colonized sediment was characterized by silt-size material with peak abundance in the range of 7.5 to 20 µm, whereas noncolonized sediment was coarser in nature (30-60 µm). Total organic carbon (TOC) and concentrations of the PCDD/PCDF congener groups and individual 2,3,7,8-substituted isomers were greater in colonized sediment. A trend was observed toward decreasing concentrations of PCDF congener groups with increased chlorine substitution in mussel tissues in contrast to increasing concentrations of PCDF congener groups with increased chlorination in sediments. The trend in the PCDD congener group profile in mussel tissues appeared more similar to the profile in sediments. Areal estimates of toxicity equivalents (TEQ) in Dreissena biomass at Port Dalhousie, Ontario, Canada (approx. 1,300 pg/m²) were about 0.9% of the TEQ in the top 3 cm of sediment (approx. 135,000 pg/m²). Differences in particle size distribution and PCDD/PCDF levels between colonized and noncolonized sediment suggest that Dreissena may influence chemical and physical properties of sediment they colonize. [ABSTRACT FROM AUTHOR]