Bioaccumulation and Food Chain Transfers

PCDDs and PCDFs are known to bioaccumulate. Some of the highest concentrations occur in the top predator species of food chains where successive stages of bioaccumulation through the food chain can result in biomagnification. PCDDs and PCDFs can be mobilized from body fat into lactating females making this an important mode of transfer to offspring.

PCDDs and PCDFs are known to accumulate in fish and are concentrated in the fatty tissues. It is important to distinguish between two mechanisms of uptake in fish: bioaccumulation and bioconcentration. Bioconcentration involves direct uptake from water across the gill membrane and is distinct from bioaccumulation, which also includes dietary uptake. Bioconcentration factors (BCFs) developed to describe this process and are derived from the ratio of uptake and depuration rate constants. Published BCFs for PCDDs vary over 3 orders of magnitude from c. 4 to as high as c. 9000. Higher-chlorinated PCDDs and PCDFs tend to have lower BCFs than less-chlorinated compounds. This is likely due to factors such as differences in membrane transport, larger molecular sizes, lower solubilities, and the possibility for preferential metabolism of certain congeners.

Fish body burdens of PCDDs and PCDFs are likely due to dietary uptake rather than membrane transport across the gill surface. Environmental monitoring data generally indicate higher body burdens in benthic organisms and bottom-dwelling fish than in pelagic fish residing in the surface water. The differences are generally attributed to the close association of sediment-dwelling organisms with sediments and the generally low opportunity for exposure to PCDDs and PCDFs in the water column. Differences in BCF between species is often explained by different feeding strategies and, perhaps, by different rates of biotransformation or excretion. Metabolic transformations of certain dioxin compounds has been suggested as an important factor in explaining low bioconcentration and bioaccumulation factors in some fish species. For example, results from PCDD and PCDF measurements in fish and fish-eating birds from the Great Lakes, USA, demonstrated that 2,3,7,8-substituted congeners preferentially accumulate to a higher degree than non-2,3,7,8-substituted congeners in the food chain. Further, there is some evidence in fish suggesting that lower-chlorinated 2,3,7,8-substituted congeners have longer half-lives (on the order of 50-100 days) than other PCDDs and PCDFs (on the order of several weeks or less).

Plant uptake of PCDDs and PCDFs from soil appears to be rare, or inefficient. Some differences in soil to plant uptake rates, however, have been observed. It is widely believed that PCDDs and PCDFs entering the plant root surface are effectively bound immediately for the life ofthe plant. Further, among the studies showing PCDDs and PCDFs bound to the root surface, few if any studies indicate migration into other plant compartments or translocation to aboveground plant tissues. For example, studies of 2,3,7,8-tetrachloro dibenzo-/>-dioxin (2,3,7,8-TCDD) uptake by carrots grown in soil contaminated after the Seveso accident in Italy showed higher concentrations in the outer root surface than in other root and above ground tissues. Dry deposition and adsorption on the surface of foliage appears the most likely pathway of contact for vegetation, and the most plausible pathway for animal exposure.

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