Bioavailability of Polychlorinated Dibenzo-p-dioxins in Lake Enclosures

The bioavailability of 1,3,6,8-tetra- (T4CDD) and octachlorodibenzo-p-dioxin (O8CDD) was examined in large (40 m3) lake enclosures at the Experimental Lakes Area in northwestern Ontario. The polychlorinated dioxins (PCDDs) were added to replicate enclosures as a sediment slurry at a nominal concentration of 58–59 ng∙L−1. T4CDD was more bioavailable to caged benthic invertebrates and fish (white sucker, Catostomus commersoni) than O8CDD immediately after the addition to the enclosures. However, as the concentration of T4CDD in the water column rapidly declined, the bioavailability of T4CDD also declined. Sorption of PCDD to organic matter and rapid partitioning to sediments might have reduced the uptake of PCDDs directly from the water column. Accumulation of PCDDs in biota appeared to shift from direct equilibrium partitioning during the first few days, when the concentrations in the water column were relatively high, to a detrital food chain transfer as the freely available PCDDs in the water declined. This conclusion is supported by the results of the simple, four-compartment food chain model of Thomann and Connolly based on the uptake kinetics of PCDDs from water and food.