Comparative toxicity and toxicokinetics of DDT and its major metabolites in freshwater amphipods

The toxicity and toxicokinetics of radiolabeled DDT and its major degradati on products, dichlorodiphenyldichloroethane (DDD) and dichlorodiphenyldichl oroethylene (DDE), were determined for the amphipods Hyalella azteca and Di poreia spp, in water-only static renewal exposures. Comparison of the water and tissue concentrations associated with decreased survival revealed larg e differences in toxicity among the three compounds. In H. azteca, the rati o of the 10-d LR50 values (median lethal tissue residue) for DDT:DDD:DDE wa s 1:24:195. In Diporeia spp., the 28-d LR50 for DDT was higher than that fo r DDD by a factor of six, and DDE did not cause significant mortality even at concentrations approaching the solubility limit. Eased on the toxicity d ata, the hazard from exposure to mixtures of DDT and its degradation produc ts should be evaluated on a toxic-units basis and not as a simple summation of the individual concentrations, which ignores the toxicity of specific c ompounds. Differences in species sensitivity were also detected. The 10-d L R50 values were higher in Diporeia spp, than in H, a:reca by a factor of 40 for DDT and eight for DDD. This difference can be only partly attributed t o differences in lipid content between H. azteca (7% dry wt) and Diporeia s pp. (24% dry wt). The uptake clearance and elimination rate constants were similar among the various compounds in both species. Uptake clearance was t ypically fourfold greater for H. azteca than for Diporeia bpp., however, an d the experimentally measured elimination rate was approximately 30-fold gr eater in H. azteca than in Diporeia spp. The larger rates of uptake and eli mination were attributed to the higher exposure temperature, greater surfac e area-to-volume ratio, and lower lipid content for H. azteca compared with Diporeia spp. In addition, extensive biotransformation of DDT by H. azteca may have contributed to a more rapid compound elimination.