Bioaccumulation and critical body residue of PAHs in the amphipod, Diporeia spp.:: additional evidence to support toxicity additivity for PAH mixtures

Polycyclic aromatic hydrocarbons (PAHs) are considered to act additively when exposed as congener mixtures. Additive internal concentrations at the site of toxic action is the basis for recent efforts to establish a sum PAH guideline for sediment-associated PAH toxicity. This study determined the toxicity of several PAH congeners on a body residue basis in Diporeia spp. These values were compared to the previously established LR50 value for a PAH mixture based on the molar sum of PAH congeners and demonstrated similar LR50 values for individual PAH. These results support the contention that the PAH act at the same molar concentration whether present as individual compounds or in mixture. Aqueous exposures were conducted for 28 d, and the water was exchanged daily to maintain the exposure concentration. The concentration in the exposures declined by an average of 22% between water exchanges across all compounds, and ranged from 11% to 32%. The toxicokinetics were determined using both time-weighted-average (TWA) and time-variable water concentrations and were not statistically different between the two source functions. Toxicity was determined for both mortality and immobility (failure to swim on-prodding) and, on both a TWA water concentration and a body residue basis. The LC50 values ranged from 1757 mug l(-1) for naphthalene after 10 d exposure to 79.1 mug l(-1) for pyrene after 28 d exposure, and the EC50 ranged from 1587 mug l(-1) for naphthalene after 10 d exposure to 38.2 mug l(-1) for pyrene after 28 d exposure. The LR50 values for all congeners at all lengths of exposure were essentially constant and averaged 7.5 +/- 2.6 mumol g(-1), while the ER50 for immobility averaged 2.6 +/- 0.6 mumol g(-1). The bioconcentration factor declined with increasing exposure concentration and was driven primarily by a lower uptake rate with increasing dose, while the elimination remained essentially constant for each compound. Published by Elsevier Science Ltd.