Lethal Body Residues for Pentachlorophenol in Zebra Mussels (Dreissena polymorpha) under Varying Conditions of Temperature and pH

Pentachlorophenol (PCP) toxicity was measured in the zebra mussel under varying conditions of pH (6.5, 7.5, or 8.5) and temperature (10, 17, or 25°C). Toxicity decreased significantly with increasing pH at all temperatures. At a given pH level, toxicity increased significantly with increasing temperature. PCP was most toxic at pH 6.5, 25°C and least toxic at pH 8.5, 10°C. Toxicokinetic parameters were determined at trace PCP concentrations under each combination of pH and temperature. Increasing temperature generally increased the PCP uptake clearance (ku) although elimination rate constants (kd) were unaffected. The effect of pH on toxicokinetic parameters was inconsistent but ku tended to decrease as pH and ionization of PCP increased. Lethal body residues (LR50s), estimated from kinetic parameters determined at trace PCP concentrations and the LC50 values, varied by a factor of 122 as a function of environmental conditions while LC50s varied by a factor of 381. LR50s were also estimated from the measured PCP tissue concentrations and varied by a factor of 8 across conditions. Calculated LR50s were always higher than measured LR50s, determined under identical conditions, by at least a factor of five. However, when LR50 values were recalculated using ku values measured at the LC25 concentration, the resulting adjusted LR50s varied only by a factor of 2.5 across the range of conditions studied and were more consistent with measured LR50 values. Thus, variance in the PCP concentration required to produce toxicity is reduced when LR50s are used in place of LC50s. Further, the method by which lethal residues (LR50 values) are determined can significantly affect the results and their interpretation.