Influences of water chemistry on the acute toxicity of lead to Pimephales promelas and Ceriodaphnia dubia

The acute toxicity of lead (Pb) was examined for fathead minnows (Pimephales promelas; 96-h) and daphnids (Ceriodaphnia dubia; 48-h) in waters modified for hardness (as CaSO₄), dissolved organic carbon (DOC; as Aldrich humic acid) and alkalinity (as NaHCO₃) for parameterization of an acute freshwater biotic ligand model (BLM). Additionally, acute (96-h) and chronic (30-d) bioassays were performed for P. promelas to more clearly define the influence of pH (5.5–8.3) on Pb toxicity as modified by addition of HCl or NaOH using an automated titration system. Results indicate that Ca²⁺ is protective against acute Pb toxicity to P. promelas but not C. dubia. Strong protection was afforded by DOC and NaHCO₃ against acute Pb toxicity to P. promelas, whereas milder protection was observed for C. dubia with both parameters. Dissolved Pb LC50s from the P. promelas pH bioassays revealed a complex effect of pH on Pb toxicity, likely explained in part by Pb speciation and the competitive interaction of H⁺ with ionic Pb²⁺. Chronic pH bioassays also demonstrated that 30-d growth is not impaired in fathead minnows at relevant Pb concentrations. The findings reported herein suggest that development of separate BLMs for P. promelas and C. dubia should be considered.