Effects of water chemistry on the chronic toxicity of lead to the cladoceran, Ceriodaphnia dubia

As the first step toward parameterization of a chronic lead (Pb) biotic ligand model (BLM) for Ceriodaphnia dubia, 7-d toxicity tests were performed in waters modified to evaluate the influences of hardness, DOM (as Suwannee River NOM and Aldrich humic acid (HA)), pH (buffered with 4mM MOPS) and alkalinity on the chronic toxicity of Pb. Calculated EC₂₀s for the control base water test and each of the most extreme modified test waters were as follows in γgL^-1 Pb (95% confidence interval): base water control=45 (14-53), 5mM CaSO₄=22 (12-30), 32mgL^-1 DOM=523 (388-573), 2.5mM NaHCO₃=73 (21-120) and pH 6.4 buffered with MOPS=3.9γgL^-1 Pb (1-5). Results indicate that hardness does not protect against chronic toxicity of Pb to C. dubia, whereas HA does protect at the highest concentration tested (597γM). Additionally, our findings suggest that low pH increases the chronic toxicity of Pb whereas increased alkalinity is protective. The findings reported herein support the need for a chronic Pb BLM as an alternative approach to hardness-based regulations.