Development of a biotic ligand model for the acute toxicity of zinc to Daphnia pulex in soft waters

The goal of this study was to develop a biotic ligand model (BLM) for the acute toxicity of zinc to the Daphnia pulex in soft water. In different tests Ca (as CaSO4), Na (as NaCl), Mg (as MgSO4), K (as KCl), pH (using the buffer 3-(N morpholino)-propanesulphonic acid (MOPS)) and dissolved organic carbon (DOC) were altered to determine possible effects on the 48 h EC50 for Zn. Increases in waterborne Ca2+ had a protective effect on Zn toxicity, suggesting that this ion competes with Zn and that they share a common site of biological uptake. Increased waterborne Mg2+ also reduced Zn toxicity, but to a lesser degree compared with Ca2+. No significant effects of other cations on EC50 for Zn were observed, indicating that the toxicity of Zn in D. pulex is not linked to Na+ and K+. Increasing DOC concentrations resulted in higher EC50 values for Zn due to the complexation of Zn by organic matter in solution and the resulting reduction of free Zn2+ ion concentrations. Tests to characterize the effect of pH on Zn toxicity showed a small rise in EC50 values between pH 6.3 and 7.1 and no further change as pH was increased to 8.0. Two existing BLM implementations for acute Zn effects on D. magna were tested to determine their applicability to D. pulex in soft water. The existing models underestimated the protective effect of Ca2+ and Mg2+ but accurately predict the effects of DOC. A modified BLM, with revised equilibrium constants for competitive cation effects was developed. This study shows that the acute effects of Zn on D. pulex in soft water can be characterized and incorporated into a predictive BLM.