The effect of water chemistry on the acute toxicity of nickel to the cladoceran Daphnia pulex and the development of a biotic ligand model

The goal of this study was to evaluate the influence of water chemistry parameters on the acute toxicity of waterborne Ni to Daphnia pulex in soft waters and using this information to develop a biotic ligand model. The effects of Ca, Mg, Na, K, Cl, pH (two differently buffered sets) and natural organic matter (NOM) from two sources were evaluated in standardized 48 h acute toxicity tests. Increases in Ca2+ had a protective effect on Ni toxicity, suggesting that this ion competes with Ni at the site of biological uptake. Increased waterborne Mg2+ also reduced Ni toxicity, but to a lesser degree compared with Ca2+. EC50 values increased at higher pH when the organic buffer 3-morpholinepropanesulfonic acid was used to adjust test pH, however in tests series where pH was varied using HCO3− the results were equivocal. Other testing showed that Na, K and Cl did not influence the toxicity response of D. pulex to Ni. Complexation of Ni by NOM reduced toxicity but Nordic Reservoir NOM was much more protective compared to Suwannee River NOM. Geochemical modeling of organic matter complexation of Ni was done using the HydroQual Biotic Ligand Model (BLM ver. 2.3.3; research mode) and the Windermere Humic Aqueous Model (WHAM ver 6.0). Results showed dramatic differences between the two models in dissolved organic matter complexation. Modelling of Ni geochemistry for test solutions other than those containing NOM showed consistent and minor differences between the WHAM and the BLM. The latter model was used to develop a comprehensive prediction model of Ni toxicity. log K values developed for competitive cationic effects showed that Ca and Mg have a much higher protective effect in soft water compared to models developed for Daphnia magna in hard water. The BLM developed for this species in soft water provided good predictions of toxicity across a wide range of Ni concentrations but also highlighted the need for an improved understanding of the effects of NOM and pH on Ni toxicity in soft waters.