Development of a biotic ligand model to predict the acute toxicity of cadmium to Daphnia pulex

The goal of this study was to develop a biotic ligand model (BLM) to predict the acute toxicity of cadmium to Daphnia pulex. Organisms were cultured in moderately soft water and standard 48 h acute toxicity tests were used to determine EC50s in various water chemistries where the effects of Ca2+, Na+, Mg2+, Cl−, K+, pH, and two sources of natural organic matter (Suwannee River and Nordic Reservoir) were evaluated. Overall, toxicity responses were consistent with the free-ion activity model and the principles inherent in the BLM. Increases in Ca2+ resulted in higher EC50s, indicating that Cd2+ competes with Ca2+ for uptake at the biotic ligand. Similar cation competition effects were observed when Mg2+ was varied but with a less pronounced protective effect relative to Ca2+. Changes in Na+ and K+ concentrations had no significant effect on Cd toxicity. EC50 values did not change significantly when pH was adjusted over a range from 8.0 to 6.1. Additions of natural organic matter resulted in elevated dissolved organic carbon (DOC) concentrations that significantly reduced Cd bioavailability via complexation of Cd2+. An existing biotic ligand model (HydroQual BLM ver 2.2.3) was tested for its ability to predict acute Cd toxicity to D. pulex. Once the BLM was adjusted for the relatively sensitivity of D. pulex the protective effects of Ca and DOC could be predicted reasonably well but other test chemistries did not match with measured EC50s. Binding constants derived from the test results (log KCaBL of 4.1, log KMgBL of 3.7, log KHBL of 6.1 and log KCdBL of 7.0) were used to develop a modified BLM for the effects of Cd on D. pulex that accounted for the moderating effect of Ca and Mg on acute toxicity but overestimated the protective effect of DOC.