Refinement and field validation of a biotic ligand model predicting acute copper toxicity to Daphnia magna

Abstract

A previously developed biotic ligand model (BLM) was validated for its capacity to predict acute 48-h EC50 values of copper to Daphnia magna in 25 reconstituted media with different pH values and concentrations of artificial dissolved organic carbon, Ca, Mg and Na. Before the BLM validation, fitting of measured (with a copper ion-selective electrode) and calculated (with the BLM) Cu 2+-activity was performed by adjusting the wham model V (i.e. the metal–organic speciation part of the BLM) copper–proton exchange constant to p K MHA=1.9. Using this value, the 48-h EC50 values observed agreed very well with BLM-predicted EC50 values for tests performed at pH<8, but not at all for tests performed at pH>8. Additional experiments demonstrated that this was due to toxicity of the CuCO 3 complex, which is the most abundant inorganic copper species at pH>8. This was incorporated into the initial BLM by allowing the binding of CuCO 3 (next to Cu 2+ and CuOH +) to the biotic ligand of D. magna. The affinity of CuOH + and CuCO 3 for the biotic ligand was approximately five- and 10-fold lower than that of Cu 2+, respectively. With the refined BLM, 48-h EC50 values could be accurately predicted within a factor of two not only in all 25 reconstituted media, but also in 19 natural waters. This validated and refined BLM could support efforts to improve the ecological relevance of risk assessment procedures applied at present.