Development of a chronic zinc biotic ligand model for Daphnia magna

The individual effects of the cations Ca(2+), Mg(2+), Na(+), and H(+) on the chronic toxicity of Zn to the waterflea Daphnia magna were investigated in different series of univariate experiments, resulting in the development of a chronic Zn biotic ligand model (BLM) for this species. Using the mathematical approach based on a linear relationship between cation activity and metal activity at the EC(x) level, the following stability constants for binding of competing cations to the biotic ligand (BL) were derived: logK(CaBL) = 3.22, logK(MgBL) = 2.69, logK(NaBL) = 1.90, and logK(HBL) = 5.77. With the derived constants and a logK(ZnBL) of 5.31, two different BLMs that predict chronic zinc toxicity (EC(50), no observed effect concentration (NOEC)) for D. magna as a function of water characteristics were developed. Fractions of binding sites occupied by Zn at the considered effect levels EC(50) and NOEC were 0.127 and 0.084, respectively. The NOEC-based model predicts the Zn toxicity within a factor of 2, while the chronic EC(50) could be predicted within a factor of 1.5. In the future, these chronic Zn BLMs for D. magna can improve the ecological relevancy of zinc risk assessments by decreasing the bioavailability-related uncertainty of zinc toxicity.