PARENT SESSION
4B Site-specific ecological risk assessment
9:00 AM to 7:00 PM, Tuesday, 08 May 2001
Session Chair:

(T/MF182) Biotic ligand model predicting acute zinc toxicity to Daphnia magna: development and validation.

Heijerick, Dagobert¹, De Schamphelaere, Karel¹, Janssen, Colin¹, ¹

ABSTRACT- The toxicity of zinc to the freshwater crustacean Daphnia magna has shown to be dependent on a variety of water quality characteristics such as pH, hardness and dissolved organic carbon (DOC). An empirical model has been developed by Heijerick and Janssen (2000, in preparation) which predicts acute zinc toxicity as a function of these parameters. In this study, a more mechanistic approach was considered and a Biotic Ligand Model predicting acute zinc toxicity for D. magna was developed. Using the method described by De Schamphelaere and Janssen (2000, submitted to Envir.Technol.Sci.), stability constants for the binding of the main competitive ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, H⁺) to the binding sites of zinc on the biotic ligand were derived. The developed BLM was validated with different testmedia containing artificial (AHA) or natural DOC. A series of zinc toxicity tests, each investigating the effect of one cation on acute zinc toxicity, were performed in standard artificial medium. All other parameters and ion-concentrations were kept constant, including the pH (use of the non-complexating buffer MOPS). Based on: 1) the slope and intercept of the linear relationship between 48h-EC₅₀ and cation-concentrations and 2) the zinc speciation calculated using the speciation model WHAM (Tipping, 1994), the BLM-constants could be derived and a model was developed. The developed BLM was subsequently validated using a series of test waters with different physico-chemical characteristics. The observed 48h-EC₅₀s ranged between 1 and 9 mg/L expressed as total zinc. The BLM was capable of predicting the observed effect concentrations within a factor 2. A similar difference was obtained with the predictions based on the empirical model developed by Heijerick and Janssen (2000, submitted). Additionally, the effect and of increased pH or hardness-ions on zinc toxicity was similar for both models. The BLM, however, is capable of taking the effect of other competitive ions into account (e.g. Na⁺, K⁺), and could therefore be more suitable for predicting acute zinc toxicity in natural waters.

Key words: BLM, Daphnia magna, metal competition