Context-dependent toxicity of diclofenac in Daphnia magna: roles of water chemistry, neonate source and preparation procedure.

Daphnia magna is a well-established reference organism for ecotoxicological studies of pharmaceuticals. Standardized acute toxicity protocols specify the composition of test media, and any alternative water type must be fully characterized and reported. In the case of acute toxicity testing for D. magna, neonates are typically employed as test organisms. Despite the strict methodological framework, reported EC50 values for diclofenac (DCF) in D. magna vary considerably, ranging from 22.4-123.3 mg L-1, with test water being the most unreported factor. Therefore, in our study, we investigated the effect of exposure medium (M4 medium and the tested natural freshwater) and the source and preparation procedure of D. magna neonates on the toxicity of DCF. The results indicate that DCF toxicity toward D. magna may differ between the tested natural freshwater and M4 medium, with higher EC50 values observed in the tested natural freshwater. Additionally, differences in sensitivity were observed between neonate groups under the tested conditions. These findings highlight the importance of considering the source and preparation history of test organisms and exposure medium when interpreting and comparing daphnid-based ecotoxicological data. Overall, the findings provide additional insight into the toxicological effects of this emerging contaminant on a model reference species under conditions that more closely reflect those of the tested natural freshwater than those prescribed in standard laboratory protocols.