Evaluating the ameliorative effect of natural dissolved organic matter (DOM) quality on copper toxicity to Daphnia magna: improving the BLM Hassan A. Al-Reasi • D. Scott Smith • Chris M. Wood Accepted: 28 October 2011 / Published online: 10 November 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Various quality predictors of seven different natural dissolved organic matter (DOM) and humic sub- stances were evaluated for their influence on protection of Daphnia magna neonates against copper (Cu) toxicity. Protection was examined at 3 and 6 mg l -1 of dissolved organic carbon (DOC) of each DOM isolate added to mod- erately hard, dechlorinated water. Other water chemistry parameters (pH, concentrations of DOC, calcium, magne- sium and sodium) were kept relatively constant. Predictors included absorbance ratios Abs 254/365 (index of molecular weight) and Abs-octanol 254 /Abs-water 254 (index of lipo- philicity), specific absorption coefficient (SAC 340 ; index of aromaticity), and fluorescence index (FI; index of source). In addition, the fluorescent components (humic-like, fulvic- like, tryptophan-like, and tyrosine-like) of the isolates were quantified by parallel factor analysis (PARAFAC). Up to 4-fold source-dependent differences in protection were observed amongst the different DOMs. Significant correla- tions in toxicity amelioration were found with Abs 254/365 , Abs-octanol 254 /Abs-water 254 , SAC 340 , and with the humic- like fluorescent component. The relationships with FI were not significant and there were no relationships with the tryptophan-like or tyrosine-like fluorescent components at 3 mg C l -1 , whereas a negative correlation was seen with the fulvic-like component. In general, the results indicate that larger, optically dark, more lipophilic, more aromatic DOMs of terrigenous origin, with higher humic-like content, are more protective against Cu toxicity. A method for incorporating SAC 340 as a DOM quality indicator into the Biotic Ligand Model is presented; this may increase the accuracy for predicting Cu toxicity in natural waters. Keywords Daphnia magna  Copper  Dissolved organic matter  Absorbance and fluorescence quality parameters  Biotic ligand model Introduction Metal toxicity is a global environmental concern in natural waters. Copper (Cu), for example, is one of the essential micronutrients required for various metabolic processes in living organisms, nevertheless it becomes toxic to many freshwater organisms at higher concentrations (Flemming and Trevors 1989). According to USEPA’s Water Quality Criteria for Copper (U.S. Environmental Protection Agency 2007), the natural background Cu concentrations may range widely from 0.2 to 30 lgl -1 in pristine freshwater and up to 100 lgl -1 or more in systems moderately contaminated by anthropogenic inputs, while in mining areas Cu levels may approach 200,000 lgl -1 . Toxicological manifestations of Cu have been linked to the interference with sodium (Na ? ) regulation and metabolism in freshwater animals (Grosell et al. 2002; Grosell and Wood 2002). In natural waters, or the exposure water of laboratory based experiments, the con- centration of Na ? and the presence of Cu-detoxifying agents such as natural dissolved organic matter (DOM) play an important role in determination of the toxic outcome. The influence of water chemistry on Cu toxicity has resulted in the development of models such as the Biotic Ligand Model (BLM), a computer program that uses water chemistry parameters (model inputs) to predict Cu toxicity to H. A. Al-Reasi (&)  C. M. Wood Department of Biology, McMaster University, Hamilton, ON L8S 4K1, Canada e-mail: alreasha@mcmaster.ca; alreasi@squ.edu.om D. Scott Smith Department of Chemistry, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada 123 Ecotoxicology (2012) 21:524–537 DOI 10.1007/s10646-011-0813-z