Comparison of the capacity of two Biotic Ligand Models to predict chronic copper toxicity to two Daphnia magna clones and formulation of a generalized BioAvailability Model

In the European Union and the United States, two differently structured bioavailability models are used in risk assessments of copper. These models, the biotic ligand models (BLM), are valuable tools based on the concept that toxicity depends on the concentration of metal bound to a biological binding site; the biotic ligand. The application of these different BLMs implies that a discrepancy exists between regulation of aquatic toxicity in the U.S. and the E.U. In this study we evaluated the capacity of these BLMs to predict chronic copper toxicity to two water flea clones (Daphnia magna). We found that one BLM performed best with one clone, while the other BLM performed best with the other clone. Furthermore, the results of our study suggested that the effect of pH on chronic copper toxicity between both clones was different, which was also confirmed by additional experiments. Based on these results we developed a generalized BioAvailability Model (gBAM), which can be considered a first step towards further improvement of the accuracy of chronic toxicity predictions of copper to aquatic biota.

Scientific abstract

While it is increasingly recognized that biotic ligand models are valuable in the risk assessment of metals in aquatic systems, the use of two differently structured and parameterized BLMs (one in the U.S. and another in the E.U.) to obtain a bioavailability-based chronic water quality criteria for copper is worthy of further investigation. Here, we evaluated the predictive capacity of these two BLMs for a large dataset of chronic copper toxicity data with two Daphnia magna clones, further denoted as K6 and ARO. We found that one BLM performed best with clone K6 data while the other performed best with clone ARO data. We also found that there is an important difference between both BLMs in how they predict bioavailability of copper as a function of pH. Our modelling results suggest that the effect of pH on chronic copper toxicity is different between the two clones considered, which was confirmed with additional chronic toxicity experiments.

Finally, as fundamental differences in model structure between both BLMs made it impossible to create an ‘average’ BLM, we developed a generalized BioAvailability Model (gBAM). Out of three developed gBAMs, we recommend the use of model gBAM-C_uni, which combines a log-linear relation between the 21-day EC50 (expressed as free Cu2+ ion activity) and pH with more conventional BLM-type competition constants for sodium, calcium and magnesium. The developed gBAM can be considered a first step in further improving the accuracy of chronic toxicity predictions of copper as a function of water chemistry (to a variety of Daphnia magna clones), even beyond the robustness of current BLMs used in regulatory applications.

Full reference (link):

Van Regenmortel T, Janssen CR, De Schamphelaere KAC. 2015. Comparison of the capacity of two Biotic Ligand Models to predict chronic copper toxicity to two Daphnia magna clones and formulation of a generalized BioAvailability Model. Environmental Toxicology and Chemistry