The effect of pH on chronic copper toxicity to Ceriodaphnia dubia within its natural pH niche.

The pH of freshwater ecosystems affects bioavailability of various metals to various organisms, including daphnids. Although it is well known that daphnid species show interclonal variation of metal sensitivity, knowledge about interclonal variation of bioavailability effects, such as the pH effect, is scarce. Here, we compared the effect of pH on chronic copper toxicity between two clones of Ceriodaphnia dubia, within its natural pH niche, which we determined to be approximately pH 6.5-8.5 based on existing experimental and biological monitoring data. Using a Bayesian modeling approach, we found that the effect of pH was not statistically significantly different between the two clones (with a credibility > 95%). Overall, we found an approximately threefold decrease in chronic Cu toxicity with increasing pH between pH 6.5 and 8.5, with 7-day 20% effect concentration (EC20) values ranging between 11.0 and 30.9 µg/L dissolved Cu. We then calibrated a preliminary generalized bioavailability model (gBAM) using these data and found a pH-effect slope parameter SpH = -0.247, which is within the range of previously reported values for Daphnia magna (-0.056 to -0.361) and similar to the SpH value of -0.220 used in the "invertebrate gBAM" for bioavailability-based Cu risk assessment under the Registration, Evaluation, Authorisation and Restriction of Chemicals. The preliminary C. dubia gBAM captured the magnitude of the observed pH effect well (mean of 1.3-fold EC20 prediction error, n = 9). It was also able to accurately predict chronic Cu toxicity in natural waters reported in an independent dataset (mean of 1.4-fold prediction error, n = 6). Also, two D. magna gBAMs (for two clones) and the invertebrate gBAM showed comparable predictive capabilities. Collectively, our work highlights the importance of studying relations between pH and metal bioavailability within the species' natural niche. It also confirms earlier findings that biological variation of pH-bioavailability relations typically does not have a large impact on predictive capacity of bioavailability models, which is important for regulatory applications.