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The effect of pH on chronic aquatic Ni toxicity is dependent on the pH itself: Extending the chronic Ni bioavailability models

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The Environmental Quality Standard (EQS) for Ni in the Water Framework Directive (WFD) is bioavailability based. Although some of the available chronic Ni bioavailability models are only validated for pH≤8.2, a considerable fraction of the European surface waters has a pH > 8.2. Therefore, we investigated the effect of a change in pH from 8.2 to 8.7 on chronic Ni toxicity to 3 invertebrate (Daphnia magna, Lymnaea stagnalis, and Brachionus calyciflorus) and 2 plant species (Pseudokirchneriella subcapitata and Lemna minor). Ni toxicity was almost always significantly higher at pH 8.7 than at pH 8.2. To test whether the existing chronic Ni bioavailability models developed for pH≤8.2, can be used at higher pH levels, Ni toxicity at pH 8.7 was predicted based on Ni toxicity observed at pH 8.2. This resulted in a consistent underestimation of toxicity. The results suggest that the effect of pH on Ni2+ toxicity is dependent on the pH itself: the slope of the pH effect is steeper above than below pH 8.2 for species for which a species‐specific bioavailability model exists. Therefore, the existing chronic Ni bioavailability models were modified to allow predictions of chronic Ni toxicity to invertebrates and plants in the 8.2–8.7 pH range, by applying a pH slope (SpH) which is dependent on the pH of the target water. These modified Ni bioavailability models resulted in more accurate predictions of Ni toxicity to all 5 species (i.e., within 2‐fold error), without the bias observed using the bioavailability models developed for pH≤8.2. The results of this study can decrease the uncertainty in implementing the bioavailability based EQS under the WFD for high pH regions in Europe. This article is protected by copyright. All rights reserved

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