Ni-EXTEND: extending the chronic nickel BLM from pH 8.2 to pH 8.7 and validation of the BLM-normalized HC5(50%) for Ni against the ecological effects observed in a 4-month microcosm experiment

In this study, we investigated if the pH range of the chronic nickel BLM could be extended above pH 8.2, by investigating effects of increasing pH on Ni toxicity to various aquatic species. Ni toxicity expressed as dissolved Ni concentration was at least similar or increased when increasing the pH from 8.2 to 8.7. However, Ni toxicity expressed as free Ni2+ activity was higher at pH 8.7 compared to pH 8.2 for all tested species. All existing Ni BLMs had the tendency to underestimate toxicity at pH 8.7. Hence, it is clear that the BLMs developed for pH<8.2, are not suitable for extrapolation to pH 8.7 and that they should be modified in terms of pH slope. A modified BLM which takes in account a mean pH slope for the invertebrates and another for plants/algae predicted Ni toxicity most accurately and without bias. As a consequence, a two-step procedure is proposed to normalize toxicity data (and calculate HC5) for target waters with pH>8.2. First, Normalize all NOECs to pH 8.2, using the BLMs that have been shown valid for pH up to 8.2, next extend the NOECs normalized from pH 8.2 to the target pH using a mean invertebrate SpH = 1.36 for all invertebrates and a mean SpH = 0.97 for all plants and algae. The NOECs from step 2 can subsequently be used to fit an SSD and calculate an HC5. This procedure is relatively simple and results in realistic normalized HC5s, that are also unbiased (i.e. neither over or under-conservative). The HC5(50%) normalized to the conditions of the microcosm experiment is protective of the overall community-level dynamics observed in a previously reported mesocosm experiment.

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