Can natural levels of Al influence Cu speciation and toxicity to Daphnia magna in a Swedish soft water lake?

- Acute Cu and Al toxicity to Daphnia magna was tested.
- Toxicity and free Cu2+ was calculated using a biotic ligand model (v.2.2.3).
- ASV labile Cu increased significantly when Al was added to the samples.
- Toxicity to D. magna increased significantly (p: 0.05) when 10 μg Al/L was added to the test.

It is well known that chemical parameters, such as natural organic matter (NOM), cation content and pH may influence speciation and toxicity of metals in freshwaters. Advanced bioavailability models, e.g. Biotic Ligand Models (BLMs), can use these and other chemical parameters to calculate site specific recommendations for metals in the aquatic environment. However, since Al is not an input parameter in the BLM v.2.2.3, used in this study, there could be a discrepancy between calculated and measured results in Al rich waters. The aim of this study was to evaluate if the presence of Al in a circumneutral (pH ∼6) soft humic freshwater, Lake St. Envättern, will affect the Cu speciation and thereby the toxicity to the cladoceran Daphnia magna. The results show a statistically significant increase in the free Cu2+ concentration with Al additions and that measured levels of Cu2+ significantly differed from BLM calculated levels of Cu2+. Furthermore, there was also a statistically significant elevated acute toxic response to D. magna at low additions of Al (10 μg/L). However, since the large difference between calculated and measured Cu2+ resulted in a significant but minor (factor of 2.3) difference between calculated and measured toxicity, further studies should be conducted in Al rich soft waters to evaluate the importance of adding Al as an input parameter into the BLM software.

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