Toxicity of Aluminum Ions to Daphnia magna Straus Depending on the Hardness of Natural and Artificial Water

The toxicity of chemical substances depends not only on their concentration but also on the hydrochemical composition of water (concentration of calcium and magnesium, humic substances, etc.), which is important to consider when extrapolating the results of bioassay to environmental conditions. A study into the acute toxicity of aluminum chloride was carried out on 1-day-old Daphnia magna crustaceans using natural and artificial water with different hardness parameters. Both artificial and natural water showed a decrease in the toxicity of aluminum ions with increasing the hardness; however, the range of changes in the toxicity in artificial water turned out to be much smaller than in natural water. Thus, in artificial water, an increase in the hardness from 0.021 to 6.46 °e led to an increase in the value of the 48-h half-lethal aluminum concentration by 1.57 times, while a similar range of changes in the hardness in natural water (0.21–8.53 °e) led to a change in this indicator by almost six times. Such differences may be associated with the influence of other hydrochemical factors typical for the studied water types. Thus, when conducting bioassay for the purpose of environmental regulation, artificial water cannot serve as a fully adequate replacement for natural water, the chemical composition of which is characterized by spatial heterogeneity.