Synergistic Effect of Chloroquine and Copper to the Euryhaline Rotifer Proales Similis

<title>Abstract</title> Chloroquine (CQ) has been widely used for many years against malaria and various viral diseases. Its important use and high potential to being persistent make it of particular concern for ecotoxicological studies. Here, we evaluated the toxicity of CQ alone and in combination with copper (Cu) to the euryhaline rotifer <italic>Proales similis</italic>. All experiments were carried out using chronic toxicity reproductive five‐day tests and an application factor (AF) of 0.05, 0.1, 0.3 and 0.5 by multiplying the 24-h LC₅₀ values of CQ (4250 µg/L) and Cu (68 µg/L). The rate of population increase (<italic>r</italic>, d^-1) ranged from 0.50 to 52 (controls); 0.19 to 0.39 (CQ); 0.09 to 0.42 (Cu); and -0.03 to 0.29 (CQ-Cu) and decreased significantly as the concentration of both chemicals in the medium increased. Almost all tested mixtures induced synergistic effects, mainly as the AF increased. We found that the presence of Cu intensifies the vulnerability of organisms to CQ and vice versa. These results stress the potential hazard that these combined chemicals may have on the aquatic systems. This research suggests that <italic>P. similis</italic> is sensitive to CQ as other standardized zooplankton species and may serve as a potential test species in the risk assessment of emerging pollutants in marine environments.