Impact of imidacloprid on Daphnia magna under different food quality regimes

This study evaluated the combined effects of imidacloprid and algae nutritional quality on Daphnia magna. Since biophysical conditions are constantly in flux in the natural environment, other factors besides pesticide concentrations can impact toxicity. D. magna neonates were exposed to nominal imidacloprid concentrations of (with the time-weighted average in brackets) 1.8 [2.0] mg/L, 25 [27.6] mg/L, 45 [44.6±3.1] mg/L, 60 [66.3] mg/L, 85 [94.0±2.5] mg/L, and 130 [158.0±6.5] mg/L for 21 days, with the test medium renewed every three days. The exposure levels were chosen based on reported NOEC and EC50 immobility endpoints. Along with varying imidacloprid levels, the daphnids were provided food with differing phosphorous levels to assess the effects of lower-quality nutrition. Results showed that mortality increased with increasing imidacloprid concentration, and that adverse effects increased with decreasing food quality. Imidacloprid affected the somatic growth rate at all concentrations, although the 2.0 mg/L treatment was negligibly different from the control. The diet containing the least phosphorous (C:P 1300) showed the lowest effective concentration for mortality after day seven of exposure, indicating that lower quality food can increase the effects of imidacloprid toxicity on D. magna. Reproduction only occurred at control and 2.0 mg/L imidacloprid concentration, indicating disruption at the higher exposure levels. These results are important because toxicity testing is typically carried out in controlled conditions that do not account for changes like food quality. Nutrient quality is often variable in natural conditions due to changes in fertilizer applications, precipitation, and other factors. The results of this study suggest that imidacloprid may have more pronounced effects in other resource-limited aquatic invertebrates, especially those that are more sensitive than D. magna and this should be accounted for in risk assessments.