Effects of Multi-generational exposure of Daphnia magna to pesticides and environmental stress

Pesticide contamination and increased temperature due to climate change have caused environmental issues and added stress on non-target aquatic organisms. The frequent occurrence of stress factors has forced organisms to adapt. However, the co-occurrence of multiple stressors may compromise the adaptation to a single stressor and increase the sensitivity towards another stressor. In this study, Daphnia magna, a planktonic crustacean commonly used in ecology and ecotoxicology, was used as the test organism. We maintained Daphnia cultures under four conditions for 19 generations, i) benign temperature (20˚C), ii) benign temperature with 0.0316µg/L esfenvalerate, iii) an elevated temperature (25˚C) and, iv) elevated temperature with 0.0316µg/L esfenvalerate. Organisms from each culture setup were exposed to a range of esfenvalerate concentrations at 20 and 25°C for 24 h, and monitored for 21 days to analyze the individual and combined effects of the esfenvalerate and the increased temperature on single and multi-stress acclimated in comparison to control culture of Daphnia magna. We recorded the effect on 7d and 21d survival and reproduction. Furthermore, we predicted the combined effects of esfenvalerate and elevated temperatures in organisms acclimated to different stress conditions using the concentration addition model (CA), the effect addition model (EA), and the stress addition model (SAM). The setup cultured with esfenvalerate at 25˚C and exposed at 20˚C showed the highest LC50 value of 0.627 µg/L on the 7th day and a value of 0.474 µg/L on the 21st day. At the same time, the setup, reared under the same conditions but exposed at 25°C, showed a lower LC50 value of 0.255 µg/L on the 7th day and 0.287 µg/L on the 21st day. The setup, reared at 20˚C without esfenvalerate and exposed at 20˚C, had an LC50 higher (0.421 µg/Lon 7d and 0.455 µg/L 21d) as compared to the setup reared under the same conditions but exposed at 25°C (0.322 µg/L on 7d, 0.43 µg/L on 21d). The lowest LC50 recorded among the setups was observed in Daphnia reared at 20˚C with esfenvalerate but exposed at elevated temperature (0.134 µg/L on 7d and 0.17 µg/L on 21d). The culture with the same conditions but exposed to esfenvalerate only had a higher LC50 (0.491 µg/L on 7d and 0.565 on 21d). Moreover, the setup reared at 25˚C without esfenvalerate but exposed to both stressors recorded lower survival (0.206 µg/L on 7d and 0.179 µg/L on 21d) than the setup reared under the same conditions but exposed at 20˚C (0.489 µg/L on 7d and 0.614 µg/L on 21d). The results revealed that the Daphnia reared at 25˚C showed higher LC50 values as compared to the Daphnia reared at 20˚C, which induction of tolerance to stressors. The results revealed that cultures exposed to both stressors always showed lower survival than those under the same rearing conditions but exposed to one stressor only. The model deviation ratio (MDR) to test synergy between the stressors was the highest for the setup reared at 20˚C with esfenvalerate but exposed to both stressors (MDR= 3.573) using CA as a null model. A slight synergistic effect was observed for all setups exposed to both stressors except for the culture reared at 20˚C without esfenvalerate (MDR= 1.348). The Daphnia reared with esfenvalerate and exposed to elevated temperature were more sensitive than those under the same conditions but exposed to 20˚C. We infer that under multiple stress conditions, Daphnia adapted to single stress induces tradeoff processes; this increases the sensitivity to any additional stress. We observed the effect of stressors on reproduction, and our results showed a significant difference between Daphnia exposed to 20˚C and those exposed to 25˚C (p-value= 0.0031). The latter had a higher reproduction rate per individual due to early maturation and other temperature-caused effects.