Pharmaceutical-induced behavioural changes in freshwater species and mesocosm insights into potential population-level consequences

Pharmaceutical contamination in aquatic environments, particularly from neuroactive drugs like antidepressants, potentially affects the behaviour of non-target organisms due to their effects on conserved neuroendocrine systems. Behavioural endpoints are increasingly investigated for risk assessment. However, their use in regulatory decision-making depends on their relevance to consequences at the population level. This research aims to bridge the gap between behavioural effects and their implications for species' populations. We investigated the effects of long-term exposure to environmentally relevant concentrations of two pharmaceuticals (0.001-100 µg/L carbamazepine and 0.01-100 µg/L citalopram) on two freshwater model organisms, the amphipod shrimp Gammarus pulex and the snail Lymnaea stagnalis, within a mesocosm setting. This approach enabled us to assess population-level responses in a natural setting. Additionally, we investigated the organisms' speed, acceleration, curvature, thigmotaxis and responses to light stimuli of individuals exposed in the mesocosms via video tracking in a laboratory setting. Our findings indicate that long-term exposure to environmentally relevant concentrations of carbamazepine and citalopram does not lead to behavioural changes in G. pulex. Its control mortality in the mesocosms was, however, substantial, most likely due to increased pH levels. This does, however, affect the reliability of the behavioural data for G. pulex in the carbamazepine treatment. On the other hand, carbamazepine and citalopram exposure can lead to minor alterations in the locomotion of L. stagnalis, particularly under citalopram exposure. These behavioural changes did not result in detectable population-level effects. This suggests that while behavioural endpoints may be sensitive indicators of pharmaceutical exposure, their ecological relevance remains uncertain under realistic environmental conditions. The findings underscore the importance of further research to connect subtle behavioural effects to population dynamics and ecosystem health, supporting more informed ecological risk assessments of pharmaceutical pollutants.