Life-stage dependent responses to microplastics and 2-methoxy-1,4-naphthoquinone in Daphnia magna

Multiple stressors threaten freshwater ecosystems, including microplastic pollution and invasive species. The invasive Himalayan balsam, Impatiens glandulifera, releases allelochemicals, such as 2-methoxy-1,4-naphthoquinone (2-MNQ), that can enter aquatic systems via runoff. While microplastics affect aquatic organisms and can alter the fate of chemical stressors, the effects of co-exposure with plant-derived allelochemicals remain largely unexplored. Using Daphnia magna, we investigated the single and combined effects of well-characterized microplastic fragments (PA 66 and PS; 500 and 1000 particles/mL) and 2-MNQ (0.75 mg/L) under chronic exposure, with kaolinite as a natural particle control. Ultraviolet-visible spectroscopy indicated that dissolved 2-MNQ concentrations declined over time in particle-free conditions, whereas the presence of particles influenced the detectable dissolved fraction. Biological responses differed across life stages. At primiparity, particle exposure reduced body length, whereas 2-MNQ and co-exposure showed no significant effects. In contrast, after 21 days, effects shifted towards 2-MNQ-containing treatments, which reduced body length and affected reproductive output, particularly during later broods. These findings demonstrate that the effects of microplastics and 2-MNQ are not static but shift over time, with particle-driven effects dominating early responses and combined effects becoming more relevant during prolonged exposure, potentially affecting long-term population dynamics.