Combined effects of polyethylene microplastics and natural stressors on Chironomus riparius life-history traits

Several studies have shown that ingestion of microplastics causes adverse effects in aquatic organisms, including sediment-dwelling invertebrates. Most studies focus on evaluating the effects of plastic particles alone without testing the mediating effects of different natural stressors and thus lacking realistic exposure scenarios. The present study addresses the interactive effects of exposure to polyethylene microplastics (PE-MPs; 2.5 g/kg) in the midge Chironomus riparius life history traits under different temperatures (15, 20 and 25 °C), a salinity gradient (0, 1 and 3 g L^-1 sodium chloride - NaCl) and different levels of food (0.5, 0.25 and 0.125 mg macerated fish food larva^-1day^-1). By the analyses of linear models and independent action models applied to different life-history traits, such as larval growth, development time and imagoes body weight, the present work reveals that under temperatures lower than 20 °C or severe food shortage (<0.25 mg macerated fish food larva^-1day^-1), microplastics' effects can be stronger than those observed at standard toxicity test conditions (20 °C and 0.5 mg food larva^-1day^-1). Additionally, we also found that, in general, toxicity induced by PE-MPs to C. riparius larvae was reduced under warmer temperature (25 °C) and salinity. As observed, MPs toxicity can be mediated by natural stressors, which underlines the importance of co-exposure studies. In this sense, these results contribute to a more accurate risk assessment of microplastics. Despite the complex interactions between microplastics and natural factors here tested, were not found evidence that the deleterious effects of PE-MPs on C. riparius life cycle history are aggravated under increased temperature, food shortage, or salinisation of freshwaters.