The combined effect of temperature and microplastics on Daphnia pulex Leydig, 1860 (Cladocera)

Microplastic contamination of freshwaters is on the rise due to anthropogenic activities. As a result, adverse effects on aquatic organisms, including cladocerans, have been observed. Cladocerans are widely used and recommended as bioassay organisms in ecotoxicological tests or as bioindicators of water pollution. Their generalist feeding habits make them capable of consuming microplastics. Here we exposed Daphnia pulex to two stressors, 30 µm microplastic beads and temperature (20 and 25 °C) and observed effects in terms of acute and sub-acute tests (LC50, filtration rates, and cardiac rhythm) and chronic tests (population growth and life table) for two generations. We hypothesized that the stressors would have a more pronounced negative effect in the F1 compared to the F0 generation. The adverse effects of microplastics were more evident in the acute toxicity tests than chronic toxicity tests and at 25 °C rather than 20 °C. Adult cladocerans consumed (in 20 min.) between 50 and 200 beads ind.-1, juveniles between 20 and 150 beads ind.-1, and neonates between 0 and 75 beads ind.-1. There was more than a 50% decrease in the ingestion rate and cardiac rhythm of the cladocerans in the presence of microplastics. Population growth rates were higher in the presence of microplastics due to greater and earlier investment in offspring production. We did not observe adverse effects of the 30 µm plastic beads on the survivorship-related variables of D. pulex. Further studies are needed to test the impact of this stressor over several generations to determine the effects of microplastics in food webs in contaminated aquatic ecosystems.