Microplastic burden in Daphnia is aggravated by elevated temperatures

Contamination of freshwater habitats with microplastic is threatening particularly filter-feeders within the aquatic community. Using Daphnia magna and Daphnia pulex as models, the effects of food supply and temperature on the ingestion of polystyrene spheres (diameter 1 μm, concentration of 200 ng*ml^-1) was analysed. The ingestion rates of microplastic beads were increased in conditions of low food and high temperatures, reflecting the complex regulation patterns of the water current generated by the animals' thoracic limbs. Maximal enrichment of 1160 times the concentration in the ambient medium was observed within one hour. Analyses of the impact of microplastic ingestion on physiological parameters used the carbohydrate concentration as an indicator for the animals' metabolic state. Exposure to the microplastic beads for three days in the presence or absence of Desmodesmus subspicatus did not affect the animals' glycogen reserves beyond the response to the prevailing food and temperature conditions. Projecting the insights from laboratory experiments to the habitat situation, increased burdens of microplastic particles can be expected in filtering zooplankton organisms in warm water and scarce supply of food, like the clear-water phase of lakes in the summer.