Feeding type and microplastic particle concentration drive the ingestion of polystyrene beads in a natural zooplankton community

The mismanagement of plastic waste in recent decades has led to global plastic pollution problem. Much of this plastic waste in the environment degrades into small particles, forming microplastics and increasing their abundance in ecosystems. These microplastics can be ingested by various microinvertebrates such as zooplankton. Using a microcosm experiment with a near-natural zooplankton community, we tested whether microplastics particle size and concentration affect the proportion of individuals ingesting polystyrene beads. Zooplankton were exposed to three bead sizes (1, 3, and 6 µm) at two concentrations (10 3 and 10 6 particles mL⁻ 1 ). Ingestion was assessed as the presence or absence of beads within individual organisms. As all beads sizes were within the ingestible range, the experiment was designed to evaluate size-dependent differences in ingestion probability rather than physical ingestion limit. At low concentrations, 40% of the cladocerans ingested microplastics, which was a higher percentage than copepods, rotifers and ostracods; suggesting that their unselective filter-feeding mode facilitates ingestion. At high microplastics concentrations, these were ingested by more than 70% of the organisms across all experimental groups, with only moderate differences in the proportion of individuals ingesting microplastics among rotifers, cladocerans, ostracods and copepods. These high percentages indicate that even raptorial feeders cannot avoid ingesting plastic at elevated concentrations. The size of the microplastics had no effect on their ingestibility. Overall, our near-natural experiment demonstrated that both feeding type and beads concentration significantly influenced microplastics ingestion by zooplankton.