Ingestion of microplastics by meiobenthic communities in small-scale microcosm experiments

Microplastics have been detected in many different environments. Nematodes are a rife meiofaunal taxon and occupy an important trophic position in benthic food webs. Laboratory-based ingestion experiments have demonstrated the susceptibility of single nematode species to microplastic uptake. However, the determinants of ingestion by meiofaunal assemblages, especially those of nematodes, have yet to be fully examined. We therefore conducted a microcosm study in which field-collected freshwater sediment was spiked with fluorescent polystyrene (PS) beads (1.0, 3.0 and 6.0 μm) in concentrations of 10³ and 10⁷ PS beads ml^-1 and the ingestion by the most dominant indigenous meiofaunal taxa (nematodes, rotifers, chironomids, copepods) was investigated after 2, 4 and 8 days using fluorescence microscopy. In additional small-scale microcosms, PS bead ingestion by nematode assemblages was quantified as a function of feeding type, exposure time (1-10 days), concentration (10³, 10⁵, 10⁷ PS beads ml^-1) and bead size (0.5, 1.0, 3.0, 6.0 μm). PS beads at 10⁷ beads ml^-1 were largely ingested by chironomids and copepods. Exposure time and concentration correlated positively with PS bead ingestion for all taxa. The most relevant size class for ingestion for the majority of meiofaunal taxa was PS beads of 1.0 μm. Nematode communities, especially deposit-feeding species, effectively ingested micropastics from sediment, as >30% of the exposed individuals and 56% of the species ingested 1.0-μm PS beads in <24 h. Ingestion rates were mainly influenced by PS bead size and nematode feeding type/habit, with the exception of a bead concentration of 10³ beads ml^-1, at which exposure time was also an important factor. Sediment particles reduced microplastic ingestion considerably for all investigated meiobenthic organisms. Our study demonstrates the ability of free-living nematodes communities to readily ingest PS beads of various sizes. If the feeding-type distribution is known, the potential exposure of nematode communities may be predicted.