Limited evidence of elevated microplastics in lower trophic levels downstream of wastewater treatment plants

Wastewater treatment plants perform an important role in removing biological oxygen demand, nutrients, and some contaminants from municipal wastewaters, yet they are suggested to be a major contributor of microplastics (plastic particles ≤ 5 mm) to aquatic systems. Once there, microplastics may accumulate in sediments and biofilms, therefore posing a risk to benthic macroinvertebrates. This study examined the abundance and characteristics of microplastics in municipal wastewater effluents, as well as sediments, biofilms, and three families of aquatic insects to assess the source of microplastics and potential exposures to primary consumers. Effluent samples were collected from eight wastewater treatment plants (sites) in the Grand River watershed, Ontario, Canada, and sediment, biofilm and aquatic insects (Hydropsychidae, Heptageniidae, Chironomidae) were taken upstream and downstream of each facility. Effluents were filtered, sediments were density separated with CaCl2, biofilm and aquatic insects were digested using H2O2, and microplastics were counted and chemically confirmed using Fourier transform infrared spectroscopy. Fibres were the most common morphology across pooled sample types (94% in effluent, 87% in sediment, 68% in biofilm, 75% in Hydropsychidae, 90% in Heptageniidae and Chironomidae) and were mainly organic material and polyester. Across the treatment plants, effluents ranged from 14.36-50.5 microplastics per L. Higher microplastic abundances were found at four and five of eight downstream locations for biofilm and sediment, respectively. For insect families Hydropsychidae and Heptageniidae, abundances were higher downstream of three of eight sites. No differences were found between up- and downstream sites for Chironomidae. No significant correlations were observed between microplastic abundances in basal resources and aquatic insect consumers. Overall, results suggest that wastewater treatment plants increased microplastics in basal resources (biofilm and sediment) in some downstream areas, but there was little evidence of increased uptake in effluent-exposed consumers.