Exploring the effects of plastic pollution on benthic macroinvertebrates in a stream mesocosm experiment

Most plastic pollution is hypothesized to be transported through urban rivers toward sinks like lakes and oceans, yet its effect on ecological communities within these systems remains poorly understood. Additionally, we do not know how changes in flow alter the exposure landscape of biota to plastic-potentially impacting effects. Here, we conducted a 28-day flow-through stream mesocosm experiment (length: 2 m, width: 0.4 m, depth: 0.3 m; N = 8, n = 2 replicate streams per treatment) to investigate the effects of plastic pollution and stream flow on the density, emergence, diversity, and function of benthic macroinvertebrates. We exposed a natural community of benthic macroinvertebrates to a single environmentally relevant mixture and concentration of microplastic (< 5 mm) and macroplastics (> 5 mm) under a high- and low-flow regime. To simulate storm-driven pulses of plastic common in urban rivers, we added additional microplastics and increased stream flow on Day 16. We deployed leaf litter bags within each mesocosm to measure macroinvertebrate density, diversity, and leaf litter decay. We also measured microplastic fate in three environmental compartments: the water column, sediment, and leaf litter. We observed significantly less benthic macroinvertebrates, a greater emergence of Ephemeroptera, and increased leaf litter decay within experimental streams exposed to plastic compared to controls, indicating both direct and indirect negative effects of plastic exposure. While high- and low-flow regimes and the simulated storm affected the fate of microplastic within streams, altering exposure, they did not lead to consistent effects. These findings suggest that mixture and concentrations of plastic pollution reported in urban rivers may be impacting resident macroinvertebrates.