Macroinvertebrate ecosystem engineering affects streambed retention of microplastics

Microplastic pollution of aquatic environments threatens human health, ecosystem processes, and biodiversity. Many existing models of microplastic movement in streams do not account for biotic effects on microplastic fate. Ecosystem engineering by net-spinning caddisflies (Hydropsychidae) has been shown to substantially affect sediment and organic matter transport as well as streambed hydrology. Caddisfly engineering may likewise affect the movement of microplastic pollution in streams. We used a controlled 11-d flume experiment to investigate the potential for caddisflies to serve as a biotic control on microplastic transport. Flumes containing a single gravel dune were randomly assigned to density treatments: control (0 caddisflies/m2) or stocked with 500, 800, or 2500 caddisflies/m2, incubated (d 1–10) to allow for caddisfly silk structure construction, inoculated (d 11) with PVC microplastics (333 µm–1 mm), and sampled (d 12). Microplastic was quantified as caught in a drift net (downstream transport), eaten by caddisflies (ingestion), or captured in caddisfly silk structures or settled into the gravel dune (i.e., total streambed retention). Mean downstream plastic transport was 9% lower than the control in the 800 caddisflies/m2 treatment and 10% lower in the 2500 caddisflies/m2 treatment (p < 0.001 and p = 0.003, respectively). Mean total streambed retention was 9% higher than control in the 800 caddisflies/m2 treatment (p < 0.001) and 910% higher in the 2500 caddisflies/m2 treatment (p = 0.004). Ingestion of plastic by caddisflies was rare and highly variable (0–0.55% of plastic particles) but did increase with caddisfly density (p = 0.002). This work represents one of the first investigations of animal ecosystem engineering as a control on the movement and fate of microplastic particles in fresh waters and establishes a foundation for future research on biotic control of microplastic transport. Our results suggest that ecosystem engineering by net-spinning caddisflies may serve as a biotic control of microplastic transport in freshwater streams.