Ingestion and impacts of microplastics on the plankton food web in a whole-lake addition experiment

Despite the prevalence of microplastics in the environment, their fate and effects on aquatic ecosystems are largely unknown. At the International Institute for Sustainable Development - Experimental Lakes Area (IISD-ELA) in northwestern Ontario, Canada, a long-term study is being conducted involving three years of microplastic additions to an entire lake. This thesis uses data collected from the experimental lake, Lake 378, and a nearby unmanipulated lake, Lake 373, to examine how microplastics may affect the planktonic food web, including phytoplankton, zooplankton, and a dominant invertebrate predator, the dipteran Chaoborus spp.. In Chapter 2, the ingestion of microplastics by zooplankton and Chaoborus spp. in the experimental lake was investigated to determine how microplastic ingestion varied temporally, spatially and across taxa. On average, 0.007 ± 0.015 microplastics/zooplankton were encountered in the guts of zooplankton. Ingestion varied among zooplankton taxa and was closely related to concentrations in the water column, emphasizing that sampling in other systems should consider these variations. Differences between the microplastics ingested by crustacean zooplankton and Chaoborus spp. suggested minimal trophic transfer. As well, we contextualized planktonic organisms as a biotic compartment for microplastics and determined that few microplastics were encountered in zooplankton as a repository compared to other matrices. In Chapter 3, data from the experimental lake and reference lake collected during four baseline years and two post- addition years were used to assess the effects of microplastics on the planktonic community using a Before-After-Control-Impact (BACI) design. Changes to all three groups of organisms were observed relative to baseline levels and the reference lake suggesting microplastic pollution may have effects on these organisms in natural settings. Changes in the plankton community were relatively small and some may have begun prior to the initiation of microplastic additions. Analyses in subsequent years will help to further elucidate the microplastic impacts. Statistically significant decreases in individual lengths and overall population biomass of Chaoborus spp. were also observed in the second year of additions (2024), suggesting a potential time lag. This whole-lake manipulation experiment is the first of its kind to use microplastics and will inform industry and government level policies to mitigate microplastic pollution.