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Zooplankton are a conduit of energy from autotrophic phytoplankton to higher trophic levels, and they can be a primary point of entry of microplastics into the aquatic food chain. Investigating how zooplankton communities are affected by microplastic pollution is thus a key step toward understanding ecosystem‐level effects of these global and ubiquitous contaminants. Although the number of studies investigating the biological effects of microplastics has grown exponentially in the last decade, the majority have used controlled laboratory experiments to quantify the impacts of microplastics on individual species. Given that all organisms live in multispecies communities in nature, we used an outdoor 1130‐L mesocosm experiment to investigate the effects of microplastic exposure on natural assemblages of zooplankton. We endeavored to simulate an environmentally relevant exposure scenario by manually creating approximately 270 000 0.015 × 1– to 1.5–mm polyester fibers and inoculating mesocosms with zero, low (10 particles/L), and high (50 particles/L) concentrations. We recorded zooplankton abundance and community composition three times throughout the 12‐week study. We found no effect of microplastics on zooplankton abundance, Shannon diversity, or Pielou's evenness. Nonmetric multidimensional scaling plots also revealed no effects of microplastics on zooplankton community composition. Our study provides a necessary and realistic baseline on which future studies can build. Because numerous other stressors faced by zooplankton (e.g., food limitation, eutrophication, warming temperatures, pesticides) are likely to exacerbate the effects of microplastics, we caution against concluding that polyester microfibers will always have no effect on zooplankton communities. Instead, we encourage future studies to investigate the triple threats of habitat degradation, climate warming, and microplastic pollution on zooplankton community health. Environ Toxicol Chem 2024;43:418–428. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

Microplastics are a global contaminant of concern, but we have little information on the characteristics and bioavailability of these pollutants in western Canadian lakes. Here, we quantify and characterize microplastics in subsurface water and zooplankton from eight lakes in BC, Canada. By sampling water and zooplankton, we provide insight into the fraction of microplastics entering the food web. We found 0.607 ± 0.153 microplastics per litre in subsurface water, 0.01 ± 0.011 microplastics per copepod, and 0.02 ± 0.014 microplastics per Daphnia. Microplastic pollution was similar in all lakes sampled and showed no relationship with local population density. Fibers were the dominant morphology observed in all lakes, and Raman spectroscopy identified polyester as the dominant polymer found both in lakes and within zooplankton. Zooplankton generally ingested microplastics that were shorter than their body length and that fell on the smaller end of the range of available microplastics. The prominence of polyester fibers and PET films and fragments suggests that the likely sources of microplastics to these lakes are recreational activities and atmospheric deposition. [ABSTRACT FROM AUTHOR]