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 towards 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 multi-species communities in nature, here we use an outdoor 1130L 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 ~270,000 0.015mm x 1-1.5mm polyester fibers and inoculating mesocosms with zero, low (10 particles/L) or 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. NMDS plots also revealed no effects of microplastics on zooplankton community composition. Our study provides a necessary and realistic baseline upon 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.

We conducted an outdoor mesocosm experiment with 15 tanks. 5 tanks received no microplastics, 5 tanks received 10 particles/L, and 5 tanks received 50 particles/L. We sampled zooplankton communities three times for 12 weeks. We measured total zooplankton, Shannon diversity, and Pielou's evenness.