The Effects of Microplastics on Zooplankton and Emerging Insect Communities in the Littoral Zone of a Boreal Lake

Abstract

Although microplastics research has rapidly accelerated in the last few years, our understanding of their effects on freshwater invertebrates, particularly under environmentally relevant conditions, remains limited. Using limnocorrals at the International Institute for Sustainable Development – Experimental Lakes Area (IISD-ELA), we investigated the in-situ effects of a microplastic mixture on zooplankton and emerging insect communities. We installed 12 open-bottom limnocorrals in the littoral zone of a boreal lake in June of 2022 and added an environmentally relevant range of microplastic concentrations in a regression-based design. Our microplastic mixture consisted of distinctively coloured polystyrene, polyethylene, and polyethylene terephthalate fragments in equal parts by count, manufactured with common plastic additives. We sampled the zooplankton community at weeks 1, 5, and 8 after microplastic addition and collected emerging insects every week for the 8-week duration of the experiment. To evaluate possible longer-term effects, we sampled the emerging insect community again in May of 2023 over the partially enclosed bases of the limnocorrals. We assessed the relationship between nominal microplastic concentration and the abundance, biomass, and community composition of zooplankton and insects, as well as the timing of insect emergence. We found some evidence that microplastic additions negatively influenced total zooplankton abundance at weeks 1 and 5, as well as total biomass at week 5. Cladocerans and cyclopoid copepods were negatively affected during the first and second half of the experiment, respectively. Microplastics had limited influence on zooplankton community composition shortly after additions, but not afterward. For emerging insects, we found weak evidence that the total abundance of insects emerged in 2022 was negatively affected by microplastic additions, and no evidence of effects on spring 2023 emergence. Microplastics did not influence insect community composition, nor did they affect the timing of emergence. Overall, our results suggest that current levels of microplastic pollution in freshwater environments may have only subtle adverse effects on zooplankton and aquatic insect emergence.