Microplastics and the freshwater plankton: Effects on grazing and mortality

When studying the effects of microplastics (MP) on zooplankton in freshwater environments, there is a knowledge gap at the community level, as most studies use monocultures under laboratory conditions with limited comparability to field studies. The aim of our study was to investigate the uptake of MP at environmentally relevant concentrations by a natural zooplankton community feeding on native phytoplankton. Rotifers and cladocerans comprised more than 96 % of total zooplankton community, while nauplii and copepodites dominated the copepod community. To test the possible change in phytoplankton grazing and zooplankton mortality after MP exposure, zooplankton were exposed to three different polyethylene MP sizes: A (1-5 µm), B (27-32 µm), and C (45-53 µm) during 96 hours. Ingestion in group A was observed in 34 % of rotifers, 20 % of cladocerans and 67 % of copepods. In group B, only cladocerans ingested MP, and to a much lower extent (2 %), which could be due to the composition of the community. None of the zooplankton studied ingested MP particles in group C. The ingestion of MP decreased phytoplankton consumption, and phytoplankton abundance varied greatly between samples. The highest phytoplankton abundance was observed in the A samples with the highest proportion of ingested MP, indicating a significant reduction in grazing pressure that allowed phytoplankton to proliferate. This result, combined with the highest mortality of cladocerans in the A samples (12.7 ± 0.91 %), indicates the negative impact of MP on the normal functioning of the freshwater plankton food web.