Size-dependent impacts from polystyrene micro- and nanoplastics on freshwater invertebrates: A mesocosm study combining environmental DNA metabarcoding and morphological identification

Micro- and nanoplastics (MNPs) are widespread in freshwater ecosystems worldwide. While numerous studies have demonstrated that exposure to MNPs may induce adverse effects at the level of individual organisms, their potential long-term impacts at the level of communities remain poorly understood. The current study comprised a 14-week outdoor mesocosm experiment in which naturally established communities of freshwater invertebrates were exposed to spherical europium-doped polystyrene (PS) particles (15 µm and 150 nm) at concentrations resembling the higher end of those reported for natural surface waters. Community-level responses were assessed by morphological identification of macroinvertebrates and emerging insects, and by environmental DNA (eDNA) metabarcoding. Morphological assessments revealed no significant treatment effects. However, eDNA metabarcoding revealed subtle but significant responses, including a temporary reduction in macroinvertebrate taxa richness and distinct community shifts in the 15 µm treatment within two weeks after exposure. These impacts were transient, with communities recovering before the end of the experiment, despite water column MNP concentrations remaining at ∼50 % of nominally applied concentrations. Our results indicate transient impacts of PS MNPs under the tested conditions, and thereby highlight the suitability of eDNA metabarcoding as a tool for detecting higher-tier impacts from stressors to which community-level responses may be subtle.