Micro- and nanoplastic effects on the reproduction of Daphnia spp. – a meta-analysis

Several micro- and nanoplastic particle (MNP) traits, like polymer type, size, and shape, have been shown to influence MNP toxicity. However, the direction and strength of these moderating effects are often unclear, and generalizations from single studies are challenging to establish. Meta-analyses increase generalizability and derive more accurate and precise effect size estimates by combining measurements from published studies. We conducted a meta-analysis to investigate the effects of MNP exposure on the reproductive output of water fleas of the genus Daphnia by aggregating 369 data points from 64 studies. We show that daphnids exposed to MNP produce, on average, 13.6 fewer neonates, a reduction of 20.8% compared with the particle-free controls (control mean = 65.37 neonates). This effect is moderated by MNP concentration, exposure duration, experimental temperature, and size category, with microplastics eliciting a stronger negative effect than nanoplastic particles. Shape category, species, age, polymer type, size (micrometers), fluorescence, modification type, presence of surfactant, and dissolved organic matter did not influence effect sizes significantly. Based on the high residual heterogeneity in the data, we suggest that additional factors likely influence observed effects and discuss how better particle characterization could improve our understanding of the drivers of MNP toxicity.