Cross-Level Responses of Freshwater Zooplankton to Micro/Nanoplastics: A Meta-Analysis Integrating Individual- to Community-Level Evidence

Despite the known threats of micro/nanoplastics (MNPs) to freshwater ecosystems, their risks to zooplankton remain poorly resolved because existing evidence is fragmented across biological levels. Consequently, associations between lower-level responses and population- and community-level outcomes remain unclear. Here, we synthesized 3042 research cases from controlled experiments worldwide covering 12 response categories. We found significant overall negative effects of MNPs in systems without predators, with predators, and with predator-conditioned medium. Although molecular-level evidence remains limited, a consistent hierarchical response pattern emerged under predator-absent conditions. We therefore propose a conceptual hierarchical effect pathway (HEP) framework integrating effect sizes and cross-level associations. Within this framework, MNPs exposure was associated with increased ingestion of MNPs, physiological damage, feeding impairment, and reduced activity and defense. These responses formed an upstream tier associated with life-history impairments, including impaired growth and development, reduced reproductive capacity, and reduced survival, although these associations do not establish temporal ordering or causation. Under predator-present conditions, negative population- and community-level effects were also observed. Effect magnitudes also varied with MNPs characteristics, exposure duration, and zooplankton group. Together, the HEP framework clarifies cross-level associations and helps identify informative endpoints and incorporate food-web context into risk assessment of MNPs.