Microplastics reduce population viability without shifting clonal structure in Daphnia pulex

total MP mass), representing experimentally elevated concentrations exceeding freshwater reports. PS-MPs significantly reduced total population density in two of three clone pairs, driven by reduced adult abundance and, in a pair-dependent manner, lower juvenile density, without changes in reproductive metrics (proportion of gravid females or clutch size). Despite these demographic effects, clonal proportions and competitive hierarchies remained stable, suggesting limited selective pressure to alter genotype frequencies within 30 days. These findings indicate that MPs do not readily drive genotype turnover over short ecological timescales under competitive, density-dependent conditions and instead reveal gradual erosion of population performance, characterised by increased adult losses, pair-specific juvenile reductions, and mild reproductive desynchronisation. Even experimentally elevated short-term MP exposure impaired demographic performance without inducing rapid evolutionary sorting. Chronic or multigenerational experiments under fluctuating environmental stress may be required to determine whether such demographic effects translate into evolutionary change in genetically diverse zooplankton populations.