Microplastics reduce population viability without shifting clonal structure in Daphnia pulex

Microplastics (MPs) are widespread aquatic pollutants with the potential to erode population viability and influence evolutionary processes, yet their effects on intraspecific competition and genotypes sorting in zooplankton remain poorly understood. Here, we present the first long-term experiment testing how MPs affect clonal composition and demographic structure in Daphnia pulex, a keystone freshwater herbivore. Mixed-habitat populations assembled from three genotype pairs (village vs. forest) were exposed for 30 days, under moderate food limitation, to a binary mixture of 1 µm and 25 µm polystyrene (PS) spheres at 8.0 × 10⁵ and 8.0 × 10 ³ particles L⁻¹ (∼72 µg L^-1 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.