Spatiotemporal Patterns, Characteristics, and Ecological Risk of Microplastics in the Surface Waters of Shijiu Lake (Nanjing, China)

Microplastics (MPs) are pervasive in freshwater and may threaten aquatic ecosystem health. We surveyed surface waters of Shijiu Lake and its inflowing tributaries during the dry (January–March) and rainy (May–July) seasons of 2024. MP abundance ranged within 17.54–30.93 items/L, with higher values in the rainy than in the dry season (28.18 ± 6.03 vs. 24.53 ± 5.68 items/L; one-way ANOVA, p < 0.05). Abundance correlated positively with turbidity (r = 0.44; R2 = 0.20; p < 0.05), whereas associations with total nitrogen, total phosphorus, and suspended solids were not significant (p > 0.05). Small particles (38–75 μm) dominated and were slightly more prevalent in the dry season, while the fraction of larger particles (>150 μm) was relatively higher in the rainy season. Granules predominated across sites, but their share decreased in the rainy season, accompanied by a notable increase in fibers. The Pollution Load Index (PLI) indicated slight but spatially uneven pollution (PLI = 1.00–1.43), generally higher during the rainy season and consistently elevated at the lake center; the Nongkan River exhibited the lowest levels. Ecologically, the patterns indicate rainfall-driven inputs and hydrodynamic controls (runoff, resuspension, residence time), identifying the lake center and tributary interfaces as priority zones for monitoring and mitigation. These results provide lake-scale evidence to refine seasonal monitoring and inform source-reduction strategies in similar inland waters.