Coexistence of Microplastics and Heavy Metals in Lake Sediments: Interaction Mechanisms and Complex Ecological Risks

Lake sediments act as important sinks for heavy metals and microplastics, yet the mechanisms governing their enrichment, coexistence, and combined ecological risks remain insufficiently understood. This study investigated sediments from Daihai Lake, China, systematically characterizing the occurrence features of microplastics (morphology, size, and composition) and their associated heavy metal contents. A composite pollution risk framework (Multi Feature Potential Ecological Risk Index) was developed to evaluate microplastics–heavy metals interactions using correlation analysis, principal component analysis, and cluster analysis. In addition, a two-dimensional pollution index was applied to assess combined ecological risks. Results showed that MP abundance ranged from 6.60 to 26.80 n·g−1, with a decreasing trend from southwest to northeast. Microplastics were dominated by fragments, with a high proportion of small particles (<0.25 mm), and were mainly composed of polyethylene terephthalate and polypropylene. The average concentrations of heavy metals in sediments were ranked as follows: Mn (863 ± 78 mg·kg−1); Cr (124 ± 28 mg·kg−1); Zn (86 ± 17 mg·kg−1); Ni (43 ± 10 mg·kg−1); Cu (36 ± 0.1 mg·kg−1); Pb (23 ± 5 mg·kg−1); As (15 ± 4 mg·kg−1); and Cd (0.20 ± 0.04 mg·kg−1). The two-dimensional comprehensive index values ranged from 124 to 1032, with an average value of 365.0, exceeding the risk threshold (>100). Approximately 70% of sampling sites exhibited high composite pollution risks. Small-sized and fibrous microplastics showed significant positive correlations with multiple heavy metals, indicating strong carrier effects.