Microplastic Pollution in China’s Inland Waters Surpasses Coastal Seas under Anthropogenic Pressures

Microplastics are emerging as a global environmental challenge, yet the role of inland waters in shaping their distribution and risks has remained largely overlooked. By synthesizing over 13,000 records from 500 studies across China’s 10 river basins and 4 seas, we show that inland waters are not only conduits but also dominant sinks─rivers exhibit 0.9–7.7 MP L–1 and reservoirs exhibit 1.2–8.3 MP L–1, both significantly exceeding adjacent seas (0.1–2.2 MP L–1). Fibers and fragments dominate (>60%) morphological profiles, while polymer compositions mirror intensive human activities, linking urban emissions and agricultural plastic use directly to freshwater burdens. Machine learning (R2 = 0.85) reveals that population density and night-time lights are the strongest predictors of abundance, surpassing climatic and geographic variables, underscoring the primacy of anthropogenic pressures in shaping spatial heterogeneity. Ecological risk assessments further demonstrate that 33.1% of freshwater samples exceed medium-risk thresholds (RQ > 0.1), and 2.4% reach high risk (RQ ≥ 1), in stark contrast to uniformly low-risk marine sites. These findings challenge the ocean-centric paradigm of plastic pollution, positioning inland waters as critical hotspots of accumulation and risk, and highlight the urgent need for integrated, source-to-sink management strategies across the land–freshwater–ocean continuum.