Using Weibull-derived λ-α parameters to diagnose physical processes controlling microplastic size dynamics in...

To elucidate the spatiotemporal evolution of microplastic particle size distributions and their controlling mechanisms in estuarine-coastal regions, this study conducted continuous monitoring in Tianjin coastal waters from 2021 to 2025. By coupling microplastic abundance and particle size analysis with Weibull distribution fitting to extract the rate parameter λ and shape parameter α, the impacts of sedimentation, fragmentation, and riverine input were investigated. The results showed a significant decline in both λ and α at all sites during 2021-2022, coinciding with extreme rainfall (978.4 mm) in Tianjin in 2021, indicating a pronounced riverine input pulse. After 2022, sites exhibited regional differentiation: northern nearshore sites showed sustained concurrent declines in λ and α (continuous input dominance); central sites exhibited stable low-level fluctuations (input-sedimentation balance); and certain sites displayed transient concurrent increases (episodic sedimentation dominance). Spatially, the multi-year mean λ increased with water depth, reflecting preferential settling of large particles. Notably, the concurrent decline in λ and α was accompanied by decreasing abundance at most sites, indicating that input particles became larger while total input flux decreased. This pattern may reflect the initial effectiveness of regional plastic control measures. This study demonstrates that the λ-α dual-parameter combination can effectively distinguish physical processes controlling microplastic size distributions, providing a new diagnostic tool for source identification and process attribution of microplastics in coastal waters.