Land-to-sea microplastic inputs via creeks, wastewater treatment plants, and atmospheric deposition: An integrated assessment in an urbanized semi-enclosed bay.

Although concern over land-based microplastic transport to the ocean is increasing, most studies have focused on individual pathways, leading to a fragmented understanding and highlighting the need for comprehensive assessments. Here, we conducted an integrated year-round assessment of microplastic inputs via creeks (seasonally), wastewater treatment plants (WWTPs, seasonally), and atmospheric deposition (monthly) to an urbanized coastal bay, aimed at characterizing seasonal variations in abundance, polymer type, size, and shape, and quantifying annual inputs and relative contributions. Annual microplastic input was approximately 1.49 trillion particles. Non-point sources, i.e., atmospheric deposition (46%) and creeks (22%), contributed more than point sources, i.e., WWTP effluents (32%). Creek inputs peaked in summer due to increased streamflow from rainfall, whereas atmospheric deposition was highest in spring and winter, influenced by seasonal wind direction and intensity. In contrast, WWTP inputs remained relatively stable year-round, reflecting consistent influent characteristics. Although overall microplastic characteristics were broadly similar, each pathway exhibited distinct features: tire wear and primary microplastics were more abundant in creek and atmospheric samples, while fibers and < 50 μm particles were enriched in WWTP effluent and atmospheric deposition. These findings provide an integrated understanding of pathway-specific microplastic pollution dynamics and characteristics, offering insights for future monitoring and management strategies to mitigate transport from land to sea.