Seasonal rainfall and land-use impacts on microplastic characteristics in an endangered salmon stream

Microplastics (MPs), increasingly common in freshwater ecosystems, pose serious ecological threats to the Formosan landlocked salmon (Oncorhynchus masou formosanus), an endangered salmonid endemic to Taiwan. This study presents the first comprehensive investigation into how seasonal variation and land-use patterns influence MP abundance, composition, and distribution in the salmon's exclusive habitats within Shei-Pa National Park, Taiwan. Using µ‑FTIR spectroscopy and fluorescence microscopy, we quantified microplastic concentrations and observed higher levels in the dry season (48-93 items/L) than in the wet season (45-72 items/L). Principal component analysis (PCA) further indicated spatial gradients aligned with land‑use contrasts, with higher concentrations associated with intensive agriculture and recreational tourism relative to pristine forest areas. Seasonal variation profoundly influenced MPs' composition, with synthetic fibers (Rayon and Polyester) predominating in the wet season and common plastic polymers (PET, PE, PP) increasing in the dry season. Smaller-sized MPs (< 25 µm), potentially more hazardous due to their capacity for trophic transfer, were predominant in upstream reaches during critical salmon breeding periods.