Dynamics of microplastics in urban rivers under varying hydrological regimes

Despite its importance in limnological studies, hydrological seasonality has been underappreciated in studies on microplastics (MPs). In this study, water and sediment samples containing MPs were collected from five different points along an urban river at eight different times throughout the year. The concentration of MPs varied between 0.14 and 6.29 MPs m^-3 in water and between 26.09 and 679 MPs kg^-1 in sediments. The daily transport of MPs in water exhibited a clear spatiotemporal pattern, with the highest values during the wet period, when inputs from artificial areas were the main source of MPs. Fibers dominated both water and sediment samples (65 %), whereas spheres (15.1 %) were exclusively present in sediments, and films (9.5 %) were found only in water. Low-density polymers, such as polyethylene (35.6 %) and polypropylene (10.6 %), were more prevalent in water, whereas high-density polymers, including polyester (35.6 %) or rayon, were predominant in sediments. Single-time samplings were inadequate for the assessment of MPs, and the role of artificial areas as a source of MPs varied across the year, being more important in wet periods. Scheduled multisampling in alignment with hydrological regimes is imperative for the accurate quantification of the daily transport of MPs. This methodological approach facilitates relatively more reliable comparisons between different rivers and within the same river over time.