Microplastic pollution differences in freshwater river according to stream order: Insights from spatial distribution, annual load, and ecological assessment

Microplastic (MP) pollution has become a pressing concern in global freshwater ecosystems because rivers serve as essential channels for the transport of terrestrial debris to the ocean. The current researches mostly focus on the large catchments, but the impact on the small catchments remains underexplored. In this study, we employed Strahler's stream order classification to delineate the catchment structure of the Beijiang River in South China. The distribution pattern of MP contamination and the factors influencing the distribution pattern, were assessed across the streams at different orders. We found that the Beijiang River was moderately polluted compare to other rivers in China, with an average MP abundance of 2.15 ± 1.65 items/L. MP abundance ranged from 3.17 to 1.45 items/L in the streams at different orders, and significantly decreased with increasing stream order (R = 0.93). This highlights the key role of small rivers as the channels for the transport of MPs from watersheds to main streams. The high abundance of PP and PE fibers, the high correlation between the stream order and the resin proportion (R = 0.89), and the significant correlation between MP abundance and proximity to urban centers (P = 0.02), indicated that MP pollution across the streams at different orders was predominantly influenced by anthropogenic activities, rather than natural environmental factors. By integrating MP data with hydrographic information, the annual MP loads for the streams at Orders 1 to Order 5 were estimated to be 4.63, 39.38, 204.63, 503.06, and 1137.88 tons/yr, respectively. Additionally, an ecological risk assessment indicates that MP pollution led to a low risk in the Beijiang River. Our findings deepen the understanding of MP pollution within freshwater river networks, and emphasize the crucial role of tributary systems in transporting MPs to main river channels.