Using microplastic-carbon to reassess pollution level of microplastics in urban rivers

Environmental microplastics (MPs) are challenging to compare due to non-harmonized sampling and quantification methods. As MPs are predominantly composed of recalcitrant organic carbon (OC), they contribute to the total organic carbon (TOC) pool in environments. The concentration of recalcitrant carbon in microplastics (MPC) can theoretically serve as a complementary, standardized mass-based index to characterize MPs pollution levels. However, the occurrence of MPC in natural river environments remain poorly understood. This study conducted a basin-scale survey to investigate the spatiotemporal distribution of MPC in urbanized river water. Results show that the annual average MPC reaches 0.27 ± 0.12 mg/L (mean ± SD), peaking during the pre-flood season, and constituting a considerable portion of TOC throughout the year. The average MPs abundance was 7.1 ± 4.5 particles/L. Multiple statistical analyses revealed a significant positive linear relationship between MPC and the traditional features of MPs, such as shape, color, and polymer type (P < 0.001). Additionally, A linear mixed regression model using TOC and hydrological season information predicted MPC concentrations (conditional R² = 0.89). These findings provide baseline information on MPC in river water and highlight its potential as a complementary indicator for rapid MP pollution assessment, underscoring the need for further research on the role of MPC in the environmental carbon pool.