Assessing the state of river plastic pollution.

Rivers play an important role in the global plastic cycle. They transport plastics from land, can store plastics for long periods of time, and export a fraction into the marine environment. Monitoring data is crucial to determine baseline pollution levels of rivers, characterize risk, and assess the impact of intervention measures. Methodological advances have been made to assess ecological risks of microplastics and the mass fluxes and storage of macroplastics. Despite increasing data availability on river plastic pollution, a comprehensive methodology to assess the overall state of river plastic pollution across size ranges and compartments is lacking. Here, we propose the Aquatic Plastic Index (API), a multimetric tool to assess the plastic pollution status of freshwater systems. We defined eight indicators across compartments (surface, water column, riverbank, sediment) and covering both microplastics (< 0.5 cm) and macroplastics (>= 0.5 cm). The API is calculated using impact-based threshold classes and takes into account both the annual average value and the percentage of observations in the worst class. We demonstrate the method by applying it to four rivers, two estuaries, and five urban water systems across the Netherlands. The state of plastic pollution shows strong temporal and spatial variability, with API scores ranging between 27 and 100 (on a scale from 0 = lowest/worst to 100 = highest/best). Our findings demonstrate that the API method can be used to quantify and compare the state of plastic pollution across locations, water bodies, and time. We show that the final API value can be driven by different indicators across systems and time. We found that despite an increase in observations and monitoring in recent years, the majority of the indicators could not be calculated for any given year. Indicators related to urban water systems and microplastics were less frequently quantified compared to other indicators. By identifying monitoring gaps and enabling consistent assessment, the API provides a foundation for application at larger geographic scales and for evaluating the urgency and effectiveness of reduction measures. Looking ahead, the API can also be adapted to incorporate policy developments, such as the establishment and refinement of micro- and macroplastic threshold concentrations in freshwater, including rivers, estuaries and urban water systems.