A practical checklist for where, when, and what to measure - A systematic review of ten years of laboratory research on microplastic transport in rivers and transitional zones

Microplastic pollution in freshwater is a global concern, yet field monitoring often lacks standardization, hindering data comparability and analysis. This systematic review bridges the gap between lab findings and field sampling by analyzing 173 peer-reviewed laboratory studies on freshwater systems, published in English and identified through Web of Science up to August 2024, to extract critical processes affecting microplastic transport and to provide recommendations on where, when, and what to sample in rivers and transitional zones. We highlight priority locations such as salinity gradients, biologically active zones, and hydrodynamically diverse areas. Temporal guidance includes the duration of hydrodynamic and biological processes, study objectives (transport vs. retention), and sudden environmental disturbances such as floods or algal bloom collapses. For particle characterization, we evaluate shape descriptors and recommend recording at least the three principal dimensions, as they can be translated into quantitative shape descriptors relevant for modelling. Finally, we present a practical checklist outlining essential measurements for microplastics, water, sediment, additional substances, and surrounding conditions. Incorporating these insights into future monitoring campaigns will improve the ecological relevance and interpretability of microplastic monitoring.