Macroplastic in urban waterways: assessing storage in Berlin's Spree River.

Over the past decade, awareness of macroplastic (MAP) pollution in rivers as a major contributor to marine litter has increased substantially. However, in urban river environments, the storage and retention dynamics of macroplastics on riverbanks remain poorly understood. This study addresses this gap by investigating MAP (and other anthropogenic material) retention across distinct compartments of the Spree River in Berlin (Germany), including riverbanks with varying substrate types, aquatic-terrestrial interfaces, and the river channel. The analysis of plastic debris identifies dominant categories and their polymer compositions. Our results indicate that in the highly engineered urban section of the Spree River, characterized by embankments 1-3 m high, MAP exchange driven by surface runoff and wind is predominantly unidirectional, from riverbanks toward the channel. These embankments act as discontinuities that inhibit reverse transport from the channel to the banks, a process commonly observed in natural rivers, and likely contribute to the distinct MAP densities and compositions observed along the riverbanks. Although direct littering cannot be excluded, the observed spatial patterns suggest that lateral transport from riverbanks represents a dominant contribution to plastics in the active channel. Polypropylene was the most prevalent polymer, accounting for 41% of all collected items, while food wrappers emerged as the dominant debris category, comprising nearly 35% of the total macroplastic load. These findings highlight the critical need to understand macroplastic retention processes in urban rivers to support the design of targeted, compartment-specific mitigation and clean-up strategies.