Hold me back! Does vegetation density shape the particle-specific fate of microplastics in stream mesocosms?

Rivers and streams are important conduits for microplastic particles from terrestrial sources to marine environments. Submerged macrophytes are expected to influence microplastic short-term retention by altering local hydrodynamics and trapping microplastics. However, their impact in longitudinal transport processes remains unclear. We investigated the fate of three particle types - polystyrene (PS), polyethylene terephthalate (PET), and tire wear particles (TWP) - within replicated 45 m stream mesocosms which varied in vegetation densities of the macrophyte Elodea nuttallii (comprising either 0%, 25%, or 100% density, n = 3). Following a controlled pulse release, microplastic concentrations were quantified in the water column, sediment, and macrophyte biomass. Retention within macrophytes was strongly particle-specific ( p = 0.007): TWP accumulated at the highest levels, with spatial variability being reduced under high vegetation density (T 100 ), whereas PET and PS consistently exhibited low accumulation across treatments. Apparent distance-decay rates revealed consistent particle-specific transport behaviors across water and sediment compartments. Streambed accumulation was significantly affected by particle type and distance ( p < 0.001), whereas vegetation density showed no significant overall effect ( p = 0.903). However, significant vegetation × distance and vegetation × particle interactions indicated that vegetation effects depended on stream position and particle identity. In both sediment and water samples, PS concentrations declined markedly with increasing distance, PET showed a moderate decrease and persisted downstream, while TWP remained constant across distance and compartments. These results indicate that particle-specific properties and transport distance dominate microplastic dynamics in streams, while vegetation modulates local retention depending on stream position and particle type. This highlights the function of small, vegetated streams with limited filtration capacity as temporary particle-specific sinks for microplastics. They may act as secondary sources of microplastics within fluvial networks, thereby affecting the timing and magnitude of downstream microplastic transport. Considering the substantial contribution of small streams to river networks in terms of total flow length, their capacity to temporarily retain and subsequently remobilize microplastics may have important implications for catchment-scale plastic budgets and in a wider context the transfer from terrestrial to marine environments.