Variation in microplastic assemblages in streams across multiple spatial scales

Microplastic concentrations and composition in freshwater systems are spatially variable, but we are just beginning to understand the nature of that variation and what drives it. Previous attempts to explain microplastic distribution in streams have mostly focused on coarse spatial scales, such as catchment-level parameters. To broaden our understanding of multi-scale patterns in microplastics, we measured the spatial variation of microplastic assemblages in stream sediments across a range of spatial scales (10 m to 6 km). Microplastic concentrations ranged from 45 to 2139 particles/kg dry sediment. Variation in assemblages was high even at small spatial scales and geographical distance between sampling points did not explain between-sample variation in assemblages, suggesting processes operating at very fine spatial scales are likely responsible for microplastic assemblage composition. Physical attributes of the stream were better predictors of microplastic distribution than land-use parameters. Water velocity most strongly influenced microplastic accumulation in stream sediments, with concentrations of plastic fragments increasing about 4 fold with decreasing water velocity. Fibers tended to accumulate in sediments with smaller grain sizes. Our results show that factors operating on a smaller spatial scale than typically considered might be important predictors of microplastic abundance and composition in streams.