Environmental factors affecting streambed microplastics in a non-perennial river catchment

Microplastics (MPs) represent an important threat to rivers and exhibit complex patterns of accumulation and transport that depend mainly on environmental characteristics, among which hydrology plays a key role. However, the effects of flow intermittence on MP transport and accumulation remain unexplored. In this study, the normalized abundance by total organic carbon (TOC) of the sediment as well as the characteristics (i.e. size and polymer type) of MPs present in sedimentation areas within intermittent and perennial reaches of a small river catchment were compared. The effects of other environmental variables known to affect MP abundance and characteristics were also considered, such as land cover, grain size distribution of the sediment, distance to the road and to the hydrological source of the sampled reaches. The results showed both a negative and positive correlation between TOC normalized MP abundance and median size of MPs with the distance to the nearest road, respectively. This was likely due to the presence of small illegal dumps located near roads, that led to localized hotspots of streambed MP pollution in nearby rivers. Flow intermittence affected the size of MPs, with a negative correlation between the median MP size and flow intermittence, but no effect was detected on TOC normalized MP abundance and polymer diversity. This study offers new insights regarding the effects of flow intermittence in combination with other factors on the abundance and characteristics of streambed MPs. • Distance from the road was the most significant factor influencing streambed microplastic pollution • Illegal dumps located in riverbanks were likely the main source of microplastics • Flow intermittence did not affect microplastic abundance and polymer diversity • Median microplastic size was lower in intermittent reaches than in perennial reaches