Microplastic fibres as active agents in mud deposition.

Clay-rich sediments are essential to the structure and function of freshwater and transitional ecosystems, affecting turbidity, stabilising channels, and providing important benthic habitats. Microplastics are widely distributed in these environments and have traditionally been seen as passive pollutant carriers, despite their well-known ability to interact with clay minerals. In this study, we explore whether microplastics can actively contribute to mud accumulation by altering the aggregation and settling behaviour of clay particles. Using natural river sediments, we compare flocculation and settling dynamics before and after adding microplastic fibres of different polymer types. Our findings indicate that microplastics facilitate the formation of larger clay flocs compared to those in microplastic-free mud, leading to increased floc settling velocities. Collectively, these processes significantly enhance mud deposition, suggesting that, beyond their recognised role as pollutant carriers, microplastics may exert a broader and previously underestimated influence on natural sedimentary dynamics and associated biomorphodynamic processes.