Transport and retention of microplastic fibers in streams are impacted by benthic algae, discharge, and substrate

Abstract

Microplastics (particles < 5 mm) are pollutants of emerging concern in aquatic ecosystems worldwide. Streams are key sites of microplastic input, retention, and transport, and empirical measurements of microplastic movement in lotic ecosystems are needed to inform global microplastic budgets. However, factors that influence microplastic retention in lotic ecosystems are not well studied. We used particle spiraling metrics to directly measure microplastic retention following pulse releases of polyester fibers using outdoor, experimental streams lined with substrates of varying sizes. We tested the impact of benthic algae, stream discharge, and substrate type on the transport of experimentally added microplastic fibers. We also quantified microplastic retention in and release from the stream benthos after an increase in discharge to simulate a storm event. Microplastic deposition rates were significantly higher with (1) well-established benthic algal biofilms, (2) higher stream discharge, and (3) larger benthic substrate. The increase in microplastic deposition rates with elevated discharge is opposite the expected trend observed for particulate organic matter, indicating distinct retention processes for microplastics. A rapid increase in discharge in our experimental streams resulted in resuspension of retained microplastic from all substrate types, suggesting that storm events could trigger microplastic release in natural streams. The results from this study provide direct measurements of the magnitude and direction of factors that drive microplastic retention in streams, which will contribute to the parameterization of models for microplastic deposition (and release) at larger spatial and temporal scales for freshwater ecosystems.