Shifting settling regimes of aquatic microplastics

Abstract Rivers are the major conveyor of plastics to the marine environment, but the mechanisms that impact microplastic (< 5mm) transport, and thus govern fate of the material in the environment, are largely unknown. This prevents progress in understanding microplastic dynamics and identifying zones of high accumulation, as well as curtailing the evolution of effective mitigation and policy measures. Using a suite of novel settling experiments here we show, for the first time, how biofilm growth significantly increases settling velocity of a range of microplastics (fragments and fibres) by > 130% and that significant increases can occur in only days. We also demonstrate how these impacts are both polymer and shape specific and that settling regimes also differ according to both salinity and sediment concentrations, which are typical of freshwater-marine boundaries found in estuaries. Our results demonstrate how existing transport formulae are inadequate to capture these impacts and highlight the importance of considering these processes within next generation predictive frameworks to understand and robustly predict the fate and impact of microplastic pollution within aquatic environments.