Leaf Litter and Benthic Sediments Are Important Reservoirs of Microplastic Pollution in Rivers

Rivers are a major pathway for microplastics to enter downstream waterbodies (e.g., lakes and oceans). However, the fate of microplastics within streams is not fully understood. To address this gap, we conducted a 28 day flow-through stream mesocosm experiment (length: 2 m, width: 0.4 m, and height: 0.3 m) with natural benthic communities under high- and low-flow conditions. Seven types of microplastics, varying in shape (fibers and fragments), size (0.1 to 1 mm), and density (0.88 to 1.38 g/cm3), were introduced at the start of the experiment; a second pulse was added during a simulated storm event on day 16. We measured microplastic fate across three environmental compartments: water, sediment, and leaf litter. Microplastic concentrations in the water declined rapidly, with <0.01% remaining after 28 days. Most particles accumulated in benthic compartments. By day 28, 18% and 19% of microplastics were retained in sediment in high- and low-flow streams, while leaf litter stored 16% and 12%, respectively. Fewer particles were retained in high-flow streams, suggesting increased export under higher discharge. Denser fragments were more likely to settle, while buoyant particles were more prone to flushing. Our findings suggest that global models may underestimate microplastic storage in rivers by overlooking instream compartments like leaf litter, a dynamic and ecologically relevant microplastic sink.