Microplastic deposition velocity in streams follows patterns for naturally occurring allochthonous particles

Accumulation of plastic litter is accelerating worldwide. Rivers are a source of microplastic (i.e., particles <5 mm) to oceans, but few measurements of microplastic retention in rivers exist. We adapted spiraling metrics used to measure particulate organic matter transport to quantify microplastic deposition using an outdoor experimental stream. We conducted replicated pulse releases of three common microplastics: polypropylene pellets, polystyrene fragments, and acrylic fibers, repeating measurements using particles with and without biofilms. Depositional velocity (v_dep; mm/s) patterns followed expectations based on density and biofilm 'stickiness', where v_dep was highest for fragments, intermediate for fibers, and lowest for pellets, with biofilm colonization generally increasing v_dep. Comparing microplastic v_dep to values for natural particles (e.g., fine and coarse particulate organic matter) showed that particle diameter was positively related to v_dep and negatively related to the ratio of v_dep to settling velocity (i.e., sinking rate in standing water). Thus, microplastic v_dep in rivers can be quantified with the same methods and follows the same patterns as natural particles. These data are the first measurements of microplastic deposition in rivers, and directly inform models of microplastic transport at the landscape scale, making a key contribution to research on the global ecology of plastic waste.