Abiotic and biotic patterns of microplastic concentration and potential for food-web uptake along a river continuum

Recent microplastic (MP) research establishes rivers as transporters of MP from terrestrial to marine environments. While studies have documented MP consumption in river food webs, few have investigated multiple consumers along a river continuum. We enumerated MP concentration in the water column, biofilm, and four macroinvertebrate families with different feeding strategies across 344 km from the Yakima River headwaters to mouth in Washington, USA. Rigorous procedural controls significantly reduced contamination, highlighting the importance of controlling exogenous plastic sources during environmental MP studies. After contamination corrections, we found MP in 93% of water samples, 96% of biofilm samples, and 68% of insect samples. MP concentration did not increase from headwaters to mouth despite cumulative human population increase, suggesting MP sources other than direct point sources or undocumented sinks in the study area. However, we found significantly different MP concentration within sites, possibly associated with flow dynamics that settle plastic from the water column. Further research is needed to positively identify plastic pollution sources not associated with urban areas and to identify temporary storage dynamics of MP in river systems. Although we found more fibers than fragments in environmental samples, insects consumed more fragments, suggesting a larger role in trophic transfer than fibers. All aquatic insect functional feeding groups had internal plastic, demonstrating that insectivores such as fish or birds are likely vectors of MP movement from the river into nearby terrestrial environments and other consumers through predation.