Document Type


Date of Degree Completion

Spring 2022

Degree Name

Master of Science (MS)



Committee Chair

Clay Arango

Second Committee Member

Jason Irwin

Third Committee Member

Paul James


Microplastic (MP) pollutes environments worldwide, but the ecological consequences remain largely unknown. While MP research has historically focused on marine environments, recent work has increasingly investigated rivers as transporters of MP from terrestrial sources to the ocean. Some studies have documented instances of MP consumption in a river food web, but few have investigated multiple consumers at different points along a river continuum. This study enumerated the MP abundance in the water column, biofilm, and four macroinvertebrate families from different functional feeding groups across nine sites spanning 344 kilometers from the headwaters to the mouth of the Yakima River in Washington state. A rigorous procedural control method was used to significantly reduce contamination, highlighting the importance of explicitly controlling exogenous plastic sources during environmental MP studies. After correcting for contamination, MP were found in 93% of water samples, 96% of biofilm samples, and 68% of insect samples. There was no discernable increase in MP abundance from the headwaters to the mouth despite a corresponding increase in cumulative human population, but there were significant differences in MP particles detected among sites. The sites with the highest MP abundance might have been influenced by recreational inputs or by flow dynamics that encouraged plastics to settle from the water column, and the lack of a relationship with human population or wastewater treatment plant outfalls suggests other sources or the presence of an undocumented sink in the study area. Further research is needed to positively identify plastic pollution sources not associated with urban areas, and more research is needed on transport and storage dynamics of MP in river systems. Fibers were more abundant in water column and biofilm samples, but there was no difference in the number of fibers or fragments consumed by insects, suggesting that fragments may play a larger role in consumption and trophic transfer than has been suggested. This study documents plastic in four different aquatic insect functional feeding groups, suggesting that insectivores such as fish or birds are also vulnerable to eating MP and or moving plastic from the river into nearby terrestrial environments by their feeding behavior.

Available for download on Wednesday, June 12, 2024