Sources of microplastics to two Southeast Michigan rivers and biofilm responses plastic substrata

Abstract Plastic pollution is an emergent global issue in freshwater and marine ecosystems. Microplastics can be introduced into waterways from a variety of sources, although the relative importance of a source for a given watershed is not well understood. To determine whether medium-sized wastewater treatment plants are significant sources of microplastics to rivers that feed into Lake Erie, we measured microplastic load up and downstream of two southeast Michigan wastewater treatment plants. We also performed experiments to test the effects of plastic on stream biofilm function. We detected a significant increase in the microplastic load downstream from the Ypsilanti wastewater treatment plant, which empties into the Lower Rouge River but not the Ann Arbor wastewater treatment plant, which empties into the Huron River. However, the background microplastic load was 10x higher in the Huron River compared to the Lower Rouge River, likely obscuring our ability to detect microplastic wastewater inputs. We found a positive relationship between river discharge and microplastic load at the Huron River site, which drains a larger watershed area than the Lower Rouge site, suggesting that watershed sources may be more important than wastewater treatment plant inputs over larger spatial scales. In biofilm experiments, biofilms grown on high-density polyethylene and polypropylene had significantly lower metabolic diversity and metabolic response, respectively. Overall, our findings indicate that attention should be directed to both point and nonpoint sources to reduce microplastic pollution and that plastics may negatively affect the function of stream biofilm communities.