Point and nonpoint sources of microplastics to two Southeast Michigan rivers and reduced biofilm function on plastic substrata

Plastic pollution is an emergent global issue in freshwater and marine ecosystems. Microplastics enter waterways from a variety of sources, although the relative importance of point and nonpoint sources for a given watershed is not well understood. To determine whether medium-sized wastewater treatment plants are significant sources of microplastics to rivers draining into Lake Erie, we measured microplastic loads up- and downstream of two wastewater treatment plants in southeastern Michigan State (USA). We detected a significant increase in the microplastic load downstream from the wastewater treatment plant discharging into the Lower Rouge River, but not downstream from the one discharging into the Huron River. However, the background microplastic load was tenfold higher in the Huron River compared to the Lower Rouge River, likely obscuring our ability to detect the wastewater treatment plant as a point source. We found a positive relationship between river discharge and microplastic load in the Huron River, which drains a larger watershed area at the effluent discharge point than the Lower Rouge site, suggesting that watershed sources may be more important over larger spatial scales. We also performed experiments to test the effects of plastic on stream biofilm function to better understand how plastic pollution alters stream metabolism. 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.