EFFECTS OF FRESHWATER BIOFILM BIODIVERSITY, BIOMASS, AND WATER QUALITY ON MICROPLASTIC ABUNDANCE IN THE POTOMAC RIVER

Microplastics (plastic particles less than 5 millimeters in length) are a ubiquitous pollutant in the environment, but research on freshwater microplastic contaminants is lacking. A possible fate of microplastics in freshwater environments is to become entangled or trapped in biofilms, which are matrices of algae, bacteria, and microinvertebrates that grow on underwater surfaces. This in-situ study at the Oasis Marina at National Harbor in Oxon Hill, Maryland, examines how the biomass, area coverage, and biodiversity of freshwater biofilms in the Potomac River are associated with the number of microplastics trapped within. Aluminum discs acting as artificial substrate for biofilm growth were deployed at the water’s surface and at 2 meters depth to survey biofilm mass and biodiversity and were sampled monthly from October 2021 – October 2022. Microplastic abundances in the water column were measured every 2 weeks over the same period. Spatial and temporal trends in trapped and suspended microplastics, biofilm biomass and biodiversity, and water quality parameters (temperature, dissolved oxygen, pH, salinity, conductivity, turbidity, nitrates, and phosphate) were measured and compared to explore factors affecting the abundance of microplastics and their partitioning between the water column and biofilms. Water quality had no measurable impact on microplastic abundance in the water column at either depth, but temperature was negatively correlated to microplastic abundance in biofilms. Temperature was also positively correlated with biofilm growth. Surprisingly, as the weather warmed and biofilms grew larger and more biodiverse, they entrapped fewer microplastics. This may have occurred because smaller, less biodiverse biofilms are mostly composed of unicellular algal colonies that provide a favorable surface for microplastic deposition. In contrast, larger, more biodiverse biofilms tend to also include microinvertebrates, whose grazing and filter feeding behavior removes visible microplastics. Understanding the seasonal changes of biofilm biodiversity and microplastic abundance may help in tracking the fate of microplastics in estuarine systems to plan mitigation strategies for the future.