Microplastics (particles < 5mm) represent an emerging aquatic pollutant of concern. These particles are ubiquitous in environments, absorb harmful chemicals, enter food webs, and, once in streams, are transported to downstream ecosystems. We used experimental releases, adapting spiraling metrics designed for measuring particulate organic matter retention, to quantify the transport of microplastics in streams in response to changing environmental conditions.
We conducted microplastic releases in replicated experimental streams at the Notre Dame Linked Experimental Ecosystem Facility (ND-LEEF). Using a fully-crossed design, we examined how microplastic retention was influenced by benthic substrate (e.g., sand, pea-gravel, cobble, mixed substrate), stream discharge (e.g., baseflow and stormflow), and benthic biofilm colonization (e.g., mature vs. post-scouring). We conducted 36 experimental pulse releases of microplastic fiber slurries, as fibers are the most common environmental microplastic. We collected grab samples of water and plastic at regular time intervals, sampling at three stations staggered distances downstream from each release point. We estimated the total number of plastics that passed each sampling location, measured the change in conductivity through time using a conservative salt release, and calculated the depositional velocity (Vdep mm/s) of microplastics. This metric represents the velocity at which an individual particle is deposited on the benthos.
Results/Conclusions
Preliminary results showed that Vdep of microplastics was approximately 50% higher in streams with mature biofilm (i.e., high algal biomass) compared to the same streams after scouring stimulated by stormflow (2-way ANOVA, F1,8 = 72.66, p < 0.001). There was no difference in Vdep between cobble and pea gravel substrate types. While retention in baseflow streams appears to be high, we found that stream algal growth and disturbance can drive microplastics retention and transport downstream. These data provide novel insights into the environmental factors controlling microplastic fate and fluvial transport and are critical to understand the role of lotic retention in global plastic budgets.