Local Monitoring Should Inform Local Solutions: Morphological Assemblages of Microplastics Are Similar within a Pathway, But Relative Total Concentrations Vary Regionally

Pathways for microplastics to aquatic
ecosystems include agricultural
runoff, urban runoff, and treated or untreated wastewater. To better
understand the importance of each pathway as a vector for microplastics
into waterbodies and for mitigation, we sampled agricultural runoff,
urban stormwater runoff, treated wastewater effluent, and the waterbodies
downstream in four regions across North America: the Sacramento Delta,
the Mississippi River, Lake Ontario, and Chesapeake Bay. The highest
concentrations of microplastics in each pathway varied by region:
agricultural runoff in the Sacramento Delta and Mississippi River,
urban stormwater runoff in Lake Ontario, and treated wastewater effluent
in Chesapeake Bay. Material types were diverse and not unique across
pathways. However, a PERMANOVA found significant differences in morphological
assemblages among pathways (p < 0.005), suggesting
fibers as a signature of agricultural runoff and treated wastewater
effluent and rubbery fragments as a signature of stormwater. Moreover,
the relationship between watershed characteristics and particle concentrations
varied across watersheds (e.g., with agricultural parameters only
being important in the Sacramento Delta). Overall, our results suggest
that local monitoring is essential to inform effective mitigation
strategies and that assessing the assemblages of morphologies should
be prioritized in monitoring programs to identify important pathways
of contamination.