Plastic pollution is well documented in marine ecosystems, with most marine plastic originating from rivers draining urban watersheds. Despite rivers being a source of plastic to oceans, freshwater plastic pollution is currently understudied. It remains unknown how much plastic emitted from terrestrial sources is retained in river networks versus exported to oceans. Factors contributing to accumulation of macroplastics (particles > 5mm) and microplastics (particles 0.1 - 5mm) in freshwaters likely vary considerably within and among river networks. Suburban watersheds are understudied compared with urbanized watersheds regarding plastic dynamics. Diversifying research across watershed scales, sizes, and urbanization gradients will improve estimates of global plastic river stock and retention rates.
We examined abundance of micro- and macroplastic in the Ipswich River (Massachusetts, USA), a watershed draining suburban Boston MA with heterogenous land-use. In this watershed, the furthest headwaters are highly suburban, while the downstream reaches drain a higher proportion of forests and wetlands. We measured plastic concentrations in flowing water and stored in the sediments in four stream reaches in the Ipswich River network in 2022: two headwaters (drainage area < 5 km2) and two larger rivers (drainage area > 150 km2). We hypothesized suburban headwaters would show higher plastic concentrations from source proximity, while mainstem rivers would show lower concentrations (i.e., distance from source and retention upstream).
Results showed greater macroplastic density in suburban headwaters (0.97 items/m2, and 42.4 g/m2) than downstream mainstem river sites (0.18 items/m2, 4.12 g/m2). Results support the hypothesis of higher concentrations in the headwaters and strengthen the current understanding that plastic concentrations are positively correlated with urbanization. Analysis of microplastic concentrations remains ongoing. The combined plastic dataset will improve our understanding of the relationship between macro- and microplastics in river systems, factors controlling plastic movement, and accumulation of plastic in river networks versus transport to oceans.