Comparing field-based microplastic observations with ocean circulation model outputs in estuarine surface waters along a human population gradient

Despite the significant ecological and human health risks posed by microplastic (MP) pollution in estuarine environments, documentation of their distribution remains absent in most water quality analyses. This study evaluated the distribution and characteristics of MPs in surface waters of Narragansett Bay, RI, the largest estuary in New England, which exhibits a strong north-to-south human population gradient along its shoreline. We implemented seasonal and tidal surface water sampling across two years coupled with simulations of MP movement using the Regional Ocean Modeling System (ROMS). We observed higher concentrations of MPs in the urbanized northern regions of Narragansett Bay (mean ± SD, 0.67 ± 0.6 particles m^-3) compared to the less populated southern regions (0.21 ± 0.1 particles m^-3), but with significant temporal variability influenced by seasonal freshwater inputs, wind, and tides. Similarities between ROMS output and field-collected data were evident when evaluating broad trends in MP movement while fine-scale accuracy was limited. Simulated counts of virtual ROMS MPs were moderately correlated to field-collected data in shorter model durations (Spearman's coefficient, 3 d, ρ = 0.64) with correlation strength decreasing with increasing model duration (7 d, ρ = 0.49; 14 d, ρ = 0.13). This study emphasizes the importance of spatially comprehensive and appropriately replicated sampling to characterize the dynamic nature of MPs in estuaries. Our results suggest that coastal pollution management strategies would benefit from adaptive monitoring programs that account for spatio-temporal fluctuations in MP concentrations and consider the drivers of MP movement unique to each estuary to mitigate inputs.