Microplastic in bivalves of an urbanized Brazilian estuary: human modification, population density and vegetation influence

Microplastics (MPs) global ubiquitously affects particularly coastal regions under significant anthropogenic pressures, where there are limited monitoring efforts. Bivalves are valuable sentinels of MPs contamination. This investigation determined MP contamination in oysters, clams, and mussels at Santos-São Vicente, a heavily urbanized estuary in Brazil, focusing on socio-environmental predictors, spatial distribution, and interspecies variations. Elevated MPs levels were linked to higher human modification (β = 0.5747, p = 0.0223) and reduced population density (β = -8.918e-06, p = 0.0443), regardless of vegetation cover (p > 0.05). Such a negative connection between population density and MP contamination occurred due to the significant presence of industrial and port activities, leading to high MP discharges despite a low population. The simultaneous presence of the three species at specific sites is an intrinsic limitation for broad spatial studies, depending on their environmental availability. Despite higher concentrations in clams, mussels and oysters also exhibited spatial variations in MP concentrations, serving as suitable sentinels. The central and left segments of the estuary displayed elevated MP concentrations, consistent with a well-known contamination gradient. Clams accumulated more MPs (1.97 ± 1.37 [0.00 – 5.55] particles.g^-1) than mussels (0.72 ± 1.07 [0.00 – 7.74] particles.g^-1) and oysters (0.70 ± 1.03 [0.00 – 7.70] particles.g^-1), suggesting higher bioavailability in sediments than the water column. MPs found across all species were predominantly smaller (<1,000 µm), fragments or fibers, and colorless. These discoveries lay a groundwork for prospective global investigations by linking MPs concentrations to socio-environmental predictors, contributing to the development of local mitigation measures and global discussions.