Microplastics in a large Brazilian bay: Multispecies bivalve approach reveals an association with population density.

Microplastic contamination is pervasive across aquatic ecosystems, including protected coastal environments. Todos os Santos Bay, the second-largest bay in Brazil, encompasses 1385 km² within a multiple-use marine protected area and supports over three million inhabitants, major port activities, petrochemical industries, mangrove forests, and coral reefs. Here, we assessed microplastic contamination using three filter-feeding bivalve species as sentinels, and evaluated its associations with population density, human modification, socioeconomic deprivation, and distance to river discharges using Crassostrea sp., the only species sampled at all 16 sites. A total of 834 microplastics were detected, with average concentrations of 3.21, 2.88, and 2.36 particles g-1 in clams, oysters, and mussels, respectively. These concentrations exceeded the literature-based Brazilian average for bivalves (2.06 particles g-1), indicating comparatively elevated contamination levels in Todos os Santos Bay. Microplastic contamination was widespread across Todos os Santos Bay despite all sampling sites being located within a multiple-use protected area. No consistent overall differences in microplastic concentrations were detected among species; however, a significant Species × Site interaction indicated that interspecific differences varied across sampling sites. Fibers, small and transparent particles predominated, whereas anthropogenic cellulose accounted for 56.8% of the spectroscopically characterized subset. Population density received the strongest statistical support among the evaluated socio-environmental predictors and was positively associated with microplastic concentrations in oysters. This spatial association highlights the importance of reducing land-based microplastic inputs in densely populated coastal areas through improved wastewater treatment, urban drainage, and solid waste management.