Microplastic in bivalves from the heavily contaminated Guanabara Bay, Rio de Janeiro, Brazil

Microplastic contamination is pervasive in urbanized coastal ecosystems, and Guanabara Bay is an ecologically relevant coastal system under intense anthropogenic use in Brazil, ranking among the most polluted estuaries in Latin America. However, spatially resolved biomonitoring efforts remain scarce in this bay. Here, we assessed microplastic contamination using multi-species, multi-site approaches to determine spatial patterns and contextualize levels with previous cross-matrix studies conducted in the bay. Across fifteen sites sampled along gradients of urbanization and environmental conditions, 678 microplastics were detected in 280 individuals, averaging 2.4 and reaching 45.0 particles g^-1 in mussels and oysters. Such values exceeded national levels, ranking among the highest reported for bivalves from Brazil and aligning with patterns related to previous cross-matrix studies in Guanabara Bay. The observed spatial gradient showed higher microplastic levels in the southern bay, near dense urban areas and port infrastructure, and lower concentrations in the eastern and northern sectors, which are surrounded by mangroves and protected areas. Despite differences in habitat use and filtration rates, microplastic concentrations did not differ significantly among species, which equally reflected spatial gradients, indicating comparable sentinel performance. This consistency among suspension-feeding species indicates comparable signals of spatial contamination, supporting their use as complementary sentinels. Fibers dominated the ingested microplastics, which were predominantly small-sized, transparent particles composed mainly of natural and anthropogenic cellulose. These findings demonstrate widespread microplastic contamination in Guanabara Bay and support the use of multiple species as robust sentinels for assessing microplastic exposure in coastal systems.