Microplastics in a mosaic of Marine Protected Areas from southeastern Brazil: An assessment based on filter-feeding bivalves

Microplastics (MPs) pose a pervasive environmental threat to coastal ecosystems, with increasing reports even within Marine Protected Areas (MPAs). Despite being designed to conserve biodiversity by reducing anthropogenic pressures, MPAs remain vulnerable to diffuse and transboundary pollutants such as plastics. This study utilized bivalves as sentinels to assess MP contamination along a mosaic of 28 MPAs located in a highly populated coastal zone in southeastern Brazil, while simultaneously examining the influence of human modification levels. A total of 687 MP particles were recovered, at a concentration of 0.70 ± 1.85 (0.00-22.14) particles g ww, with levels ranging from 0.08 ± 0.08 to 3.81 ± 6.56 among the 30 studied sites. The MP contamination levels were overall low compared to other studies carried out within unprotected areas. However, even low MP levels in MPAs raise concerns about conservation goals and potential threats to local biodiversity due to potential trends of increased contamination. MPs were predominantly <500 μm, fibrous (55.3%), transparent (58.8%), and composed mainly of rayon, polyamide, and polyester, reflecting global qualitative trends observed in bivalves and other matrices. The contamination was significantly influenced by anthropic modification levels. These patterns highlight how environmental factors shape MP distribution and underscore the need for focused conservation efforts in urban and riverine zones. Our results further highlight bivalves as effective sentinels and reinforce the need for continuous monitoring to strengthen the role of MPAs in mitigating anthropogenic impacts.