Microplastic contamination in bivalves from urban estuaries: Are they sentinels for differences in pollution levels?

Bivalves are often used as sentinel organisms for assessing microplastic (MP) pollution in marine ecosystems. However, MP contamination in estuaries remains under-studied despite their high vulnerability to this type of pollution. This study aimed at evaluating MP contamination in Scrobicularia plana from two estuaries in SW Spain (Guadiana and Guadalquivir), subjected to different levels of human pressure (low vs. high, respectively). A total of 231 suspected MP items were detected, with an average abundance of 1.96 +/- 1.22 items per individual in the Guadalquivir estuary and 3.09 +/- 1.96 items per individual in the Guadiana estuary, but no significant differences were found between them. GLMM models showed a significant negative effect of bivalve weight on MP accumulation. Micro-FTIR analysis confirmed that 59.8 % of the particles were plastic, 26.8 % were anthropogenic non-plastics, and 13.4 % were organic materials. The majority of MPs found in both estuaries were dark fibers composed of PET. The PERMANOVA analysis revealed significant differences in polymeric composition between the two estuaries, with PET and cellulose being more prevalent in Guadiana compared to Guadalquivir. Our findings indicate that MP concentrations in the two estuaries are similar despite varying human pressures, suggesting that factors like hydrodynamics and seasonal dynamics could play a key role in MP transport and deposition beyond local pollution sources. This study highlights how the occurrence of MPs in bivalves is a matter of concern, particularly given their importance as a food source. Further research is needed to comprehensively assess the spatial and temporal dynamics of MP contamination in estuarine ecosystems.