The role of organisms' size in microplastic pollution monitoring: Insights from Mytilaster lineatus and Amphibalanusimprovisus.

Marine organisms can serve as valuable bioindicators, providing insights into the spatial and temporal distribution of microplastics (MPs) in aquatic environments. While barnacles and mussels have shown promise as bioindicators of MP pollution in coastal environments, recent studies have questioned the efficacy of bivalves in this role. To address this, our research investigated the suitability of Mytilaster lineatus and Amphibalanus improvisus as biomonitors of MP pollution in the Caspian Sea through a comprehensive survey of these filter-feeding organisms. Sediment, seawater and organisms samples were collected from 9 stations during July to September 2022. MPs were detected in all compartments with an average of 0.57 ± 0.59 items/L for water, 72.66 ± 29.29 items/kg dry weight for sediment, 1.69 ± 0.79 items/individual or 7.96 ± 3.231 items/g wet weight (ww) for M. lineatus and 1.8 ± 0.9 items/individual or 35.18 ± 35.33 items/g ww for A. improvisus. MPs were prevailed by class 1000-3000 μm in size, black in color, fiber in shape and polyamide in polymer. The pollution load index (PLI) for seawater and sediment stations were 2.11 and 2.22, respectively, confirm low level risk of MP pollution at the sampling stations. There was a positive correlation between the MP abundance isolated from seawater and those extracted from the small species (p < 0.05). Bioconcentration factor (BCF) exhibited that both organisms absorbed and accumulated MPs from their surrounding water (BCF >1). A negative correlation was detected between the number of MPs extracted from organisms and their soft tissues (p < 0.01). Therefore, smaller individuals of M. lineatus and A. improvisus showed greater potential for biomonitoring. Our findings underscore the importance of species size in assessing MP pollution using biomonitors and recommend that future studies incorporate size-related variables for a more comprehensive understanding of MPs biomonitors.