Microplastic (MP) pollution is increasingly recognized as a global environmental concern, both in marine and freshwater ecosystems, with the latter acting both as MP transport pathways and accumulation zones. Despite the growing interest and research, there is still a lack of information about MP pollution in freshwater systems, especially for lowland hydrological networks, where small spring-fed watercourses are particularly vulnerable, due to their proximity to human settlements and agricultural areas, where the risk of continuous MP input increases, leading to potential long-term environmental accumulation. Moreover, regarding the biotic compartment, organisms may exhibit different degrees of MP accumulation, depending on their feeding strategies, habitat use, and ecological roles.

In this study, we investigate MP pollution in a lowland spring watercourse from Northwestern Italy (Sacchetti Ditch, Friuli Venezia Giulia), assessing temporal MP trends both in abiotic (water and sediment) and biotic compartments. For this purpose, the mollusk Unio elongatulus was chosen as it is widely distributed in the study area and may be heavily affected by MP contamination due to its filtering activity.

Bimonthly sampling activities highlighted MP contamination both in abiotic matrices and mollusks, while polymer analysis identified different MP types, such as polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE), and polyamide (PA). No significant temporal trends were detected in abiotic/biotic matrices, suggesting continuous MP inputs from untreated wastewater discharge, runoff, and agricultural/industrial effluents. U. elongatulus showed close association with MP content in water, and the Strauss selectivity index showed that PP was preferentially ingested, while PE and PA were significantly avoided, suggesting that polymer properties (such as density) can affect bioavailability and uptake.

Oure results reinforce the role of filter-feeding bivalves as bioindicators of MP pollution and highlight the need for targeted management strategies to mitigate MP pollution in small but highly contaminated water bodies.