Microplastics Interception from Riverine Ecosystems and Translocation to Fish Internal Organs.

Microplastics (MPs) are widespread in aquatic environments, yet the links between environmental contamination, fish interception, and translocation to internal organs remain poorly understood. This study provides the first integrated assessment of MP contamination in an urban river system, quantifying concentrations in the water column and tracing their interception and organ-specific accumulation in a bioindicator freshwater fish, the planktivorous bleak (Alburnus arborella). MPs were characterized by concentration, shape, and size in six organs: gastrointestinal tract, liver, kidney, muscle, brain, and gonads. Remarkable loads of MPs were detected downstream of a metropolitan area and in all examined organs, with the liver and kidney showing the highest concentrations. Trophic ecology emerged as a key driver of active-indirect MP uptake via contaminated prey. Organ-specific translocation pathways were identified, with long fibers dominating in muscle, fragments in the kidney, and small spheres in the brain, the latter suggesting a higher capacity to cross the hematoencephalic barrier. These results demonstrate that size and shape of MPs, together with feeding ecology, govern MP interception and distribution dynamics, highlighting smaller MPs as a particular risk. The findings fill a critical gap in freshwater MP contamination research, underscore the need for further field-based studies on bioaccumulation and long-term exposure effects, and highlight the importance of incorporating MP morphology and ecological interactions into environmental risk assessments.