Hydrodynamic stress exacerbates the Trojan horse effect of microplastics and toxicity of co-occurring lead in grass carp (Ctenopharyngodon idella).

Hydraulic conditions in aquaculture environments are rarely static, yet their influence on contaminant dynamics remains poorly understood. This study investigated how water flow alters the bioavailability and toxicity of co-occurring microplastics and lead in grass carp. Fish were exposed to polyamide microplastics (100 μg/L) and lead (50 μg/L) under hydrodynamic stress (2.0 BL/s water velocity). Flow exposure increased lead accumulation in fish blood (0.006 ± 0.001 ppm) and liver (1.853 ± 0.096 mg/kg) compared to still water conditions, suggesting that hydrodynamic stress may enhance the proposed 'Trojan horse' role of microplastics as carriers for lead uptake. Fish exhibited a sharp rise in cumulative mortality at Day 7 reaching 75.0% in the triple exposure group, driven by severe hematological disruption marked by anemia (Hb: 3.9 ± 0.85 g/dL; RBC: 1.65 ± 0.1 × 106/mm3), lymphocytopenia (59.33 ± 1.52%), and elevated inflammatory cytokines. Hepatic antioxidant defenses collapsed under combined stress, with suppressed enzyme activities alongside intensified lipid peroxidation and reactive oxygen species production (4.66-fold increase in ROS intensity). These findings demonstrate that hydrodynamic forces are not merely physical background variables but active regulators of contaminant bioavailability and toxicity in aquaculture systems. The results highlight a critical need to move beyond single-stressor risk assessments and incorporate realistic hydraulic conditions into aquaculture safety frameworks.