Seasonal shifts in phthalate ester composition and ecological risks along the Mudanjiang River–Jingpo Lake continuum

Phthalate esters (PAEs) are widespread contaminants in freshwater ecosystems, yet how temporal shifts in congener composition relate to environmental gradients and ecological-risk patterns remains poorly understood in connected river–lake systems. In this study, 16 PAEs were investigated in 36 surface-water samples collected from 12 sites along the Mudanjiang River–Jingpo Lake system during spring, summer, and autumn 2024. Water-quality conditions and surrounding landscape pressures were also examined to evaluate the environmental context of PAE variation. Thirteen PAEs were detected, with Σ16PAEs concentrations ranging from 210.90 to 852.11 ng L −1 and averaging 513.35 ± 144.86 ng L −1 . DBP was the predominant congener, accounting for 31.9% of Σ16PAEs, whereas DNP showed the highest detection frequency (86.1%). Σ16PAEs concentrations were highest in summer. The congener profile shifted from DBP dominance in spring and summer to a more even composition of DNP, DNPP, DPhP, and DNOP in autumn. PAE composition was associated with water-quality and landscape gradients, indicating that contaminant profiles varied across the observed environmental conditions. Σ16PAEs concentrations were positively associated with cumulative fish risk, but concentration dominance did not correspond directly to congener-specific risk contribution. DBP dominated the PAE burden, whereas DPhP contributed most strongly to fish risk. Non-priority PAEs also contributed substantially to both mixture composition and ecological risk. These results show that ecological-risk patterns in connected freshwater systems are shaped not only by total contaminant burden but also by temporal changes in mixture composition and environmental context. Considering congener-specific composition, non-priority contaminants, and multiple environmental pressures may therefore improve ecological-risk assessment and monitoring of river–lake ecosystems.