Stage-dependent molecular transformation of DOM during Vallisneria natans decomposition affects DBP formation potential

The decomposition of submerged plants releases significant dissolved organic matter (DOM) into freshwater, serving as a precursor for potentially toxic disinfection by-products (DBPs) during drinking water chlorination. However, the molecular mechanisms linking decomposition phases to DBP formation remain poorly understood. Here, we tracked the temporal molecular evolution of DOM during decomposition of Vallisneria natans , a dominant macrophyte widely distributed in Asian freshwater and commonly used for ecological restoration. We annotated molecules significantly correlated with haloacetic acids (HAAs), halogenated hydrocarbons and chloroacetonitrile (CAN). The results revealed a distinct "stable-unstable-restabilized" succession pattern in DOM composition. The "unstable" DOM was characterized by elevated molecular instability and an enrichment of aromatic compounds. Moreover, simulated drinking water disinfection experiments revealed that HAAs formation potential was associated with "unstable" DOM. Although halogenated hydrocarbons and CAN also reached their maximum formation potentials during the same stage, their correlations with specific DOM features were weak. These findings from a controlled laboratory study suggest that DBP formation potential of DOM released during plant decomposition may vary with decomposition stage, highlighting the importance of considering such temporal dynamics when assessing DBP risks in plant-rich freshwater.