Seasonal climate and reservoir regulation drive coupled dynamics of dissolved organic matter and heavy metals

Cascade reservoirs serve as active biogeochemical reactors, yet their impacts on pollutant migration and transformation processes remain unclear. Here, we propose an attribution analysis framework to reveal the spatiotemporal patterns, coupled dynamics mechanisms, and key drivers of dissolved organic matter (DOM) and heavy metals in the upper Yellow River cascade reservoirs. The results indicate that synergistic photodegradation-microbial processes shift DOM composition from terrestrial humic-like to autochthonous protein-like components. This process reduces aqueous heavy metal concentrations but increases eutrophication risk, exhibiting strong seasonal variability. Reservoir discharge also risks secondary heavy metal release via sediment remobilization. Cascade reservoirs thus present dual “purification” and “pollution promotion” effects with cumulative downstream impacts. Quantitative models explain 75.1% of DOM and 69.8% of heavy metal variation, driven mainly by seasonal climate and reservoir operations. DOM transformation critically regulates heavy metal fate (33.8%). Clarifying these biogeochemical interactions is essential for watershed water security and targeted pollution control.