Seasonal hydrological regulation drives coupled dynamics of dissolved organic matter and heavy metals in cascade reservoirs

Cascade reservoirs have drastically altered natural hydrological cycles and material transport in rivers, yet their effects on pollutant migration remain unclear. Here, we present an attribution analysis framework based on field sampling to elucidate the spatiotemporal patterns, coupled dynamics, and drivers of dissolved organic matter (DOM) and heavy metals in the upper Yellow River cascade reservoirs. The results demonstrate that cascade reservoirs exert pronounced seasonal hydrological regulation on DOM and heavy metals. Wet-season impoundment facilitates biogeochemical transformation of surface DOM from humic-like to protein-like components, with concurrent heavy metal purification. Dry-season release primarily acts through physical dilution, decreasing outflow metal concentrations while increasing total flux. Hydrochemical and DOM variables jointly explain 58% of heavy metal variation, with DOM-related variables contributing 67.78%; hydrochemical factors mainly affect heavy metals indirectly via DOM transformation. Additionally, cumulative reservoir purification and external inputs from intervening river reaches result in longitudinal fluctuations in DOM and heavy metal distributions along the cascade. Understanding the transport and driving mechanisms of DOM and heavy metals in these reservoir systems is critical for ensuring watershed water security and effective pollution control.