Chemodiversity and molecular traits of dissolved organic matter driven by cascade reservoirs regulations in the upper Yangtze River

Dissolved organic matter (DOM) plays a fundamental role in biogeochemical cycles within riverine ecosystems. However, the construction and impoundment of dams disrupt the natural biophysical gradients in rivers, potentially leading to alterations and turnover of DOM compositions. This research investigated the composition and chemodiversity of DOM in the cascade reservoirs along the upper Yangtze River. Our findings reveal that DOM in the cascade reservoirs are predominantly composed of lignin-like, lipid-like, and protein-like compounds. The DOM chemodiversity exhibited a significant decrease during the flood season, but recovered during the dry season. Additionally, a general decline in DOM chemodiversity was observed along the longitudinal gradient in the cascade reservoirs. The molecular traits of DOM results indicated high bioavailability and lability of DOM molecules during the flood season. A random forest analysis identified physicochemical indicators as the most important factor influencing DOM chemodiversity. During the flood season, the selected variables were mostly strongly correlated with DOM molecular traits and categories. Specifically, Q_in and Q_out showed significant negative correlations with easily degradable compounds such as carbohydrates and lipids, as well as with molecular traits. Conversely, during the dry season, few physicochemical indicators and hydrological parameters show significant correlations with proteins and aromatic compounds. Structural equation model further demonstrated that hydrological parameters exert a strong positive influence on DOM molecular categories. This study provides a foundational framework for further evaluation cascade damming effects on DOM biogeochemical processes.