Hydrological conditions regulate dissolved organic matter transport in the Yangtze River

The third longest Yangtze River plays an important role in transporting organic matter from the land to the global oceans, but the impacts of hydrological conditions remain unclear. Based on weekly measurements during 2020-2023, this study demonstrated the regulating effects of hydrological conditions on the transport of dissolved organic matter (DOM) in the Yangtze River. The results showed that DOM concentration and composition seasonally fluctuated along with the hydrological addition-dilution effects. Weekly DOC concentration at the Datong station had a variation range of 0.45-6.58 mg/L. In the rainy summer, high water discharge flushed humic substance output from the land and phytoplankton also produced protein-like DOM; water discharge was positively correlated with DOC concentration (p < 0.01). However, extremely high water discharge diluted DOC concentration when water volume increase exceeded DOM supply. By constructing a piecewise concentration-discharge quantitative model, we identified a water discharge threshold of 25,000 m³/s for the transition of addition (b = 0.23) and dilution (b = -0.45) effects. Additionally, potential hydrological regulation and watershed-scale human activities may further modify downstream DOM transport, although their individual contributions require further research. In sum, this study reveals the synergistic regulation effects of hydrological conditions on DOM transport in the Yangtze River, which are of great significance for managing river water quality and optimizing carbon cycle models.