Dual Regulation of Natural Hydrological Process and Human Activities on the Dissolved Organic Carbon Interception-Transport Balance in Cascade Reservoirs

This study investigated the spatiotemporal variation, transport mechanism and source characteristics of dissolved organic carbon (DOC) in the Longyangxia, Laxiwa, Lijiaxia and Liujiaxia cascade reservoirs of the upper Yellow River. Samples were collected throughout a hydrological year (low water, normal water and high water period). Fluorescent components were identified by using three-dimensional excitation-emission matrix fluorescence spectroscopy coupled with parallel factor analysis (EEM-PARAFAC). These analyses were integrated with long term hydrological data (1956-2020) to assess cascade reservoirs regulation impacts. Results showed that DOC concentration followed the order: normal water period > high water period > low water period, which was influenced collectively by glacial meltwater inputs, reservoir interception, and human activities. Water temperature, nitrogen, and phosphorus were key factors influencing DOC distribution. DOM composition varied distinctively across hydrological periods. In the low water period, it was characterized by predominant autochthonous accumulation. The normal water period exhibited a transitional signature, mixing terrestrial inputs with autochthonous production. During the high water period, human influence led to structurally complex DOM dominated by significant allochthonous inputs. Consequently, the cascade system constituted a weak net carbon source, with a net delivery efficiency of 2.3%. This interception-transport balance is dually controlled by natural hydrology and human activities, and projections suggest a potential decline in the system’s carbon sink function as it ages.