Dissolved inorganic carbon (DIC)-rich surface waters in the karst regions of southwest China (CKR) do not necessarily emit CO₂ to the atmosphere during carbonate mineral precipitation, as DIC can be partially utilized during photosynthesis by aquatic plants under the biological carbon pump (BCP) effect, leading to autochthonous organic carbon (Auto-OC). However, the operation of BCP in surface waters of the Chinese Loess Plateau (CLP), which is also enriched in DIC, has not been well established, which limits our ability to accurately assess carbon sinks in inland surface waters. We combined stable isotope (δ²H, δ¹⁸O, and δ¹³C), C/N, ultraviolet absorption spectroscopy, and excitation-emission matrix  fluorescence approaches to investigate the sources and compositions of particulate and dissolved OC in two river–reservoir ecosystems (Wulihe (WLH)-river to WLH-reservoir and Honghe (HH)-river to HH-reservoir) on the CLP. We found that from the inflow river to reservoir areas along the course of river flow, both DOC and POC signals from autochthonous production gradually increased, especially during the wet season (WS) with the same duration of rain and heat. Compared with the HH-reservoir, the WLH-reservoir had higher δ²H, δ¹⁸O, and δ¹³C values, protein-like fluorescence components, chlorophyll-a (Chl-a) and DOC concentrations, and lower CO₂ efflux. Our results revealed that the BCP intensity of the WLH-reservoir was higher than that of the HH-reservoir, potentially due to the long water residence time of the WLH-reservoir. Additionally, although the amount of autochthonous dissolved organic matter (Auto-DOM) produced by BCP in the surface water on the CLP was lower than that in the CKR, the proportion of Auto-DOM in the former was close to that in the latter, indicating the significance of Auto-DOM in the surface water on the CLP. Furthermore, a significant negative correlation was found between the BCP-derived Auto-DOM and pCO₂, while WLH-reservoir with stronger BCP effect exhibited CO₂ influx in the WS (-1.47±0.02 g m^-2 d^-1), indicating that the enhanced BCP process contributed to the water–air interface C sink. Our research emphasizes the high BCP potential of surface water in the CLP, and provides a valuable elementary background to better understanding of the carbon cycle in the aquatic ecosystem of the CLP.  
 

Biological carbon pump; River–reservoir ecosystems; Autochthonous organic carbon; Chinese Loess Plateau; Stable isotope; EEM-PARAFRAC