The primary sources of dissolved inorganic carbon (DIC) in water are carbonate materials and CO₂ produced during the biological processing of organic matter. The application of carbon isotope techniques to terrestrial and aquatic ecosystems can accurately elucidate carbon fluxes and other carbon cycle processes in these systems. Lake ecosystems on the Qinghai-Tibetan Plateau are fragile and sensitive to changes in climate and environment. This study explored the relationship between the carbon isotopic composition (δ¹³C) of the DIC (δ¹³C_DIC) in the Genggahai Lake, the lake environment, and the climate of the watershed based on the observed physicochemical parameters of water in areas with different types of submerged macrophyte communities, combined with changes in the temperature and precipitation during the same period. Overall, the δ¹³C_DIC of the Genggahai Basin exhibited a large range of values, with an average δ¹³C_DIC for inflowing spring water (δ¹³C_DIC-I) of –11.1 ‰, which was the most negative, followed by an average δ¹³C_DIC value of –10.8 ‰ for that from the Shazhuyu River (δ¹³C_DIC-R) and an averageδ¹³C_DIC value of –6.91 ‰ for lake water (δ¹³C_DIC-L). Variations in the photosynthetic activity intensity of different aquatic plants yield significantly changing δ¹³C_DIC-L values in areas with varied aquatic plant communities. Hydrochemical observations revealed that δ¹³C_DIC-I and aquatic plant photosynthesis primarily affected the differences in the δ¹³C_DIC-L values of the Genggahai Lake, thereby identifying them as the key components of the lake carbon cycle.