In-lake dissolved carbon turnover is an important component of the global carbon cycle. However, previous studies largely focused on the hydro-chemical characteristics of dissolved inorganic carbon, with limited understanding of the internal linkages between dissolved inorganic carbon (DIC) and organic carbon (DOC). In this study, we aim to explore the mutual transformation among dissolved carbon components under seasonal changes in the Lugu Lake, Southwest China. We examined seasonal variations in DIC species (DIC, HCO3-, CO32-, and CO2) and DOC optical parameters (S275-295, SUVA254, SUVA280, URI, BIX, HIX and FI). In particular, we performed Fluorescence Region Integration (FRI) and Excitation-Emission Matrix Parallel Factor Analysis (EEM-PARAFAC), and measured partial pressure of CO2 (pCO2) and CO2 flux (F), to comprehensively assess dissolved carbon. Our results showed that carbonate dissolution caused significant seasonal differences in DIC species (P < 0.05). The significant seasonal variations in the spectral slope (S275-295) (P < 0.05) indicated the presence of allochthonous high-molecular-weight dissolved organic carbon inputs during the rainy season, and the high level of biological index (BIX) yet low level of humification index (HIX) suggested significant biological activity. Soluble microbial by-products were the primary DOC component, accounting for 44.53% to 71.97% on average, with humic-like and tryptophan-like DOC being dominant in the lake. During the rainy season, Lake Lugu acts as a pronounced CO2 source driven by intense microbial mineralization of DOC（pCO2 > 470 μatm，F > 0）. Overall, this study elucidates the variations in DIC and DOC in Lake Lugu across rainy and dry seasons, revealing the transformation patterns driven by seasonal microbial metabolism within the dissolved carbon pool. Specifically, respiratory processes dominate during the rainy season, mineralizing DIC into CO2, while photosynthetic activity prevails during the dry season. These findings offer novel insights into natural aquatic carbon fixation and sequestration in lakes.