Hydroelectric reservoirs have been increasingly concerned with respect to their contributions to GHGemissions which are poorly understood due to data paucity particularly in monsoon region. This studypresents the spatiotemporal changes of the carbonate system and CO2flux of a hydroelectric reservoir(Dangjiankou Reservoir) in the subtropical monsoon climate region. A total of 21 field surveys conductedduring 2004–2011 revealed significant spatial and monthly variations of aqueous partial pressure of CO2(pCO2) in the Reservoir. pCO2showed higher concentrations in the wet and warm seasons with an aver-age of 595 ± 545 [1]atm (ranging from 53 to 3751 [1]atm). Substantially higher pCO2(1132 ± 1220 [1]atm)was observed in the river below the dam. Pronounced seasonality of pCO2was controlled by seasonalmonsoon rainfall, while photosynthetic CO2uptake dominated spatial patterns and dry-month variabil-ity of pCO2. Correlative analyses demonstrated that pCO2had strong positive correlations with Si and Pspeciations, TOC and DOC, negative correlations with DO saturation, TN and Chl-a. The estimated CO2fluxof 9 mmol m？2d？1from the Reservoir surface was relatively low, and river downstream the dam had ahigher flux of 119 mmol CO2m？2d？1, implying that water release from a reservoir would be an importantchannel for atmospheric CO2sources. The annual CO2emission from the Danjiangkou Reservoir was esti-mated at 3.4 × 109mol C y？1. Information in the spatial and temporal variabilities in CO2flux from China’shydroelectric reservoirs is urgently needed for revision of global models of reservoirs’ carbon emissions.