The consumption of CO2 due to carbonate weathering is thought to be an important terrestrial sink for atmospheric CO2. However, the reaction and fate of weathering products may significantly affect the magnitude of the sink. Carbon mass balances were determined in Hongfeng Lake, a small artificial lake located in the carbonate plateau of SW China. This was accomplished by measuring the concentrations of DIC (dissolved inorganic C), DOC (dissolved organic C), POC (particulate organic C), and CO2 partial pressure in lake waters, inflows and outflows, and in surrounding spring waters. Results show that maximum DIC (mainly HCO 3 derived from carbonate weathering by CO2) to DOC ratios in the catchment export load reach 21, implying that as much as 80% of the exported primary production is in the form of HCO 3 , and, therefore, the CO2 consumed by carbonate weathering and subsequent DIC export is a critical component of the C balance in this ecosystem. This exported HCO 3 subsidizes C cycling of aquatic ecosystems in the catchment, and 32% (excluding carbonate-derived C) returns to the atmosphere as CO2 evasion through the fluvial and lacustrine water–air interface and 10% is stored in lake sediment. This lake has a small CO2 emission rate (13.2 ± 3.9 mmol m2 d1 , or 0.097 mol m2 a1 relative to catchment area) compared to mean values for many reservoirs found in temperate and tropical climatic zones. This is likely due in part to low organic C loads supplied by the karst-dominated catchment, and low lake water DOC concentrations. The estimate indicates that more C (39%) returns to the atmosphere and much less (13%) is stored in sediments, implying that the C sink of carbonate weathering may not be as significant as once thought. 2011 El