Ecosystem metabolism determines organic matter cycling and trophic structure in aquatic ecosystems. Ecosystem metabolism is well studied in ponds, lakes, estuaries, streams, and rivers, but it is seldom investigated in large reservoirs. Here, we report daily ecosystem metabolism (26 January-10 May 2009), measured with the free-water technique, in the surface layer of Xiangxi Bay, Three Gorges Reservoir. Further, we investigated the potential factors influencing variation in metabolic processes by systematically testing 4 hypotheses based on phytoplankton biomass, water temperature, nutrient concentration, and light availability. Daily dynamics of respiration (R), gross primary production (GPP), and net ecosystem production (NEP) had high variability during the spring phytoplankton blooms in Xiangxi Bay. GPP ranged from 0.57 to 9.79 mg O-2 L-1 d(-1); R ranged from 0.27 to 7.80 mg O-2 L-1 d(-1); and NEP ranged from -4.24 to 1.96 mg O-2 L-1 d(-1) (mean = -0.26 mg O-2 L-1 d(-1)). Further analysis showed that: 1) GPP is determined by phytoplankton biomass and maximum daily photosynthetically active radiation (PAR); 2) R is determined by phytoplankton biomass and water temperature; and 3) NEP has a significant positive correlation with soluble reactive P. However, the potential mechanism explaining the relationship between NEP and P requires further research. Our study reveals that the metabolic processes in Xiangxi Bay are determined mainly by phytoplankton biomass, water temperature, and maximum daily PAR. This work improves our understanding of ecosystem metabolism in Three Gorges Reservoir.