<p>Shanzai Reservoir, a subtropical monsoon reservoir in southeastern China, was investigated as a spatially heterogeneous carbon-processing system using monthly observations from five hydrogeomorphic zones between April 2023 and March 2024. Gross phytoplankton carbon fixation was estimated using a depth-integrated model, while the diffusive CH₄ flux potential was estimated from an empirical dissolved CH₄ relationship combined with wind-based gas-transfer calculations. Both areal rates and area-weighted zonal totals were evaluated. Estimated carbon fixation and diffusive CH₄ flux potential showed pronounced temporal and spatial variability, but their seasonal patterns differed. Estimated carbon fixation was highest during Peak Monsoon, with a mean of 2.51 × 10⁶ mg C m⁻²&#xa0;month⁻¹, whereas diffusive CH₄ flux potential was highest during Typhoon Wet, reaching 6.75 × 10³&#xa0;mg CH₄ m⁻²&#xa0;month⁻¹. Reservoir Center contributed the largest area-weighted annual totals for estimated carbon fixation and CH₄ flux potential because of its larger surface area, while River Channel and Rixi Entrance showed stronger areal process intensity. Environmental associations indicated that estimated carbon fixation was most strongly related to Chl-a, whereas diffusive CH₄ flux potential was associated with water temperature, transparency, COD, pH, and Chl-a. After accounting for shared Chl-a dependence, the results did not indicate a simple positive relationship between estimated carbon fixation and diffusive CH₄ flux potential. These findings emphasize the need to distinguish gross carbon fixation from net carbon sequestration and model-estimated diffusive CH₄ flux potential from directly measured methane emission.</p> Graphical abstract <p></p>