Daytime Photosynthesis Reshapes the Carbon Emission Pattern of a Regulated Reservoir Water-Level Fluctuation Zone

Reservoir water-level fluctuation zones (WLFZs) are important in carbon budgets, but opaque chambers omit daytime photosynthesis. We measured CO2 and CH4 fluxes at the air–water and air–soil interfaces of Hanfeng Reservoir (water-level amplitude 4.72 m), with three-chamber partitioning in the WLFZ during the growing season. The air–water interface was a weak CO2 source (2.1 ± 40.9 mg m−2 h−1) and a persistent CH4 source (0.71 ± 1.36 mg m−2 h−1). In the WLFZ, daytime gross primary production (1665 mg m−2 h−1) exceeded ecosystem respiration (1280 ± 757 mg m−2 h−1; daytime net ecosystem exchange NEE, −385 ± 1180 mg m−2 h−1), whereas the estimated 24 h NEE indicated a weak net CO2 source (+153 to +343 mg m−2 h−1). Air–soil CH4 emissions averaged 0.70 ± 1.51 mg m−2 h−1, with no significant net plant-mediated transport. At this site, and under modeled night-time scenarios, the opaque chamber overestimated daily net CO2 emissions 3.7 to 7.1-fold. In 24 h measurements of two dominant species at one site in August 2024, 09:00–13:00 sampling represented the daily mean CH4 flux within 15%, but not the CO2 flux. These site-specific results indicate that accounting for vegetation and diel variability can reduce bias in WLFZ carbon budgets.