Intertidal seagrass beds can shift rapidly between photosynthetic carbon uptake and respiratory carbon release during low-tide exposure, yet how light, seagrass biomass, and sediment oxidation-reduction (redox) conditions jointly influence short-term sediment-air greenhouse gas (GHG) exchange remain unclear. We measured CO2 and CH4 fluxes across a natural seagrass biomass gradient using closed dynamic transparent and opaque chambers during morning low tides. We combined full-model hypothesis testing with exploratory model averaging and sensitivity analyses to evaluate how chamber light conditions, seagrass biomass, and sediment biogeochemical context jointly influenced flux variation. CO2 flux differed strongly between chamber types: transparent chambers showed net CO2 uptake, whereas opaque chambers showed net CO2 release, indicating photosynthesis and respiration respectively. However, net CO2 uptake negatively related to seagrass biomass, suggesting that microphytobenthos on the soil surface rather than seagrass biomass drove photosynthesis under no-water condition. Natural variation in photosynthetically active radiation had no detectable main effect on CO2 flux, CH4 emissions increased with seagrass biomass, particularly under lower sediment redox conditions. In contrast, CH4 flux showed little evidence of an overall light-dark effect but increased significantly with seagrass biomass and was further affected by sediment redox condition. Sensitivity analyses showed that the main CO2 light-dark contrast and the overall CH4 biomass effect were robust, and redox-dependent interaction terms should be interpreted more cautiously. These findings indicate that low-tide CO2 and CH4 exchange are shaped by seagrass biomass, light conditions, and sediment redox status, rather than by biomass alone, highlighting the need to account for short-term exposure conditions when interpreting GHG exchange in intertidal blue-carbon ecosystems.