Diel Variability of CO2 and CH4 from Inland Waters: Patterns, Drivers, and Implications

Inland waters exhibit pronounced temporal variability in CO2 and CH4 emissions. However, existing research has predominantly focused on seasonal to interannual scales, with most field measurements conducted during daytime hours. Consequently, diel (24 h) emission dynamics remain poorly characterized. This study synthesizes current knowledge on diel variations in CO2 and CH4 fluxes across inland water bodies—including rivers, lakes, reservoirs, and ponds—with particular attention to day–night contrasts. Based on the limited available data, the average day-to-night flux ratios are 0.87 for CO2 and 1.44 for CH4. Exclusive reliance on daytime sampling may therefore underestimate daily CO2 emissions by approximately 13% while overestimating CH4 emissions by a similar magnitude. Diel fluctuations in water temperature and wind speed—driving corresponding changes in surface water gas concentrations and air–water gas transfer velocity—represent primary controls on these emission patterns. Secondary influences include solar radiation, dissolved oxygen, pH, and nutrient availability. Future efforts should prioritize high-frequency monitoring of diel carbon emission cycles and mechanistic analysis of their drivers, ultimately enabling the development of large-scale models that explicitly incorporate diel dynamics. Such advances are essential for accurate quantification and correction of carbon emissions from inland waters at regional to global scales.