Metalimnetic CO2 and N2O Maxima in a Deep Hydropower Reservoir: Implications for Degassing and Downstream Emissions

The vertical profiles of dissolved greenhouse gas (GHG) concentrations provide pivotal information about their dynamics in reservoir water columns; however, our knowledge remains constrained. This study quantified the vertical distributions of GHG concentrations and their diffusive fluxes in a major tributary of a large and deep reservoir on the Lancang River, China. The metalimnetic oxygen minima phenomenon was observed, corresponding to the peak values of carbon dioxide (CO₂) and nitrous oxide (N₂O) concentrations along the water column. The concentrations of CO₂ and N₂O were significantly correlated to the saturation of dissolved oxygen, suggesting that the aerobic respiration and nitrification processes play crucial roles in driving their vertical patterns. The relative water column stability significantly correlated with the chemical stratification index of physiochemical water variables as well as CO₂ and N₂O concentrations, indicating the importance of water stratification in regulating both the vertical patterns of water variables and the concentrations of CO₂ and N₂O. Moreover, the sediment within the transition zone between fluvial and lacustrine phases may be a hotspot for GHG production, which was supported by the high GHG concentrations in the deep water. Overall, the GHG diffusive fluxes from the studied area were much lower than global reservoir averages. Although our results show that vertical GHG concentrations do not necessarily correlate with surface concentrations, the large variations in the vertical distributions of GHG concentrations still emphasize the importance of selecting the reservoir outlet depth to manage GHG emissions through degassing and downstream pathways.