Magnitudes and drivers of greenhouse gas fluxes in floodplain ponds during drawdown and inundation by the Three Gorges Reservoir

Hydropower reservoirs are well-known emitters of greenhouse gases to the atmosphere. This is due in part to seasonal water level fluctuations that transfer terrestrial C and N from floodplains to reservoirs. Partial pressures and fluxes of the greenhouse gases CH4, CO2, and N2O are also a function of in situ biological C and N cycling and overall ecosystem metabolism, which varies on a diel basis within inland waters. Thus, greenhouse gas emissions in hydropower reservoirs likely vary over seasonal and diel time scales with local hydrology and ecosystem metabolism. China's Three Gorges Reservoir is among the largest and newest in the world, with a floodplain that encompasses approximately one third of the reservoir area. We measured diel partial pressures and fluxes of greenhouse gases in ponds on the Three Gorges Floodplain. We repeated these measurements on the submerged floodplain following inundation by the Three Gorges Reservoir. During reservoir drawdown, CH4 ebullition comprised 60-68% of emissions from floodplain ponds to the atmosphere. Using linear mixed effects modeling, we show that partial pressures of CH4 and CO2 and diffusive CO2 fluxes in floodplain ponds varied on a diel basis with in situ respiration. Floodplain inundation by the Three Gorges Reservoir significantly moderated areal CH4 diffusion and ebullition. Diel pCO(2), pCH(4), pN(2)O, and diffusive fluxes of CO2 on the submerged floodplain were also driven by in situ respiration. The drawdown/inundation cycle of the Three Gorges Reservoir therefore changes the magnitudes of aquatic greenhouse gas fluxes on its floodplain. Plain Language Summary Considered to be clean sources of energy, reservoirs emit greenhouse gases like other inland waters. Reservoir water levels fluctuate seasonally, introducing terrestrial organic matter to dammed rivers. Greenhouse gases such as methane and carbon dioxide are produced daily when organic matter is respired by aquatic microbes. This is balanced by consumption of carbon dioxide during daytime photosynthesis. We measured concentrations and fluxes of methane, carbon dioxide, and nitrous oxide over 24 hr in ponds on the Three Gorges Floodplain. We repeated these measurements in the overlaying Three Gorges Reservoir following its inundation of the floodplain. Among the different flux pathways, methane bubbles comprised the bulk of greenhouse gas emissions from floodplain ponds. Daily methane and carbon dioxide concentrations and carbon dioxide fluxes in these floodplain ponds varied with microbial respiration. By contrast, methane fluxes were much lower per unit area following floodplain inundation by the Three Gorges Reservoir. Daily concentrations of these gases and carbon dioxide fluxes were also driven by microbial respiration on the submerged floodplain. Seasonal water level fluctuations in the Three Gorges Reservoir therefore change how greenhouse gases move to the atmosphere on its floodplain.