Magnitudes and Drivers of Greenhouse Gas Fluxes in Floodplain Ponds During Drawdown and Inundation by the Three Gorges Reservoir

Abstract

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 CH 4 , CO 2 , and N 2 O 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, CH 4 ebullition comprised 60-68% of emissions from floodplain ponds to the atmosphere. Using linear mixed effects modeling, we show that partial pressures of CH 4 and CO 2 and diffusive CO 2 fluxes in floodplain ponds varied on a diel basis with in situ respiration. Floodplain inundation by the Three Gorges Reservoir significantly moderated areal CH 4 diffusion and ebullition. Diel pCO 2 , pCH 4 , pN 2 O, and diffusive fluxes of CO 2 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.

Drivers of Methane Flux Differ Between Lakes and Reservoirs, Complicating Global Upscaling Efforts

“…We did not evaluate methane emissions from natural versus artificial ponds due to the low ( n = 23) sample size and geographical bias of artificial ponds, but this is an important future research question. Preliminary evidence suggests that artificial ponds may have higher methane fluxes, even in cases where diffusion was the only pathway measured (Gorsky et al., 2019; Ollivier et al., 2018, but see; Miller et al., 2019), and that artificial pond type matters (Grinham et al., 2018). In cases where ponds are constructed, methane emissions may be managed through morphometric design as deeper ponds often emit less methane than shallower ones (Gorsky et al., 2019; Webb et al., 2019).…”

Small artificial waterbodies are widespread and persistent emitters of methane and carbon dioxide

“…For instance, substantially larger night‐time fluxes and concentrations of CH 4 and CO 2 have been observed in arable ditches, urban ponds and channelized streams (Deng et al, 2020; Harrison et al, 2005; Natchimuthu et al, 2014) which are often attributed to variations in photosynthesis and O 2 concentrations. However, some pond and lake studies have found lower CH 4 fluxes during night‐time due to decreased wind speeds reducing gas exchange (Natchimuthu et al, 2014; Sieczko et al, 2020) and others have found no clear diel pattern in pond CH 4 or CO 2 (Miller et al, 2019), especially during winter sampling (Ollivier et al, 2019b). No diel measurements were made during our study and so we cannot be sure to what extent, if any, our annual fluxes have been under‐ or overestimated, but night‐time measurements should be a priority for future work.…”

Spatial Patterns of Diffusive Methane Emissions Across Sediment Deposited Riparian Zones in Hydropower Reservoirs

“…Previous studies have identified significant methane emissions in the riparian zones (Miller et al, 2019;Zhou et al, 2017). For example, Miller et al (2019) found elevated riparian methane emissions relative to the reservoir during drawdown and inundation by the Three Gorges reservoir, China.…”

“…Previous studies have identified significant methane emissions in the riparian zones (Miller et al, 2019;Zhou et al, 2017). For example, Miller et al (2019) found elevated riparian methane emissions relative to the reservoir during drawdown and inundation by the Three Gorges reservoir, China. In this study, reduced methane emissions were found at the edge of the riparian zone under reservoir operation, which advanced our knowledge on methane emissions from riparian zones.…”

“…In the dammed river, sediments are often deposited on shorelines, in addition to the riverbed. Previous studies have mainly focused on methane emissions from sediments deposited on the riverbed (DelSontro et al, 2010(DelSontro et al, , 2011Yang et al, 2013), and a few studies have concerned methane emissions from sediment deposited riparian zones at the land-water interface (Miller et al, 2019;Zhou et al, 2017), while our understanding of the spatial patterns of methane emissions across the zone remains incomplete (Maeck et al, 2013).…”