Imbalanced Stoichiometric Reservoir Sedimentation Regulates Methane Accumulation in China's Three Gorges Reservoir

Abstract

The river-reservoir continuum drives a separation of sedimentation along the longitudinal hydrodynamic gradients, potentially causing imbalanced stoichiometric sedimentation. However, there are still uncertainties regarding the contribution of imbalanced stoichiometric sedimentation to methane emissions along this continuum. A 2-year field survey and in situ experiments were conducted in China's Three Gorges Reservoir (TGR), a river-valley-dammed reservoir. Sediments were trapped and collected to analyze for particulate carbon (C), nitrogen (N), and phosphorus (P) concentrations of different sizes in the summer flooded and winter dry seasons along the mainstem. Large amounts of sediments were deposited in the middle part of the TGR, particularly in the summer flood season. Hydrodynamic gradients structured imbalanced stoichiometric sedimentation patterns. Averaged particulate C:N:P proportions in the sediment layer along the upper, middle, and lower parts of the TGR in the first 30 years of reservoir operation were estimated to be 68:7:1, 97:8:1, and 151:13:1, respectively. The middle part of the TGR, being the "control point" of methane accumulation based on seasons, exhibited a shift in stoichiometry and stable isotope signatures, indicating that there was likely a shift in sources of POM. P, specifically in smaller size particles (<10 μm), seemed to be the potential key driver that regulates particulate C:N:P in the sediment of the TGR. Its sedimentation, primarily in the middle part of the TGR, contributed to the significant decreases in particulate C/P ratios and could possibly regulate long-term CH 4 accumulation in the reservoir.

Spatial and Seasonal Variations in Dissolved Methane Across a Large Lake

“…Greater TP concentrations will result in high‐chlorophyll a , low‐O 2 , eutrophic conditions (DelSontro et al., 2018), with higher rates of methanogenesis in sediments (West et al., 2016). The negative correlation between CH 4 and C:P (Figure 6e) shows that more C relative to P leads to lower CH 4 , and has been reported in reservoirs (Z. Li et al., 2020). This finding agrees with a growing body of work arguing that nutrient stoichiometry, and not just absolute concentration, is important in controlling aquatic biogeochemistry (Graeber et al., 2021; Peacock et al., 2022; Stutter et al., 2018; Taylor & Townsend, 2010), including GHG emissions (Peacock et al., 2017; Webb et al., 2021).…”

Three Gorges Dam: friend or foe of riverine greenhouse gases?

“…In addition, the pre-impoundment clearance also reduced decomposition of organic carbon and inhibited the significant increase in CH 4 emissions in the TGR. During reservoir flushing, degassing would occur because of rapid depressurization and strong aeration, resulting in increased emissions of dissolved CH 4 and lowering of CH 4 concentration downstream [ 6 , 41 ]. Overall, the TGD regulated the CH 4 emission regime of the Yangtze, causing dissolved CH 4 to increase in the upper reach and decrease in the lower reach.…”

Methane accumulation affected by particulate organic carbon in upper Yangtze deep valley dammed cascade reservoirs, China

“…The distribution of POC in a river can be significantly influenced by dams. At first, damming increased losses of river connectivity and reduced water flow in the main channel, which possibly decreased the transport of POC after damming due to the long HRT, further resulting in bulk accumulation and burial of POC in reservoirs (Ulseth and Hall Jr, 2015;Li et al, 2017a;Almeida et al, 2019;Li et al, 2020a;Wang et al, 2020).…”

“…The largest share of CH4 emissions may be due to OC retention and mineralization in the reservoir (Shi et al, 2017). Recent research showed that imbalanced stoichiometric sedimentation mediated CH4 accumulation in the mid-part of China's Three Gorges Reservoir (Li et al, 2020a). However, the findings supported that reservoir operation strategy significantly impacts the patterns of CH4 accumulation.…”