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

Abstract. Methane (CH4) emissions from freshwaters to the atmosphere have a profound impact on global atmospheric greenhouse gas (GHG) concentrations. Anthropogenic footprints such as dam construction and reservoir operation significantly changed the fate and transport of CH4 in freshwaters. The type of particulate organic carbon (POC) in reservoirs is a critical factor controlling CH4 production and emissions. However, little is known of how reservoir operation mediates the distribution of POC and regulates CH4 accumulation in cascade hydroelectric reservoirs. Here, spatial and temporal variations in POC and CH4 were explored in the Xiluodu (XLD) and Xiangjiaba (XJB) reservoirs which are deep valley dammed cascade reservoirs located in the main channel of the upper Yangtze River. Based on the δ13C-POC and N / C mole ratios of particulate organic matter, the results of multi-endmember stable isotope mixing models by a Bayesian model show that terrestrial POC and autochthonous POC accounted for approximately 56 ± 19 % and 42 ± 19 % (SD, n = 181) of POC, respectively. CH4 concentrations and δ13C-CH4 in the cascade reservoirs were potentially influenced by CH4 oxidation. Together with other physicochemical parameters and structural equation model, these results suggested that the input of terrestrial POC was dominantly influenced by water level variations and flow regulation due to reservoir operation. The cumulative effect of POC caused by cascade reservoirs was not apparent at a bimonthly scale. Terrestrial POC was more likely to dominate CH4 accumulation in cascade reservoirs under reservoir operation.