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.
Highlights
Methane (CH4) is widely recognized as the second most important greenhouse gas after carbon dioxide (CO2) (Saunois et al, 2020)
Based on the N/C mole ratios and δ13C-particulate organic carbon (POC) values, POC mostly originated from terrestrial POC in the XLD and XJB reservoirs
This study showed that the proportion of POC in total particulate matter (TPM) during the flood season was lower than that in the dry season in cascade reservoirs (Figure S9)
Summary
Methane (CH4) is widely recognized as the second most important greenhouse gas after carbon dioxide (CO2) (Saunois et al, 2020). The annual increase in global atmospheric CH4 between 2007 and 2020 fell within the range of 7.99 ppb⋅yr-1 to 14.81 ppb⋅yr-1 (Dlugokencky, 2021). There is a very high level of confidence that the atmospheric CH4 increase during the Industrial Era was caused by anthropogenic activities, which caused approximately 60% CH4 emissions (Ciais et al, 2014; Saunois et al, 2016; Saunois et al, 2020). The production of CH4 in lakes and reservoirs is an important process in the global methane cycle. This is partly because freshwater systems are closely linked to and manipulated by anthropogenic activities, e.g., hydrological process regulation, geomorphological alternation, large inputs of organic carbon, and nutrients from surrounding communities. Anthropogenic footprints significantly change the fate and transport of CH4 in freshwaters
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