Air–water CO2 and CH4 fluxes along a river–reservoir continuum: Case study in the Pengxi River, a tributary of the Yangtze River in the Three Gorges Reservoir, China

Authors: HUANG, YANG - Chinese Academy Of Sciences; Witthaus, Lindsey; LI, ZHE - Chinese Academy Of Sciences; STURM, BELINDA - University Of Kansas; ZHANG, ZENGYU - Chinese Academy Of Sciences; GUO, JINSONG - Chinese Academy Of Sciences; SHEN, YU - Chinese Academy Of Sciences

Submitted to: Environmental Monitoring and Assessment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2017
Publication Date: 4/21/2017
Publication URL: http://handle.nal.usda.gov/10113/5700669
Citation: Huang, Y., Yasarer, L.M., Li, Z., Sturm, B.S., Zhang, Z., Guo, J., Shen, Y. 2017. Air–water CO2 and CH4 fluxes along a river–reservoir continuum: Case study in the Pengxi River, a tributary of the Yangtze River in the Three Gorges Reservoir, China. Environmental Monitoring and Assessment. 189:223. doi:10.1007/s10661-017-5926-2.

Interpretive Summary: There is a global effort to understand the role of reservoirs in the carbon cycle and to both quantify and mitigate their emissions of greenhouse gases. As the largest reservoir in China, the Three Gorges Reservoir (TGR) and its tributaries on the Yangtze River represent a critical study area. The TGR is operated for both flood control and hydropower generation, which creates a 30 meter difference in water level throughout the year and a fluctuating aquatic environment. This study examined the spatial and temporal patterns of carbon dioxide and methane fluxes from the Pengxi River, a tributary of the TGR, using monthly measurements from seven locations along the tributary. Measurements began in the upstream portion of the Pengxi River that was unaffected by reservoir levels and continued downstream to the point where the tributary converges with the Yangtze River. Carbon dioxide (CO2) fluxes showed a decreasing trend from upstream to downstream due to phytoplankton, which created CO2 sinks during active periods of production. Algal populations were able to thrive in the downstream environments as a result of thermal stratification that occurred in the deeper waters and high nutrient concentrations during spring and summer months. Methane fluxes did not show a distinct spatial trend, but were affected by water temperature and concentrations of dissolved organic carbon. This study suggests that nutrient enrichment and algal growth are important factors in regulating reservoir carbon emissions and should be considered in future studies.

Technical Abstract: Water surface greenhouse gas (GHG) emissions in freshwater reservoirs are closely related to limnological processes in the water column. Affected by both reservoir operation and seasonal changes, variations in the hydro-morphological conditions in the river–reservoir continuum will create distinctive patterns in water surface GHG emissions. A one-year field survey was carried out in the Pengxi River–reservoir continuum, a part of the Three Gorges Reservoir (TGR) immediately after the TGR reached its maximum water level. The annual average water surface CO2 and CH4 emissions at the riverine background sampling sites were 6.23 ± 0.93 and 0.025 ± 0.006 mmol h-1 m-2, respectively. The CO2 emissions were higher than those in the downstream reservoirs. The development of phytoplankton controlled the downstream decrease in water surface CO2 emissions. The presence of thermal stratification in the permanent backwater area supported extensive phytoplankton blooms, resulting in a carbon sink during several months of the year. The CH4 emissions were mainly impacted by water temperature and dissolved organic carbon. The greatest water surface CH4 emission was detected in the fluctuating backwater area, likely due to a shallower water column and abundant organic matter. The Pengxi River backwater area did not show significant increase in water surface GHG emissions reported in tropical reservoirs. In evaluating the net GHG emissions by the impoundment of TGR, the net change in the carbon budget and the contribution of nitrogen and phosphorus should be taken into consideration in this eutrophic river–reservoir continuum.