Partial pressure of CO2 and CO2 emission in a monsoon-driven hydroelectric reservoir (Danjiangkou Reservoir), China

•Reservoir water is supersaturated with CO2 relative to atmospheric CO2.•A clear pCO2 drawdown in the reservoir as water flows.•Seasonal monsoon rainfall and nutrients control aquatic pCO2.•The Reservoir has a bottom average of CO2 flux (9 mmol m−2 d−1) in China's reservoirs.•High downstream CO2 flux (119 mmol m−2 d−1) is compared to global rivers’ average.

Hydroelectric reservoirs have been increasingly concerned with respect to their contributions to GHG emissions which are poorly understood due to data paucity particularly in monsoon region. This study presents the spatiotemporal changes of the carbonate system and CO2 flux of a hydroelectric reservoir (Dangjiankou Reservoir) in the subtropical monsoon climate region. A total of 21 field surveys conducted during 2004–2011 revealed significant spatial and monthly variations of aqueous partial pressure of CO2 (pCO2) in the Reservoir. pCO2 showed higher concentrations in the wet and warm seasons with an average of 595 ± 545 μatm (ranging from 53 to 3751 μatm). Substantially higher pCO2 (1132 ± 1220 μatm) was observed in the river below the dam. Pronounced seasonality of pCO2 was controlled by seasonal monsoon rainfall, while photosynthetic CO2 uptake dominated spatial patterns and dry-month variability of pCO2. Correlative analyses demonstrated that pCO2 had strong positive correlations with Si and P speciations, TOC and DOC, negative correlations with DO saturation, TN and Chl-a. The estimated CO2 flux of 9 mmol m−2 d−1 from the Reservoir surface was relatively low, and river downstream the dam had a higher flux of 119 mmol CO2 m−2 d−1, implying that water release from a reservoir would be an important channel for atmospheric CO2 sources. The annual CO2 emission from the Danjiangkou Reservoir was estimated at 3.4 × 109 mol C y−1. Information in the spatial and temporal variabilities in CO2 flux from China's hydroelectric reservoirs is urgently needed for revision of global models of reservoirs’ carbon emissions.