Partial pressure of CO<sub>2</sub> and CO<sub>2</sub> emission in a monsoon-driven hydroelectric reservoir (Danjiangkou Reservoir), China

Abstract. Hydroelectric reservoirs have been under sampled to establish them as sources or sinks of the atmospheric carbon dioxide (CO2). Such poor coverage is well known for subtropic, particularly monsoon driven reservoirs in China. Our study presented the spatiotemporal changes of the carbonate system and CO2 flux in a hydroelectric reservoir (Dangjiankou Reservoir) locating in a subtropical monsoon climate region. Our 21 filed surveys conducted during 2004–2011 revealed significantly spatial and monthly variations of surface water partial pressure of CO2 (pCO2) in the Reservoir. pCO2, showing higher concentrations in the wet and warm seasons, averaged 595 ± 545 µatm (ranging from 53–3751 µatm) in the reservoir surface, while substantially higher pCO2 (1132 ± 1220 µatm) was observed in the river downstream the dam. A clear pCO2 drawdown in the reservoir as water flows demonstrated a significantly descending order of Dan Reservoir > site close to dam > Han Reservoir. This spatial contrast can also be seen in the distributions of dissolved inorganic carbon and total alkalinity. Pronounced seasonality in pCO2 was controlled by seasonal monsoon rainfall, while photosynthetic CO2 uptake dominated spatial patterns and dry-month variability of pCO2. We further related pCO2 to water chemical properties and indicated that pCO2 had strong positive correlations with Si, TP and DOC, negative correlations with DO saturation, TN and Chl a, while weak correlations with other variables including biogenic elements. CO2 flux from the Reservoir surface showed a bottom average of 9 mmol m–2 d–2 in comparison with other hydroelectric reservoir in China. River downstream the dam had quite high flux of CO2 (119 mmol m–2 d–2), which was intermediate between temperate rivers and compared to global rivers' average. This means that water releasing from reservoir would be an important channel for atmospheric CO2 sources. The annual CO2 emission from the Danjiangkou Reservoir was estimated to be 3.4 × 109 mol C. Remarkably spatial and temporal heterogeneities in CO2 flux from China's hydroelectric reservoirs are urgently included for advancing global models of reservoirs' carbon emissions.