Partial pressure of CO₂ and CO₂ 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.