Spatio-temporal variations of carbon dioxide and its gross emission regulated by artificial operation in a typical hydropower reservoir in China

Supersaturation and excess emission of greenhouse gases in freshwater reservoirs have received a great deal of attention in recent years. Although impoundment of reservoirs has been shown to contribute to the net emission of greenhouse gases, reservoir age, geographical distribution, submerged soil type and artificial regulation also have a great impact on their emissions. To examine how large scale reservoir operation impact the water column CO 2 and its air–water interface flux, a field study was conducted in 2010 to evaluate potential ecological processes that regulate the partial pressure of CO 2 ( p CO 2 ) in the water column in the Pengxi River backwater area (PBA), a typical tributary in the Three Gorges Reservoir, China. Measurements of total alkalinity (TA), pH and water temperature were applied to compute the p CO 2 . And this approach was also validated by calculation of p CO 2 from the dissolved inorganic carbon data of samples. Partial least squares (PLS) regression was used to determine how the dynamics of the water p CO 2 were related to the available variables. The estimated p CO 2 in our sample ranged from 26 to 4,087 μatm in the surface water. During low water operation from July to early September, there was an obvious p CO 2 stratification, and p CO 2 in the surface was almost unsaturated. This phenomenon was also observed in the spring bloom during discharge period. Conversely, there was no significant p CO 2 stratification and the entire water column was supersaturated during high water operation from November to the following February. Significant correlation was observed between the magnitude of p CO 2 , DO and chlorophyll a , suggesting that phytoplankton dynamics regulate p CO 2 in the PBA. The average areal rate of CO 2 emissions from the Pengxi River ranged from 18.06 to 48.09 mmol m −2 day −1 , with an estimated gross CO 2 emission from the water surface of 14–37 t day −1 in this area in 2010. Photosynthesis and respiration rates by phytoplankton might be the dominant processes that regulated p CO 2 in the water column. We conclude that p CO 2 values in the surface water of Pengxi River could be regarded as potential sources of CO 2 to the atmosphere were smaller or similar to those that have been reported for many other reservoirs to date.