Seasonal variations and intricate diel differences in the physio-chemical parameters and CO₂ emissions from a typical karst groundwater-fed reservoir in southern China

The quantification of CO₂ flux of water reservoirs is still largely uncertain due to complex temporal variations in physio-chemical parameters and CO₂ emissions. To address this issue, the diel-scale characteristics of physio-chemical parameters, δ¹³C_DIC, and CO₂ emission flux from Dalongdong reservoir (DLD), a karst groundwater-fed reservoir, in southern China were measured covering July, August, and November of 2014 and March and June of 2015. The results showed that the physio-chemical parameters, δ¹³C_DIC, and CO₂ emission flux displayed significant seasonal variations and intricate diel differences in the DLD reservoir. At the seasonal scale, the DLD reservoir is a CO₂ source because the mean value of the CO₂ emission flux is higher than zero. The highest CO₂ emission flux was in November of 2014 due to reservoir overturn and the lowest flux was in August of 2014 due to reservoir thermal stratification. Diel differences in pH, DO, DIC, and pCO₂ were inconspicuous during the study periods, while the CO₂ emission and δ¹³C_DIC showed higher values during the daytime than the nighttime during July of 2014 and March and June of 2015. In August of 2014, mean differences between the daytime and nighttime CO₂ emission were inconspicuous due to a negative CO₂ emission flux occurrence. Reservoir overturn in November of 2014 resulted in a higher mean CO₂ emission flux and larger data range during the daytime than at nighttime. Future studies that calculate CO₂ emissions in water reservoirs should consider monitoring temporal frequencies to obtain more accurate estimations of CO₂ emission fluxes.