Varying thermal structure controls the dynamics of CO₂ emissions from a subtropical reservoir, south China

Thermal stratification and mixing are important to the physicochemical composition of reservoirs and lakes and impact their water quality and biogeochemical cycles. However, it remains unclear how thermal stratification and mixing process control the exchange of CO₂ between surface water and the Earth's atmosphere. To address this issue, we examine the temporal characteristics of some physicochemical parameters, partial pressure of CO₂ (pCO₂), the δ¹³C_DIC, and CO₂ emission from a typical karst groundwater-fed reservoir (Dalongdong reservoir). During the 23 month study (2016-2018) thermal stratification limited CO₂ emission, in part from photosynthetic uptake of CO₂, from early April to late October, while mixing processes stimulated CO₂ emission of CO₂ generated from organic matter remineralization in bottom water from October to April. The Dalongdong reservoir is an atmospheric source of CO₂ for most of the study period; however, during periods of stratification, approximately 0.37 ± 0.44 Gg CO₂ (1 Gg = 10⁹g) dissolved into the water from the atmosphere, while approximately 6.24 ± 3.73 Gg CO₂ was lost to the atmosphere during periods lacking stratification. Limited emissions during stratified period may thus represent a negative feedback to CO₂ contributions to global warming, which has increased lengths of stratified periods. These study results are important to optimize sampling monitoring strategies to reduce errors of regional CO₂ emission estimation.