Effect of buoyant turbulence and water quality factors on the CO2 net atmospheric flux changes in a stratified reservoir

The temporal variations in CO₂ net atmospheric flux (NAF) in stratified reservoirs are controlled by both physical and biological factors. However, research on the factors and processes affecting CO₂ NAF variability over time is insufficient, and as a result, there is considerable uncertainty in present estimations of global reservoir CO₂ emissions. In the present study, we analyzed the effects of hydrodynamic and water quality factors on CO₂ NAF variability in a stratified reservoir based on field studies and data modeling. Three empirical and four surface renewal gas transfer models were used to characterize the effects of hydrodynamic factors on gas transfer rate and CO₂ NAF at the air–water interface. Buoyant turbulence notably affected CO₂ NAF when the stratification strength was reduced. As a result, the CO₂ NAF (1485 mg-CO₂ m⁻² day⁻¹) estimated using surface renewal models that considered the effects of buoyant turbulence were twice greater than the NAFs estimated using empirical models that only considered wind force (724 mg-CO₂ m⁻² day⁻¹). The best linear regression model explained 81.6% of the temporal variation in CO₂ NAF using water temperature (Tw), electrical conductivity (EC), pH, chlorophyll a, total organic C (TOC), and alkalinity. The nonlinear parsimonious random forest model explained 84.4% of the temporal change in CO₂ NAF using only three independent variables (EC, dissolved oxygen, and TOC). Principal component analysis revealed that the CO₂ NAF tended to be large under low Tw, weak stratification, and low pH. These results indicate that the temporal variability of CO₂ NAF in the stratified reservoir can be predicted using data-driven modeling with minimal water quality variables and selection of an appropriate gas exchange model. The findings improve the accuracy of estimates of CO₂ emissions and monitoring activities in stratified reservoirs.