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

Abstract

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.