Dynamics of oxygen evolution in a thermally stratified reservoir under climate warming

Dissolved oxygen (DO) is a key variable in water quality management of lakes and reservoirs. The prediction of DO dynamics is challenging due to interactions of ecological, biogeochemical, and physical processes strongly influenced by climate warming. We observed a pronounced metalimnetic oxygen minimum (MOM) in the eutrophied Panjiakou Reservoir, China. In this study, we simulated the vertical DO distribution with a well-established three-dimensional water quality model, combined with multi-point synchronous monitoring data to accurately reproduce the spatial and temporal extent of the MOM and explore the responses of DO stratification structure and the MOM to climate warming. We found that: From the perspective of physical structure, thermal stratification was the initial force driving DO vertical stratification, which showed a certain lag compared with thermal stratification. From the perspective of biochemical processes, oxygen production by photosynthesis, oxygen consumption by respiration, and microbial decomposition of algal biomass affected the vertical distribution of DO; Increases in temperature led to an earlier formation of thermal stratification, as well as increases in duration and the enhancement of stability. The intensification of the vertical density gradient further limited the vertical exchange of DO. In addition, increases in temperature led to earlier outbreaks of algal blooms and increases in concentrations of algae-derived particulate organic matter sinking to the metalimnion, leading to decreases in DO concentrations. The MOM usually occurs from July to September every year. A 2 °C increase in atmospheric temperature advances the MOM onset by 8 days and extends its duration by 12 days. Therefore, climate warming has had a substantial effect on the DO regime in Panjiakou Reservoir. A deeper understanding of the DO dynamics of stratified lakes and reservoirs under the influence of climate change is needed for optimizing reservoir management.