Summer drought enhances diurnal amplitude of CO2 in two German rivers of different size

Drought is becoming increasingly prevalent globally, stimulating research into its effects on river ecosystems. However, our understanding of how droughts affect riverine CO₂ dynamics on a daily scale remains limited, particularly considering the likelihood of future drought occurrence. Here, we seize the opportunity to compare daily CO₂ cycles between a non-drought summer and an unprecedented drought summer. We developed a new diel CO₂ process model to examine how droughts affect diel change in riverine CO₂. Our findings reveal that summer drought amplifies diurnal CO₂ fluctuations and the pattern holds true across rivers of varying sizes, with increases of 62% for the stream and 24% for the river during drought conditions. We demonstrate that, in comparison to higher radiation and temperature induced by droughts, diel amplitude is more sensitive to low water depths. A decrease in water depth by 43% and 44% corresponded to 13% and 25% less gas exchange in the studied stream and river, respectively, while decreasing ecosystem respiration by 26% and 57%. Our model effectively captures diel CO₂ variations driven by drought considering river size, contributing valuable insights into aquatic ecosystem behavior and refining CO₂ emission estimates. We emphasize the vulnerability of shallow rivers to drought, and carbon emissions from shallower waters should be explicitly assessed at sub-daily scales to improve the estimates of daily CO₂ emissions.