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.