Summer droughts amplify the diel CO2 variations in temperate rivers

As droughts become more frequent, understanding their impact on diel CO2 variations in river ecosystems—particularly at sub-daily scales—becomes increasingly crucial. To explore diel CO2 fluctuations and carbon dynamics at fine temporal scales, we deployed in situ sensors to monitor CO2 concentrations in two rivers in Germany during both non-drought and drought summers, aiming to quantify how drought affects sub-daily CO2 dynamics.Our results show that summer drought significantly amplifies diel CO2 amplitude, with increases of 62% in the stream and 24% in the river under drought conditions. However, daily mean CO2 concentrations did not differ significantly between drought and non-drought summers. Shallower water depths are the main cause of the amplified diel CO2 fluctuations, which reduce gas exchange and ecosystem respiration. Specifically, a 43% decrease in water depth in the stream and 44% in the river resulted in 13% and 25% reductions in gas exchange, respectively, and a corresponding 26% and 57% decline in ecosystem respiration.These findings suggest that diel CO2 amplitude is more sensitive to changes in water depth than to the increased radiation and temperature associated with drought. Our study highlights the vulnerability of shallow rivers to drought and underscores the importance of high-frequency CO2 monitoring to capture sub-daily variations more accurately. As droughts become more frequent, single daily measurements may become highly uncertain, making high-frequency monitoring essential for improving CO2 emission estimates.