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.