Seasonal and diel variability of CO2 emissions from a semiarid hard-water reservoir

Reservoirs represent a key component of the global carbon cycle. However, estimates of carbon dioxide (CO2) emissions from reservoirs remain poorly constrained due to the absence of spatially and temporally resolved measurements. We performed high-resolution monitoring of CO2 emissions (FCO2) in a semiarid hard-water reservoir to examine its seasonal and diel variability. Our results suggest that dissolved inorganic carbon input plays a central role in sustaining the surface water CO2 partial pressure (pCO2), which varies from 1076 to 4587 μatm. Although the reservoir is moderately to highly productive throughout the year, it is a net CO2 source with FCO2 values in the range of 308–1753 mg C m−2 d–1. This high CO2 efflux indicates that productive waters are not necessarily CO2 sinks. Both pCO2 and FCO2 exhibit clear seasonal and diel patterns. Surface water pCO2 is highest in March and presents a consistent diurnal/nocturnal pattern with the daytime pCO2 6–13% lower than the nighttime pCO2. High CO2 efflux is observed during the ice-thaw period, indicating the release of CO2 that was accumulated during the winter. CO2 effluxes are typically higher during the nighttime driven by aquatic metabolism, but episodic weather events (e.g., rainfall and strong winds) can significantly enhance CO2 emissions and even reverse the diel pattern. Our study also shows that using only daytime measurements to estimate daily CO2 emissions would underestimate it by 9–25%. Hence, future global assessments should incorporate CO2 emissions from hard-water reservoirs and account for their seasonal and diel variability.