Seasonal and diel variability of CO2 emissions from a semiarid hard-water reservoir
2022
Ran, Lishan | Yue, Rong | Shi, Hongyan | Meng, Xiangdong | Ngai Chan, Chun | Fang, Nufang | Shi, Zhihua
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.
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