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Revealing the magnitude, dynamics, and influencing factors of CO 2 emissions across the water-air interface in karst water with high frequency is crucial for accurately assessing the carbon budget in a karst environment. Due to the limitations of observation methods, the current research is still very insufficient. To solve the above problems and clarify the main influencing factors of CO 2 emission in karst water, this study selected Dalongdong (DLD) Reservoir, located in the typical karst peak and valley area in southwest China, to carry out a multi-parameter high-frequency monitoring study from January to December 2021, and used the thin boundary model method to estimate the CO 2 flux across the water-air interface (CF). The average annual flux of DLD reservoir is 84.48 mmol·(m 2 ·h) -1 , which represents a CO 2 source overall. However, during the stratification period in August, there is a transient carbon sink due to negative CO 2 emission. The alteration of thermal stratification in water is crucial in regulating the seasonal variation of CF. Meanwhile, the diurnal variation is significantly influenced by changes in hydrochemical parameters during the thermal stratification stage. Compared to low wind speeds (<3 m/s), high wind speeds (≥3 m/s) have a greater impact on the CO 2 flux. Furthermore, high-frequency continuous data revealed that the reservoir triggered a CO 2 pulse emission during the turnover process, primarily at night, leading to unusually high CO 2 flux values. It is of great significance to monitor and reveal the process, flux, and control factors of CO 2 flux in land water at a high-frequency strategy. They will help improve the accuracy of regional or watershed carbon budgets and clarify the role of global land water in the global carbon budget., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

The chamber method is widely used to measure CO 2 and CH 4 flux in inland water. However, the designs of chamber used in various studies are different and lack unified standards, which would affect the observation results. To clarify the impacts of chamber characteristics, including light transmittance, air pressure difference inside and outside the chamber, and gas mixing degree in the chamber, on CO 2 and CH 4 flux measurements at the water-air interface, we compared the effects of transparent/opaque chamber, the chamber with/without air pressure equalizing device and fan on CO 2 and CH 4 flux measurements in the aquaculture pond, based on the multi-channel closed dynamic chamber system. The results showed that, during the daytime in summer, compared with the transparent chamber which could measure the actual CO 2 flux, when CO 2 was emitted from the pond, the opaque chamber overestimated the CO 2 flux by 90%; when CO 2 was absorbed by the pond, the opaque chamber underestimated the CO 2 flux by 50%. The CH 4 diffusion flux measured by the opaque chamber was 40% lower than that measured by the transparent chamber. There was no significant difference between CO 2 and CH 4 flux measured by the chamber with and without air pressure equalizing device. CO 2 flux observed by the chamber without fan had poor representativeness, being 20% higher than that observed by the chamber with fan. Moreover, CH 4 flux emitted through different pathways could not be distinguished using the chamber without fan. Therefore, when the chamber method was used to observe the CO 2 and CH 4 flux at the water-air interface, the chamber shall be transparent and be installed with fan.

The static chamber-gas chromatography method was applied to observe CO 2 fluxes across the water-air interface from different aquaculture systems, including four groups of mesocosms, i.e., blank control system (CK), monoculture system of Sebastes schlegelii (F), monoculture system of Chlamys farreri (B) and polyculture system of S. schlegelii and C. farreri (FB). Meanwhile, the physical, chemical and biological indices of the water were measured. The results showed that the group CK was a stable CO 2 source, with the mean flux of 12.42 mg·m -2 ·h -1 . The group B was a CO 2 source during the early and middle periods of the experiment and a CO 2 sink at the end of the experiment, with the mean flux during the experiment being 10.95 mg·m -2 ·h -1 . The change trends of CO 2 flux in the group F and FB were generally consistent, acting as CO 2 sources in the early period and as CO 2 sinks subsequently. The mean fluxes of those two groups during the expe-riment were -3.53 and -10.49 mg·m -2 ·h -1 , respectively. Water pH was a good predictor for CO 2 flux across the water-air interface according to the result of regression analysis, with pH value of 8.25 as the critical threshold between efflux and influx. The net primary production of water column was the main factor influencing the CO 2 flux, indicating that phytoplankton might be the main internal force regulating the CO 2 flux across the water-air interface. Our results indicated that C. farreri at present stocking density in the polyculture system could promote phytoplankton biomass as well as primary production and therefore enhance the CO 2 sink function.

The high-intensity partial pressure of CO2 (pCO2) in karst underground rivers rapidly releases in open-flow sections. This is an important process affecting the global karst carbon cycle. This study focuses on the diurnal variation patterns and driving factors of CO2 exchange flux at the water–air interface in the open-flow sections of typical karst underground rivers in southwestern China. The inorganic carbon in water and water–air interface exchange fluxes are observed. Three representative survey stations, i.e., the outlet of the underground river (Q1), the river sections without submerged plants (H1), and the river sections with submerged plants (H2), are selected to study the CO2 exchange process and its influencing factors. The results show that the CO2 release flux at Q1 exhibits high pressure in the daytime and low pressure in the nighttime, while H1 and H2 exhibit the opposite pattern. The photosynthesis of submerged plants significantly inhibits the carbon release flux of the river, and in the river sections where submerged plants are distributed, their biological effects have inhibited approximately 0.131 Tg C/yr of carbon emissions. This study emphasizes the significant contribution of submerged plants in restraining the release of CO2, thereby promoting carbon sequestration and storage in karst water systems. [ABSTRACT FROM AUTHOR]

The significance of CO2 emissions at the water–air interface from inland water bodies in the global carbon cycle has been recognized and is being studied more and more. Although it is important to accurately assess CO2 emission flux in a catchment, little research has been carried out to investigate the spatio-temporal variations in CO2 emissions in view of a water continuum. Here, we systematically compared the differences and control factors of CO2 degassing across the water–air interface of a spring–river–lake continuum in the discharge area of Baotuquan Spring in July 2017, which is a typical temperate karst spring area in Jinan city, northern China, using hydrogeochemical parameters, stable carbon isotope values, and CO2 degassing flux. Affected by the pCO2 concentration gradient between the water and ambient air, the spring water showed a high CO2 degassing flux (166.19 ± 91.91 mmol/(m2 d)). After the spring outlet, the CO2 degassing flux in the spring-fed river showed a slight increase (181.05 ± 155.61 mmol/(m2 d)) due to river flow rate disturbance. The river flow rate was significantly reduced by the "blockage" of the lake, which promoted the survival and reproduction of phytoplankton and provided favorable conditions for aquatic plant photosynthesis, increasing the plankton biomass in the lake to 3383.79 × 104/L. In addition, the significant decrease in the dissolved inorganic carbon (DIC) concentration and the increase in the δ13CDIC values in the lake also indicated that the photosynthesis of the lake's aquatic plants resulted in a significant decrease in the pCO2 concentration, thus limiting the amount of CO2 off-gassing (90.56 ± 55.03 mmol/(m2 d)). [ABSTRACT FROM AUTHOR]