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[Display omitted] • δ13C, δ15N, and δ18O were analyzed to reveal carbon–nitrogen coupled absorption. • Stability regulation mechanism of karst reservoir was indicated by diel monitoring. • The stability promotes the biological carbon pump development of karst reservoir. • Biological carbon pump caused carbon burial in karst reservoir. While river damming has been extensively investigated, there remains unclear in the diel biogeochemical processes and carbon control mechanisms in karst reservoirs, under the influence of the biological carbon pump (BCP) effect. We investigated the diel variation of Hongjiadu (HJD) and Wujiangdu (WJD) reservoirs, which exhibit typical periodic thermal stratification, in the Wujiang River located in Southwest China. We found that diel variation of dissolved inorganic carbon (DIC) (HJD: 1668.6 ± 10.4 µmol/L; WJD: 1777.9 ± 31.5 µmol/L), δ13C DIC (HJD: −3.1 ± 0.1 ‰; WJD: −4.6 ± 0.4 ‰), and other parameters in the lentic zones were relatively stable. Moreover, the Revelle factor in the lentic zones (HJD: 44.3-49.0; WJD: 44.3-53.9) were substantially higher than those of the fluvial zones (HJD: 24.4-33.4; WJD: 14.2-35.1). The stable diel variations indicate that the HJD and WJD reservoirs may have distinct stability regulatory mechanisms. In the lentic zones, δ13C DIC , nitrogen–oxygen isotope (δ15N and δ18O), and the correlation of C-N indicated that the conversion of DIC and NO 3 – is primarily influenced by primary production, and there was coupled absorption of carbon and nitrogen nutrients during photosynthesis. Meanwhile, the C/N weight ratio (6.0-10.1) and the δ13C POC (−30.0 ‰ to −24.7 ‰) also revealed a gradual transition from soil organic matter (OM) in the fluvial zone to phytoplankton source in the intermediate-lentic zone. Thus, two reservoirs had a significant BCP effect and converted a significant amount of DIC to autochthonous organic carbon (AOC) burial in the lentic zones. The BCP effect in subtropical deep karst reservoirs buried lots of carbon, increased carbon sequestration and decreased CO 2 emissions in the lentic zone. Simultaneously, the stability of reservoirs promotes BCP effect and intensifies carbon sequestration, which is of great significance for the study of the global carbon sources-sinks pattern. [ABSTRACT FROM AUTHOR]

• Unveiling the dissolved inorganic carbon cycle in a karst reservoir through hydrodynamic modeling and δ13C. • The DIC retention capacity is more pronounced under weak hydraulic conditions in HJD. • Significant CO 2 emissions during periods of intense hydrodynamics necessitate close monitoring. • Quantification of DIC flux across diverse hydrodynamic scenarios in a deep subtropical reservoir. • The method (hydrodynamic model and δ13C) is applicable to karst canyon-type reservoir worldwide. Dam construction significantly impacts river hydrodynamics, subsequently influencing carbon biogeochemical processes. However, the influence of hydrodynamic conditions on the migration and transformation of Dissolved Inorganic Carbon (DIC) remains uncertain. To bridge this knowledge gap, we integrated hydrochemistry, isotopic composition (δ13C DIC), and a hydrodynamic model (CE-QUAL-W2) to examine the distinctions, control mechanisms, and environmental effects of DIC biogeochemical processes in a typical large and deep reservoir (Hongjiadu Reservoir) under different hydrodynamic conditions. We evaluated hydrodynamic alterations through the Schmidt stability index and relative water column stability. The analysis disclosed that during weak hydrodynamics periods, the energy necessary for complete mixing the surface and deep water was 34 times higher (3615.32 J/m2 vs.106.86 J/m2), and stability was 13 times greater (312.96 vs. 24.69) compared to periods of strong hydrodynamics. Additionally, the spatiotemporal heterogeneity of DIC concentrations (1.4 % to -9.1 %) and δ13C DIC (-1.7 % to -19.5 %) from the dry to wet seasons reflected disparities in DIC control mechanisms under varied hydrodynamic conditions. Based on model simulations, our calculations indicate that during weak hydrodynamics periods, the enhancement of the biological carbon pump effect resulted in substantial sequestration of DIC, reaching up to 379.6 t-DIC·d−1 in the water. Conversely, during strong hydrodynamics periods, DIC retention capacity decreased by 69.2 t·d−1, resulting in reservoir CO 2 emissions of 22.7 × 104 t, which were more than 7 times higher than during weak hydrodynamics periods (3.2 × 104 t). Our findings emphasize the discernible impact of hydrodynamic conditions on reservoir biogeochemical processes related to DIC. Considering the increasing construction of reservoirs globally, understanding and controlling hydrodynamic conditions are crucial for mitigating CO 2 emissions and optimizing reservoir management. [Display omitted] [ABSTRACT FROM AUTHOR]

• Carbon transport and transformation varies in two reservoirs with different geological settings. • Karst reservoirs exhibit a higher Revelle factor (RF) and low CO 2 emissions. • Attention should be paid to the high CO 2 emission effect of non-karst reservoirs. • Consider geological settings when estimating the global budget for reservoir CO 2 emissions. Carbon migration, transformation, and emissions as CO 2 in reservoir and lake systems have been extensively studied. However, uncertainties persist regarding carbon cycling variations in both karst and non-karst regions within large thermal stratified river-reservoir systems. To address this knowledge gap, we combined measurements of water chemistry, isotopic compositions (δ13C), partial pressure of CO 2 (p CO 2), and CO 2 fluxes (F CO2) to elucidate the differences, control mechanisms, and environmental effects of various carbon biogeochemical processes in the Hongjiadu reservoir (HJD, karst reservoir) and Shangyoujiang reservoir (SYJ, non-karst reservoir), Yangtze River basin, China. Our results demonstrate that key biogeochemical processes are associated with CO 2 production and emissions. The stronger biological carbon pump (BCP) effect in the lentic area of the HJD reservoir limited CO 2 emission (0.9 ± 5.0 mmol m−2 d−1) compared to the SYJ reservoir (50.9 ± 33.4 mmol m−2 d−1), leading to low dissolved inorganic carbon (DIC) and high saturation state of carbonate minerals in the lentic epilimnion. Although the released water from both reservoirs acted as hotspots for CO 2 emissions, the results of higher Revelle factor (RF) in the HJD reservoir than SYJ reservoir suggest the greater buffer capacity and lower sensitivity of OC degradation changes in DIC. Despite the shorter hydraulic retention time (HRT) of SYJ reservoir (120 d) compared to HJD reservoir (368 d), F CO2 was 3.5 times higher than that of the HJD reservoir (652.9 mmol m−2 d−1). Moreover, upon analyzing selected reservoirs worldwide, it becomes apparent that karst reservoirs demonstrate a reduced potential for CO 2 emissions. Our results indicate that the impact of varying geologic settings should be considered to minimize errors in regional and global CO 2 emission estimates. [Display omitted] [ABSTRACT FROM AUTHOR]

• First measurements of PIC radiocarbon age in a karst reservoir system. • Photosynthesis-induced PIC precipitation is a source of PIC sink in the lentic zones. • The dissolution of autochthonous PIC is responsible for the build-up of DIC in the hypolimnion. • Discharge of the DIC-rich hypolimnion water through the dam causes downstream CO 2 emission to the air. Damming alters the biogeochemical cycling of carbon along river systems. However, the effects of river damming on the fate and transport of particulate and dissolved inorganic carbon (PIC and DIC) remain unclear. Here, we use dual isotope (13C and 14C) compositions, water chemistry, and electron microscopy to unravel the processes controlling the in-reservoir cycling of inorganic carbon in a river-reservoir system of a karst region of southwest China. From the reservoir's fluvial to lentic zone, δ13C PIC decreases from −2.2 ‰ to −7.6 ‰, whereas the PIC radiocarbon age drops from 11,512 to 2,027 years B.P. Electron microscopy images and water chemistry provide further evidence of in-reservoir photosynthesis-induced carbonate precipitation of autochthonous PIC in the water column. Although part of PIC is dissolved into DIC in the reservoir's hypolimnion, the contribution of carbonate minerals (allochthonous PIC) accounted for 41 % of PIC based on an isotopic mass balance model. This indicates that the precipitation rate of autochthonous PIC was greater part of carbon cycle in the reservoir. Moreover, the autochthonous PIC accounted for 61 % of the PIC and its dissolution accounted for 50 % of the DIC and the high CO 2 efflux in the water discharging from the reservoir. The results shed light on the important role of autochthonous PIC in carbon cycling in karst river-reservoir systems. With continued dam construction in karst areas, the role of in-reservoir PIC production and dissolution on the riverine carbon cycle deserves more attention. [ABSTRACT FROM AUTHOR]