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 δ¹³C_DIC 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)).

In order to improve the understanding of the global carbon cycle and the stability of karst carbon sinks, it is necessary to better understand the source, distribution and transformation characteristics of organic matter (OM) in aquatic ecosystems. Here, stable isotope ratios (δ¹³C and δ¹⁵N), elemental analysis (C/N ratios), and lipid biomarkers were analyzed for dissolved organic matter (DOM) (0.7 μm) of water, and organic matter from sediment cores (SCOM) to identify the sources, distribution, and transformation of OM in a subtropical karst reservoir. The results showed that short-chain (C14–20) n-alkyl lipids were more abundant than long-chain (C21–34) n-alkyl lipids in both the DOM and SCOM samples, indicating that bacteria were the primary sources of these lipids, while terrestrial organic matter (OM) made only a minor contribution to the n-alkyl lipid pool, and aquatic plants (macrophytes) OM contributed major contribution to the n-alkyl lipid pool in POM. Microbial activity and lipid degradation were more pronounced in the DOM. Furthermore, terrigenous and macrophyte-derived lipids were found to be more abundant in POM than in DOM and SCOM, suggesting that they are relatively resistant to degradation compared with phytoplankton-derived OM.

Hydrogen and oxygen stable isotopes serve as essential tracers for deciphering atmospheric circulation patterns and driving mechanisms of extreme climatic events. However, their utility in high-resolution climate reconstruction remains constrained by their inherent spatiotemporal heterogeneity. This study conducted event-scale observation on the summer precipitation in the Yellow River Delta (YRD), aiming to better understand the isotopic characteristics and controlling mechanisms. The Local Meteoric Water Line (LMWL) in the YRD exhibited low slope and intercept values, indicating strong influences of sub-cloud evaporation and contributions from continental air masses. Furthermore, pronounced interannual variations were observed in the LMWL, reflecting unstable moisture sources and fluctuating local meteorological conditions. D-excess-based estimation demonstrated pronounced interannual differences in the proportions of losses from sub-cloud evaporation and contributions from locally recycled moisture. The isotopic composition in summer precipitation was negatively correlated with the proportion of low-latitude marine air masses and positively correlated with that of high-latitude continental air masses. The relationships between isotopes and local meteorological variables suggest that the “amount effect” could be an important local factor influencing the isotopic composition of precipitation in the YRD. Global and national isoscape products for precipitation isotopes exhibited deviations of varying degrees in simulating the isotopic variation in the YRD, which may be associated with the complexity of local meteorological conditions and moisture sources. This study demonstrates that relying on single-year precipitation isotopic signatures as the regional context for the hydrological tracing may introduce non-negligible uncertainties in coastal monsoon areas, and indicates that the use of historical data and simulation results of precipitation isotopes requires particular caution.