Hydrogeochemistry and Carbon Cycling in the Lijiang River Basin during the Dry Season: Coupling Control of Geological Background and Biological Processes

To systematically elucidate the intricate carbon cycling processes and identify their primary controlling factors within karst river systems during the hydrologically stable dry season, this empirical study selected the Lijiang River Basin as a representative research domain. We established a monitoring network comprising four distinct cross-sections characterized by a progressively increasing spatial proportion of karst geomorphology: a non-karst area (HJ_n), a karst contact area (CT_min), a half karst area (JC_mid), and a typical karst area (XL_max). Routine monthly hydrochemical sampling was initially conducted during the dry season. Furthermore, to explicitly prevent rainfall-induced surface runoff from masking the metabolic activity signals of aquatic organisms, a specific environmental window was deliberately targeted. A continuous, completely rainless period from November 12 to 24, 2024, was selected, ensuring the absolute absence of precipitation for both the two weeks preceding the monitoring and during the sampling phase itself. Utilizing this optimal temporal window, high-frequency continuous diel monitoring was executed across the four cross-sections at strictly two-hour intervals over a consecutive 48-hour span. We systematically analyzed a comprehensive suite of variables, including general hydrochemical parameters (specifically water temperature, dissolved oxygen, electrical conductivity, and total dissolved solids), major cations and anions, dissolved inorganic carbon (DIC), and its stable carbon isotopic composition (δ¹³C_DIC). Additionally, the calcite saturation index (SIc), the partial pressure of carbon dioxide (P_CO2) and Net Ecosystem Productivity (NEP) were calculated to thoroughly explore the coupled controlling effects of the underlying geological background and aquatic biological processes.The comprehensive results demonstrate three major findings: (1) Based on the projection of hydrochemical data onto a Piper trilinear diagram, the dominant dissolved cation across all evaluated river cross-sections is exclusively calcium (Ca²⁺), while the dominant dissolved anion is universally bicarbonate (${\rm{HCO}}_3^{-}$). Other constituent ions account for comparatively marginal proportions; consequently, the fundamental hydrochemical facies characterizing all four monitoring sites are uniformly classified as a typical HCO₃-Ca water type. Concurrently, the ionic composition of the water bodies is overwhelmingly governed by natural rock weathering mechanisms. Specifically, the overarching hydrochemistry of the studied watershed is jointly controlled by the concurrent weathering and dissolution processes of both silicate and carbonate rocks. Moreover, the aqueous concentrations of Ca²⁺, ${\rm{HCO}}_3^{-}$, magnesium ions (Mg²⁺), and the aggregate metric of Total Dissolved Solids (TDS) systematically increased in direct conjunction with the expanding areal proportion of karst landscapes. The specific solute concentrations documented in the non-karst cross-section were significantly and consistently lower than those recorded in the karst cross-sections, definitively constituting a well-defined spatial pattern of aquatic hydrochemistry. (2) The vast majority of evaluated parameters exhibited exceptionally strong diel variation patterns. In the karst areas, intense photosynthetic activities executed by aquatic biological communities during the daytime generated profound geochemical shifts, resulting in significant increases in dissolved oxygen (DO), pH, and SIc. Simultaneously, this process caused a precipitous decline in the aqueous concentrations of ${\rm{HCO}}_3^{-}$, Ca²⁺ and P_CO2, while inducing a pronounced positive shift in the δ¹³C_DIC signature. Conversely, during the nighttime hours, biological respiration became the dominant metabolic process, dictating completely inverse geochemical trajectories: DO, pH, and SIc decreased, whereas the concentrations of ${\rm{HCO}}_3^{-}$, Ca²⁺ and P_CO2 rebounded substantially, accompanied by a distinct negative shift in the δ¹³C_DIC values. In stark contrast, the amplitude of diel variations in the non-karst area (HJ_n) was notably smaller than that observed in the karst regions. Although parameters such as DO, pH, and δ¹³C_DIC in the non-karst area also exhibited corresponding diurnal fluctuations in response to aquatic biological metabolism, the underlying drivers were fundamentally inconsistent with the dry-season karst areas, where variations were primarily governed by robust aquatic photosynthesis. Instead, the dynamics in the non-karst area were simultaneously constrained by physical degassing processes typical of the dry season, the strict limitation imposed on photosynthesis due to an insufficient supply of inorganic carbon sources, and the characteristically low geochemical buffering capacity of mountainous headwater streams. Under these compounding conditions, the observed diel variations were far less pronounced. (3) The quantitative results derived from the NEP calculations revealed that the cross-sections located within the karst areas consistently functioned as autotrophic ecosystems throughout the monitoring period, definitively indicated by positive NEP values (NEP > 0). This robust autotrophy signifies that biological carbon fixation operates as an exceptionally active carbon sink process within karst regions. In direct contrast, the non-karst area persistently exhibited a heterotrophic state characterized by negative NEP values (NEP < 0), effectively acting as a net carbon source. This spatial dichotomy effectively substantiates the "fertilization effect" exerted by naturally high concentrations of ${\rm{HCO}}_3^{-}$on the photosynthetic capabilities of aquatic organisms in karst areas. Essentially, the karst geological background directly enhances the operational efficiency of the biological carbon pump by consistently providing an abundant supply of inorganic carbon sources, although this process is also dynamically modulated by the local spatial richness of aquatic biological communities. In summary, this empirical investigation profoundly elucidates the coupled mechanistic framework governing the carbon cycle in karst rivers: the underlying geological background fundamentally dictates the capacity for carbon source supply, whereas dynamic biological metabolic processes directly drive the ultimate transformation of that carbon.