This study focuses on the Xibeikou Reservoir in the Huangbaihe River Basin of western Hubei, where carbonate rocks are exposed. In-situ sampling and monitoring as well as laboratory experimental analyses were conducted to obtain basic physicochemical parameters and major ions of the surface water and vertical profiles across different seasons and sections. The spatial and temporal variations, driving factors, and carbon sink capacity of the biological carbon pump (BCP) effect were analyzed from the perspective of changes in hydrochemistry. The results indicated that water in Xibeikou Reservoir was weakly alkaline with the hydrochemical type of HCO₃-Ca·Mg. The spatiotemporal variations of basic physicochemical factors were significant. Water temperature, chlorophyll-a, and pH were generally higher in summer and lower in winter, with higher values observed in the reservoir area compared to the riverine area, and electrical conductivity, total dissolved solids, and partial pressure of carbon dioxide (pCO₂) showed the opposite behavior. The ion concentrations in the inflow river were generally higher than those in the reservoir. Within the reservoir, the concentrations of HCO^-₃, Ca²⁺, Mg²⁺, and K⁺ were lower in summer while higher in winter. The BCP effect appeared to be strongest in summer, followed by spring and autumn, and weakest in winter. Spatially, BCP effect was more pronounced in the reservoir area compared to the riverine area, and stronger in the tail area of the reservoir compared to the head. The stable thermal stratification in the reservoir enhanced the BCP effect and suppressed carbon emissions. The correlations between chlorophyll-a and pCO₂, HCO^-₃ concentrations to some extent suggested significant carbon control in the Xibeikou Reservoir. Preliminary estimates of the BCP carbon sink flux of Xibeikou Reservoir ranged from 0.04 to 0.07 t/(km²·d), comparable to other karst reservoirs. Overall, the estimated BCP carbon sink flux of karst reservoirs across China accounted for approximately 4% to 26% of national reservoir carbon emission flux, highlighting its importance in carbon source-sink accounting and carbon sequestration enhancement.