This study focuses on Xibeikou Reservoir in the Huangbaihe River Basin of western Hubei, where carbonate rocks are exposed. In-situ sampling and laboratory analysis were conducted to monitor and analyze the basic physicochemical parameters and major ions in the surface water and vertical profiles for different seasons and different characteristic cross 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 hydrochemistry changes. The results indicated that water in Xibeikou Reservoir was weakly alkaline with a water chemistry 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, while electrical conductivity, TDS, and pCO₂ showed the opposite trends. The ion concentrations in the inflow river were generally higher than those in the reservoir. Within the reservoir, concentrations of HCO_3-^, Ca₂₊^, Mg₂₊^, and K₊ were lower in summer while higher in winter. The BCP effect was strongest in summer, followed by spring and autumn, and weakest in winter. Spatially, it 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 of the reservoir enhanced the BCP effect and suppressed carbon emissions. The correlations between water chlorophyll-a and pCO₂ and HCO_3- ^{concentrations suggest}ed significant carbon control in the Xibeikou Reservoir. It was preliminarily estimated that the BCP carbon sink flux of Xibeikou Reservoir ranged from 0.04 to 0.07 t/(km2.d), which was comparable to other karst reservoirs. Overall, the estimated BCP carbon sink flux in karst reservoirs in China accounted for approximately 4% to 26% of national reservoir carbon emission flux, highlighting their importance in carbon source-sink accounting and carbon sequestration enhancement.