Water scarcity and pollution pose a threat to the sustainable development of cities and society. Therefore, it is crucial to analyze the hydrochemical characteristics and carbon dynamics of water-deficient areas. Taking the Taiyuan section of Fenhe River as the research object, we systematically explored the hydrochemical characteristics of surface water and its evolutionary processes, as well as the ecological effect of algal carbonic anhydrase in carbon cycle using the hydrochemical evolution method and correlation analysis. The ternary diagram demonstrates that the main water chemical type in Fenhe River was SO 4 2− ·Cl−-Na+. The Gibbs and end-member diagrams of each ion display that the chemical composition of surface water was mainly controlled by silicate decomposition. The chemical ions originated mainly from dissolution of some minerals, such as plagioclase, halite, dolomite, calcite, and gypsum. The diatoms had a lower CO2 requirement because they exhibited a higher abundance at a lower partial pressure of CO2 (pCO2). However, high CO2 concentration had a positive effect on cyanobacteria, which reduced the active transport of HCO 3 − , saved the energy needed for this part of active transport, and indirectly improved the overall photosynthetic efficiency of algae. Carbonic anhydrase (CA) activity was significantly negatively correlated with pCO2 and positively correlated with HCO 3 − concentration, indicating that CA in water promoted the conversion of CO2 to HCO 3 − . The HCO 3 − generated from this process continued to participate in the erosion of silicate rocks, sequestering CO2 in the form of CaCO3, which has a non-negligible impact on the carbon sink in the Fenhe River. These consequences may have important implications for the biogeochemical cycling occurring in urban water.
