The Karasuhara Reservoir, where abundant phytoplankton proliferate, is a typical eutrophic lake. In mesotrophic lakes, previous studies have shown that aquatic plants utilize photosynthesis to absorb CO₂ from the atmosphere. However, research on CO₂ in eutrophic lakes remains unsolved, particularly regarding the long-term emission and absorption of CO₂. Dissolved inorganic carbon (DIC), which determines the CO₂ flux at the water surface, varies depending on the water quality environment, thus necessitating an understanding of its controlling factors. Therefore, observational data from 2010 to 2018 were used to investigate correlation and attribution (Structural Equation Modeling analysis). Furthermore, a box model was proposed to analyze the factors governing DIC. As a result, DIC's internal production is significantly influenced by a decrease due to photosynthesis and an increase due to the mineralization of dissolved organic carbon.