Aragonite saturation states ( arag) were estimated for the surface and bottom waters of Jinhae Bay over four seasons to assess the major factors controlling the arag. The surface and bottom waters of Jinhae Bay were highly supersaturated with respect to aragonite in spring, autumn, and winter. In summer, however, most of the bottom waters are undersaturated with respect to aragonite during summer, even though the surface waters are supersaturated with respect to aragonite. This phenomenon can be explained by the phytoplankton-carbonate saturation state interaction high primary production increases the arag in surface waters, while vertical export and subsequent remineralization of organic matter decreases the arag in subsurface waters. The strong linear correlation between the arag values and dissolved oxygen (DO) concentrations supports that the biological activity is a major factor controlling the arag in Jinhae Bay. The ocean acidification induced by the invasion of anthropogenic CO2 also affects considerably the aragonite saturation state in all seasons, particularly in winter when the arag values have reduced since the pre-industrial era by 32~35% at the surface waters and 33~37% at the bottom waters. In Jinhae Bay, the undersaturation with respect to aragonite in summer may have large impacts on the calcifying benthic organisms mussel calcification may bepersaturated with respect to aragonite in spring, autumn, and winter. In summer, however, most of the bottom waters are undersaturated with respect to aragonite during summer, even though the surface waters are supersaturated with respect to aragonite. This phenomenon can be explained by the phytoplankton-carbonate saturation state interaction high primary production increases the arag in surface waters, while vertical export and subsequent remineralization of organic matter decreases the arag in subsurface waters. The strong linear correlation between the arag values and dissolved oxygen (DO) concentrations supports that the biological activity is a major factor controlling the arag in Jinhae Bay. The ocean acidification induced by the invasion of anthropogenic CO2 also affects considerably the aragonite saturation state in all seasons, particularly in winter when the arag values have reduced since the pre-industrial era by 32~35% at the surface waters and 33~37% at the bottom waters. In Jinhae Bay, the undersaturation with respect to aragonite in summer may have large impacts on the calcifying benthic organisms mussel calcification may be