Dissolved calcium ion (Ca²⁺) is one of the eleven major ions in the ocean. Unlike the conservative ones, such as sodium ion (Na⁺) and potassium ion (K⁺), Ca²⁺ has small but systematic variations in the ocean, likely related to the formation and dissolution of calcium carbonate (CaCO₃) particles. High precision measurements of Ca²⁺ can thus provide insights of CaCO₃ cycling and fluxes. Moreover, due to the fact that CaCO₃ precipitation and/or dissolution also affect the carbonate system, we can potentially link the Ca²⁺ variation to discuss the oceanic sink of atmospheric CO₂. This paper attempts to summarize the reported excess Ca and other non-conservative calcium behaviors and to discuss their controlling processes and impacts on the seawater composition and the carbonate system. The potential changes and the role of Ca²⁺ in the context of ocean acidification are also touched upon.