Underestimation of calcium carbonate saturation state in marginal seas due to the disregard of calcium ion addition: A case study of the Bohai sea, China.

Seawater calcium ion (Ca) concentration was investigated based on the potentiometric titration method during the summer of 2018 in the Bohai Sea, China. The measured Ca concentration ranged from 7760 to 9739 μmol kg and deviated from the theoretical Ca values, which were estimated from the calcium/salinity ratio. The excess calcium (Ca) ranged from 186 to 1229 μmol kg, showing a decreasing trend from the estuary to the nearshore, and then the offshore areas.