Non-conservative Characteristics of Seawater Calcium Ion in Offshore Aquaculture Areas: A Case Study in the Muping Marine Ranch, Yantai, China

Calcium (Ca 2+ ) is a major ion in the ocean, and its ratio to salinity is generally considered to be relatively constant. However, its conservativeness is often compromised in offshore waters, particularly in marine shellfish cultured areas, where the Ca 2+ concentration could be affected by biological activities. Therefore, a deviation might occur in evaluating ocean acidification in an aquaculture area when the aragonite saturation (Ω arag ) is calculated based on the Ca 2+ concentration via the calcium-salinity ratio. This study measured the Ca 2+ concentration, and explored the distribution of Ca 2+ and the causes of excess calcium (Ca 2+ excess ) as well as the degree of acidification based on a field survey from 2020 to 2021 in the Muping Marine Ranch, Yantai. The results showed that the Ca 2+ concentrations in both surface and bottom seawater and vertical differences were higher in summer than in other seasons. The measured Ca 2+ concentrations in seawater were on average 0.372 mmol/kg higher than those calculated by the calcium-salinity ratio. The riverine input was the leading cause for the formation of Ca 2+ excess , which was reduced to some extent by the activity of cultured calcifying organisms. The Ω arag obtained from the measured Ca 2+ was on average 0.10 higher than that derived from the calculated Ca 2+ via the calcium-salinity ratio, indicating that the ocean acidification level might be somewhat overestimated if Ω arag is derived from the calculated Ca 2+ in coastal areas, especially in estuarine environments that are heavily influenced by riverine input. Therefore, the investigation of the non-conservative characteristics of Ca 2+ in offshore seawater should help to accurately assess the acidification level of aquaculture areas.