Dissolved calcium issues in estuaries and marine areas: Review of the Chinese Coast

Dissolved calcium plays a critical role in coastal environments, influencing marine ecosystems and fisheries. This review comprehensively examines current research on dissolved calcium along China's coast. Despite its importance, research on dissolved calcium in China's estuarine and coastal areas is still in its early stages, spanning estuaries, continental shelf seas, and bays, yet with limited literature available. This review advocates for the use of automated EGTA (C₁₄H₂₄N₂O₁₀) potentiometric titration for accurate determination and quality control of dissolved calcium. Alternative techniques, such as ion chromatography and ICP-MS, are also viable options, while methods like fluorescence, capillary zone electrophoresis, and pulsed constant current control should be approached cautiously. Thermodynamics, terrestrial inputs, biological activity, and calcification influence dissolved calcium dynamics, resulting in its excess presence. Investigating the distribution and contributing processes of this excess calcium poses significant research challenges. Special scenarios including sea ice, submarine groundwater discharge, and hydrothermal vents warrant further investigation for their impact on excess calcium. Discrepancies between Ω_arag calculated from measured dissolved calcium and carbonate (CO₃²⁻), and apparent solubility product (K_sp) versus calcium-salt ratios, may introduce inaccuracies in acidification assessments. The intricate nature of calcium ions and their geochemical implications should be carefully considered when studying coastal acidification effects on the calcium carbonate system. While focusing on China's coastal regions, insights from these studies could substantially contribute to global research and management of coastal acidification.