Deciphering seawater acidification in the Muping Marine Ranch in summer: pH vs. aragonite saturation state

Coastal seasonal acidification has severely impacted marine aquaculture, particularly shellfish farming. However, factors controlling seawater acidification vary in different aquaculture conditions. This study conducted summer cruises of a typical shellfish farming in China, from June to July 2022 to analyze the spatiotemporal distributions of seawater pH and aragonite saturation state (Ω_arag), and their response to temperature, sea-air CO₂ exchange, mixing, CaCO₃ formation/dissolution, and biological effects. The pH and Ω_arag of bottom seawater were both lower than surface seawater, with the pH of surface and bottom seawater ranging from 8.02 to 8.09 and 7.87 to 8.02, while the Ω_arag ranging from 2.80 to 3.80 and 2.21 to 2.43, respectively. The bottom seawater pH and Ω_arag both show a decreasing trend, with Ω_arag at certain stations close to or below 2. Biological (surface seawater: 51 % ± 4 %; bottom seawater: -39 % ± 8 %) and temperature (surface seawater: -28 % ± 6 %; bottom seawater: -41 % ± 8 %) effects were crucial on regulation of seawater pH. Biological effects were the dominant factor in the variation of Ω_arag in both the surface (64 % ± 7 %) and bottom (-48 % ± 9 %) seawater. Additionally, the dissolution of calcium carbonate in bottom seawater contributed to an increase in Ω_arag to some extent (15 % ± 27 %). Ω_arag better reflects the influence of non-temperature processes (e.g., CaCO₃ formation/dissolution and biological effects). This study advocates for Ω_arag as a more suitable indicator of seawater acidification in coastal aquaculture areas.