Processes Controlling Sea Surface pH and Aragonite Saturation State in a Large Northern Temperate Bay: Contrasting Temperature Effects

Understanding the natural variability of pH and aragonite saturation state (Ωarag) is important for assessing ocean acidification (OA) impacts especially in the coastal ocean since anthropogenic CO2 increase‐induced OA is often superimposed by their natural variability. Here, we report the seasonal variability of sea surface pH and Ωarag from spring to summer in the Jiaozhou Bay (JZB) and compare their controls based on two cruises conducted in April and August 2018. Results show that sea surface pH on the NBS scale slightly increases from 8.10 ± 0.05 in spring to 8.13 ± 0.04 in summer, whereas surface Ωarag substantially increases from 2.05 ± 0.18 in spring to 3.34 ± 0.25 in summer. The difference in pH and Ωarag seasonal increase is related to the contrasting temperature effects on them, which can be divided into the first temperature effect associated with acid‐base equilibrium of the CO2 system and the second temperature effect associated with CO2 solubility‐driven air‐sea exchange. The two temperature effects have opposite influences on pH, canceling each other and causing a relatively small seasonal variability of pH, while they have consistent influences on Ωarag, reinforcing each other and causing a relatively large variability of Ωarag. Also, through both qualitative analyses and a 1‐D model, we identify the processes controlling the seasonal variability of pH and Ωarag. We find air‐sea exchange dominates the seasonal variability of pH and Ωarag in nearshore areas, while biological production is the most important in the central part of the JZB. Plain Language Summary Both pH and saturation state of calcium carbonate (CaCO3) minerals (Ω) are good metrics for ocean acidification (OA). Understanding their natural variability is important for assessing OA impacts especially on calcifying organisms in the coastal ocean since anthropogenic CO2 increase‐induced OA is often superimposed by their natural variability. Although a lot of work has been done, their controlling processes have not been well known. For example, how temperature would influence the seasonal variability of pH and Ω is poorly understood. Here, we divide temperature effects into two aspects: the first temperature effect associated with acid‐base equilibrium of the CO2 system and the second temperature effect associated with CO2 solubility‐driven air‐sea exchange. We take the Jiaozhou Bay as an example to show that the two temperature effects have opposite influences on pH, canceling e