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

Abstract

Abstract Understanding the natural variability of pH and aragonite saturation state (Ω arag ) is important for assessing ocean acidification (OA) impacts especially in the coastal ocean since anthropogenic CO 2 increase‐induced OA is often superimposed by their natural variability. Here, we report the seasonal variability of sea surface pH and Ω arag 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 Ω arag substantially increases from 2.05 ± 0.18 in spring to 3.34 ± 0.25 in summer. The difference in pH and Ω arag 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 CO 2 system and the second temperature effect associated with CO 2 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 Ω arag , reinforcing each other and causing a relatively large variability of Ω arag . Also, through both qualitative analyses and a 1‐D model, we identify the processes controlling the seasonal variability of pH and Ω arag . We find air‐sea exchange dominates the seasonal variability of pH and Ω arag in nearshore areas, while biological production is the most important in the central part of the JZB.