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

Abstract

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.

Why are Surface Ocean pH and CaCO3 Saturation State Often out of Phase in Spatial Patterns and Seasonal Cycles?

“…(2019) and Xue et al. (2020) show that in this simplified ocean system the thermal and nonthermal components of pH are comparable in magnitude, while Ω is dominated by nonthermal components. For example, if we could move a body of water from the polar area (∼0°C) to the tropical area (∼30°C), the thermal and nonthermal components would make pH change by −0.47 and 0.46 units, respectively, thus nearly fully canceled each other.…”

“…They mainly refer to changes in pH, Ω and CO 2 partial pressure ( p CO 2 ) due to temperature effects on the CO 2 solubility constant (K 0 ), the apparent dissociation constants of carbonic acid (K 1 and K 2 ) and the apparent solubility product (K sp ) of calcium carbonate (CaCO 3 ) in a closed system with no gas exchange with the atmosphere. They are calculated under conditions of constant total alkalinity (TA), total dissolved inorganic carbon (DIC) and salinity (e.g., Cai, Xu et al., 2020; Jiang et al., 2019; Xue et al., 2020). For example, the pH correction made from measurement temperature to in situ temperature is special for adjusting the influence from thermal components (Gieskes, 1969).…”

“…Note that the separation of thermal and nonthermal components does not imply that the nonthermal components are not temperature sensitive. Rather, for example, the air‐sea CO 2 flux is very much driven by the temperature dependent solubility constant particularly in oceans with shallow mixed layer depths (e.g., Xu et al., 2020; Xue et al., 2020).…”

Contrasting Controls of Acidification Metrics Across Environmental Gradients in the North Pacific and the Adjunct Arctic Ocean: Insight From a Transregional Study

“…A community consensus and effort are therefore reached for systematic observations in time and space of seawater partial pressure of CO 2 ( p CO 2 ), pH, and carbonate ion concentration to interpret the biogeochemical impacts of OA and validate model projections (Bakker et al., 2016; Cai et al., 2020; Gregor & Gruber, 2021; Jiang et al., 2015; Key et al., 2015; Lauvset et al., 2020; Olsen et al., 2020; Terhaar et al., 2020). Through these achievements, scientists are being aware of the characteristic of changing surface ocean chemistry and OA (in terms of pH and aragonite saturation state [Ω arag ]) at local and regional scale (Cai et al., 2020; Feely et al., 2016; Ono et al., 2019; Qi et al., 2017; Sutton et al., 2014; Tynan et al., 2016; Xue et al., 2020) and at global scale (Fassbender et al., 2017; Hagens & Middelburg, 2016a; Jiang et al., 2015, 2019; Sutton et al., 2019; Takahashi et al., 2014).…”

“…The process-driven variability imprints on top of the temperature-related effects which ultimately control the large-scale patterns of pH and Ω arag . Temperature effects are previously divided into internal and external effects, where the former one impacts the seawater CO 2 chemistry through chemical speciation changes (also known as thermal effects) and the latter one impacts the air-sea CO 2 gas exchange through CO 2 solubility changes (also known as nonthermal effects) (Cai et al, 2020;Jiang et al, 2015Jiang et al, , 2019Ouyang et al, 2020;Xue et al, 2020). In terms of pH, the external temperature effect offsets the internal one and they result in only a small pH variation along temperature gradient.…”