Human-induced changes in carbon fluxes across the land-ocean interface can influence the global carbon cycle, yet the impacts of rapid urbanization and establishment of wastewater treatment plants (WWTPs) on coastal ocean carbon cycles are poorly known. This is unacceptable as at present ∼64% of global municipal wastewater is treated before discharge. Here, we report surface water dissolved inorganic carbon (DIC) and sedimentary organic carbon concentrations and their isotopic compositions in the rapidly urbanized Jiaozhou Bay in northeast China as well as carbonate parameters in effluents of three large WWTPs around the bay. Using DIC, δC and total alkalinity (TA) data and a tracer model, we determine the contributions to DIC from wastewater DIC input, net ecosystem production, calcium carbonate precipitation, and CO outgassing. Our study shows that high-DIC and low-pH wastewater effluent represents an important source of DIC and acidification in coastal waters. In contrast to the traditional view of anthropogenic organic carbon export and degradation, we suggest that with the increase of wastewater discharge and treatment rates, wastewater DIC input may play an increasingly more important role in the coastal ocean carbon cycle.

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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.

Both natural processes and human activities affect seawater calcium carbonate saturation state (Ωarag), while the mechanisms are still far from being clearly understood. This study analysed the seawater surface Ωarag during summer and winter in Jiaozhou Bay (JZB), China, based on two cruises observations performed in January and June 2017. The ranges of Ωarag values were 1.55~2.92 in summer and 1.62~2.15 in winter. Regression analyses were conducted to identify the drivers of the change of Ωarag distribution, and then the relative contributions of temperature, mixing processes and biological processes to the spatial differences in Ωarag were evaluated by introducing the difference between total alkalinity (TA) and dissolved inorganic carbon (DIC) as a proxy for Ωarag. The results showed that biological processes were the main factor affecting the spatial differences in Ωarag, with relative contributions of 70% in summer and 50% in winter. The contributions of temperature (25% in summer and 20% in winter) and the mixing processes (5% in summer and 30% in winter) were lower. The increasing urbanization in offshore areas can further worsen acidification, therefore environmental protection in both offshore and onshore is needed.

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