Emergence of Summertime Hypoxia and Concurrent Carbonate Mineral Suppression in the Central Bohai Sea, China

Abstract

The Bohai Sea to the west of the North Yellow Sea in the Northwest Pacific is a semienclosed shallow‐water marginal sea on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism‐induced summertime dissolved oxygen (DO) decline and acidification, eight field surveys were conducted between August 2011 and September 2017, investigating seasonal and interannual variations in DO and carbonate system parameters in the central Bohai Sea. The historically lowest Bohai Sea DO was recorded at 66 μmol O2/kg in subsurface waters in early September 2015. By comparing our data set with literature data obtained in the 1980s, 1990s, and 2000s, the emergence history of summertime hypoxia in this marginal sea was shown. Associated with the seasonal DO shortage, the metabolism‐enhanced CO2 accumulation in subsurface waters in summer resulted in critical aragonite saturation states of close to or even less than 1.5, threatening calcifiers with potentially severe consequences for valuable shellfish fisheries. Based on model results combining the aerobic respiration (as indicated by apparent oxygen utilization) and the given wintertime/springtime atmospheric CO2 level of the time, this study suggests that combined stresses of the seasonal DO shortage and CO2 acidification capable of threatening Bohai Sea ecosystems could develop within decades.

Exploring Sources and Biogeochemical Dynamics of Dissolved Methane in the Central Bohai Sea in Summer

“…It is expected that the sea gales weakened the stratification in the Bohai Sea, leading to basin-wide bottom water ventilation. A similar result was observed in August 2012 in the Bohai Sea (Zhai et al, 2019). However, the locations of DO-deficit areas detected in this study were similar to the DO-deficient zones previously outlined by Zhao et al (2017) and Wei et al (2019).…”

“…In this study, DO was relatively abundant as compared to our previous reports (Zhai et al, 2012(Zhai et al, , 2019Zhao et al, 2017), likely due to disturbance by typhoons. During 2-30 days prior to our field survey, three typhoon events (named Ampil, Yagi, and Rumbia, respectively 1 ) affected the Bohai Sea.…”

“…If the stratification is episodically disturbed (e.g., by sea gales), the CH 4 store in bottom waters will be rapidly vented into the atmosphere. At the same time, the water column can be reoxygenated (Zhai et al, 2019), and the aerobic CH 4 oxidation will resume at a relatively high rate. This hypothesis can explain why CH 4 showed a net loss in the oxygenated bottom water in this study (Figure 4), whereas it showed a net increase in hypoxic bottom waters in the East China Sea (Ye et al, 2016).…”

“…The Bohai Sea is a semi-enclosed shallow-water marginal sea of the northwest Pacific Ocean with seasonal stratification (Huang et al, 1999;Zhao et al, 2017;Zhou et al, 2017) and a summertime dissolved oxygen (DO) deficit in its bottom waters (Zhai et al, 2012(Zhai et al, , 2019Zhao et al, 2017;Wei et al, 2019). Spatiotemporal variations in [CH 4 ] and thereby sea-to-air CH 4 fluxes have been previously investigated by Li et al (2010), Zhang et al (2014), and Zang (2018), showing that the Bohai Sea is usually supersaturated with CH 4 , with moderate sea-to-air CH 4 fluxes from 0.8 to 53.9 µmol m −2 d −1 .…”

Comparing Subsurface Seasonal Deoxygenation and Acidification in the Yellow Sea and Northern East China Sea Along the North-to-South Latitude Gradient

“…To quantify the effect of net community metabolism on DIC, we calculated the air-equilibrated DIC (corresponding to a mean air-equilibrated f CO 2 value of 415 ± 5 µatm during our seasonal cruises conducted in 2017-2018) from corresponding field-measured seawater temperature, salinity and TAlk (Zhai, 2018). The air-equilibrated f CO 2 was calculated from the flask analysis data of atmospheric CO 2 mole fraction at the adjacent Tae-ahn Peninsula (TAP) site (36 • 44'N 126 • 08'E), which varied from 406 ppm (ppm = parts of CO 2 per million dry air) in August to 419-421 ppm during January to May in 2017-2018 (Supplementary Figure S1B, data from NOAA/ESRL's Global Monitoring Division) 1 , and corrected to the survey-based barometric pressure and 100% humidity at water temperature and salinity (Zhai et al, 2019). Similar to the definition of AOU (section "Sampling and Analyses"), the DIC departure from the air-equilibrated DIC was defined as the excess DIC (ExcessDIC).…”