The contrasting behaviors of CO2 systems in river-dominated and ocean-dominated continental shelves: A case study in the East China Sea and the Peter the Great Bay of the Japan/East Sea in summer 2014
Introduction
While it only accounts for 7% of the world's oceanic surface area, the continental shelf plays a crucial role in the global carbon cycle by linking terrestrial, oceanic, and atmospheric reservoirs (Walsh, 1991, Mackenzie et al., 2004). Although the continental shelf is generally viewed as a significant sink of atmospheric CO2 at the global scale, with recent estimations converging toward 0.1 to 0.4 PgC year− 1 (Borges et al., 2005, Cai et al., 2006, Chen and Borges, 2009, Chen et al., 2013, Dai et al., 2013, Laruelle et al., 2014, Bourgeois et al., 2016), it remains under-sampled to derive precise information for climate change predictions because of the large diversity and heterogeneity of individual shelf systems (Cai, 2011). Therefore, better global coverage of CO2 studies in continental shelves are still needed to improve global carbon budgets.
Earlier studies suggest a latitudinal trend in carbon sources or sinks, i.e., the low latitude shelf systems are sources of atmospheric CO2, while those in high latitudes are sinks. However, recent research has shown that such a latitudinal trend may not fully withstand the observed global pattern of carbon sinks and sources. For instance, the Adriatic Sea shelf (45.5°N; Turk et al., 2010), Scotian shelf (42°N to 47°N; Shadwick et al., 2010), and southern Bering Sea (53°N to 55°N; Fransson et al., 2006) are CO2 sources, and they are all located in high latitudes (40°N is defined as the boundary separating high/low latitudes in this study). In contrast, the tropical coastal Northwestern Atlantic Ocean acts as a CO2 sink (3°N to 15°N; Cooley and Yager, 2006). This discrepancy demonstrates that the fundamental question of why some shelf systems are sources to the atmosphere, while others are sinks is, to date, still poorly understood.
To acquire a better mechanistic understanding of coastal carbon cycles, Dai et al. (2013) recently proposed that the coastal ocean can be classified into the following two distinct settings: River-dominated Ocean Margins (RiOMar) and Ocean-dominated Ocean Margins (OceMar). The CO2 sink/source status of several RiOMar shelf systems adjacent to the world's large rivers, including the plumes of the Changjiang (Tseng et al., 2011), Amazon (Cooley et al., 2007), Mississippi (Guo et al., 2012, Huang et al., 2015), and Pearl (Cao et al., 2011) Rivers, has been extensively studied. These RiOMar shelf systems have all shown to act as major CO2 sinks at high river discharges, suggesting that CO2 consumption through enhanced primary productivity may surpass CO2 production from the remineralization of terrestrial organic matter, driving the RiOMar system to be a sink during high discharge periods.
OceMar shelf systems did not show any consistency in CO2 sink/source status. Dai et al. (2013) hypothesized that this is because OceMar systems are characterized by dynamic interactions with the open ocean, and their CO2 sink/source status may largely depend on the CO2/nutrients balance between the contributions from the open ocean and the subsequent biological consumption in the surface layer of OceMar systems. If the contribution ratio of CO2/nutrients in the source water from the open ocean is higher than the subsequent biological consumption ratio in the surface mixed-layer, there would be an excess of CO2 accumulated. The excess CO2 in surface waters would ultimately be removed by degassing into the atmosphere, driving the OceMar to be a source of atmospheric CO2. A lower CO2/nutrient contribution ratio would lead to a CO2 deficiency, making the OceMar into a CO2 sink. The OceMar hypothesis has been verified by Dai et al. (2013) using data from the tropical South China Sea and the Caribbean Sea and has successfully explained their CO2 source status. However, it has never been examined using the data from the continental margin in the higher latitudes.
In this study, within the framework of a Taiwan-Russia joint research project, we concurrently investigated the CO2 system and pertinent hydrographic parameters in two distinct continental shelf systems in the Northwest Pacific in the summer of 2014. The East China Sea (ECS) is recognized as a river-dominated system, and the Peter the Great Bay (PGB) of the Japan/East Sea represents an ocean-dominated system. While the CO2 system of the ECS has been extensively studied (Tsunogai et al., 1999, Wang et al., 2000, Chou et al., 2009a, Chou et al., 2011, Tseng et al., 2014, Guo et al., 2015), the availability of CO2 data in the PGB consists of a sole survey of autumn data (Tishchenko et al., 2012). Therefore, the present work reports the first comprehensive summertime CO2 data from the PGB. Furthermore, the concurrent investigations in the RiOMar ECS and the OceMar PGB provide us a rare opportunity to demonstrate how terrestrial and oceanic contributions can affect CO2 dynamics in two contrasting continental shelf systems. Finally, data from the PGB were utilized to test the applicability of the OceMar hypothesis to continental margins in high latitudes.
Section snippets
Study sites
The subtropical East China Sea (ECS), located between 25°N and 34°N, is one of the largest marginal seas in the northwest Pacific (Fig. 1a). It is characterized by a broad continental shelf (500 × 103 km2) and enormous runoff from the Changjiang River (Yangtze River). The water discharge of the Changjiang River is ranked fourth in the world after the Amazon, Zaire, and Orinoco Rivers, with an annual average discharge of 940 km3. The highest monthly discharge occurs in July, which is approximately
Hydrological settings
The distributions of sea surface temperature (SST) and salinity (SSS) in the ECS and the PGB are shown in Fig. 3. SST varied from 24.2 to 30.4 °C in the ECS, and generally increased in the offshore direction from the Changjiang estuary toward the Ryukyu Island chain (Fig. 3a). SST in the PGB, varying from 16.3 to 22.5 °C, was significantly lower than in the ECS (Table 1) and revealed a contrasting offshore decreasing trend (Fig. 3b). The lower SST in the PGB is expected because it is located in
Factors controlling the variability of pCO2 in the ECS and the PGB
It is known that factors controlling the pCO2 variation in surface ocean include (1) temperature, (2) horizontal and vertical mixing of water masses, and (3) biological production/respiration. The temperature dependence of pCO2 was simulated using CO2SYS with the average TA and DIC data in the ECS (DIC = 1890 μmol kg− 1, and TA = 2185 μmol kg− 1) and the PGB (DIC = 1972 μmol kg− 1, and TA = 2181 μmol kg− 1). The results show that the relationship between pCO2 and SST in the PGB generally follow the simulated
Summary and concluding remarks
In this study, we have described the spatial distributions of TA, DIC, pCO2, nitrate, and Chl a, and examined the factors controlling pCO2 variability in two distinct continental shelf systems in the Northwest Pacific Ocean, namely the RiOMar ECS and the OceMar PGB, in the summer of 2014. The results showed that the RiOMar ECS acted as a sink of atmospheric CO2 in which the dominant controlling factor of pCO2 variation was biological production. The OceMar PGB was a CO2 source in which the
Acknowledgments
We are grateful to the Captains, crew and technicians of R/V Ocean Researcher I and R/V Professor Gagarisky for assistance with deck operations and shipboard sampling, and to C.Y. Yang and R.W. Syu for laboratory assistance. Constructive comments from two anonymous reviewers have greatly improved the manuscript. This work was supported by the Ministry of Science and Technology of Taiwan to W.C. Chou (NSC 102-2923-M-019-001-MY3 and MOST 104-2611-M-019-018-MY3) and the grants of Russian
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