Elsevier

Estuarine, Coastal and Shelf Science

Volume 151, 5 December 2014, Pages 355-360
Estuarine, Coastal and Shelf Science

Looming hypoxia on outer shelves caused by reduced ventilation in the open oceans: Case study of the East China Sea

https://doi.org/10.1016/j.ecss.2014.08.010Get rights and content

Highlights

  • Kuroshio Intermediate Water increases in nutrients yet decreases in dissolved oxygen.

  • Changes are due to reduced ventilation in North Pacific Intermediate Water.

  • More severe eutrophication and hypoxia in the East China Sea may occur in the future.

  • Other shelves in contact with the intermediate water may be affected too.

Abstract

The discharge of nitrate and phosphate from Changjiang (Yangtze River) has increased in recent decades. Eutrophication off the mouth of Changjiang has subsequently become a serious problem, as evidenced by the hypoxia area reaching 12,000 km2. This study demonstrates that in the wide East China Sea (ECS) the nitrate and phosphate concentrations in the Kuroshio Intermediate Water (KIW) have also increased, but the dissolved oxygen (DO) concentration has decreased since as early as 1982, most likely owning to reduced ventilation in the North Pacific Intermediate Water (NPIW). Conversely, the Kuroshio Tropical Water (KTW) has decreased in the nitrate and phosphate concentrations yet increased in DO concentration. As KIW contributes substantially to the upwelling, the nitrate and phosphate concentrations in the bottom water on the outer shelf of the ECS appear to have increased as well, but the DO has decreased. Given that the nutrient inputs from both the land and the Kuroshio Current have increased, yet the input of DO from the Kuroshio has decreased, more severe eutrophication and hypoxia may occur in the entire ECS. Similar processes may also affect other shelves that come into contact with NPIW.

Introduction

As is widely recognized eutrophication and related hypoxia (dissolved oxygen (DO) < 2 mg L−1 or <30%) adversely impact marine ecosystems (Howarth, 2008, Rabalais et al., 2010, Lui and Chen, 2012). As the most sensitive and first to be affected, coastal regions suffer from increasing terrestrial inputs of nutrients and organic matter. Recent decades have witnessed a several folds increase in riverine nitrate and phosphate fluxes over their original values (Howarth, 2008, Conley et al., 2009). Such nutrients are largely consumed by phytoplankton in the river plume from spring to summer. Stratification of seawater during the summer reduces bottom seawater ventilation. Consequently, in addition to consuming DO at the bottom, decomposition of the settled organic matters causes hypoxia when the oxygen at the bottom is insufficiently replenished. In 1970, 60 coastal ecosystems were reportedly under hypoxia, subsequently increasing to over 400 in 2007, when they covered more than 245,000 km2 of the sea bottom (Diaz and Rosenberg, 2008). Strengthening stratification of seawater under global warming and increased nutrient fluxes from terrestrial sources are expected to exacerbate the above situation (Diaz and Rosenberg, 2008, Chen, 2008a, Rabalais et al., 2010).

In the case of coastal eutrophication, incoming offshore seawater has rarely been addressed. Although surface seawater offshore generally contains low nutrient concentrations, the upwelled subsurface offshore seawater is a major source of nutrients and DO to coastal regions and marginal seas (Feely et al., 2008, Bauer et al., 2013, Yang et al., 2013), such as in the East China Sea (ECS), one of the world's most productive marginal seas (Chen, 1996, Chen, 2008b).

Changjiang (Yangtze River) contributes roughly 90% of all freshwater discharge to the ECS, and possibly the same proportion of land-derived nutrients as well (Chen and Wang, 1999). However, it has been reported that the dominating source of nutrients to the ECS, even all the way to the coast, comes from the upwelling of subsurface waters from the Kuroshio Current (Chen, 1996, Yang et al., 2013). Consider phosphate (PO43−) as an example, in which the upwelled subsurface seawater from the Kuroshio Current contributes ten times more than all rivers entering the ECS combined (Chen, 1996, Chen and Wang, 1999). Correspondingly, any change in the nutrient concentration in the subsurface of the Kuroshio seawater could significantly impact the nutrient dynamics on the ECS continental shelf. According to a previous study, the nitrate (NO3) concentration in the middle layers across the Kuroshio Current in the Okinawa Trough in the ECS is also increasing, yet with a decreasing DO (Guo et al., 2012). This study attempts to quantify the temporal rates of changes in DO, apparent oxygen utilization (AOU), NO3, PO43− and chlorophyll-a (Chl-a) concentrations, potential temperature (θ), and salinity (S) along the PN-line section (Fig. 1) with the first-order simple linear regression (SLR) method. The distinguishing patterns of changes between Kuroshio Intermediate Water (KIW) and Kuroshio Tropical Water (KTW) are also described. Implications for the changes in the coastal eutrophication and hypoxia are discussed as well.

Section snippets

Study area, dataset and methods

As one of the largest marginal seas and one of the most productive fishing grounds worldwide, ECS is supported by nutrients largely from the upwelled KIW and the Yangtze River (Chen, 1996). In the eastern region of ECS, oceanographic observations have been made regularly along a section, i.e. the PN line, by the Nagasaki Marine Observatory of the Japan Meteorological Agency (JMA). The PN-line data are generally collected on a quarterly basis each year. Generally, time-series of each parameter

Spatial distributions and upwelling of KIW

Fig. 2 shows the plot of the annual average θ vs. average S at each depth between 1982 and 2010. Obviously, the θS distribution of seawater along the PN-line is between that of the West Philippine Sea (WPS) and the South China Sea (SCS). Generally speaking, the WPS seawater mixes with the SCS seawater off the Luzon Strait (Chen and Wang, 1998; Chen, 2005). Those waters flow off the eastern coast of Taiwan into the ECS as the Kuroshio Current. Mixed with surface and subsurface seawaters of the

Conclusions

In summary, the NO3 and PO43− concentrations in the wide ECS away from the Changjiang River mouth appear to be increasing, along with a decreasing DO yet increasing AOU. This event is due to the upwelling of KIW along the continental slope and onto the bottom of the shelf. This process has undoubtedly increased nutrient inventories, yet reduced the DO concentration of bottom waters on the shelf. Hurricanes during the summer and fall, and winter cooling mix the increased nutrients to the

Acknowledgments

The authors would like to thank the Nagasaki Marine Observatory of the Japan Meteorological Agency for providing the invaluable data. The Aim for the Top University Plan (03C 0302 04) and the Ministry of Science and Technology of Taiwan are acknowledged for financially supporting this research under contracts NSC 101-2611-M-110-010-MY3 and 102-2611-M-110-003. D.K. Chen, J.L. Zhou, X.H. Wang and two anonymous reviewers provided valuable comments that strengthened the manuscript.

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