Quantifying the contributions of riverine vs. oceanic nitrogen to hypoxia in the East China Sea

Abstract

Abstract. In the East China Sea, hypoxia (oxygen ≤ 62.5 mmol m−3) is frequently observed off the Changjiang (or Yangtze River) estuary covering up to about 15 000 km2. The Changjiang is a major contributor to hypoxia formation because it discharges large amounts of freshwater and nutrients into the region. However, modeling and observational studies have suggested that intrusions of nutrient-rich oceanic water from the Kuroshio Current also contribute to hypoxia formation. The relative contributions of riverine vs. oceanic nutrient sources to hypoxia have not been estimated before. Here, we combine a three-dimensional physical-biogeochemical model with an element-tracing method to quantify the relative contributions of nitrogen from different riverine and oceanic sources to hypoxia formation during 2008–2013. Our results suggest that the hypoxic region north of 30∘ N is dominated by Changjiang inputs, with its nitrogen loads supporting 74 % of oxygen consumption. South of 30∘ N, oceanic nitrogen sources become more important, supporting 39 % of oxygen consumption during the hypoxic season, but the Changjiang remains the main control on hypoxia formation also in this region. Model scenarios with reduced Changjiang nitrogen loads and reduced open-ocean oxygen levels suggest that nitrogen load reductions can significantly reduce hypoxia in the East China Sea and counteract a potential future decline in oxygen supply from the open ocean into the region.

A numerical model study of the main factors contributing to hypoxia and its interannual and short-term variability in the East China Sea

“…This aspect was neglected in previous studies, which only emphasized the role of advection as an oxygen sink promoting hypoxia formation (Ning et al, 2011;Qian et al, 2017). The Taiwan Warm Current originates from the subsurface of the Kuroshio Current northeast of Taiwan and thus represents an intrusion onto the continental shelves from the open ocean (Guo et al, 2006). In addition to oxygen advection, nutrients are transported, supporting PP on the ECS shelves (Zhao and Guo, 2011;Grosse et al, 2020).…”

“…The Taiwan Warm Current originates from the subsurface of the Kuroshio Current northeast of Taiwan and thus represents an intrusion onto the continental shelves from the open ocean (Guo et al, 2006). In addition to oxygen advection, nutrients are transported, supporting PP on the ECS shelves (Zhao and Guo, 2011;Grosse et al, 2020). The intrusion of the Taiwan Warm Current and the Kuroshio Current accompanied by relatively cold and saline water, and nutrient and oxygen transport, is thought to influence hypoxia development (Li et al, 2002;Wang, 2009;Zhou et al, 2017), but no quantification of the relative importance has occurred until now (see companion paper by Grosse et al, 2020, using the same model).…”

“…The Kuroshio Current is stronger in summer than in winter. The model captures the seasonal pattern of the current system and resolves currents in the ECS and Yellow Sea (also see Grosse et al, 2020).…”

“…We also present an oxygen budget to quantify the relative importance of SOC and the influence of lateral advection of oxygen in the model. The companion study by Grosse et al (2020) used the same model to quantify the importance of intrusions of nutrientrich oceanic water from the Kuroshio Current for hypoxia development off the CE. The physical model is based on the Regional Ocean Modeling System (ROMS; Haidvogel et al, 2008) and was implemented for the ECS by Bian et al (2013a).…”

The Globalization of Cultural Eutrophication in the Coastal Ocean: Causes and Consequences

“…Intrusions of nutrient-rich oceanic water from the Kuroshio also contribute to the development of hypoxia. The hypoxic region north of 30 • N is dominated by Changjiang inputs, with its N loads supporting 74% of oxygen consumption; south of 30 • N, oceanic nitrogen sources become more important, supporting 39% of oxygen consumption during the hypoxic season, but the Changjiang remains the main control on hypoxia formation also in this region (Große et al, 2020). The importance of oceanic nutrient supply distinguishes hypoxia in the ECS from the otherwise comparable situation in the NGM, where a similar spatial extent of hypoxia is fueled by riverine inputs of anthropogenic nutrients from the Mississippi-Atchafalaya River system (Fennel and Testa, 2019).…”