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

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 km 2 . The Changjiang River is a major contributor to hypoxia formation because it discharges large amounts of freshwater and nutrients into the region. However, modelling 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 versus oceanic nutrient sources to hypoxia have not been estimated before. Here, we combine 5 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 River 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 River remains the main control of hypoxia formation also in this region. Model scenarios 10 with reduced Changjiang River 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.

Wind-driven stratification patterns and dissolved oxygen depletion off the Changjiang (Yangtze) Estuary

“…Higher wind speed causes stronger vertical mixing and vanishing of hypoxia (Zheng et al, 2016). In July 2015, 1.5 months before our survey, hypoxia was terminated by a typhoon, but 2 d later, hypoxic conditions were reestablished (Guo et al, 2019). Our field measurements showed that the upper boundary of the DO depletion is strongly linked to the upper boundary of KSSW (Fig.…”

“…Intensive hypoxia can start from very shallow depths (at 4-8 m) in the northern area (Fig. 5g), and it can develop very fast under favorable conditions (Guo et al, 2019). At the same time, the bottom layer there can be easily ventilated by wind stirring or downwelling as we observed at section N15 in 2015 (Fig.…”