Hypoxia off the Yangtze River estuary:
Observations and
simulations
Feng Zhou1,2, Fei Chai3, Daji Huang1,2, Huijie Xue3,4, Jianfang Chen1,2,
Peng Xiu4, Jiliang Xuan1, Jia Li1, Dingyong Zeng1, Xiaobo Ni1 and Kui
Wang1
1
State Key Laboratory of Satellite Ocean Environment Dynamics, Second
Institute of Oceanography, State Oceanic Administration, Hangzhou,
China. E-mail: zhoufeng@sio.org.cn
2
Ocean College, Zhejiang University, Hangzhou, China
3
School of Marine Science, University of Maine, Orono, ME, U.S.A.
4
State Key Laboratory of Tropical Oceanography, South China Sea
Institute of Oceanology, Chinese Academy of Sciences, Guangzhou,
China
Onset and extent of hypoxia off the Yangtze River estuary and the
adjacent coast changes from year to year, and the mechanism is poorly
known due to varied nutrient sources and the complex circulation
systems. Regional Ocean Modeling Systems (ROMS) coupled with a
11-component biogeochemical model has been applied to the East China
Sea to reproduce the historically large hypoxia extent observed in 2006.
The physical part of the model is driven by climatological forcing and
2006 real time forcing is verified, before the biological part is coupled.
The biological model is based on the Carbon, Silicate, Nitrogen
Ecosystem Model (CoSiNE). The modelled chlorophyll a, nutrients and
oxygen are all in an agreement with observations. A set of sensitivity
tests are designed to identify the relative contributions of riverine nutrient
sources and upstream Kuroshio nutrients (nitrate, phosphate, silicate) in
controlling the extent of the hypoxic water. All cases produce hypoxia
extent that extends farther north of the Changjiang River mouth,
indicating the significant impacts from an enhanced inward transport
through the shelf break and the Taiwan Strait in 2006. The temporal
variations of hypoxia and the minimum dissolved oxygen concentration
off the estuary are also captured reasonably. Experiments with two types
of benthic layer treatments suggest the importance of sediment oxygen
consumption to the size of the hypoxic zone. Hypoxia extends tens of
meters above the seabed up to the seasonal pycnocline which is shallow
and thin (approximately 10 m) being suppressed upward by strong tidal
mixing at lower layers. Hypoxia is very sensitive to the stratification, that
is episodically relaxed at synoptic scales by strong wind stirring related
with events like typhoons.