1. Title
Coupling of the shelf process and the Kuroshio in the development of hypoxia off the Changjiang
Estuary
2. Members’ names and affiliations
Feng Zhou

Second Institute of Oceanography, Ministry of Natural Resources, China

3. Aim(1) Understand how many Kuroshio-related nutrients are transported to the mid-shelf and the estuary,
and their contributions to the phytoplankton blooms(2) Make clear how much low dissolved oxygen (DO) water is transported to the mid-shelf and theestuary and thus reduces the concentration of DO before and during the period when hypoxia usuallyoccurs(3) Promote the exchanges and cooperation between the Second Institute of Oceanography, Ministryof Natural Resources, China and the Center for Marine Environmental Studies (CEMS) in Ehime
University.

4. Procedure
Both observational data and numerical modelling are used in this study. First, in situ DO and remotelysensed Chl a dataset (Fig. 1) are used to give a seasonal variation (Fig. 2). Then a coupled numericalmodel ROMS-CoSiNE is used to simulate the physical process and ecosystem in the East China Sea(ECS), and a series of sensitivity experiments are set to examine the different effects of nutrient
loadings from rivers and Kuroshio, advection, diffusion, and biological on the variation of hypoxia off
the Changjiang Estuary.

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5. Results
The simulated seasonal-mean Chl a concentrations show similar spatial patterns and seasonal
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variations as the observed (Fig. 2). They were high in the inner shelf where water depth is less than 50m, decreasing towards the outer shelf and slope with local maximum concentrations oﬀ the Changjiang
Estuary. Although the satellite data coverage in winter (December–February) was poor, the available
data still show low Chl a because phytoplankton growth was limited by low temperature andinsuﬃcient light. In winter and spring, Fig. 2 shows large Chl a compared to the observations southeastof the 100-m isobath. There was a trend of oﬀshore extension of mid-shelf water with highconcentration of Chl a at the surface at 28°N–30°N (Fig. 2b). Close to the suspected location, weanalyzed the observations along Transect PN. Fig. S4 shows a strong nitrate-cline below 200 m withlow concentration of nitrate near the surface in January 2006. There was a dome of high nitrate at theshelf edge, resulting in an upward tilt of nitrate-cline. A similar low-oxygen dome also occurred at theshelf edge, above which an oxygen minimum appeared near surface. The oﬀshore extension wasprobably caused by the oﬀshore branch of the TWC, which also induced upwelling at the shelf edge.
A signiﬁcant increase of Chl a appeared in spring (March–May), when the satellite Chl a concentration
reached the highest value of 17.3 mg m−3 oﬀ the Zhejiang coast, while the simulated value was 18.4
mg m−3 oﬀ the Subei Shoal.
The proﬁles in Fig. 7 show large variability of DO structure in the vertical. The observed oxygen-cline
becomes shallower and intensiﬁes from June to August. The DO below the oxygen-cline is as low as
4.0 mg L−1 in June and decreases to less than 2.0 mg L−1 in August. In addition to the mid-depth
minimum, the observed DO proﬁle on August 13 also shows a local maximum at 15 m (Fig. 7b), likely
due to episodic entrainment of phytoplankton from the upper layer into pycnocline.

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Large variability of hypoxia in the ECS can also be seen from the horizontal distribution of DO,
particularly in terms of hypoxic water extent. Hypoxia is observed in June–October and in two
diﬀerent regions (Fig. 8). In June hypoxia appears only oﬀ the Zhejiang coast (Fig. 8a), while in Julyit appears mainly oﬀ the changjiang estuary with a small area at the eastern edge of the study region(Fig. 8b). In August, hypoxia is widely observed but most signiﬁcantly north of thechangjiang estuary (Fig. 8c). In September-October, the hypoxia center occurs south of thechangjiang estuary. The observed hypoxia area increased from 0.46 × 104 km2 in June to 0.74 ×104 km2 in July, and then to 7.70 × 104 km2 in August. The extent in September is approximately 1/7of that in August (note that we had less observational data in September) and increases slightly inOctober. The simulated hypoxic water appears southeast of the changjiang estuary in late June.
From July to August, both observations and simulations show that hypoxic water expands signiﬁcantly.
Though large misﬁt of hypoxic zones occurred in August, the model reproduced the signiﬁcantnorthward spread of hypoxia north of the changjiang estuary, resembling the observations fromJuly to August. The model captured the essential intraseasonal and seasonal variability of hypoxia,
except for that in October, when hypoxia disappeared unrealistically. Note that from July to September
the simulated hypoxic water was separated into two parts with the main body oﬀ thechangjiang estuary and the northern part southeast of the Subei Shoal (Fig. 8g–i). The northern partof the hypoxia is likely associated with the detached low-salinity water from the Changjiang diluted
water plume, which carries nutrient-rich water northward (Xuan et al., 2012). It could also be related
to the seaward transport of nutrients and phytoplankton from the Subei Shoal. The northern hypoxic
zone appears in waters deeper than 30 m rather than in the Subei Shoal where water is shallower than
20 m and usually well mixed vertically.

6. Publication/Conference presentation
Publication[1]Jiang, Z., J. Chen, H. Zhai, F. Zhou, X. Yan, Y. Zhu, J. Xuan, S. Lu, and Q. Chen, 2019,
Kuroshio shape composition and distribution of filamentous diazotrophs in the East China Sea
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and

Southern

Yellow

Sea,

Journalof

GeophysicalResearch:

Oceans,doi:

10.1029/2019JC015413.
[2]

Ye, W., G. Zhang, P. Li, F. Zhou, and C. Liu, 2019, Variations in dissolves methane in theYellow Sea during the spring algal blooms of 2009, Journal of Ocean University of China, 18,896-912. Doi:10.1007/s11802-019-3993-4.

[3]

Jiang, Z., J. Chen, Y. Gao, H. Zhai, H. Jin, F. Zhou, S. Lu, X. Yan, and Q. Chen, 2019,Regulation of spatial changes in phytoplankton community by water column stability andnutrients in the Southern Yellow Sea, Journal of Geophysical Research: Biogeosciences, doi:
10.1029/2018JG004785.

[4]Xuan, J., Y. He, F. Zhou, C. Tang, X. Zheng, H. Liu, L. Yu, and J. Chen, 2019, Aquacultureinduced boundary circulation and its impact on coastal frontal circulation, Environmental
Research Communications, 1, 051001.

[5]

Hao, Q., F. Chai, P. Xiu, Y. Bai, J. Chen, C. Liu, F. Le, and F. Zhou, 2019, Spatial and temporalvariation in chlorophyll a concentration in the Eastern China Seas based on a locally modified
satellite dataset, Estuarine, Coastal and Shelf Science, 220, 220-231.

Conference presentation
Zhou F., 2019, Coupling of the shelf process and the Kuroshio in the development of hypoxia off theChangjiang Estuary, International symposium on coastal ecosystem change in Asia: hypoxia,eutrophication, and nutrient conditions, Ehime University, Matsuyama, Japan, Oral, November, 2019.
7. Perspectives in Future(1) More integrated observations are expected to improve the prediction of hypoxia variability bymonitoring hypoxia and related important parameters, sch as the PAR at the estuary and nutrient
loading at the river mouth.
(2) An improved algorithm of under-water light availability incorporating into the coupled model isalso necessary to capture reasonable distribution of phytoplankton, particularly in turbid coastal waters.
(3) Enhance the cooperation between Ocean University of China and CEMS of Ehime University.

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