Interdisciplinary Studies on Environmental Chemistry—Marine Environmental Modeling & Analysis,
Eds., K. Omori, X. Guo, N. Yoshie, N. Fujii, I. C. Handoh, A. Isobe and S. Tanabe, pp. 19–30.
© by TERRAPUB, 2011.

Response of Nutrients and Primary Production over the Shelfin the East China Sea to the Reductionof Oceanic Nutrient Supply
Xinyu GUO 1 and Liang ZHAO2
1

Center for Marine Environmental Studies (CMES), Ehime University,
2-5 Bunkyo-cho, Matsuyama 790-8577, Japan2
Physical Oceanography Laboratory, Ocean University of China,
238 Songling Road, Qingdao 266100, P.R. China(Received 24 December 2010; accepted 12 January 2011)

Abstract—A three-dimensional coupled biophysical model was used to examine
the response of oceanic nutrients and primary production over the continental
shelf to reduction of the oceanic nutrient supply in the East China Sea (ECS).
The model consisted of two parts: the hydrodynamic module was based on anested model with a horizontal resolution of 1/18 degree, whereas the biologicalmodule was a low trophic level ecosystem model including two types ofphytoplankton, three elements of nutrients, and biogenic organic material.
After completing a control calculation designed to reproduce the climatologicalconditions of this low trophic level ecosystem, the concentrations of nutrientsin the Kuroshio water were artificially reduced by 30%. A reduction in
nutrients over the shelf was then observed in the bottom layer from spring tosummer, and in the surface layer from autumn to winter. A reduction in primary
production over the shelf was found not only in the surface layer in winter but
also in the subsurface layer over the shelf from spring to autumn.
Keywords: ecosystem modelling, oceanic nutrients, Kuroshio, East China Sea,
primary production

INTRODUCTION

The East China Sea (ECS) is one of the major marginal seas of the northwestern
Pacific (Fig. 1). Many rivers, including the Changjiang River (Yangtze River),provide a substantial input of freshwater and nutrients into adjacent seas (Zhang,1996). On the other hand, the ECS also receives water and associated nutrientsfrom the Taiwan Strait, whose volume transport is greater than river discharge
into the ECS by as much as two orders of magnitude (Isobe, 2008). The Kuroshio
in the ECS flows along the shelf break (see 200 m isobath in Fig. 1), and theonshore volume transport of the Kuroshio across the break has been reported to
be within the same order as the transport through the Taiwan Strait (Guo et al.,2006; Isobe, 2008).

19

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X. GUO and L. Z HAOLuan

Liaohe R.

he

Haihe R.

R.

Yalujiang R.

Bohai Sea
n
Hua

ghe

Han R.

R.

Korea
Tsushima St.

Yellow Sea
Huaihe R.

Jeju Is.

KyushuChina
Changjiang R.

East China Sea

PN

Tokara St.

Minjiang R.

Taiwan St. Taiwan
Fig. 1. Model domain and bathymetry. Rivers are denoted by dots along the coastline; contours withnumbers are isobaths in meters. The 200 m isobath along the shelf break is overlapped by a linewith dots and numbers, across which the fluxes of volume and nutrients are calculated andpresented in Table 1.

Using a box model to calculate the annual nutrient budget over the shelf ofthe ECS, Chen and Wang (1999) suggested the Kuroshio subsurface water has amore important role in supplying nutrients onto the ECS shelf than rivers do.
Zhang, J. et al. (2007) revisited this calculation for budget in summer and winterand confirmed the important role of the nutrients supplied by Kuroshio subsurface
water intrusion onto the shelf. These findings are supported by a passive tracer
experiment following Kuroshio water with a three-dimensional numerical model

SIL

SIL

SPM CHLa

SIS

SIS

SIS

SIL

DIA

DIA

DIA

FLA

FLA
N2
DET

DET

DIP

DET

DIN

DIN

DIN

DIN

DIP

DIP

DIP

FLA

SIL

SPM

SPM

SPM

Diffusion

Temperature

Elevation

Velocity

Hydrodynamic Model (POM)

river input

Fig. 2. Schematic illustration of the biophysical model that has a hydrodynamic module and a biological module. The hydrodynamic
module is based on the Princeton Ocean Model (POM) (Mellor, 2003). The biological module includes three elements of nutrients(dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and silicate (SIL)), two types of phytoplankton (diatoms(DIA) and flagellates (FLA)), and two types of biogenic organic materials (dead organic matter containing nitrogen and phosphorus(DET) and biogenic silica (SIS)). See Zhao and Guo (2011) for the processes included in this module.

BottomEuphotic depth

Surface

atmospheric deposition

Nutrients and Primary Production over the ECS Shelf
21

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X. GUO and L. Z HAOTable. 1. Change in onshore flux of volume, DIN, DIP and silicate across the shelf break definedby the 200 m isobath shown in Fig. 1.

Case (ctrl)
Case (-30%)
Ratio

Volume (Sv)DIN (kmol s– 1 )DIP (kmol s– 1 )SIL (kmol s– 1 )

1.531
1.531
1

9.361
6.270
0.670

0.718
0.476
0.663

18.21911.8370.650(Guo et al., 2006).
Recently, we re-evaluated the nutrient flux across the shelf break using athree-dimensional numerical biophysical model and demonstrated that oceanic
nutrients were readily utilized by phytoplankton over the shelf in the ECS (Zhaoand Guo, 2011). In this study, we applied the same model to examine theresponses of nutrients and primary production over the shelf to a reduction in theonshore flux of oceanic nutrients across the shelf break.
MODEL CONFIGURATION AND CALCULATION PLAN

Our model consisted of two parts: a hydrodynamic module and a biologicalmodule. The hydrodynamic module provided physical parameters, such as watertemperature, velocities, and diffusivity coefficients, to the biological module.
The two modules were run simultaneously in coupled runs.
The hydrodynamic module is based on the Princeton Ocean Model (Blumberg
and Mellor, 1987; Mellor, 2003) and configured with a nesting method to obtaina high horizontal resolution (1/18 degree) for the ECS, as described in detail by
Guo et al. (2003). In the vertical direction, 21 sigma levels were used. Differencesfrom the previous version were the explicit inclusion of freshwater input from sea
surface and rivers and the addition of tidal forcing (M2, S2, K1 and O1 tides) along
the lateral boundary (Wang et al., 2008).
The biological module (Fig. 2) is based on the biological part of NORWECOM(Aksnes et al., 1995; Skogen et al., 1995; Skogen and Søiland, 1998) andreconstructed for the ECS. The model components include three elements ofnutrients (dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus(DIP), and silicate (SIL)), two types of phytoplankton (diatoms (DIA) andflagellates (FLA)), and two types of biogenic organic material (dead organic
matter (DET) and biogenic silica (SIS)). For the details including parameters andsome equations used in this model, please refer to Zhao and Guo (2011).
The model domain covers the Bohai Sea, Yellow Sea, and ECS with an open
boundary along the southern and eastern boundaries (Fig. 1). With the objectiveof reproducing seasonal variation, the hydrodynamic module was driven bymonthly river runoff, monthly heat flux, monthly evaporation and precipitation
rates, and monthly ocean currents at the open boundary. During model integration,
the monthly data were linearly interpolated to every time step. The tidal currents
were specified at the open boundary as described by Wang et al. (2008). A

Nutrients and Primary Production over the ECS Shelf

Case(–30%) - Case(ctrl)

23
DIN(mmol m–3)

Fig. 3. Anomaly of monthly DIN (mmol m–3) in the surface layer (2 m depth) between Case (-30%)
and Case (ctrl).

regression relation with nine coefficients (annual mean, two harmonic constants
each for the annual period, for the semi-annual period, for the 4-month period,and for the 3-month period) in the Scatterometer Climatology of Ocean Winds(Risien and Chelton, 2008) was used to describe the wind fields at every time step.
In order to reproduce the seasonal variations of nutrients and chlorophyll a, thesame monthly conditions were prescribed at river mouths, at the open boundary,
and at the air-sea interface for the biological module.
The calculation was started with winter initial conditions and was spun up
for two years; model results in the third year were analyzed. The initial conditions
for three elements of nutrients followed Chen (2009), while those along open
boundaries were from the World Ocean Atlas 2005 (WOA2005) (Garcia et al.,2006a, b). Runoff from ten major rivers (Fig. 1) was from the Marine Atlas ofBohai Sea, Yellow Sea, East China Sea, Hydrology (Chen, 1992). The
concentrations of nutrients in rivers and those for atmospheric dry and wet
deposition of nutrients were obtained from published data (Zhang, 1996; Wan etal., 2002; Zhang, G. S. et al., 2007; Liu et al., 2009). Solar radiation was

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X. GUO and L. Z HAO

Case(–30%) - Case(ctrl)

DIN(mmol m–3)

Fig. 4. Same as Fig. 3 but for DIN (mmol m–3) in the bottom layer, defined as the deepest sigma layer.
The color range is different from that used in Fig. 3.

calculated using the model given by Dobson and Smith (1988), and cloud cover
data was from the NCEP/NCAR reanalysis (Kalnay et al., 1996). With the above
configuration, the model has been demonstrated to be able to reproduce seasonalvariations in nutrients and chlorophyll a found in observational data from the
ECS (Zhao and Guo, 2011). Hereafter, we call this simulation the control
experiment and denote it as Case (ctrl).
To see the response of nutrients and chlorophyll a to a reduction in theoceanic nutrient supply across the shelf break of the ECS, we applied sensitivity
experiments in which we multiplied the concentration of nutrients (DIN, DIP, andsilicate) in Kuroshio water from the sea surface to bottom along the southernboundary (from Taiwan to 124°E) by a constant of 0.7, but kept the otherboundary conditions, initial conditions, and model integration schedule the sameas in the control experiment. As shown in Table 1, this method reduced the supplyof oceanic nutrients across the shelf break from the Kuroshio but did not affectthe flow fields. Hereafter, we call this case a sensitivity experiment and denoteNutrients and Primary Production over the ECS Shelf

Case(–30%) - Case(ctrl)

25
Chla(mg m–3)

Fig. 5. Same as Fig. 3 but for chlorophyll a (mg m–3) in the surface layer.

it as Case (-30%). The difference in results between Case (-30%) and Case (ctrl)
is only caused by the change in the supply of oceanic nutrients across the shelf
break, and therefore it can give us some insight as to the potential role of changing
oceanic nutrient concentrations in the low trophic level dynamics of the ECS
ecosystem.
CHANGES IN SURFACE AND BOTTOM NUTRIENTS AND
SURFACE CHLOROPHYLL A OVER THE SHELF OF THE ECSThe decrease in nutrients (e.g., DIN) in the surface layer appeared mostly
offshore of the Changjiang estuary (Fig. 3). In summer (June–August), a negative
anomaly in DIN concentration (Case (-30%) – Case (ctrl)) was found along thecoast from 29°N to 34°N, but offshore the DIN anomaly was positive. After
September, the area with a negative DIN anomaly grew and extended to the Jeju
Strait (the channel between Jeju Island and the southern coast of Korea) andfarther to the Tsushima Strait in winter (December–February).
The decrease in nutrients (e.g., DIN) in the bottom layer over the shelfFig. 6. Anomaly of monthly DIN (mmol m–3) along the PN line between Case (-30%) and Case (ctrl).
The white line denotes euphotic depth.

Case(–30%) - Case(ctrl)
DIN(mmol m–3)

26
X. GUO and L. Z HAOCase(–30%) - Case(ctrl)

Fig. 7. Same as Fig. 6 but for chlorophyll a (mg m –3).

Chla(mg m–3)

Nutrients and Primary Production over the ECS Shelf
27

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X. GUO and L. Z HAO

appeared mostly northeast of Taiwan and southwest of Kyushu (Fig. 4). In thearea northeast of Taiwan, a negative anomaly in bottom layer’s nutrients can be
identified throughout the year. However, its distribution over the shelf grew from
April to September. In the area southwest of Kyushu, the area with negative
anomaly in bottom layer’s nutrients grew from June to September, after which anorthwestward branch of negative nutrient anomaly appeared and reached the
Yellow Sea.
The response of chlorophyll a in the surface layer to reduction of oceanic
nutrients in winter appeared in Kuroshio pathway, in particular, northeast of
Taiwan where a local maximum in the chlorophyll a anomaly can be identified(Fig. 5). The reduction of chlorophyll a over the shelf in winter was difficult to
identify, although that in nutrients was apparent (Fig. 3), indicating that primary
production over the shelf in winter was not limited by nutrients.
CHANGES IN SUBSURFACE NUTRIENTS AND CHLOROPHYLL a
ACROSS THE SHELF OF THE ECSThe reduction of oceanic nutrients in the sensitivity experiment depended on
the original concentration in the control experiment, and therefore it was small inthe surface layer and large in the subsurface and bottom layers. From January to
March, contours of reduced oceanic DIN concentration were distributed
horizontally along the PN section; from April to June, the 2 mmol m–3 contour of
reduced DIN concentration moved from the shelf break to the middle and innershelf and stayed over the shelf (Fig. 6). An important point shown in Fig. 6 is thatsome of the nutrient reduction can enter the euphotic zone.
The reduction in nutrients caused a decrease in chlorophyll a in the surface
layer in cold months (November–March) and in the subsurface in warm months(June–October) (Fig. 7). April through May appears to be a transition period
when the decrease in chlorophyll a moves from the surface to the subsurface
layer.
CONCLUDING REMARKSBy artificially reducing the concentration of oceanic nutrients in the Kuroshio
water, we confirmed that it influences nutrients and primary production over the
shelf of the ECS. The area with a nutrient decrease is in the bottom layer from theshelf break to the offshore region of the Changjiang estuary from spring tosummer, and it is in the surface layer mainly from autumn to winter. The decreasein primary production over the shelf can be found not only in the surface layer(mainly at the outer shelf and shelf break in winter and in the region outside theturbidity zone of the Changjiang estuary in summer), but also in the subsurface
layer over the shelf from spring to autumn. All these results are consistent with
the new understanding of the role of oceanic nutrients in primary production overthe shelf of the ECS (Zhao and Guo, 2011).
Acknowledgments—The authors thank Prof. Jing Zhang at East China Normal University
for discussions about model results. Xinyu Guo was supported by Global COE ProgramNutrients and Primary Production over the ECS Shelf

29

from the Japanese Ministry of Education, Culture, Sports, Science and Technology, JSPS
KAKENHI (21310012), and Overseas, Hong Kong & Macao Scholars Collaborated
Researching Fund from National Science Foundation of China (No. 41028006). Liang
Zhao was supported by the National Science Foundation of China (Nos. 40806001 and40830854), National Basic Research Program of China (973 Program 2010CB428904)
and Global COE Program from the Japanese Ministry of Education, Culture, Sports,
Science and Technology.
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X. Guo (e-mail: guoxinyu@sci.ehime-u.ac.jp)