Journal of Oceanology and Limnology
Vol. 36 No. 1, P. 48-61, 2018https://doi.org/10.1007/s00343-017-6233-z

Simulating the responses of a low-trophic ecosystem in the
East China Sea to decadal changes in nutrient load from the
Changjiang (Yangtze) River*
WANG Yucheng (王玉成)1, GUO Xinyu (郭新宇)1, 2, 3, **, ZHAO Liang (赵亮)41

Center for Marine Environmental Studies, Ehime University, Matsuyama 7908577, Japan

2

Key Laboratory of Marine Environment and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100,

3College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China

4

College of Marine and Environmental Sciences, Tianjin University of Science and Technology, Tianjin 300457, China

China

Received Aug. 30, 2016; accepted in principle Oct. 15, 2016; accepted for publication Nov. 16, 2016
© Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract
Using a three-dimensional coupled biophysical model, we simulated the responses of a lowtrophic ecosystem in the East China Sea (ECS) to long-term changes in nutrient load from the Changjiang(Yangtze) River over the period of 1960–2005. Two major factors aﬀected changes in nutrient load: changes
in river discharge and the concentration of nutrients in the river water. Increasing or decreasing Changjiang
discharge induced diﬀerent responses in the concentrations of nutrients, phytoplankton, and detritus in the
ECS. Changes in dissolved inorganic nitrogen (DIN), silicate (SIL), phytoplankton, and detritus could be
identiﬁed over a large area of the ECS shelf, but changes in dissolved inorganic phosphate (DIP) werelimited to a small area close to the river mouth. The high DIN:DIP and SIL:DIP ratios in the river water werelikely associated with the diﬀerent responses in DIN, DIP, and SIL. As DIP is a candidate limiting nutrient,perturbations in DIP resulting from changes in the Changjiang discharge are quickly consumed throughprimary production. It is interesting that an increase in the Changjiang discharge did not always lead to anincrease in phytoplankton levels in the ECS. Phytoplankton decreases could be found in some areas close
to the river mouth. A likely cause of the reduction in phytoplankton was a change in the hydrodynamic ﬁeld
associated with the river plume, although the present model is not suitable for examining the possibility indetail. Increases in DIN and DIP concentrations in the river water primarily led to increases in DIN, DIP,
phytoplankton, and detritus levels in the ECS, whereas decreases in the SIL concentration in river water led
to lower SIL concentrations in the ECS, indicating that SIL is not a limiting nutrient for photosynthesis, basedon our model results from 1960 to 2005. In both of the above-mentioned cases, the sediment accumulation
rate of detritus exhibited a large spatial variation near the river mouth, suggesting that core sample data
should be carefully interpreted.
Keyword: East China Sea; Changjiang (Yangtze) River; low-trophic ecosystem

1 INTRODUCTION
The East China Sea (ECS), with a board shelf areaof about 0.6×106 km2, is one of the largest marginalseas in the northwestern Paciﬁc Ocean. The ECS
receives a large amount of nutrients from atmosphericdeposition (e.g., Kim et al., 2011; Zhang et al., 2011),
nearby rivers (Zhang, 1996; Liu et al., 2009), theTaiwan Strait (Chung et al., 2001), and upwelling
Kuroshio waters (Chen, 1996; Zhao and Guo, 2011)* Supported by the National Basic Research Program of China (973
Program) (No. 2012CB956004), the National Natural Science Foundationof China (NSFC) (No. 41576010), the JSPS KAKENHI (Nos. JP26241009,
JP26287116, JPH05821), the Fundamental Research Funds for Central
Universities from the Ministry of Education of China (No. 201512004) to
X. Guo, the Strategic Priority Research Program of Chinese Academy of
Sciences (No. XDA11020305), the NSFC (No. 41276016), the National
Key Research and Development Program of China (No. 2016YFA0601301),
and the Ministry of Education, Culture, Sports, Science and Technology,
Japan (MEXT), under a Joint Usage/Research Center, Leading Academia
in Marine and Environment Pollution Research (LaMer) Project to L. Zhao** Corresponding author: guo.xinyu.mz@ehime-u.ac.jp

No.1

WANG et al.: Response of the ECS ecosystem to decadal changes in Changjiang

and thus exhibits annual mean primary production as
high as 155 g C/(m2·yr), with a maximal rate of 939 g
C/(m2·yr) in summer (Gong et al., 2003). Both nutrient
conditions and primary production in the ECS varyseasonally. For example, nutrient fronts are generally
strong in winter and weak in summer (Chen, 2009);
primary production in summer is about 3 times higher
than in other seasons (Gong et al., 2003). Besides
seasonal variations, long-term variations in nutrient
levels and primary production in the ECS are also
important. Based on satellite data from 1998 to 2007,
Yamaguchi et al. (2013) reported that there was asigniﬁcant increase in surface chlorophyll a in coastal
areas of the ECS during this period.
Rivers serve as an important source of nutrients for
the ECS. Among the rivers emptying into the ECS, theChangjiang (Yangtze) River is the largest in terms ofdischarge and sediment and nutrient loads (Beardsley
et al., 1985; Zhang, 1996; Liu et al., 2009). The annual
mean discharge of the Changjiang is 9×1011 m3, which
accounts for 90% of the total freshwater input for the
ECS and Yellow Sea (Beardsley et al., 1985). Similar
to the variations in nutrient distribution and primary
production in the ECS, discharge from the Changjiang
also varies seasonally. Changjiang diluted water
covers most of the northern area of the ECS (Bai et al.,2014) and even inﬂuences the water mass of the
Tsushima Strait in summer (Morimoto et al., 2012).
Similar to the seasonal variations in the Changjiang
discharge, the Changjiang nutrient load is 3–4 times
higher in the summer than in the winter (Liu et al.,2009). Bulge-shaped nutrient fronts form in theoﬀshore area of the Changjiang estuary in summer,
and a high-concentration band of nutrients forms
along the coast of China in winter (Chen, 2009).
The Changjiang nutrient load also exhibits decadal
variations. Wang (2006) reported that between the
1960s and 1990s, the concentrations of dissolved
inorganic nitrogen (DIN) and phosphate (DIP) in
Changjiang river water increased by a factor of ﬁve,
whereas the concentration of silicate (SIL) decreasedby two-thirds. These signiﬁcant changes in nutrient
concentrations were attributed to fertilizer application,eﬄuent from cites, and the construction of large dams
along the river (Duan and Zhang, 2001).
The impacts of changes in Changjiang
characteristics on nutrient conditions and theecosystems of the ECS have been conﬁrmed by many
previous studies (e.g., Wang, 2006), but these studies
were primarily based on discrete ﬁeld observations
that focused on coastal areas around the Changjiang

49

estuary. How do changes in Changjiang characteristics
impact nutrient conditions and primary production inother areas of the ECS? Changes in the nutrient loadof the Changjiang are aﬀected by yearly changes inriver discharge (although long-term discharge trends
can be ignored) (Zhang et al., 2014) and by decadalchanges in the concentrations of nutrients in
Changjiang water (Wang, 2006; Li et al., 2007).
In this study, we simulated the changes in nutrient
conditions and primary production in the ECS in
response to changes in Changjiang discharge andnutrient concentrations using a three-dimensional
low-trophic biophysical model. This model has beenused to simulate climatologic conditions of nutrient
levels and primary production in the ECS (Zhao andGuo, 2011). By varying Changjiang discharge andnutrient concentrations, we were able to model thecorresponding responses of low-trophic ecosystems
in the ECS.

2 MODEL DESCRIPTION
The numerical model used in this study is based ona three-dimensional low-tropic biophysical modelthat has been used to calculate onshore oceanic
nutrient ﬂuxes from the Kuroshio across the shelf
break of the ECS (Zhao and Guo, 2011). The model
domain covers the Bohai Sea, Yellow Sea, and ECS(as shown in Fig.1). The model includes ahydrodynamic module and a biogeochemical module.
The hydrodynamic module is based on that describedby Guo et al. (2003) and Wang et al. (2008), which
provides physical parameters such as watertemperature, diﬀusion coeﬃcients, and currents to thebiogeochemical module. The biogeochemical moduleis based on NORWECOM (e.g., Aksnes et al., 1995;
Skogen et al., 1995; Skogen and Søiland, 1998) andincludes three diﬀerent nutrients (DIN, DIP, and SIL),
two types of phytoplankton (diatoms [DIA] andﬂagellates [FLA]), and two types of biogenic organic
material (detritus [DET] and biogenic silica [SIS]).
Information regarding equations and parameter values
can be found in the report of Zhao and Guo (2011).
Climatological forcing was used to reproduce
seasonal variations in nutrient concentrations andprimary production in the ECS, including monthly
river discharge, heat ﬂux, evaporation andprecipitation rates, and ocean currents at open
boundaries in the hydrodynamic module; river
nutrient concentrations, atmospheric deposition, andnutrient concentrations at open boundaries in thebiogeochemical module (Zhao and Guo, 2011).

50

J. OCEANOL. LIMNOL., 36(1), 2018

Vol. 36

N
40°

38°

36°

34°
s.

Jeju I
Cha
ngjia
ng R
iver

32°

30°

28°

26°

Taiwan
Is.
24°
118°

120°

122°

124°

126°

128°

130° E

Fig.1 Model domain and bathymetry
Numbers with lines denote isobaths.

Changjiang discharge was monitored at the Datong
Hydrological Station, which is located along thelower reaches of the Changjiang. The annual meandischarge of the Changjiang from 1960 to 2005
changed year to year, although there were no
signiﬁcant diﬀerences in the long-term trend (Fig.2a).
For example, the lowest annual mean Changjiang
discharge (~23 400 m3/s) occurred in 1978, and thehighest annual mean discharge (~38 700 m3/s)

occurred in 1998. The multi-year mean Changjiang
annual discharge was ~28 500 m3/s from 1960 to2005. The highest monthly mean discharge from 1960
to 2005 was in July, and the lowest was in January(Fig.2b). Similarly, the standard deviation of themonthly discharge was highest in summer and lowest
in winter (Fig.2b). Most of the Changjiang discharge
data (~95%) fell within ±2 standard deviations of themonthly mean (Fig.2b).

No.1

WANG et al.: Response of the ECS ecosystem to decadal changes in Changjiang

Discharge (m3/s)

45000

51

a

40000
35000
30000
25000
20000
15000
10000
1960

1965

1970

1975

1980

Year

1985

1990

1995

2000

2005

11

12

80000

Discharge (m3/s)

70000

b

60000
50000
40000
30000
20000
10000
01

2

3

4

5

6

Month

7

8

9

10

2

400
DIN
DIP

300

1.5

SIL

250
200

1

150
100

0.5

DIP Conc. (mmol/L)

DIN and SIL Conc. (µmol/L)

350

c

50
0
150

0

Ratios of DIN:DIP and SIL:DIN

d
120
90
DIN:DIP60

SIL:DIN

30
0
1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

Year

Fig.2 Time series of annual mean discharge of the Changjiang from 1960 to 2005 (a); monthly discharge of Changjiang from1960 to 2005 (b); nutrient concentrations from 1963 to 2003 derived from Fig.2a of Wang (2006) (c); ratios of DIN to
DIP and SIL to DIN from 1963 to 2003 (d)In a: red dashed line denotes the multi-year average annual discharge, with value of ~28 500 m3/s; black dashed lines from top to bottom are averageplus two standard deviations (std), average plus one std, average minus one std, and average minus two std of Changjiang discharge from 1965 to
2005, respectively; In b: thin black lines are monthly discharge of Changjiang from 1960 to 2005; red line denotes the multi-year average monthly
discharge; blue and green lines represent average±std and average±2 std of monthly Changjiang discharge, respectively; In c: the middle year ofeach 5-year period was chosen to represent the mean over 5 years. For example, the nutrient concentration in 1963 denotes the average nutrient
concentration from 1961 to 1965.

52

40°
N

J. OCEANOL. LIMNOL., 36(1), 2018abVol. 36

c

d
33
32
31
30
28
26
24
22
20
18
15
10

35°
Feb.

May

Aug.

Oct.

30°

..

..

.

..

.

..

.

.

25°
40°
N

e

f

g

h

-0.5
-1
-2
-2.5
-3.5

35°
Feb.

May

Aug.

Oct.

-4.5
-6
-6
-7

30°

..

..

.

..

.

..

.

.

25°
120°

125°

130°E

120°

125°

130°E

120°

125°

130°E

120°

125°

130°E

Fig.3 Horizontal distribution of sea surface salinity for case 10 (upper panel), and diﬀerence between cases 18 and 10 (lower
panel) over four seasons
Area enclosed by red lines in (h) represents the area in which salinity diﬀerences were less than -0.5. This area was used as the integral area for
calculating standing stocks of biogeochemical variables as discussed in Section 3.

Concentrations of nutrients (DIN, DIP, SIL) from1963 to 2003 were deduced from the report of Wang
(2006). As shown in Fig.2c and as reported by Wang
(2006), the concentrations of DIN and DIP increased
from the 1960s to 2000s, whereas that of SIL
decreased. Increasing of DIN to DIP ratio anddecreasing of SIL to DIN ratio were apparent in thisperiod (Fig.2d).
In this study, we used the monthly average
Changjiang discharge (red line in Fig.2b) andintermediate nutrient concentrations (i.e., the nutrient
concentrations in 1983 representing the period of
1981–1985) as the control case (case 10, Table 1).
After 2 years of spin-up, when the nutrient
concentrations and phytoplankton density in the ECS
became seasonally stationary, we changed theChangjiang discharge and nutrient concentrations in
the river water case by case (Table 1) and continued
the simulations with the new conditions for one more
year. These results were used to examine how nutrient
conditions and primary production in the ECS respondto changes in Changjiang discharge and nutrient

concentrations. The results of these modelingexperiments in the third year are presented in section
3 below. It should be noted that only conditions
pertaining to the Changjiang were changed in theseexperiments, and other forcing conditions remained
the same in all of the cases.

3
MODELINGDISCUSSIONSRESULTAND

As shown in Fig.3a–d in the control case, thesurface salinity was much lower in the ECS shelfregions than in the outer shelf regions. Here, we used
31 isohaline as the salinity front for Changjiang
diluted water. In the control case, Changjiang diluted
water exhibited obvious seasonal variations: it is
conﬁned to the coastal area in winter (Fig.3a),
gradually expands oﬀshore to cover most areas of thenorthern ECS in summer (Fig.3b–c); and retreats
toward the coast of China in autumn (Fig.3d). If the
Changjiang discharge is increased by only two
standard deviations (case 18), the change in salinity inthe ECS in response to changes in river discharge can

No.1

WANG et al.: Response of the ECS ecosystem to decadal changes in Changjiang

Table 1 Numerical experiments designed for this study
Case

Changjiang
discharge

10

Mean

DIN conc. DIP conc. SIL conc.
(μmol/L) (μmol/L) (μmol/L)
40.00

0.53

185.00Period1981–1985

11

Mean−2.0 std

40.00

0.53

185.001981–1985

12

Mean−1.5 std

40.00

0.53

185.001981–1985

13

Mean−1.0 std

40.00

0.53

185.001981–1985

14

Mean−0.5 std

40.00

0.53

185.001981–1985

15

Mean+0.5 std

40.00

0.53

185.001981–1985

16

Mean+1.0 std

40.00

0.53

185.001981–1985

17

Mean+1.5 std

40.00

0.53

185.001981–1985

18

Mean+2.0 std

40.00

0.53

185.001981–1985

21

Mean

10.00

0.23

338.00

1961–1965

22

Mean

15.00

0.27

290.00

1966–1970

23

Mean

21.00

0.35

252.001971–1975

24

Mean

30.00

0.43

216.00

1976–1980

25

Mean

58.00

0.67

160.001986–1990

26

Mean

85.00

0.82

140.001991–1995

27

Mean

120.001.04

120.00

1996–2000

28

Mean

162.001.30

105.002001–2005“Mean” and “std” denote the multi-year average and standard deviation ofmonthly Changjiang discharge from 1960 to 2005. “Period” refers to thetime period exhibiting the corresponding nutrient conditions.

be examined based on the diﬀerence between cases
18 and 10 (Fig.3e–h). The salinity response also
shows seasonal variations. Changes in salinity occur
primarily near the river mouth in winter (Fig.3e). In
spring, the area inﬂuenced by changes in riverdischarge becomes a little larger (Fig.3f), and bysummer, the aﬀected area covers most of the northern
ECS, including the Tsushima Strait (Fig.3g). In
autumn, increased Changjiang discharge not only
decreases the salinity of the northern ECS, but also
aﬀects the southeast coast of China (Fig.3h). The
eﬀect observed in the northern ECS may be the resultof cumulative changes occurring over the previous
months, whereas that along the southeast coast of
China is related to northerly winds during autumn.
Therefore, the seasonal variations in Changjiang
diluted water in our model are closely related to
Changjiang discharge and wind direction, as suggested
by Bai et al. (2014) in their explanation of interannual variations in ECS salinity as recorded bysatellite observations.
Seasonal variations in DIN and total phytoplankton(PHY, sum of DIA+FLA) in the control case were
examined by Zhao and Guo (2011). Here, we provide
only a brief description of these variables in thesurface layer, as the eﬀects of Changjiang diluted

53

water are primarily concentrated in the surface layer.
In winter, concentrations of DIN, DIP, and SIL werehigh in the coastal areas of the Changjiang estuary(Fig.4a, e, and i). Phytoplankton concentrations werelow (below 0.2 mg Chl-a/m3) in most of these areas,
except for nearshore regions (Fig.4m). DET levels
were high in the northern area of the Changjiang
estuary due to intensiﬁed vertical mixing, whereas the
SIS distribution was similar to that of DIN (Fig.4q
and u). In May, wedge-shaped fronts of DIN and SIL
formed oﬀ the Changjiang estuary (Fig.4b and j) due
to consumption of DIN and SIL by phytoplankton in
the central Yellow Sea and middle shelf of the ECS inearly spring (Zhao and Guo, 2011), whereas DIP wasdepleted in most areas of the ECS (Fig.4f). Highphytoplankton concentrations could be also found oﬀ
the Changjiang estuary (Fig.4n), where DET and SIS
levels began to increase due to phytoplanktonmortality (Fig.4r and v). In summer, high DIN and
SIL levels could be found in the Tsushima Strait(Fig.4c and k) that corresponded to the low salinity
distribution (Fig.3c). Phytoplankton levels in the
Changjiang estuary were high at this time comparedwith the spring (Fig.4o). In autumn, highconcentrations of DIN could be found north of Jeju
Island, whereas the phytoplankton level began to
decline. Levels of DET and SIS oﬀ the Changjiang
estuary were highest in autumn.
3.1 Changes in nutrient concentrations andecosystem conditions in the ECS in response tochanges in Changjiang discharge
By comparing cases 11–18 with case 10 (Table 1),
we could examine the inﬂuence of Changjiang
discharge on nutrient concentrations and ecosystem
conditions. As an example, we discuss the diﬀerences
between cases 18 and 10 (Fig.5).
When the Changjiang discharge was increased by
two standard deviations, three elements of nutrients(i.e., DIN, DIP, and SIL), PHY, DET, and SIS in thesurface layer of the ECS responded diﬀerently (Fig.5).
Horizontal distributions similar to that for salinity(Fig.3e–h) were observed with respect to increased
DIN (Fig.5a–d) and SIL (Fig.5i–l) in all four seasons,
whereas only minimal changes in DIP were observed(Fig.5e–h). This indicates that DIN and SIL arerelatively passive and can be transported oﬀshore like
freshwater. However, the increase in DIP was conﬁned
to an area near the Changjiang estuary (Fig.5e–h).
Moreover, the horizontal distributions of PHY, DET,and SIL diﬀered (Fig.5m–x).

54

J. OCEANOL. LIMNOL., 36(1), 2018

Vol. 36

N
35°

a

b

c

d

30°

Feb.
DIN

May
DIN

Aug.
DIN

Oct.
DIN

N
35°

e

f

g

h

30°

Feb.
DIP

May
DIP

Aug.
DIP

Oct.
DIP

N
35°

i

j

k

l

30°

Feb.
SIL

May
SIL

Aug.
SIL

Oct.
SIL

N
35°

m

n

o

p

Feb.
PHY

May
PHY

Aug.
PHY

Oct.
PHY

N
35°

q

r

s

t

30°

Feb.
DET

May
DET

Aug.
DET

Oct.
DET

u

v

w

x

Feb.

May
SIS

Aug.
SIS

Oct.
SIS

30°

N
35°

30°

SIS

120°

125°

130°E

120°

125°

130°E

120°

125°

130°E

120°

50
30
20
15
10
8
5
2

2
1.5
1
0.8
0.5
0.4
0.3
0.2

300
100
50
30
20
15
10
5

15
10
8
5
2
1
0.5
0.2

20
15
12
10
8
5
2
1

20
15
12
10
8
5
2
1
125°

130°E

Fig.4 Horizontal distribution of (a)–(d) surface DIN (units: μmol/L N), (e)–(h) DIP (units: μmol/L P), (i)–(l) SIL (units:
μmol/L Si), (m)–(p) PHY (units: mg Chl-a/m3), (q)–(t) DET (units: μmol/L N), and (u)–(x) SIS (units: μmol/L Si) for
case 10 over four seasons

No.1
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WANG et al.: Response of the ECS ecosystem to decadal changes in Changjiang

a

b

30°

d

Feb.

May

Aug.

Oct.

DIN

DIN

DIN

DIN

e

f

g

h

Feb.
DIP

May
DIP

Aug.
DIP

Oct.
DIP

30°

N
35°

c

55

30
20
N
35°

k

j

i

10

l

5
2
30°

Feb.
SIL

Aug.
SIL

May
SIL

Oct.
SIL

-2
-5
-10

N
35°

n

m

-20

p

o

-30

30°

Feb.
PHY

May
PHY

Aug.
PHY

Oct.
PHY

N
35°

q

r

s

t

Feb.
DET

May
DET

Aug.
DET

Oct.
DET

u

v

w

x

Feb.
SIS

May

Aug.
SIS

Oct.
SIS

30°

N
35°

30°

120°

SIS

125°

130°E

120°

125°

130°E

120°

125°

130°E

120°

125°

130°E

Fig.5 Horizontal distribution of the change in the percentage of (a)–(d) DIN, (e)–(h) DIP, (i)–(l) SIL, (m)–(p) PHY, (q)–(t)DET, and (u)–(x) SIS in the surface layer for case 18 compared with the control case (case 10) over four seasons
Black line with numbers in each panel denotes ±20% contour.

56

J. OCEANOL. LIMNOL., 36(1), 2018

The diﬀerences in the response of each variable
can be attributed to high DIN to DIP and SIL to DIN
ratios in Changjiang freshwater, leading to high DIN
to DIP and SIL to DIN ratios around the Changjiang
estuary. Therefore, DIP is the limiting nutrient forphotosynthesis, whereas DIN and SIL are in excess.
DIP from the Changjiang is quickly consumed byphytoplankton, such that there is no excess to betransported far oﬀshore.
Interestingly, phytoplankton levels did not always
increase around the Changjiang estuary withincreasing Changjiang discharge. Reductions in
phytoplankton levels could be found in some areas
close to the river mouth. Changes in temperature andcurrent ﬁelds, which both directly and indirectly
inﬂuence biological processes, could be responsible
for lower phytoplankton levels. However, thehydrodynamic conditions of the river plume aroundthe Changjiang estuary are very complex and cannot
be addressed in detail by our model.
3.2 Changes in nutrient concentrations andecosystem conditions in the ECS in response tochanges in nutrient concentrations in Changjiang
freshwater
By varying the nutrient concentrations from the
1960s to 2000s with an interval of 5 years while
keeping the Changjiang discharge the same, we
examined the responses of low-trophic ecosystems of
the ECS to decadal changes in nutrient concentrationsin Changjiang freshwater (cases 21–28). Figure 6
shows the diﬀerences between case 28 (nutrient
concentrations in the 2000s) and case 10 (nutrient
concentrations in the 1980s). Similar to the diﬀerences
between cases 18 and 10, a similar pattern of increased
DIN and similar seasonal variation (Fig.6a–d) wereobserved, but the range was large. This is because the
DIN from the Changjiang in case 28 was ~4 times
higher than that in case 10. Positive responses in PHY,
DET, and SIS were observed around the Changjiang
estuary (Fig.6m–x).
Without changes of Changjiang discharge,
increases in DIP in river water were conﬁned to near
the river mouth (Fig.6e–h). The area of increased DIP
was even smaller than the diﬀerence between cases
18 and 10. SIL was generally lower, and the
distribution of percent change was similar but a little
larger than the distribution of percent change in DIN.
As mentioned in the last subsection, higher DIP
concentrations were only observed in a limited area
near the Changjiang mouth, and no excess DIP wasVol. 36

available to be transported far oﬀshore due to rapid
consumption by phytoplankton for photosynthesis.
Furthermore, transformation of PHY into detritus due
to mortality occurs on a time scale of days. As a result,
we found similar monthly distributions of PHY, DET,and SIS oﬀ the Changjiang estuary.
3.3 Changes in the standing stock of eachbiogeochemical variable in the ECS in response tochanges in the Changjiang
The area enclosed by the red line shown in Fig.3h
was considered to be aﬀected by the Changjiang diluted
water and was thus used to calculate standing stocks ofbiogeochemical variables in our model. With changes
in Changjiang discharge, the responses of DIN and SIL
were almost linear, indicating that these two nutrientswere in excess for photosynthesis (Fig.7). For example,
for cases 11–18, which represent variations in
Changjiang discharge from -2 standard deviations to+2 standard deviations, changes in the percentage ofstanding stock varied from approximately -4 to +4%
for DIN, -7 to +7% for SIL, and -0.4 to +0.4% for DIP(Fig.7). Other variables showed non-linear responses,
especially DET and SIS (Fig.7e and f). It is interesting
to note that phytoplankton levels did not increase withincreasing Changjiang discharge. As mentioned in
Section 3.1 above, although an increase in Changjiang
discharge results in higher load of dissolved nutrients,
it also leads to a redistribution of the dissolved nutrientsand changes the pre-existing relative ratios of nutrients,
thus changing the composition of phytoplankton(results not shown). Changes in dissolved nutrient
concentrations and phytoplankton levels could havecontributed to the non-linear responses of DET and
SIS. In addition, changes in hydrodynamic conditions
that directly redistribute DET and SIS might also havecontributed to this non-linear response, as shown in
Fig.5. Moreover, increases in the DIN to DIP and SIL
to DIN ratios (Fig.7g and h) from cases 11 to 18 alsosuggest that increases in Changjiang discharge aﬀect
the biogeochemical environment in the ECS, especiallynear the river mouth.
For cases 21–28, which were characterized byhigher DIN and DIP loads and lower SIL load, thestanding stocks of all variables except SIL increased
substantially from the 1960s to 2000s (Fig.8). The
decrease in SIL standing stock was caused by adecrease in SIL load from the Changjiang. Comparedwith the nutrient conditions in the 1980s, the DIN
standing stock in the ECS increased by ~30%,
although the DIN load from the Changjiang increased

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WANG et al.: Response of the ECS ecosystem to decadal changes in Changjiang

a

b

30°

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35°

d

Feb.

May

Aug.

Oct.

DIN

DIN

DIN

DIN

e

f

g

h

Feb.
DIP

May
DIP

Aug.
DIP

Oct.
DIP

i

j

k

l

30°

N
35°

c

57

100
50
30

30°

Feb.
SIL

Aug.
SIL

May
SIL

Oct.
SIL

20
10
-10

N
35°

n

m

-20

p

o

-30
-50
30°

Feb.
PHY

May
PHY

Aug.
PHY

Oct.
PHY

N
35°

q

r

s

t

Feb.
DET

May
DET

Aug.
DET

Oct.
DET

u

v

w

x

Feb.

May

SIS

SIS

Aug.
SIS

Oct.
SIS

30°

N
35°

30°

120°

125°

130°E

120°

125°

130°E

120°

125°

130°E

120°

-100

125°

130°E

Fig.6 Horizontal distribution of the change in the percentage of (a)–(d) DIN, (e)–(h) DIP, (i)–(l) SIL, (m)–(p) PHY, (q)–(t)DET, and (u)–(x) SIS in the surface layer for case 28 compared with the control case (case 10) over four seasons
Black line with number in each panel denotes ±20% contour.

J. OCEANOL. LIMNOL., 36(1), 2018
0.6

10

6
c. SILd. PHY-0.2

Percent change (%)

b. DIP-4
10000 20000 30000 40000 50000
Changjiang discharge (m3/s)-0.4
10000 20000 30000 40000 50000
Changjiang discharge (m3/s)-1010000 20000 30000 40000 50000
Changjiang discharge (m3/s)-410000 20000 30000 40000 50000
Changjiang discharge (m3/s)

0.4

0.3

18

2.6

-2

e. DET

0.2
0f. SIS

0.2
0
-0.2

0.2
0.1
0
-0.1

-0.4
10000 20000 30000 40000 50000
Changjiang discharge (m3/s)

5
0
-5

4
2
0

-2

g. DIN:DIP

Ratio of SIL:DIN

0

0.4

Ratio of DIN:DIP

2

Percent change (%)

a. DIN

Percent change (%)

Percent change (%)

Percent change (%)

4Vol. 36

Percent change (%)

58

17.5

17

h. SIL:DIN
2.55
2.5
2.45
2.4

16.5
2.35
-0.2
10000 20000 30000 40000 50000 10000 20000 30000 40000 50000 10000 20000 30000 40000 50000
3
3
Changjiang discharge (m /s)
Changjiang discharge (m /s)
Changjiang discharge (m3/s)

Fig.7 Response of the annual mean standing stock of each variable in the ECS to changes in Changjiang discharge
Red lines denote ﬁrst-order linear regression.

1

20
10
0
50
100
150
DIN conc. (mmol/L)

200

-0.5
0

0

-10

0.5
1
DIP conc. (mmol/L)

1.5

-20
400

6
e. DET

Percent change (%)

Percent change (%)

6
4
2
0
-2
-4
0

0

10

50
100
150
DIN conc. (mmol/L)

200

100

2
0
-2
300
200
SIL conc. (mmol/L)

100

22
20
18
16
0

d. PHY
5

0

-5
0

50
100
150 200
DIN conc. (mmol/L)

3.5
g. DIN:DIP

4

-4
400

300
200
SIL conc. (mmol/L)

24
f. SIS
Raito of DIN:DIP

-10
0

0.5

10
c. SIL

Percent change (%)

30

20
b. DIP

Percent change (%)

Percent change (%)

Percent change (%)

a. DIN

Raito of SIL:DIN

40

50
100
150
200
DIN conc. (mmol/L)

h. SIL:DIN
3
2.5
2
1.5
0

50
100
150 200
DIN conc. (mmol/L)

Fig.8 Response of the annual mean standing stock of each variable in the ECS to changes in nutrient concentrations in
Changjiang freshwater
Red lines denote ﬁrst-order linear regression. It should be noted that the axes in (c) and (f) are reversed in order to follow the decline in silicate
concentration in Changjiang water over time (Fig.2c)

more than 4 fold. Phytoplankton levels increased by~7%, with diﬀering ratios of DIA and FLA. The levelsof DET and SIS increased by only approximately 5
and 6%, respectively. This means that except for theincreased DIN concentration in the ECS, a portion of
the increased DIN load from the Changjiang wasconverted to phytoplankton via photosynthesis, while
the other portion was converted to detritus due tophytoplankton mortality.

In our model, increased DIN and DIP loads anddecreased SIL load enhanced photosynthesis in the
Changjiang estuary, consistent with other reports (Liet al., 2007). As mentioned above, long-term changes
in nutrient concentrations in Changjiang freshwater
may be suﬃcient in and of themselves to explain
degradation of the ECS ecosystem, although the
responses of the ecosystem to changes in Changjiang
discharge are more complex.

No.1

WANG et al.: Response of the ECS ecosystem to decadal changes in Changjiang
4
3

59

6
b

a
4

Percent change (%)

Percent change (%)

2
1
0
-1

2

0

-2
-2
-3
-4
10000

20000
30000
40000
Annual Changjiang discharge (m3/s)

50000

-4

0

50

100
150
DIN conc. (mmol/L)

200

Fig.9 Response of annual mean sediment accumulation in (a) cases 10–18 and (b) case 10 and cases 21–28
Red lines denote ﬁrst-order linear regression.

4 INTEGRATION OF DATA FROM CORE
SAMPLES COLLECTED AROUND THE
CHANGJIANG ESTUARY
In our model, terrestrial sediment (include organic
materials and other suspended materials) was notconsidered. Nevertheless, changes in oceanic
sediment in the ECS, especially around the Changjiang
estuary, were sensitive to changes in Changjiang
discharge and nutrient concentrations. Although theannual mean sediment accumulation rate onlychanged ±4% (Fig.9a) with changes in Changjiang
discharge, most areas of the ECS and Yellow Sea
were aﬀected (Fig.10e–h). Responses as a result ofchanges in only nutrient concentrations of Changjiang
freshwater were relatively simple and were distributed
primarily near the Changjiang estuary (Fig.10i–l).
That is to say, the horizontal distribution of thesediment accumulation rate was more complex whenrealistic year to year conditions of the Changjiang
were examined.
Most previous studies used a mean sedimentaccumulation rate in the ECS for the interpretations ofcore sample data. For example, Chen and Wang
(1999) used a value of 0.2±0.06 g cm2/yr to calculate
the nutrient and carbon budgets for the ECS. There
are two problems with this methodology, however.
First, the sediment accumulation rate in the ECS haslarge spatial variations, as suggested by Deng et al.
(2006) and also by our modeling results. Sedimentaccumulation in the Changjiang estuary and innershelf regions accounted for 59% of the total sedimentaccumulation in the ECS, whereas that in the outer

shelf regions accounted for only 7% (Deng et al.,2006). Second, the sediment accumulation rate also
has large seasonal and interannual variations,especially near the Changjiang estuary and inner shelfregions, as suggested by our results. For example, thesediment accumulation rate was low in winter due tolow primary production and high in spring andsummer as a result of intensiﬁed primary production.
Moreover, sediment accumulation rate was sensitive
to change in the Changjiang river discharge andnutrient concentration therein. Therefore, estimates ofsediment accumulation rates over such large area of
the ECS must take into careful consideration spatialtemporal variations in the ECS.
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