Journal of Oceanology and Limnology
Vol. 36 No. 1, P. 62-76, 2018https://doi.org/10.1007/s00343-017-6234-yInterannual variation of nutrients along a transect across theKuroshio and shelf area in the East China Sea over 40 years*
HU Yingying (胡莹英)1, GUO Xinyu (郭新宇)1, 2, 3, **, ZHAO Liang (赵亮)41
2

Center for Marine Environmental Studies, Ehime University, Matsuyama 7908577, JapanKey Laboratory of Marine Environment and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100,
China

3

College 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

Received Aug. 31, 2016; accepted in principle Oct. 15, 2016; accepted for publication Nov. 22, 2016
© Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractLong-term datasets of water temperature, salinity, dissolved oxygen, phosphate and nitratefrom 1973 to 2013 were used to study interannual variations of these parameters along a transect (sectionPN) across the Kuroshio and shelf area in the East China Sea. Water temperature and salinity at depths of
0–200 m showed a low-high-low-high pattern over the 40-year period. Water temperature and salinity at
500 m were relatively high from 1985 to 1990 and decreased continuously thereafter. Salinity at 800 m
was lowest around 1993 and increased thereafter. Nutrients were highest between 1980 and 1985. After
1985, nutrients at 500 m were increasing. The range of variation in nutrients was large before 1985 andthe magnitude increased with greater depths. Apparent oxygen utilization at 500 m depth was decreasing
before 1990 and started to increase after 1992. Signiﬁcant changes after 1990 at 500 m depth may be dueto the upward trend of the upper boundary of Kuroshio intermediate water. Interannual variation of watertemperature, salinity, dissolved oxygen, phosphate and nitrate along section PN has a signiﬁcant correlation
with the Paciﬁc Decadal Oscillation index.
Keyword: Kuroshio; East China Sea; interannual variation; nutrient; phosphate; nitrate

1 INTRODUCTION
Marginal seas of the Northwest Paciﬁc Ocean
exhibit high primary and secondary productivity,
which is supported by high nutrient concentrations(Hama et al., 1997; Gong et al., 2003). These are
generally important ﬁshing grounds, and millions ofpeople along the coasts make a living from the seas.
However, ﬁsheries cannot be maintained without new
production, which relies upon external source ofnutrients (Chen, 2008). Therefore, studies of spatialtemporal variations in nutrient concentration in themarginal seas of the North Paciﬁc Ocean makesigniﬁcant contributions to both the shore-side
ecosystem and the economy.
The important role of nutrients in the ocean has
spurred much attention from social and scientiﬁcstandpoints, and much scientiﬁc research on nutrientsin the ocean has been conducted starting several

decades ago. Taking the East China Sea as an example,
early research on nutrient concentration was mostly
conﬁned to riverine exports or to the estuaries andcoastal areas (Lü et al., 1985; Huang et al., 1986; Huet al., 1990). Wang (1991) produced a Marine Atlas,which is one of the most comprehensive reports on* 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, JP15H05821), the Fundamental Research Funds for Central
Universities from the Ministry of Education of China (No. 201512004) to
X. Guo, the National Key Research and Development Program of China(No. 2016YFA0601301), the Strategic Priority Research Program ofChinese Academy of Sciences (No. XDA11020305), the National Natural
Science Foundation of China (NSFC) (No. 41276016), and the Ministryof Education, Culture, Sports, Science and Technology, Japan (MEXT)
through a project on Joint Usage/Research Center, Leading Academia in
Marine and Environment Pollution Research (LaMer) to L. Zhao.
** Corresponding author: guo.xinyu.mz@ehime-u.ac.jp

No.1

HU et al.: Interannual variation of nutrients in Kuroshio

Table Locations and water depth at stations along section
PN
Station No.

Latitude (°N)

Longitude (°E)

Range (°)

Depth (m)

1

27.50

128.25

±0.05

1 020

2

27.80

127.80

±0.05

950

3

28.10

127.35

±0.05

1 020

3’

28.25

127.14

±0.05

970

4

28.42

126.89

±0.06

280

4’

28.55

126.68

±0.05

155

5

28.70

126.45

±0.05

130

6

29.00

126.00

±0.05

110

7

29.31

125.56

±0.05

100

8

29.61

125.09

±0.05

80

9

30.00

124.50±0.05

70

the spatial distribution of nutrients in the East China
Sea, the Bohai Sea, and the Yellow Sea. Based on thisatlas, Chen (2009) produced several maps of nutrient
distributions (nitrate, phosphate and silicate) in thesurface and bottom layers in summer and winter inthe East China Sea with additional information from
Korea, Taiwan and Japan.
Although we have some fundamental informationon the distribution of nutrients and seasonal variations,
we have little information on long-term trends. There
are only a few published reports that address thisissue. Guo et al. (2012) used in-situ data from 88
cruises along transect PN from 1987 to 2009 andfound an increasing nitrate concentration averaged
along the transect. Lui et al. (2014) analyzed the samedata and found that while nitrate and phosphateconcentrations in the Kuroshio Intermediate Water(KIW) have increased, the dissolved oxygen (DO)concentration has decreased starting as early as 1982.
Aoyama et al. (2008) found that nutrient concentrationin the upper 200 m has been decreasing while
dissolved oxygen content and sea water temperature
have been increasing in the upper 200 m in the mainstream of the Kuroshio in the East China Sea during
the period of 1950 to 2004. Moreover, almost none of
published reports examine interannual variation ofnutrients.
This study examines seasonal and interannual
variation of temperature, salinity, DO, nitrate andphosphate levels from 1970 to 2013 along the transect
in the East China Sea that is identiﬁed as section PN.
Correlations between PDO and the other variables are
made to ﬁnd relationships in interannual variations
between North Paciﬁc Ocean and East China Sea.

63

2 DATA AND METHOD
Oceanographic and marine meteorologicalobservation data from Japan Meteorological Agency(JMA) surveys conducted in the western North Paciﬁcand in the seas adjacent to Japan by JMA research
vessels from 1970 to 2013 were used in this study. In
consideration of the study purpose, 11 stations along
the classical PN section having locations and depths
shown as Table 1 were chosen (Fig.1a).
Surveys were conducted four times per year alongsection PN starting from April 1972, except for Stn. 3’
and 4’ where observations were started from July
1977. From October 1996, observations at Stn. 7 to 9
were discontinued (Fig.1b).
Most observations included measurements ofwater temperature, salinity, dissolved oxygen,
phosphate, nitrate and chlorophyll a. We usedtemperature, salinity, dissolved oxygen, phosphateand nitrate in this study. Before further analysis of thedata, several data processing steps were conducted to
all parameters. Here we chose temperature as an
example to explain the data processing procedure:
1. Linear interpolation of data into standard levels.
The observation data were not taken at exactly
standard depths. So we used data at depths less than
10 m as surface data. And linear interpolated data tostandard depths of 10 m, 20 m, 30 m, 50 m, 75 m,
100 m, 125 m, 150 m, 200 m, 250 m, 300 m, 400 m,
500 m, 600 m, 700 m and 800 m.
2. Calculation of seasonal variation. All data fromthe same depth and collected in the same calendar
month were averaged (pink line in Fig.2b).
3. Seasonal variation correction. Monthly averages(Fig.2b) were subtracted from raw data (Fig.2a) fromthe same month. In Section 3.1, we used results inJanuary, April, July and October from this procedure
to represent spatial distributions of each parameter inwinter, spring, summer and autumn.
4. Annual variation correction. The average ofmonthly anomalies in the same year gives the annual
average anomalies (pink line in Fig.2c).
5. Smoothed averages (5-year smoothed averages)
were calculated to evaluate long-term variation ofannual average anomalies (pink line in Fig.2d).

3 RESULT
3.1 Spatial distributions of parameter values
Seasonal variations of temperature and salinity(Fig.3a–h) are almost identical to the ﬁndings ofHinata (1996). In winter (Fig.3a), water temperature

64

J. OCEANOL. LIMNOL., 36(1), 2018
N
40°

Vol. 36

a

38°

36°

34°

50 m
100 m

32°

200 m
9

30°

5
28°

1

26°

24°

Stn. 1

118°

124°
Stn. 2

Stn. 3'Stn. 4

Stn. 4'

Stn. 5

Stn. 6

Stn. 7

Stn. 8Stn. 9

Month

12
10
8
6
4
2

Month

12
10
8
6
4
2

Month

12
10
8
6
4
2

Month

b

12
10
8
6
4
2
1970

1980

1990
Year

2000

2010

120°

1970

122°

1980

1990
Year

126°

128°

130° E

Stn. 3

1970

2000

1980

1990
Year

2000

2010

2010

Fig.1 a. map of study area showing station locations (ﬁlled circles) along section PN. Contours indicate isobaths of 50, 100
and 200 m; b. timing of temperature observations over the range of years

was vertically uniform due to strong mixing from Stn.
5 to 9 on the continental shelf. A warm core
corresponding to the Kuroshio was observed in layers
shallower than 100 m from Stn. 2 to 4’. Compared tothe mean water temperature during 1972 to 1993
(Hinata, 1996), the 22°C isotherm expanded eastward
to Stn. 2. In the layer deeper than 200 m from Stn. 2

to 4, the thermocline was slightly elevated toward thecontinental shelf. In spring, the water temperature
was nearly the same as in winter in the surface layeron the continental shelf while it decreased considerably
in the bottom layer (Fig.3b). In summer, a clear
thermocline formed over the continental shelf(Fig.3c). Sea surface temperature was more than 28°C

No.1

HU et al.: Interannual variation of nutrients in Kuroshio
Raw SST data of Stn. 1

31
Temperature (°C)

65

28
25
22
a
19
1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Year
Seasonal variation

Temperature (°C)

31
Raw data
Monthly average

28
25
22
b
19

Feb.

Temperature anomalies (°C）

Temperature anomalies (°C）

Jan.

Mar.

Apr.

May

3

Jun.
Jul.
Month
Interannual variation

Aug.

Sep.

Oct.

Nov.

Dec.

2
1
0
-1
-2
-3
1970

Monthly anomalies
Annual average anomalies

c
1975

1980

1.5

1985

1990

1995
2000
Year
Interannual variation (5-year smoothed averaged)

2005

2010

2015

2010

2015

1.0
0.5
0.0
-0.5
-1.0
-1.5
1970

Annual average anomalies
5-year smoothed average

d
1975

1980

1985

1990

1995

2000

2005

Year

Fig.2 Temperature data with data processing
a. raw sea surface temperature data at Stn. 1; b. seasonal variation; c. interannual variation showing monthly anomalies; d. smoothed interannual variation
showing long-term variation.

and showed less horizontal diﬀerence (Fig.3c)
compared to in spring (Fig.3b) and winter (Fig.3a). In
autumn, water temperature became vertically uniform
in layers shallower than 50 m over the continental
shelf due to the increase in vertical mixing (Fig.3d).
As solar input decreased in autumn, sea surface
temperature decreased compared to that in summer.
Mean salinity along section PN is shown in
Fig.3e–h. Salinity over the continental shelf wasvertically uniform in winter when it exceeded 34, themaximum of the four seasons (Fig.3e). Maximum
salinity occurred in layers between 100 and 250 m,
which represent the Kuroshio subsurface water(Fig.3e). In spring, salinity on the continental shelfdecreased due to the increase of river discharge andincrease of precipitation (Fig.3f). In summer, salinity
above the halocline was much lower compared to in
winter and spring while salinity below the halocline
was almost the same as that in other seasons (Fig.3g).
In autumn, the eﬀect of Changjiang diluted water was
smaller, and salinity over the continental shelf
increased compared to that with summer (Fig.3h).
DO results (Fig.3i–l) also showed clear seasonalvariation on both the continental shelf and Kuroshio
region. DO was highest in spring on the continental
shelf, especially at the surface (more than 260 mol/L)
due to high primary production (Fig.3j). The second
highest DO levels appeared in winter because of
mixing and lower water temperature, which leads tohigher saturated dissolved oxygen (Fig.3i). In deeper

Depth (m)Depth (m)Depth (m)Depth (m)

66

0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700

J. OCEANOL. LIMNOL., 36(1), 20189

Temperature (°C)
7 6 5 4' 4 3' 3

8
32
29
26
23
20
17
14
11
8
5
2

2

1

9

8

7
35
34.8
34.6
34.4
34.2
34
33.8
33.6
33.4
33.2
33

a

Salinity
6 5 4' 4 3' 32Vol. 36

1

9DO (μmol/L)
7 6
5 4' 4 3' 3

8
280
260
240
220
200
180
160
140
120
100
80

e

2

1

i

b

f

j

c

g

k

d

h

l

Depth (m)Depth (m)Depth (m)Depth (m)Fig.3 Average values over 1973 to 2013 along section PN and depth for (a–d) temperature, (e–h) salinity, (i–l) DO in January(row 1), April (row 2), July (row 3) and October (row 4)

0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700

9

NO3ˉ (μmol/L)
7 6 5 4' 4 3' 3

8
50
45
40
35
30
25
20
15
10
5
0

a

2

1

9

8

7
2.5
2.25
2
1.75
1.5
1.25
1
0.75
0.5
0.25
0

PO43- (μmol/L)
6 5 4' 4 3' 3

e

2

1

9

AOU (μmol/L)
7 6
5 4' 4 3' 3

8
270
240
210
180
150
120
90
60
30
0
-30

2

1

i

b

f

j

c

g

k

d

h

l

Fig.4 Average values over 1973 to 2013 along section PN and depth for (a–d) nitrate, (e–h) phosphate, (i–l) AOU in January(row 1), April (row 2), July (row 3) and October (row 4)
Contour lines in a-d show nitrate concentration at 5 μmol/L intervals with the interval level reduced to 1 μmol/L when nitrate concentration is less than
5 μmol/L. Contour lines in e-h show phosphate concentration at 0.25 μmol/L intervals with the interval level reduced to 0.05 μmol/L when phosphateconcentration is less than 0.25 μmol/L.

water, DO decreases and is the lowest (<100 mol/L)
at Stn. 3’ (depth, 800 m). Similar to distributions
observed for water temperature and salinity, DO
contours increased slightly toward the continental

shelf.
For nitrate, higher concentrations were observed in
winter on the shelf region (>1 mol/L, Fig.4a). As
water temperature increases, production becomes

No.1
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HU et al.: Interannual variation of nutrients in Kuroshio

67

10 m

Standard deviation: 0.12 (at Stn. 7)–0.36 (at Stn. 6)

Stn. 1
Stn. 2
Stn. 3Stn. 3'Stn. 4
Stn. 4'
Stn. 5
Stn. 6
Stn. 7
Stn. 8Stn. 9

0.5
0.0
-0.5

a

-1.0
1.0

50 m

Standard deviation: 0.15 (at Stn. 7)–0.44 (at Stn. 9)

0.5
0.0
-0.5

b

-1.0
1.0

100 m

Standard deviation: 0.19 (at Stn. 4')–0.39 (at Stn. 3')

Temperature anomalies (°C)

0.5
0.0
-0.5

c

-1.0
0.8

200 m

Standard deviation: 0.18 (at Stn. 2)–0.25 (at Stn. 3)

0.4
0.0
-0.4

d

-0.8
0.8

500 m

Standard deviation: 0.14 (at Stn. 1)–0.18 (at Stn. 3)

0.4
0.0
-0.4

e

-0.8
0.4

800 m

Standard deviation: 0.07 (at Stn. 3')–0.10 (at Stn. 1)

0.2
0.0
-0.2

f
-0.4
1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Year

Fig.5 Interannual variations of temperature along section PN by depth
a. 10 m; b. 50 m; c. 100 m; d. 200 m; e. 500 m; f. 800 m.

higher, and nitrate concentration in the euphotic layer
decreases (Fig.4b). On the shelf bottom, high nitrateconcentration is evident closer to the continental shelfthan from the Kuroshio region. In spring, summer andautumn, the bottom water of the shelf had a higher
nitrate concentration of 5–10 mol/L, which is larger
than in the winter. In deeper water, nitrate increases
and even exceeds 10 times that in the surface layer.
Spatial distribution of phosphate was similar tothat of nitrate. Over the four seasons, shelf region(Stn. 6–9) had the highest phosphate concentration(0.25–0.5 mol/L) in winter, and shelf bottom water

had the highest phosphate concentration (0.5–0.75 mol/L) in summer and autumn.
Apparent oxygen utilization (AOU) is deﬁned as
the diﬀerence between O2' and O2, where O2' is theamount of oxygen the water would hold if it is inequilibrium with the atmosphere at the temperatureand salinity of the water and O2 is the dissolved
oxygen actually measured in the water sample. AOU
can represent how much dissolved oxygen respiration
occurred in the water. Results show that a low valueon the surface, which can be less than 0 mol/L, is a
result of primary production being greater than68

J. OCEANOL. LIMNOL., 36(1), 2018
0.40

Vol. 36

10 m

Standard deviation: 0.042 (at Stn. 3')–0.191 (at Stn.9)

Stn. 1
Stn. 2
Stn. 3Stn. 3'Stn. 4
Stn. 4'
Stn. 5
Stn. 6
Stn. 7
Stn. 8Stn. 9

0.20
0.00
-0.20

a

-0.40
0.16

50 m

Standard deviation: 0.046 (at Stn. 3')–0.068 (at Stn.2)

0.08
0.00
-0.08

b

-0.16
0.16

100 m

Standard deviation: 0.028 (at Stn. 6)–0.046 (at Stn.4)

0.08
0.00

Salinity anomalies

-0.08

c

-0.16
0.06

200 m

Standard deviation: 0.023 (at Stn. 2)–0.024 (at Stn.1)

0.03
0.00
-0.03

d

-0.06
0.06

500 m

Standard deviation: 0.012 (at Stn. 3')–0.016 (at Stn.2)

0.03
0.00
-0.03

e

-0.06
0.04

800 m

Standard deviation: 0.006 (at Stn. 3')–0.009 (at Stn.2)

0.02
0.00
-0.02

f
-0.04
1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Year

Fig.6 Interannual variations of salinity along section PN by depth
a. 10 m; b. 50 m; c. 100 m; d. 200 m; e. 500 m; f. 800 m.

consumption. There was no signiﬁcant seasonalvariation in the deep layers.
3.2 Interannual variation
Using 5-year smoothed averages for eachparameter, interannual variations of each variable
were examined at depths of 10, 50, 100, 200, 500 and
800 m in this study. Water temperature at depths of
10–200 m showed a similar pattern. Water was
relatively warm around 1975 to 1980 and 1990 to2005 while relatively cold in 1972, 1980 to 1990 andafter 2008. Water temperature on the continental shelf

showed a slightly diﬀerent pattern, especially aroundyear 1985, when water temperature at 50 m at Stn. 9reached a low value. Water temperature at 500 m wasincreasing before the late 1980s and decreasing after
the late 1980s.
The magnitude of interannual variation isrepresented by the standard deviation. Largeinterannual variation in water temperature was foundat the surface (largest value, 0.44°C at 50 m at Stn. 9)
and lowest (0.07°C) at 800 m at Stn. 3’.
Interannual variation of salinity is shown in Fig.6.
For depths shallower than 100 m, there were three

No.1
10

HU et al.: Interannual variation of nutrients in Kuroshio

69

10 m

Standard deviation: 1.3 (at Stn. 5)–2.5 (at Stn. 6)

Stn. 1
Stn. 2
Stn. 3Stn. 3'Stn. 4
Stn. 4'
Stn. 5
Stn. 6
Stn. 7
Stn. 8Stn. 9

5
0
-5

a

-10
10

50 m

Standard deviation: 1.6 (at Stn. 3')–2.9 (at Stn. 7)

5
0
-5
b
-10
10

100 m

Standard deviation: 2.0 (at Stn. 3')–3.7 (at Stn. 4)

5

DO anomalies (μmol/L)

0
-5

c

-10
10

200 m

Standard deviation: 1.5 (at Stn. 3')–3.5 (at Stn. 3)

5
0
-5
d
-10
10

500 m

Standard deviation: 3.6 (at Stn. 3')–4.5 (at Stn. 2)

5
0
-5

e

-10
6

800 m

Standard deviation: 0.9 (at Stn. 3')–2.1 (at Stn. 1)

3
0
-3
f
-6
1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Year

Fig.7 Interannual variations of DO along section PN by depth
a. 10 m; b. 50 m; c. 100 m; d. 200 m; e. 500 m; f. 800 m.

high salinity periods around 1980, 1993 and 2004.
Despite these three short high salinity periods, salinity
showed an obvious increasing trend before 1993 anda decreasing trend after 1993. At deeper layers,changes in salinity showed diﬀerent patterns: at 500
m, increasing before 1985 and decreasing afterward;
but at 800 m, a high in 1976 and a low in 1993
followed by an increasing trend. If we focus on theyears after 2005, salinity in shallow layers was lessthan that at 100 m increasing slightly, while salinity indeeper layers (more than 200 m) was decreasing.
The largest magnitude of interannual variation insalinity was found on the surface at Stn. 9 because
this area can be inﬂuenced by Changjiang diluted
water, which has a much lower salinity than Kuroshio
water. At greater depths, the magnitude of thevariation decreases and is at the smallest at 800 m.
DO in Kuroshio surface and subsurface water was
lowest around 1979, similar to the pattern of DO in
surface water in the shelf region (Fig.7). DO increased

70

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

until 1990. In the bottom water of the shelf region(50 m at Stn. 7–9), DO anomalies suddenly decreased
after 1988/1989 from +5 mol/L to -5 mol/s within
3 years, while at 50 m in Stn. 1–6, DO decreasedslightly from 1990 to end of the 1990s. DO at 0–200 m
was the highest around 2005, decreased from 2005 to2010 and increased after the 2000s. From 200 to
800 m, the same pattern of high (1972) to low (1977)
to high (1992/1993) to low (1998/1999) to high(2004) to low (2009) was observed. However, forlong-term variation, an increasing trend was observed
before 1992/1993 and decreased thereafter. The
decreasing trend after 1992 was most obvious at
500 m as a ratio was around -0.43 mol/(L∙yr).
Similar results were also found in the North Paciﬁc
Intermediate Water (NPIW) by Takatani et al. (2012)as a particularly signiﬁcant decreasing trend in DO
after the mid-1980s with a ratio of (-0.36±0.08) mol/(kg∙yr). This signiﬁcant decreasing trend of DO in the
NPIW is occurring a few years in advance of the trend
observed here.
The magnitude of interannual variation of DO was
greatest at 500 m and smallest at 800 m (Fig.7), eventhough the spatial distribution of DO showed a lower
value in deep layers than in shallow layers (Fig.3i–l).
The nitrate dataset is the shortest available. Before
1985, only Stn. 1, 4 and 9 had nitrate observations andonly twice per year. After 1996, no more observations
were made at Stn. 7 to 9. From this limited data, we
ﬁnd that the nitrate concentration before 1985 wasincreasing but decreased from 1985 to 1989. After
1989, nitrate in the surface and subsurface water was
not changing much, while nitrate at 500 m showed anincreasing trend mean of 0.124 mol/(L∙yr).
The magnitude of nitrate variation (Fig.8) wasmuch larger before 1989, and the diﬀerence inmagnitude before and after 1989 is larger at shallow
depths. At 500 m, the magnitudes before and after
1989 were similar to each other. After 1989, the nitrateconcentration was tending to increase at 500 m, andthe trend is most pronounced of the ﬁve depths.
Phosphate observation was longer and more
complete than for nitrate observation (Fig.9). Surface
phosphate was high around 1982 and 2003 and low
around 1990. After 1989, an interannual variation
pattern similar to that of nitrate was observed at 500 m
with an increasing trend mean of 0.005 1 mol/(L∙yr).
The magnitude of phosphate variation (Fig.9) was
smallest for surface layers (0.01 mol/L at Stn. 4’ 0.022 mol/L at Stn. 5) and largest at 800 m(0.072 mol/L at Stn. 3’ -0.139 mol/L at Stn. 1).

Vol. 36

AOU at 200 m decreased from 1975 to 2003 andincreased after 2004 (Fig.10). At 500 and 800 m,
AOU showed a pattern of decreasing and thenincreasing. The diﬀerence is that the lowest value at
500 m appeared around 1992, while it appeared
around 1988 at 800 m. At 800 m, AOU was evenhigher after 2010 than it was in 1975.

4 DISCUSSION
The hydrologic character along section PN section
is determined by Kuroshio surface water, Kuroshio
subsurface water, KIW, deep water, coastal water andmixed water.
Figure 11 shows the correlations between variation
at 200 m at Stn. 2 (represents Kuroshio subsurface
water) and that at other depths and stations for eachparameter. For DO, nitrate, phosphate and AOU, there
is a signiﬁcant positive correlation across almost theentire section. This suggests that DO, nitrate,
phosphate and AOU show synchronous interannual
variation in the Kuroshio subsurface water. However,water temperature in the surface and Kuroshio
subsurface water (T>14°C) shows an opposite pattern
to that in the KIW (T<14°C). Salinity variation shows
an opposite pattern between shallower than 600 m
and deeper than 600 m.
From Fig.12 we can see that interannual variationof water temperature and salinity shows a positive
relationship in the KIW and a negative relationship in
Kuroshio surface and subsurface water. This meansthat when water temperature is high, salinity at thesurface is low and salinity in the KIW is high, whichleads to lighter water on the surface. The correlation
between water temperature and the concentrations of
nitrate, phosphate and DO matches well in Kuroshio
intrusion water on the shelf slope. This is because
when the intrusion is strong, intrusion water carries
cold water with high nutrient and low DO to the shelfregion.
Trends in the long-term data sets show that nutrientsin the KIW increased and DO decreased since as early
as 1982 (Lui et al., 2014). As for interannual variation,
we see results similar to those of Lui et al., (2014) if
we set the study period from 1988 to 2010. For results
prior to 1988, DO was increasing from 1973 to 1988
and phosphate was increasing before 1983 thendecreasing from 1983 to 1988. This suggests that
long-term trends might diﬀer, depending on theselected study period and that interannual variationsof nitrate, phosphate and DO might be correlated tolong-period variations.

No.1
1.0

HU et al.: Interannual variation of nutrients in Kuroshio

71

10 m

Standard deviation: 0.05 (at Stn. 3')–0.31 (at Stn. 9)

Stn. 1
Stn. 2
Stn. 3Stn. 3'Stn. 4
Stn. 4'
Stn. 5
Stn. 6
Stn. 7
Stn. 8Stn. 9

0.5
0.0
-0.5

a

-1.0
3.0

50 m

Standard deviation: 0.05 (at Stn. 2)–1.18 (at Stn. 9)

1.5
0.0
-1.5

b

-3.0
2.0

100 m

Standard deviation: 0.12 (at Stn. 3')–0.51 (at Stn. 4)

1.0

Nitrate anomalies (μmol/L)

0.0
-1.0

c

-2.0
2.0

200 m

Standard deviation: 0.29 (at Stn. 3)–0.86 (at Stn. 4)

1.0
0.0
-1.0

d

-2.0
3.0

500 m

Standard deviation: 0.86 (at Stn. 3)–1.04 (at Stn. 3')

1.5
0.0
-1.5

e

-3.0
6.0

800 m

Standard deviation: 0.56 (at Stn. 3)–1.85 (at Stn. 1)

3.0
0.0
-3.0

f
-6.0
1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Year

Fig.8 Interannual variations of nitrate along section PN by depth
a. 10 m; b. 50 m; c. 100 m; d. 200 m; e. 500 m; f. 800 m.

Most interannual variation can be explained by PDO(Fig.13). Water temperature is negatively correlatedwith PDO (R>-0.2 with a signiﬁcant level of P<0.1) at
depths shallower than 300 m and a positively correlated(R>0.6 with a signiﬁcant level of P<0.1) at deeper
depths. As for salinity, a positive correlation (R>0.8
with a signiﬁcant level of P<0.1) was found at depths
shallower than 700 m and negative correlation was
found at depths deeper than 700 m. We can ﬁnd asigniﬁcant positive correlation between PDO andphosphate as well as DO, a signiﬁcant negative
correlation between PDO and nitrate as well as AOU in

the KIW and a portion of the bottom water of the shelfregion.
However, discharge of the Changjiang Rivercannot explain interannual variations along section
PN very well (Fig.14). The ﬁrst possible reason is that
Changjiang diluted water is light and mainly ﬂows in
the shallow layers. The second reason is that the
movement of Changjiang diluted water is mainly
controlled by wind. Bai et al. (2014) studied
summertime Changjiang River plume variation
during 1998 to 2010 and found that the plume
extension was more inﬂuenced by the southwesterly

72

J. OCEANOL. LIMNOL., 36(1), 2018
0.10

Vol. 36

10 m

Standard deviation: 0.010 (at Stn. 4')–0.022 (at Stn. 5)

Stn. 1
Stn. 2
Stn. 3Stn. 3'Stn. 4
Stn. 4'
Stn. 5
Stn. 6
Stn. 7
Stn. 8Stn. 9

0.05
0.00
-0.05

a

-0.10
0.10

50 m

Standard deviation: 0.010 (at Stn. 2)–0.033 (at Stn. 8)

0.05
0.00
-0.05

b

-0.10
0.10

100 m

Standard deviation: 0.012 (at Stn. 3')–0.029 (at Stn. 5)

0.05

Phosphate anomalies (μmol/L)

0.00
-0.05

c

-0.10
0.10

200 m

Standard deviation: 0.021 (at Stn. 3')–0.038 (at Stn. 3)

0.05
0.00
-0.05

d

-0.10
0.50

500 m

Standard deviation: 0.043 (at Stn. 3')–0.089 (at Stn. 3)

0.25
0.00
-0.25

e

-0.50
0.50

800 m

Standard deviation: 0.072 (at Stn. 3')–0.139 (at Stn. 1)

0.25
0.00
-0.25

f
-0.50
1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Year

Fig.9 Interannual variations of phosphate along section PN by depth
a. 10 m; b. 50 m; c. 100 m; d. 200 m; e. 500 m; f. 800 m.

wind during periods of smaller discharge. The
inﬂuence of Changjiang River to the area around the
PN section should be considered to be a combinationof nitrate concentration in water carried by ChangjiangRiver, discharge from the Changjiang River and wind.
From these results, we identiﬁed an interesting
phenomenon. From the late 1980s and thereafter,
water characteristics at 500 m all showed a trend
diﬀerent than that at shallow depths. Temperature,salinity and DO decreased while phosphate, nitrateand AOU increased. At 500 m to 800 m, the KIW (Luiet al., 2014) has lower temperature, salinity, DO andhigher phosphate, nitrate and AOU compared to atshallow depths. Lui et al. (2015) suggested thatenhanced vertical movement of the seawater may be acause of the increased AOU concentration but
decreased DO concentration and θ in the KIW. Timedepth distribution of salinity at Stn. 3 (Fig.15) showsthat the depth of upper boundary of KIW (S≤34.35) is
becoming shallower after the late 1980s. A possible
reason for this upward trend is the lower density of
the NPIW (Kouketsu et al., 2009).

No.1

HU et al.: Interannual variation of nutrients in Kuroshio
8.0

73

10 m

Standard deviation: 1.43 (at Stn. 2)–2.28 (at Stn. 4)

Stn. 1
Stn. 2
Stn. 3Stn. 3'Stn. 4
Stn. 4'
Stn. 5
Stn. 6
Stn. 7
Stn. 8Stn. 9

4.0
0.0
-4.0

a

-8.0
8.0

50 m

Standard deviation: 1.54 (at Stn. 3')–3.09 (at Stn. 8)

4.0
0.0
-4.0

b

-8.0
10.0

100 m

Standard deviation: 2.29 (at Stn. 4')–4.23 (at Stn. 4)

5.0

AOU anomalies (μmol/L)

0.0
-5.0

c

-10.0
8.0

200 m

Standard deviation: 1.42 (at Stn. 3')–3.33 (at Stn. 3)

4.0
0.0
-4.0

d

-8.0
14.0

500 m

Standard deviation: 4.31 (at Stn. 1)–5.14 (at Stn. 2)

7.0
0.0
-7.0

e

-14.0
8.0

800 m

Standard deviation: 1.26 (at Stn. 3')–2.48 (at Stn. 1)

4.0
0.0
-4.0

f
-8.0
1970

1975

1980

1985

1990

1995

2000

2005

2010

2015

Year

Fig.10 Interannual variations of AOU along section PN by depth
a. 10 m; b. 50 m; c. 100 m; d. 200 m; e. 500 m; f. 800 m.

5 CONCLUSION
This study describes the vertical patterns intemperature, salinity, DO, nitrate, phosphate and
AOU along a transect (section PN) that crosses theKuroshio in East China Sea. The characteristics of
seasonal variation in temperature and salinity arealmost consistent with the previous studies (Hinata,
1996). DO in shallow layers shows seasonal variation
with the highest value in spring and lowest value inautumn. Both nitrate and phosphate have a highest

value in winter on the continental shelf region.
Interannual variation of water temperatures andsalinity changed diﬀerently between the Kuroshio
surface water and KIW. In shallow layers, watertemperature showed two relatively warm periods
around 1975 to 1980 and 1990 to 2005 while otherthree periods are relatively cold. For depths shallower
than 500 m, salinity increased to the highest value
around 1993 and decreased thereafter. At 800 m, thelowest salinity was observed around 1993 and showschanges that are opposite to those at depths shallower

74

J. OCEANOL. LIMNOL., 36(1), 20189

Depth (m)

0
50
100
150
200

8

7

6

5 4' 4 3' 3

2

1

9

8

7

6

5 4' 4 3' 32

1Vol. 3698

7

6

5 4' 4 3' 3

2

1

300
400
500
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1

600
700

Depth (m)800 a. Temperature
900
0
50
100
150
200

b. Salinity

c. DO

e. PO43-

f. AOU

300
400
500
600
700
800 d. NO3ˉ900

Fig.11 Correlations coeﬃcient with a signiﬁcance level of P<0.1 (color) between a. temperature, b. salinity, c. DO, d. nitrate,e. phosphate, f. AOU at all observation points with the same variable at depth of 200 m of Stn. 2 (star)

Depth (m)

0
50
100
150
200

9

8

7

6

5 4' 4 3' 3

2

1

9

500

800 a. Salinity

Depth (m)8

7

6

5 4' 4 3' 3

2

1

6

5 4' 4 3' 3

2

1

400

700

9

7

300

600

0
50
100
150
200

8

b. DO

900

300

1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1

400
500
600
700
800 c. NO3ˉd. PO43-

e. AOU

900

Fig.12 Correlations coeﬃcient with a signiﬁcance level of P<0.1 (color) between water temperature and a. salinity, b. DO, c.
nitrate, d. phosphate, e. AOU at the same observation points

than 500 m. For nutrients and DO from 200 to 800 m,
all have a pattern of high (1972) to low (1977) tohigh (1992/1993) to low (1998/1999) to high (2004)
to low (2009). For the long-term pattern of DO, an
increasing trend was observed before 1992/1993 anddecreasing one after that. Nitrate and phosphate wereincreasing up to 1985 and decreasing from 1985 to
1989. After 1992, DO at 500 m shows a signiﬁcantdecreasing trend because of the shallower upperboundary of KIW and is possibly related to a similarchange of DO in NPIW. Conditions at the PN section
are mainly controlled by the open ocean, and a good
relationship is shown with the PDO index.
Importantly, further research on the impact ofinterannual variation to ecosystem of East China Sea
is necessary.

No.1

0
50
100
150
200

HU et al.: Interannual variation of nutrients in Kuroshio
9

8

7

6

5 4' 4 3' 3

2

1

9

8

7

6

5 4' 4 3' 3

2

1

9

8

75
7

6

5 4' 4 3' 32

1

Depth (m)

300
400
500

1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1

600
700

Depth (m)800 a. Temperature
900
0
50
100
150
200

b. Salinity

c. DO

e. PO43-

f. AOU

300
400
500
600
700
800

d. NO3ˉ

900

Fig.13 Correlations coeﬃcient with a signiﬁcance level of P<0.1 (color) between PDO index and a. temperature, b. salinity,
c. DO, d. nitrate, e. phosphate, f. AOU

Depth (m)

0
50
100
150
200

9

8

7

6

5 4' 4 3' 3

2

1

9

8

7

6

5 4' 4 3' 3

2

1

9

8

6

5 4' 4 3' 3

2

1

300
400
500
600
700
800 a. Temperatureb. Salinity

c. DO

e. PO43-

f. AOU

900
0
50
100
150
200
Depth (m)

7

300
400

1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1

500
600
700
800 d. NO3ˉ900

Fig.14 Correlations coeﬃcient with a signiﬁcance level of P<0.1 (color) between Changjiang discharge and a. temperature,b. salinity, c. DO, d. nitrate, e. phosphate, f. AOU

6 ACKNOWLEDGEMENTHU Y. Y. thanks the China Scholarship Council(CSC) for supporting her stay in Japan.
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34.7
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