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Detailed monitoring of nutrient supply through tidal front in Seto Inland Sea, Japan
Tomohiro Komorita
(Prefectural University of Kumamoto)
X. Guo, N. Yoshie, H. Takeoka
(Ehime University)
N. Fujii
(Saga University)
Characteristics of Tidal Front
Tidal Front: transition zone
Medium Nutrients
Medium Light
High productivity
ISSUES: Limited number of sampling times
(almost one times per year)
–> Seasonal fluctuation of both Nutrient and
phytoplankton dynamics is largely unknown.
+
To develop management for coastal ecosystem,
appropriate investigation method is necessary.
Several mixed region around narrow strait
(Takeoka 2003)
Tidal front in Seto Inland Sea, Japan
Objective
(1)
We determined the relation between phytoplankton and nutrient
dynamics from beginning to the end of tidal front formation.
(2)
We conducted the fixed-point observation to measure the short-term
fluctuation of the nutrients concentration.
–> To propose the appropriate investigation method for tidal front.
Nutrient supplied from Mixed region
(Yamamoto et al., 2000)
Region: 2 regions
(Iyo-Nada & Hoyo Strait)
Period: ’09 Apr.–Nov., 10times
Item: Temp., Sal.,
PO4-P, NO3+NO2-N, Si(OH)4-Si,
Chl-a
R/V “ISANA” of CMES, Ehime Univ.LOA: 17.5m, Cruise speed: 43km/h
Shikoku
island
Honsyuisland
Kyusyu
island
Iyo-Nada
Bungo Channel
Location
Hoyo Strait
Fixed point
Vertical Mixing periodStratified period
Seasonal variation of water temperature
23 Apr
3 Jun
25 Jun
22 Jul
23 Aug
29 Aug
17 Sep
1 Oct
14 Oct
20 Nov
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100 20 km
Iyo
(Stratified)Hoyo
(Mixed)
De
pth
(m) 20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
0
Hoyo: mixed
Iyo-Nada: stratified
Cold-Water-Dome
Tidal Front
Vertical Mixing periodStratified period
Seasonal variation of NO3+NO2-ND
ep
th (m
)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
Iyo
(Stratified)Hoyo
(Mixed)
23 Apr
3 Jun
25 Jun
22 Jul
23 Aug
29 Aug
17 Sep
1 Oct
14 Oct
20 Nov20 km
Nut in tidal front was moderate high.
Vertical Mixing periodForming period of Stratification
Seasonal variation of Chl-aD
ep
th (m
)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
23 Apr
3 Jun
25 Jun
22 Jul
23 Aug
29 Aug
17 Sep
1 Oct
14 Oct
20 Nov20 km
Chl-a conc. in tidal front was high.
Typical vertical profiles of stratified and mixed periodD
ep
th (
m)
0
20
40
60
80
NO3+NO2-N
(µmol L-1)
Chl-a (µg L-1)
Temp (°C)
Dep
th (
m)
0
20
40
60
80
Dep
th (
m)
0
20
40
60
80
NO3+NO2-N
(µmol L-1)
Chl-a (µg L-1)
Temp (°C)
29 Aug (Stratified) 20 Nov (Mixed)
Tidal front occurred
Medium nut. conc.
Subsurface Chl-a Maximum
(SCM)
No Tidal front
High nut. Conc.
No SCM
(& relative low Chl-a conc.)
We consider that SCM was sustained nutrient supply
from mixed region.
Contribution of water from mixed area to tidal front
Iyo-Nada Hoyo Strait Bungo Channel
I-S B-S
I-B B-B
(Takeoka et al., 1993)
I-S B-S
I-BB-B
Subsurface water consisting:
only 2 end-member (surface and bottom) -> Mixing line should be a straight.
3 end-member (+mixed region) -> Mixing line should be a curve.
We can estimate the contribution of the subsurface intrusion
from mixed region by using 3 end-member mixing model.
Subsurface intrusion would occur.
TS-diagram (22 July)
18
20
22
24
32.0 32.5 33.0 33.5 34.0Salinity
Tem
pera
ture
(°C
)
24
22
I5-60m
I6-0m
Mean of
HB & HC
23
We defined 3 end-member at each occasion.
Vertical Mixing periodForming period of Stratification
Contribution of mixed region to the stratified region
Contribution (%)
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
De
pth
(m
)
0
20
40
60
80
100 0
20
40
60
80
100
23 Apr
3 Jun
25 Jun
22 Jul
23 Aug
29 Aug
17 Sep
1 Oct
14 Oct
20 Nov20 kmTransition zone of subsurface water
was affected by mixed water.
Stratified period (29 Aug)
NO3+NO2-N
(µmol L-1)
Chl-a (µg L-1)
Relationship between nitrate conc. of mixed region
and Chl-a conc. of tidal front
Vertical mean of Chl-a
y = 0.356x + 0.330
r2 = 0.455
Maximum of Chl-a
y = 1.01x - 0.585
r2 = 0.545
0
1
2
3
4
5
0 1 2 3 4 5
Mean nitrate conc. on mixed region (µM)
Ch
l-a
co
nc
. o
n t
ida
l fr
on
ts(µ
g/L
)
Stratified period
NO3-N conc. of
mixed region
Chl-a conc. of
tidal front
NO3-N conc. of mixed water affected on Chl-a of tidal front.
–>We can estimate the Chl-a conc. during stratified period.
0
1
2
3
4
5
6
A M J J A S O N DC
on
c. o
f n
itra
te (
µM
)
an
d C
hl-
a (
µg
/L)
Max of Chl-a
Mean of Chl-a
NO3-N
Relationship between Fixed-point observation and spatial
distribution
y = 0.719x + 0.743
r2 = 0.516
0
1
2
3
4
5
6
0 1 2 3 4 5 6Nitrate con. of spatial observation (µM)
Nit
rate
co
nc
. o
f fi
xe
d-p
oin
t (µ
M)
Fixed-point observation was representative of mixed water.
Mean value of 3 stations around Hoyo Strait
0
2
4
6
8
10
A M J J A S O N D J F MN
O3+
NO
2-N
(µ
M)
2009
Summary
1. We describe from the beginning to
the end of the tidal front.
2. Tidal intrusion was occurred
during stratified period.
1. Nitrate of mixed water would
affect significantly the Chl-a conc.
of tidal front area.
4. Our fixed-point observation suggested
the NO3-N conc. was representative for
the mixed water.
5. Fixed-point observation should be
useful method to monitor the
magnitude of bloom and to manage the
coastal environment.
0
2
4
6
8
10
A M J J A S O N D J F M
NO
3+
NO
2-N
(µ
M)
2009
Future work: Continuous observation for the nutrient
concentration on the mixed region
We will clarify the high resolution and long-term succession
of the coastal productivity.
Nutrients regeneration within the Cold Water Dome
y = -14.212x - 15.913
r2 = 0.872
-80
-40
0
40
80
120
-8 -6 -4 -2 0 2 4
DIN wt (mmol m-2 d-1)
DO
wt
(mm
ol m
-2 d
-1) y = -99.868x - 17.122
Ｒ2 = 0.768
-80
-40
0
40
80
120
-1 0 1
DIP wt (mmol m-2 d-1)
DO
wt
(mm
ol
m-2
d-1
)
y = -2.772x - 20.570
r2 = 0.643
-80
-40
0
40
80
120
-40 -30 -20 -10 0 10
DSi wt (mmol m-2 d-1)
DO
wt
(mm
ol m
-2 d
-1)
NO3+NO2-N vs DO DIP vs DO
DSi vs DORedfield ratio (Redfield et al., 1963)
O/N=17.2, O/P=276, O/Si= 18.4
Deconstruction of the organic matter from PHY↓
Increasing Nut Conc
Characteristics of nutrient supply in Iyo-Nada
205 mmolN m–2
最高時（8/22）の平均現存量（海底より30 m）
Primary production in autumn：0.7 gC m-2 d-1 (Tada et al. JO 1998)
Bloom was sustained At least 23 days
205 mmol N m-2
（29 Aug.）
Decomposition
Nutrients ni CWD：16.2 gC m–2 (C/N = 6.6)
Sinking
Mixed region
Seasonal variation of Phytoplankton communiy
Iyo-Nada Hoyo strait Bungo channel
Diatom%Nano%Pico%
Chl.a (7Sts. 0-40m avr.)
Jan. Apr. Jul. Oct.
4
3
2
1
0
Ch
l.a0
-40
m a
vr. (
mg
m-3
)
Season Iyo-Nada Hoyo strait Bungo channel
Spring Pico & Nano dom. Pico & Nano dom. Pico & Nano dom.
Sum.-Fall Diatom bloom Diatom bloom 3 groups coexist.
Winter Diatom dom. 3 groups coexist. 3 groups coexist.
100
75
50
0
Frac
tio
n o
f 3
gro
up
s0
-40
m a
vr. (
%)
25
Chl.a (4Sts. 0-40m avr.)
Chl.a (16Sts. 0-40m avr.)
Jan. Apr. Jul. Oct. Jan. Apr. Jul. Oct.
Jan. Apr. Jul. Oct. Jan. Apr. Jul. Oct. Jan. Apr. Jul. Oct.
Diatoms dominated during stratified period.
Early studies about tidal fronts and productivity
•Nutrient dynamics and plankton bloom(Holligan et al., 1984, Fever 1986, Son et al., 2006)
•Phytoplankton activity, community structure and productivity(Maguer et al., 2000, Moore et al., 2003, Weston et al., 2005)
•Bacterial abundance(Harrison & Wood 1988, Li et al., 2007)
•Zooplankton community(Yamamoto et al., 2000, Wishner et al., 2006)
•Construction of the ecosystem model(Franks & Chen 1996, Jie et al., 2008)
ISSUES: Limited number of sampling times
(almost one times per year)
–> Seasonal fluctuation of Nutrient and phytoplankton
dynamics is largely unknown.
+
To develop management for coastal ecosystem,
appropriate investigation method is necessary.
各種栄養塩類の比較S
urf
ac
e (
0 m
)Su
bsu
rfac
e (
20
m)
Bo
tto
m
AJ
AO
DM
JS
NA
JA
OD
MJ
SN
AJ
AO
DM
JS
NNO3+NO2-N (µM) PO4-P (10-1 µM) Si(OH)4-Si (µM)
123456 123456 510
15 0
Comparison of each nutrients
y = 10.533x - 0.603r２ = 0.833, n = 1113
0
1
2
3
4
5
6
7
0.0 0.2 0.4 0.6 0.8
PO4-P (µmol L-1)
NO
3+
NO
2-N
(µ
mo
l L
-1)
y = 0.863x + 5.302r2 = 0.140, n = 1113
0
5
10
15
20
25
0 2 4 6 8
NO3+NO2-N (µmol L-1)
Si(
OH
) 4-S
i (µ
mo
l L
-1)
海峡部から成層域・亜表層への貫入の影響
AJ
AO
D混合域 伊予灘
MJ
SN
AJ
AO
DM
JS
N -1
0
1
密度差
密度
混合域の平均値からの差を算出
各層における時系列変化(Isoplot)
密度差表
層(0
m)
亜表
層(2
0 m
)底
層NO3+NO2-N (µM) Chl-a (µg/L)
AJ
AO
D
混合域 伊予灘
MJ
SN
AJ
AO
DM
JS
NA
JA
OD
MJ
SN
-1 0 1 0123456 01234
Seasonal changes in Iyo-Nada
Shi-koku
Kyu-syu
Diatom%
[SS]
Dep
th (
m)
HonsyuIyo
[Chl.a]
[Si(OH)4]
[NO3]
CWD
Temp. Spr.-Sum.Low [Nut.]Low [Chl.a]Nano & Pico dom.
Sum.-FallOn the CWD
High [Chl.a]Nut. supply
→Diatom bloom
Sum.-Win.Diatom dom.Pico in surface
Nano-phy.%
Pico-phy.%
Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Apr. May Jun. Jul. Aug. Sept. Oct. Nov.
06/16
Seasonal changes in Hoyo strait
Shi-koku
Kyu-syu
Diatom%
[SS]
Dep
th (
m)
Honsyu
[Chl.a]
[Si(OH)4]
[NO3]
Temp. Spr.-Sum.Low [Nut.]Low [Chl.a]Nano & Pico dom.
Sum.-FallHigh [Chl.a]Nut. supply
→Diatom bloom
Win.Nano & Pico
increase
Low light limit.of diatom? Nano-phy.%
Pico-phy.%
Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Apr. May Jun. Jul. Aug. Sept. Oct. Nov.
Hoyo
07/16
Seasonal changes in Bungo channel
Shi-koku
Kyu-syu
Diatom%
[SS]
Dep
th (
m)
Honsyu
[Chl.a]
[Si(OH)4]
[NO3]
Temp. 3 groupscoexistence
Spr.-Sum.Low [Nut.]Low [Chl.a]Nano & Pico dom.
Sum.-FallHigh [Chl.a]in upper layer
3 groups coexistence
Diatom in subsurface
Win.3 groups coexistence
Nano-phy.%
Pico-phy.%
Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Apr. May Jun. Jul. Aug. Sept. Oct. Nov.
Bungo
08/16
底部冷水塊内における栄養塩現存量の季節変化
0
100
200
300
400
500
A M J J A S O
Si(
OH
) 4-S
i (m
mo
l m
-2)
0
2
4
6
8
10
12
14
16
18
PO
4-P
(m
mo
l m
-2)
0
50
100
150
200
A M J J A S O
NO
3+
NO
2-N
(m
mo
l m
-2)
0
1
2
3
4
5
6
7
8
9
Dis
so
lve
d o
xyg
en
(m
g L
-1)
8/23
冷水塊内の栄養塩現存量を算出
Si
P
N
O
1日当たりの変化量
成層形成期 成層期 鉛直混合期
密度の季節変化
4/23
6/3
6/25
7/22
8/23
8/29
9/17
10/1
10/14
11/20
De
pth
(m)
0
20
40
60
80
100
De
pth
(m
)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
De
pth
(m)
0
20
40
60
80
100
BungoIyo
(Stratified)Hoyo
(Mixed)