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)