Detailed monitoring of nutrient supply through tidal front in Seto Inland Sea, Japan

Transcription

1 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)

2 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.

3 Tidal front in Seto Inland Sea, Japan Several mixed region around narrow strait (Takeoka 23) Objective Nutrient supplied from Mixed region (Yamamoto et al., 2) (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.

4 Location Honsyu island Region: Period: 2 regions (Iyo-Nada & Hoyo Strait) 9 Apr. Nov., 1times Iyo-Nada Item: Temp., Sal., PO 4 -P, NO 3 +NO 2 -N, Si(OH) 4 -Si, Chl-a Fixed point Hoyo Strait Shikoku island Kyusyu island Bungo Channel R/V ISANA of CMES, Ehime Univ. LOA: 17.5m, Cruise speed: 43km/h

5 km Seasonal variation of water temperature Tidal Front Hoyo: mixed Stratified period 23 Apr 3 Jun 25 Jun 22 Jul Iyo-Nada: stratified Cold-Water-Dome 23 Aug 29 Aug 17 Sep 1 Oct Vertical Mixing period Hoyo (Mixed) 14 Oct 2 Nov Iyo (Stratified)

6 Seasonal variation of NO 3 +NO 2 -N 2 km Stratified period 23 Apr 3 Jun 25 Jun 22 Jul 23 Aug 29 Aug Nut in tidal front was moderate high. 17 Sep 1 Oct Vertical Mixing period Hoyo (Mixed) 14 Oct 2 Nov Iyo (Stratified)

7 Seasonal variation of Chl-a Forming period of Stratification Vertical Mixing period 23 Apr 23 Aug 14 Oct 2 km 3 Jun 29 Aug 2 Nov Chl-a conc. in tidal front was high. 25 Jun 17 Sep 22 Jul 1 Oct

8 Typical vertical profiles of stratified and mixed period Aug (Stratified) 2 Nov (Mixed) Tidal front occurred Medium nut. conc. Temp ( C) NO 3 +NO 2 -N (µmol L -1 ) Subsurface Chl-a Maximum (SCM) Chl-a (µg L -1 ) No Tidal front High nut. Conc. Temp ( C) NO 3 +NO 2 -N (µmol L -1 ) We consider that SCM was sustained nutrient supply from mixed region. No SCM (& relative low Chl-a conc.) Chl-a (µg L -1 )

9 Contribution of water from mixed area to tidal front Iyo-Nada Hoyo Strait Bungo Channel I-S B-S I-S B-S I-B B-B Subsurface intrusion would occur. I-B B-B (Takeoka et al., 1993) 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.

10 Temperature ( C) TS-diagram (22 July) 24 I6-m Mean of HB & HC 2 23 We defined 3 end-member at each occasion I5-6m Salinity

11 Contribution of mixed region to the stratified region Forming period of Stratification Apr 23 Aug Transition 2 kmzone 3 Jun of subsurface water 29 Aug was affected by mixed water Jun 17 Sep Jul 1 Oct Vertical Mixing period Contribution (%) 14 Oct 2 Nov

12 Relationship between nitrate conc. of mixed region and Chl-a conc. of tidal front Conc. of nitrate (µm) and Chl-a (µg/l) Chl-a conc. on tidal fronts (µg/l) Stratified period (29 Aug) NO 3 -N conc. of mixed region Stratified period NO 3 -N Max of Chl-a NO 3 +NO 2 -N (µmol L -1 ) Chl-a conc. of tidal front Maximum of Chl-a 4 y = 1.1x r 2 =.545 Chl-a (µg L -1 ) 3 Vertical mean of Chl-a y =.356x +.33 NO 2 3 -N conc. of mixed water affected on Chl-a of tidal front. r 2 = >We can estimate the Chl-a conc. during stratified period. Mean of Chl-a A M J J A S O N D Mean nitrate conc. on mixed region (µm)

13 Nitrate conc. of fixed-point (µm) NO 3 +NO 2 -N (µm) Relationship between Fixed-point observation and spatial distribution 1 8 Mean value of 3 stations around Hoyo Strait A M 29 J J A S O N D J F M 6 5 y =.719x r 2 = Fixed-point observation was representative of mixed water Nitrate con. of spatial observation (µm)

14 NO 3 +NO 2 -N (µm) Summary 1. We describe from the beginning to the end of the tidal front. 2. Tidal intrusion was occurred during stratified period. 4. Our fixed-point observation suggested the NO 3 -N conc. was representative for the mixed water Nitrate of mixed water would affect significantly the Chl-a conc. of tidal front area. 2 A M J J A S O N D J F M Fixed-point observation should be useful method to monitor the magnitude of bloom and to manage the coastal environment.

15 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.

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17 DO wt (mmol m-2 d-1) DO wt (mmol m-2 d-1) Nutrients regeneration within the Cold Water Dome DO wt (mmol m-2 d-1) NO 3 +NO 2 -N vs DO y = x r 2 = DIP vs DO y = x R 2 = DSi vs DO DIN wt (mmol m-2 d-1) y = x r 2 = DIP wt (mmol m-2 d-1) Redfield ratio (Redfield et al., 1963) O/N=17.2, O/P=276, O/Si= DSi wt (mmol m-2 d-1) Deconstruction of the organic matter from PHY Increasing Nut Conc

18 Characteristics of nutrient supply in Iyo-Nada Mixed region Sinking Decomposition 25 mmol N m -2 (29 Aug.) Primary production in autumn:.7 gc m -2 d -1 (Tada et al. JO 1998) Bloom was sustained At least 23 days Nutrients ni CWD: 16.2 gc m 2 (C/N = 6.6) 25 mmoln m 2 最高時 (8/22) の平均現存量 ( 海底より3 m)

19 Fraction of 3 groups -4m avr. (%) Chl.a -4m avr. (mg m -3 ) Seasonal variation of Phytoplankton communiy 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. Iyo-Nada Hoyo strait Bungo channel Chl.a (7Sts. -4m avr.) Chl.a (4Sts. -4m avr.) Chl.a (16Sts. -4m avr.) Jan. Apr. Jul. Oct Pico% Nano% Diatom% Jan. Apr. Jul. Oct. Jan. Apr. Jul. Oct. 25 Diatoms dominated during stratified period. Jan. Apr. Jul. Oct. Jan. Apr. Jul. Oct. Jan. Apr. Jul. Oct.

20 Early studies about tidal fronts and productivity Nutrient dynamics and plankton bloom (Holligan et al., 1984, Fever 1986, Son et al., 26) Phytoplankton activity, community structure and productivity (Maguer et al., 2, Moore et al., 23, Weston et al., 25) Bacterial abundance (Harrison & Wood 1988, Li et al., 27) Zooplankton community (Yamamoto et al., 2, Wishner et al., 26) Construction of the ecosystem model (Franks & Chen 1996, Jie et al., 28) 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.

21 Bottom A M J J A S O N D Subsurface (2 m) A M J J A S O N D Surface ( m) A M J J A S O N D 各種栄養塩類の比較 NO 3 +NO 2 -N (µm) PO 4 -P (1-1 µm) Si(OH) 4 -Si (µm)

22 Si(OH) 4 -Si (µmol L -1 ) NO 3 +NO 2 -N (µmol L -1 ) Comparison of each nutrients 7 6 y = 1.533x -.63 r 2 =.833, n = PO4-P (µmol L-1) 25 2 y =.863x r 2 =.14, n = NO3+NO2-N (µmol L-1)

23 A M J J A S O N D 海峡部から成層域 亜表層への貫入の影響 A M J J A S O N D 混合域 伊予灘 密度 密度差 -1 混合域の平均値からの差を算出 1

24 亜表層 (2 m) 底層 表層 ( m) A M J J A S O N D A M J J A S O N D A M J J A S O N D -1 混合域 伊予灘 各層における時系列変化 (Isoplot) 密度差 NO 3 +NO 2 -N (µm) Chl-a (µg/l)

25 Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Temp. CWD Seasonal changes in Iyo-Nada [NO3] Diatom% Apr. May Jun. Jul. Aug. Sept. Oct. Nov. [Chl.a] 6/16 Spr.-Sum. Low [Nut.] Low [Chl.a] Nano & Pico dom. Sum.-Fall On the CWD High [Chl.a] Nut. supply Diatom bloom [Si(OH)4] Nano-phy.% Sum.-Win. Diatom dom. Pico in surface [SS] Pico-phy.% Honsyu Iyo Shikoku Kyusyu

26 Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Temp. Seasonal changes in Hoyo strait Apr. May Jun. Jul. Aug. Sept. Oct. Nov. [Chl.a] 7/16 Spr.-Sum. Low [Nut.] Low [Chl.a] Nano & Pico dom. [NO3] Diatom% Sum.-Fall High [Chl.a] Nut. supply Diatom bloom Win. Nano & Pico increase [Si(OH)4] Nano-phy.% Low light limit. of diatom? Honsyu [SS] Pico-phy.% Hoyo Shikoku Kyusyu

27 Seasonal changes in Bungo channel Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Temp. Apr. May Jun. Jul. Aug. Sept. Oct. Nov. [Chl.a] 8/16 3 groups coexistence [NO3] [Si(OH)4] Diatom% Nano-phy.% Spr.-Sum. Low [Nut.] Low [Chl.a] Nano & Pico dom. Sum.-Fall High [Chl.a] in upper layer 3 groups coexistence Diatom in subsurface Win. 3 groups coexistence [SS] Pico-phy.% Honsyu Shikoku Kyusyu Bungo

28 Si(OH) 4 -Si (mmol m -2 ) 底部冷水塊内における栄養塩現存量の季節変化 PO 4 -P (mmol m -2 ) NO 3 +NO 2 -N (mmol m -2 ) Dissolved oxygen (mg L -1 ) 冷水塊内の栄養塩現存量を算出 8/ Si 3 P 2 1 A M J J A S O N O 1 日当たりの変化量 A M J J A S O

29 Bungo Hoyo (Mixed) 密度の季節変化 成層形成期成層期鉛直混合期 Iyo (Stratified) 4/23 6/ /25 7/22 8/23 8/29 9/17 1/1 1/14 11/2