Primary production and potential for carbon export in naturally iron-fertilized waters in the

Southern Ocean

Anne-Julie CavagnaFrank DehairsStéphanie H.M. JacquetFrédéric Planchon

Antarctic Session

Gaining information on C-sequestration efficiency using a production / export /

remineralisation toolbox: the S.O. naturally Fe-fertilized areas study-case

Natural Fe-fertilized open ocean zones in the S.O.

Constraint of blooms by circulation & topography

SeaWiFS chl-a images in October and December 1998 (from Pollard et al., 2007)

KEOPSleg 1 (Jan.-Feb. 2005)

SUMMERleg 2 (Oct.-Nov. 2011)

SPRING

CROZEXleg 1 (Nov. 2004-mid-Dec. 2004)leg 2 (mid-Dec. 2004-Jan. 2005)

SAZ-SenseJan.-Feb. 2007

SUMMER

What do we learn from comparative study of Fe-

replete / Fe-deplete areas & time series located in FeNX

sites ?

CROZEX (Spring – early Summer 2004/05)

North areaLARGE LONG EARLY BLOOM

High surface chl-ahigh productivity zone

Defined as “bloom / Fe-replete”

South areaSMALL SHORT LATE BLOOM

Low surface chl-alow productivity zone

Defined as “HNLC control / Fe-deplete”

N

S

Surface Chl a (mg m-3)

CROZEX (Spring – early Summer 2004/05)

Morris and Sanders, 2011 (GBC)

- Significant increased level of integrated PP in the N. compared to the S.

-- shallow seasonally integrated export, annually integrated deep water POC flux and core-top organic carbon accumulation enhanced 2 to 3 fold as a result of the iron-fertilized bloom (Pollard et al., 2009 - Nature)

Seasonal integrationHide shorter timescale events

CROZEX (Spring – early Summer 2004/05)

Morris et al., (2007) DSR2

234Th derived export rate:

Post-bloom EP insensitive to size of

bloom

Leg 1 Leg 2

Why similar export in high productive & low productive zone during Leg 2 ?

North = High Biomass Low Export zone ? (HBLE – Lam & Bishop, 2007 DSR II)

Miss the high export rate at bloom peak ?

New and export production are not equivalent, with this lack of equivalence being particularly pronounced in the north (Fe-replete area)

≈ 180 mgC m-2 d-1

≈ 60 mgC m-2 d-1

No N-S gradient seen once the modest bloom occurred in the south

N-S gradient

Nov. => mid-Dec. Mid-Dec.. => Jan.

The toolbox – production / export / remineralisation

NetPP(mgC m-2 d-1)

EP (mgC m-2 d-1)

MR (mgC m-2 d-1)

100 m

0 m

1000 m

Export

Export

Net primary

production

POC (µM)

POC attenuation curve

Remin.

Remin.

Remin.

Fe, nutrients, light, stratification

Based on the idea of Buesseler & Boyd L&O

(2009)

Carbon sequestration efficiency(deep carbon export relative to surface netPP)

SAZ-Sense (Summer period 2007)

Surface Chl-a (mg m-3)

AZ

PFZ

SAZ-S

SAZ-N

STZ

P1P3

STF

SAF-NSAF-S

EAC

ZC

0.0

0.4

0.8

1.2

1.6

2.0

0 25 50 75 100 125 150

integrated GPP (mmolC m-2 d-1)

chl a

sur

face

(µg

l-1)

P1 #3

P3

P1 #3

P1 #2

P2

3 repeat measurement / station in 1 week

-euphotic layer-

P2

SAZ-Sense (Summer period 2007)

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.2 0.4 0.6 0.8 1.0T600 = EP600/EP100

EP10

0/GPP

1% 5% 10% 20% 30%

P1

P3

P2

P1 = 929 ± 808 mgC m-2 d-1 => 70 mgC m-2 d-1

P2 = 424 ± 18 mgC m-2 d-1

=> 32 mgC m-2 d-1

P3 = 680 ± 96 mgC m-2 d-1

=> 5.4 mgC m-2 d-1

Export 600 m vs. export 100 m

Export

10

0 m

vs.

pro

duct

ion

P3 => High Biomass Low Sequestration system ?Stable system less efficient than versatile system for carbon export + sequestration

P1

P2

P3

KEOPS (KEOPS 1 Summer period - 2005)

A3 siteINSIDE THE BLOOMHigh surface chl-a

high productivity zoneDefined as “bloom / Fe-replete”

C11 siteOUTSIDE THE BLOOM

Low surface chl-alow productivity zone

Defined as “HNLC control / Fe-deplete”

KEOPS (KEOPS 1 Summer period - 2005)

Highly active bacterial communityOn-shelf

15.2% 28.3%

Prevalence of regenerated production and low uptake of NO3 above the Plateau proportionally low export.

Plateau surface waters operate as a High Biomass Low Export system, but since subsurface remineralisation is relatively limited there still is an important fraction of C left for deep sequestration. However overall the off-shelf system appears as the most efficient site for C sequestration

11 novembre 2011

R

E5

E3

E1 F

A3

E4EE4W

KEOPS (KEOPS 2 Spring period - 2011)

Reference station (HNLC and low Fe) : R Cluster 1 (productive sites south of PF) : A3-2 and E4W Cluster 2 (stationary permanent meander south of PF): E stations: E1, E3, E4E, E5 Cluster 3 (productive site on to north of the Polar Front): NPF

From expedition & first workshop data analysis: 3 clusters + reference station:

Courtesy from Y-H. Park (MODIS Chl-a biomass + data from surface buoy and altimetry (Nov. 2011)

Toolbox data KEOPS 1 & KEOPS 2Net PP

(mgC m-2 d-1)

132 ± 22

300

3380 ± 145

3287 ± 83

2172 ± 230

1460

578 ± 54

748 ± 103

1037 ± 130

1064 ± 126

R

C11 (summer)

NPF

E4W

A3-2 (spring)

A3 (summer)

E1 (day 0)

E3 (day 5)

E4E (day 14)

E5 (day 20)

C-export production234Th proxy

(mgC m-2 d-1)

23 ± 17

120

53 ± 07

87 ± 12

47 ± 22

250

156 ± 18

159 ± 16

On going

99 ± 11

Meso-remin.Particulate Baxs proxy

(mgC m-2 d-1)

86.3

36

41.2

65.5

17.6

28

42.0

32.3

57.4

62.0

C-sequestrationEfficiency

(mgC m-2 d-1)

To be investigated

85

16.9

32.9

21.7

222

115.6

130

On going

74.5

• 234Th derived integrated export below 100m exceeds 200m trap C-export (T. Trull pers. communic.) by 20 to 60%

0

500

1000

1500

2000

2500

3000

0 1000 2000 3000 4000

New

pro

duct

ion

/ Exp

ort pr

oduc

tion

Net primary production

New prod/Net prod

Exp.prod/Net prod.

NPF

A3-2

E4W

E4EE5E3E1

R

In accordance with CROZEX (Morris et al., 2007 – DSR2), we observe for KEOPS 2 an evidence for a decoupling of new and export production. With also the effect being most apparent in the high productive area (for CROZEX the effect was most apparent within the northern bloom area)

Toolbox data KEOPS 2

KEOPS Integrated Information

ThE

= E

P/N

etP

P

EP700/EP = 1 – MR/EP

A3-2E4W

E3 (day 5)

E1 (day 0)

E5 (day 20)

KEOPS 2 (spring period) and KEOPS 1 (summer period)

High surface chl-a sites = high production – low sequestrationMeander E & A3 site at keops 1 and 2 = highlight a seasonal cycle

A3 (K1)

C11 (K1 HNLC)

Spring

Summer

Early spring

NPFRK2 (50°S-66°E)

Eddy

A3-2

NPF

RK2 (50°S-66°E)

Eddy

A3-2

NPFMeander E

A3

C11

KEY-POINTS

Deep carbon sequestration efficiency is related to the type of production regime

Low Biomass systems (E stations at K2 in early season; C11 at K1) seem to be more efficient in terms of C-sequestration than High Biomass systems (K2: E5 cluster 1 and 3; K1: A3)** High Biomass Low Sequestration vs. Low Biomass High Sequestration

**=> Not in contradiction with Fe-replete areas exporting more than Fe-deplete areas

Example for K1: PP at C11 (Fe-deplete area / HNLC) is only 20% of PP at A3 (Fe-replete area) => C11 sequestration = 38% A3 sequestrationIs there evidence for a temporal succession from LBHS to HBLS over the

season ?

LBHS at the early stage of the productive seasonRapid transition to HBLS was ongoing for E stations, while clusters 1 and 3 were already HBLS at the start of the study=>At the end of the season HBLS conditions (A3 Keops2) returned to LBHS (A3 Keops1)

QUESTION :Do systems keep the ‘LBHS’ status during winter ? What is the strength of the biological pump in winter?

Putting the pieces together

Natural Fe-availability and enhanced surface Chl a does not always reflect enhanced integrated production and

deep carbon export

3 / Primary production & potential for carbon export 17

100 m

0 m

600 m

Export

Export

Gross primary production

POC (µM)

POC attenuation curve

different systems can have the same deep export

efficiency

remineralization

remineralization

remineralization

Fe, nutrients, light, stratification

Natural Fe-availability and enhanced surface Chl a does not always reflect enhanced integrated production and deep carbon export, especially at the end of the productive season

What do we learn from previous FeNXs inter-comparison ?9

100 m

0 m

600 m

Export

Export

Gross primary production

POC (µM)

POC attenuation curve

different systems can have the same export export efficiency

and inversely

remineralization

remineralization

remineralization

Fe, nutrients, light, stratification

End of the productive season, naturally Fe-fertilized sites seems

to function as HBLE systems => Needs further investigations

These 2 studies occurred at the end of the productive season

Key observations 12

High surface productivity in the Kerguelen Islands area is perhaps not only due to natural iron fertilization but also to vicinity with Polar Front (mesoscale frontal dynamics boost primary production- Strass et al. 2002 – DSR II)If nutrient consumption efficiency is increased by iron artificial addition, what will remain for the low latitude regions nutriently supplied by Antarctic Intermediate Water (Sarmiento et al., 2004 - Nature) ?Tamburini et al. (2009 –DSR II) demonstrate from 200 to 1500m that pressure decrease the number of prokaryotes attached to aprticles and the apparent activity of free-living prokaryotes. This helps to explain why fast sinking particles such as fecal pellets, but possibly also including fast sinking marine snow aggregates, can fall through the water column with minimal degradation.

Looking on A station, we join one of the De Brauwère et al. 2013 conclusion being that increasing analytical information throughout the duration of the bloom would strongly help to upgrade and tune modelsDeep carbon export efficiency using the proposed toolbox is an encouraging way to gain information on the biological carbon pump. The important point is to carefully take MLD and EZD into account in order to avoid dangerous misestimation.

KEOPS 2: Raw information is available to mature the 3 flux estimation needed to obtain the relative global view of studied systems

R station shows a peculiar functioning: leads to the question of winter primary production

Preliminary results. Have to be carefully validated together (depth layers).

The toolbox – production / export / remineralisation13C-assimilation (Net PP) and 15NO3 / 15NH4-uptake rates (f-ratio – New production)

Euphotic zone depth integrated parameters (7 depths measurements between 75 and 0% light attenuation)

24 h incubation experiments (daily Net PP = Gross PP + C-loss) 15N-NO3

- dilution experiment to measure nitrification in the euphotic zone

Carbon export below the surface water using ISP sampling 234Th proxy

Mesopelagic carbon remineralisation using particulate Baxs proxy

100 m

0 m

700 m

Export

Export

Net primary production

POC (µM)

POC attenuation curve

Remin.

Remin.

Remin.

Fe, nutrients, light, stratification

Baxs ICP-MS measurements Dehairs et al. (1997) DSRII algorithm to

convert Baxs content into final POC mineralization rate

(S.H.M. Jacquet – poster 361)

234Th deficit / excess depth profile measurement

C-export conversion using C/234Th ratio in particle (2 size classes at each sampling depth)

(See Savoye et al. 2008 DSR2)

KEOPS CROZEX

Isotopic model of oceanic silicon cycling: the Kerguelen Plateau case study (de Brauwere et al., in revision for DSR I)

Having additional measurement during the season would tremendously help to constrain the bloom peak and hence the rate parameters

A puzzling result of this modeling exercise is that seasonally-integrated Si-uptake flux above the plateau is lower than off the plateau while it might be expected that above the plateau more production occur due to the fertilization effect.

Natural Fe-fertilized open ocean zones in the S.O. xx

S.O. species have overcome the antagonistic iron-light relationship by increasing size rather than number of photosynthetic units under low irradiance resulting in an acclimatation strategy that does not increase their cellular iron requirement.

Planchon et al. 2013 BGH transect (summer period from South Africa to northern Weddell gyre) => same range than Exp. Prod. at KEOPS 2 R station

C-flux at 100m (SS model)(mgC m-2 d-1)

21,610,820,427,631,239,642,056,461,251,639,6

Regime of production – surface water (euphotic zone)Net PP

(mgC m-2 d-1)Exportable prod.Net PP x f-ratio

f-ratioU-NO3/(U-NH4+U-NO3)

132 ± 22

3380 ± 145

3287 ± 83

2172 ± 230

578 ± 54

748 ± 103

1037 ± 130

1064 ± 126

0.41

0.81

-

0.87

0.70

0.59

0.67

0.64

50

2738

-

1890

405

441

695

681

Euphotic Zonenitrification

R station => control HNLC with low Net PP

A3 => KEOPS 2 = 181.0 ± 19.2 mmolC m-2 d-1 (f-ratio = 0.9) EARLY SPRING KEOPS 1 = 80.6 ± 5.6 mmolC m-2 d-1 (f-ratio = 0.6) SUMMER

E stations => Effective temporal variation through 3 to 4 weeks monitoring

R

NPF

E4W

A3-2

E1 (day 0)

E3 (day 5)

E4E (day 14)

E5 (day 20)

yes

yes

no

Yes

no

yes

no

no

11

Carbon export – below the surface water (100m horizon)

Net PP(mgC m-2 d-1)

R

NPF

E4W

A3-2

E1 (day 0)

E3 (day 5)

E4E (day 14)

E5 (day 20)

132 ± 22

3380 ± 145

3287 ± 83

2172 ± 230

578 ± 54

748 ± 103

1037 ± 130

1064 ± 126

C-export production234Th proxy

(mgC m-2 d-1)

23 ± 17

53 ± 07

87 ± 12

47 ± 22

156 ± 18

159 ± 16

On going

99 ± 11

• Evidence for carbon export in pre-bloom conditions.• 234Th derived integrated export below 100m exceeds 200m trap C-export (T. Trull pers. communic.) by 20 to 60%• K2 C-export fluxes (early spring) are generally smaller than during K1 (summer)

ThE-ratio (%)EP:NetPP

17

1.6

03

02

27

21

On going

09

Euphotic layer depth (m)0.3% (0%)

PAR

116 (-)

33 (52)

42 (67)

49 (78)

80 (126)

86 (137)

42 (67)

69 (110)

Mixed layer depth (m)

107

29

57

163

64

27

70

58

=

=

<

<

>

>

<

>

12

Remineralisation – mesopelagic zone (MLD - 700m)

Net PP(mgC m-2 d-1)

R

NPF

E4W

A3-2

E1 (day 0)

E3 (day 5)

E4E (day 14)

E5 (day 20)

132 ± 22

3380 ± 145

3287 ± 83

2172 ± 230

578 ± 54

748 ± 103

1037 ± 130

1064 ± 126

C-export production234Th proxy

(mgC m-2 d-1)

23 ± 17

53 ± 07

87 ± 12

47 ± 22

156 ± 18

159 ± 16

On going

99 ± 11

Meso-remin.Particulate Baxs proxy

(mgC m-2 d-1)

86.3

41.2

65.5

17.6

42.0

32.3

57.4

62.0

Meso-remin:EP(0<value<1)

3.75

0.78

0.75

0.37

0.27

0.20

On going

0.63

R : meso-remineralization strongly exceeds C-export below the euphotic zone / mixed layer.Though same magnitude of temporal scale integration for 234Th and Baxs proxies (several weeks), EZ C-export & meso-remineralization seems to be decoupled. If ambient mesopelagic water are saturated in BaSo3, barytine cristals will not be dissolved: to be checked.

13